Marine Mining Technologies

and Mitigation Techniques

A detailed analysis with respect to

the mining of specific offshore mineral commodities

July 1996

Contract No. 14-35-0001-30723

U.S. Department of the Interior

Minerals Management Service

Office of International Activities and Marine Minerals (INTERMAR)

OCS Report

MMS 95-0003
The Department of the Interior Mission

As the nations Principal conservation agency, the Department of the Interior has responsibility for

most of our nationally owned public lands and natural resources. This includes fostering sound

use of our land and water resources; protecting our fish, wildlife, and biological diversity; preserving

the environmental and cultural values of our national parks and historical places;and providing for

the enjoyment of life through outdoor recreation. The Department assesses our energy and

mineral resources and works to ensure that their development is in the best interests of all our

people by encouraging stewardship and citizen participation in their care. The Department also

has a major responsibility for American Indian reservation communities and for people who live in

island territories under U.S. administration.

The Minerals Management Service Mission

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary

responsibilities are to manage the mineral resources located on the nation's Outer Continental

Shelf (OCS), collect revenue from federal OCS and onshore Federal and Indian lands, and

distribute those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administrators the OCS competitive leasing program and oversees the safe and environmentally

sound exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The MMS Royalty Management Program meets its responsibilities by ensuring the

efficient, timely and accurate collection and disbursement of revenue from mineral leasing and

production due to Indian tribes and allottees, States and the U.S. Treasury.

The MMS strives to fulfil its responsibilities through the general guiding principles of: (1) being

responsive to the public's concerns and interests by maintaining a dialogue with all potentially

affected parties and (2) carrying out its programs with an emphasis on working to enhance the

quality of life for all Americans by lending MMS assistance and expertise to economic development

and environmental protection.
DISCLAIMER

This report has been reviewed by the Minerals Management

Service and approved for publication. Approval does not signify

that the contents necessarily reflect the views and policies of the

Service, nor does mention of trade names or commercial

products constitute endorsement or recommendation for use.
Proposed Marine Mining Technologies

and Mitigation Techniques:

A detailed analysis with respect to

the mining of specific offshore mineral commodities

Contract Report

Prepared for

U.S. Dept. of the Interior, Minerals Management Service

Prepared by

C-CORE

C-CORE Publication 96-C15

July 1996

R E

Centre for Cold Ocean Resources Engineering

Memorial University of Newfoundland

St. John's, NF, A1B 3X5, Canada

Tel. (709) 737-8354 Fax. 709-737-4706
The correct citation for this report is:

C-CORE "Proposed marine mining technologies and mitigation techniques: a detailed analysis with

respect to the mining of specific offshore mineral commodities.' Contract report for U.S. Dept. of

the Interior, Minerals Management Service. C-CORE publication 96-C 15
QUALITY CONTROL REPORT

Client: U.S. Dept. Of the Interior, Minerals Management Service

Project: Marine Mining Technologies and Mitigation Techniques

Client's Contract Ref.: 14-35-0001-30723

C-CORE Cost Centre: 340280

Document Title: Marine Mining Technologies and Mitigation Techniques: A detailed analysis with

respect to the mining of specific offshore mineral commodities

C-CORE Pub. No.: 96-C15

Prepared By: C. Woodworth-Lynas and L. Davis

Date: July, 1996

Reviewers

Document Accepted

Date Signature

Technical Accuracy /6 ...,

(

Syntax Av I.._" 2? _ . . Layout & Presentation G 17

General Evaluation

Approval for Release

Date: (/9 President & CEO
TABLE OF CONTENTS

Page

List of Tables ix

List of Figures Xiii

1.0 Introduction 1 1.1 Purpose and Objectives 1 1.2 Project Team 1 1.3 Approach and Methodology 2

1.3.1 Marine Mining Scenarios 2

1.3.2 Mitigation Analysis 4

1.4 Structure of the Report 4

2.0 Environmental Issues 6

2.1 Physical Environmental Issues 6

2.1 .1 Physical Environment Definition 6

2.1.2 Physical Environmental Effects on Marine Mining Operations 7

2.1.3 Required Physical Environmental Knowledge 8

2.2 Geochemistry 9

2.2.1 Diagenetic Zonation and Production of Hydrogen Suiphide 9

2.2.2 Potential Consequences of H2S Generation 10

2.3 Seabed Biodiversity 11

2.3.1 Extraction Risks 12

2.3.1.1 Resource Losses 12

2.3.1.2 Anoxic Sediments 12

2.3.1.3 Organism Depth Within Deposits 13

2.3.1.4 Sediment Suspension and Settlement 13

2.3.1.5 Benthic Primary Biological Production 13

2.3.1.6 Organism Exposure 13

2.3.2 Discharge Risks 13

2.3.2.1 Smothering 14

2.3.2.2 Crustacean Stocks 14

2.3.3 Benthos of Gravel and Sand Deposits 14

2.3.3.1 Formatting Benthic Data 15

2.3.3.2 Scope of Reports Reviewed 16

2.3.3.3 Kenny and Rees (1994; 1993) - Appendix 3 17

2.3.3.4 Salomon et al. (1982) - Appendix 4 17

2.3.3.5 Padan (1977) - Appendix 5 18

2.3.3.6 ENSR(1991)-Appendix6 18

2.3.3.7 Seabed Biodiversity Patterns 19

2.3.4 Biology of Marine Deposits Relevant to Dredging 20
2.3.5 Benthic Recovery After Impact 22

2.3.5.1 Recolonization 22

2.3.5.2 Tailings Deposition 22

2.3.5.3 Benthic Sensitivity to Substrate Change 23

2.3.6 Design and Analysis of Monitoring Programs 24

2.4 Biological Oceanography 25

2.4.1 Suspended Sediments/Turbidity 25

2.4.2 Release of Toxic Plumes 27

2.4.3 Monitoring 27

2.4.5 Fishery and Wildlife Resources 28

2.5 Dredge-derived Risk Factors 28

2.5.1 Turbidity 28

2.5.2 Noise 28

2.6 Biologically-derived Risk Factors for Wildlife 28

2.6.1 Territoriality and Homing 28

2.6.2 Mass Migrations 29

2.6.3 Noise 29

2.7 Quality Assurance/Quality Control 31

2.7.1 Database Quality 31

2.7.2 Sampling and Analytical Protocols 31

2.7.3 Environmental Guidelines 32

2.8 Summary of Environmental Impacts, Mitigation Objectives and Achievement Criteria

33

3.0 Mining Scenarios 34

3.1 Regional Marine Conditions for East Coast Mining Scenarios 34

3.1.1 Regional Meteorological Conditions 35

3.1.2 Regional Oceanographic Conditions 38

3.2 Scenario 1: Precious Metal Mining, Alaska - Bucket Ladder Dredge 41

3.2.1 Mining Scenario 41

3.2.1.2 Location 41

3.2.1.3 Marine Conditions 41

3.2.1.4 Geological Background 43

Dredged Material 43

Size Variations in Dredged Material 44

Distribution of Gold 44

Geotechnical Data 45

3.2.2 Dredging System 45

3.2.2.1 Design Criteria 47

3.2.2.2 Treatment Plant and Products 47

3.2.2.3 Support Facilities 47

3.2.2.4 Dredge Products and Effluents 47

3.2.3 Geochemistry 48

3.2.4 Seabed Biodiversity and Resources 49

11
3.2.4.1 Locality and Dredging Characteristics Affecting Benthos 49

3.2.4.2 Benthos Pre-dredging 50

3.2.4.3 Benthos Post-dredging 50

3.2.5 Fisheries and Wildlife Resources 51

3.2.6 Biological Oceanography 51

3.2.7 Summary of Environmental Impacts, Mitigation Objectives and Assessment

Criteria for Scenario 1 53

3.2.7.1 Environmental Impacts 53

3.2.7.2 Mitigation Objectives 53

3.2.7.3 Mitigation Achievement Criteria 53

3.3 Scenario 2: Precious Metal Mining, Alaska - Underwater Miner 54

3.3.1 Mining Scenario 55

3.3.1.1 Location 55

3.3.1.2 Marine Conditions 55

3.3.1.3 Geological Background 55

3.3.1.4 Mined Ground 56

3.3.1.5 Size Variations in Excavated Material 56

3.3.1.6 Distribution of Gold 57

3.3.2 Mining System 57

3.3.2.1 Underwater Miner 57

3.3.2.2 Deployment 57

3.3.2.3 Treatment Plant 58

3.3.2.4 Support Facilities 58

3.3.3 Operations 58

3.3.3.1 Mining Products and Effluents 60

3.3.4 Geochemistry 60

3.3.5 Seabed Biodiversity and Resources 60

3.3.5.1 Locality and Mining Characteristics Affecting Benthos 60

3.3.5.2 Benthos Pre-mining 60

3.3.5.3 Benthos Post-mining 60

3.3.5.4 Fisheries and Wildlife Resources 61

3.3.6 Biological Oceanography 61

3.3.7 Summary of Predicted Environmental Impacts, Resulting Mitigation

Objectives, and Achievement Criteria for Scenario 2 61

3.3.7.1 Predicted Environmental Impacts 61

3.3.7.2 Mitigation Objectives 61

3.3.7.3 Achievement Criteria 61

3.4 Scenario 3: Marine Aggregate Mining, Massachusetts Bay 63

3.4.1 Mining Scenario 63

3.4.1.1 Marine Conditions 63

3.4.1.2 Market-based Determination of Annual Output 65

3.4.1.3 Deposit Details 65

3.4.1.4 Daily Production Rate 66

3.4.1.5 Project Life 66

111
3.4.1.6 Dredging and Production Methods 66

3.4.2 Dredging System 67

3.4.2.1 Dredge Pumping System 67

3.4.2.2 Effluent 67

3.4.3 Dredging Cycle Times 67

3.4.3.1 Loading 67

3.4.3.2 Unloading 68

3.4.3.3 Travelling Time 68

3.4.3.4 Total Time 68

3.4.4 Number of Dredging Units Required 68

3.4.5 Costs 68

3.4.6 Possible Problems 68

3.4.7 Seabed Biodiversity and Resources 69

3.4.7.1 Locality and dredging characteristics affecting benthos 69

3.4.7.2 Benthos Pre-dredging 69

3.4.7.3 Benthos Post-dredging 70

3.4.8 Biological Oceanography 71

3.4.9 Fisheries and Wildlife Resources 71

3.4.10 Prediction of Environmental Impacts, Setting of Mitigation Objectives and

Achievement Criteria for Scenario 3 72

3.4.10.1 Prediction of Environmental Impacts 72

3.4.10.2 Mitigation Objectives 72

3.4.10.3 Achievement Criteria 72

3.5 Scenario 4: Heavy Mineral Sands, Virginia 73

3.5.1 Mining Scenario 73

3.5.1.1 Marine Conditions 73

3.5.1.2 Deposit Details 75

3.5.1.3 Deposit Thickness 75

3.5.1.4 Grade 75

3.5.1.5 Grain-Size Distribution 75

3.5.2 Dredging and Production Methods 75

3.5.2.1 Dredge 76

3.5.2.2 Wet Mill 76

3.5.2.3 Dry Miii 77

3.5.2.4 Tailings Disposal 77

3.5.3 Seabed Biodiversity and Resources 77

3.5.3.1 Locality and Dredging Characteristics Affecting Benthos 77

3.5.3.2 Benthos Pre-dredging 77

3.5.3.3 Benthos Post-dredging 78

3.5.4 Fisheries and Wildlife Resources 78

3.5.5 Biological Oceanography 78

3.5.6 Prediction of Environmental Impacts, Mitigation Objectives and Achievement

Criteria 79

3.5.6.1 Prediction of Environmental Impacts 79

iv
3.5.6.2 Mitigation Objectives 79

3.5.6.3 Achievement Criteria 79

3.6 Scenario 5: Beach Nourishment, Maryland 80

3.6.1 Mining Scenario 80

3.6.1.1 Marine Conditions 80

3.6.1.2 Dredging Area 81

3.6.1.3 Resource Specification 81

3.6.2 Dredging and Sand Transport Method 81

3.6.2.1 Cutter Suction Dredge and Pipeline 82

3.6.2.2 Trailing Suction Hopper Dredge 82

3.6.3 Effluent 82

3.6.4 Emplacement on the Beach 82

3.6.5 Costs 82

3.6.6 Other Features 83

3.6.7 Geochemistry 83

3.6.8 Seabed Biodiversity and Resources 83

3.6.8.1 Locality and Dredging Characteristics Affecting Benthos 83

3.6.8.2 Benthos Pre-dredging 83

3.6.8.3 Benthos Post-dredging 83

3.6.9 Biological Oceanography 84

3.6.10 Fisheries and Wildlife Resources 84

3.6.11 Prediction of Environmental Impact, Mitigation Objectives and Achievement

Criteria 85

3.6.11.1 Prediction of Environmental Impact 85

3.6.11.2 Mitigation Objectives 85

3.6.11.3 Achievement Criteria 85

4.0 Mitigation of the Environmental Impacts of Marine Mining 86

4.1 Environmental Effects 87

4.2 Mitigation Objectives and Techniques 89

4.3 Impact Avoidance 90

4.3.1 Location-based Constraints 91

4.3.2 Time-based Constraints 91

4.3.3 Scale-based Constraints 92

4.3.4 Offshore Treatment 92

4.4 Impact Minimization 93

4.4.1 Use of Appropriate and Best Technology 93

4.4.1 .1 Appropriate Dredging Technology 93

Bucket Ladder Dredge versus Underwater Miner 93

Stationary Dredger versus Trailing Suction Hopper Dredge 94

4.4.1.2 Excavation System 95

4.4.1.3 Noise Reduction 96

4.4.1.4 On-Shore Treatment 97

General 97

V
Aggregates 97

4.4.2 Operational Restrictions 98

4.4.2.1 Dredge Course Planning 98

Dredge Course Boundaries 98

Dredge Course Overlaps and Gaps 99

4.4.2.2 Reduction of Throughput Rate 100

4.4.3 Discharge Restrictions and Control 100

4.4.3.1 Optimization of Effluent Characteristics 101

Chemical Characteristics 101

Degassing and De-aeration 101

Dewatering and Solids Retention 101

4.4.3.2 Optimization of Effluent Discharge System 102

Discharge Design 103

Sub-surface Discharge 103

Discharge Location 103

4.4.3.3 Control of Discharged Effluent 104

Polymers 104

Silt Curtains 105

4.5 Remediation 106

4.5.1 Re-Dredging 106

4.5.2 Bed Leveller 106

4.6 Compensation: Alternative Enhancement 106

4.7 Regulatory Mechanisms 107

4.7.1 International Examples 108

4.7.1.1 United Kingdom 108

4.7.1.2 Japan 109

4.7.2 Requirements 109

4.7.2.1 Clear Jurisdiction 109

4.7.2.2 Data Availability 110

Application Review 110

Data Collection 110

4.8 Economics 112

4.8.1 Marine Mining Costs 112

4.8.2 Investment 113

4.8.3 Mitigation Costs 114

5.0 Application of Mitigation Techniques 115

5.1 Scenario 1: Precious Metal Mining, Alaska - Bucket Ladder Dredge 115

5.1.1 Background 115

5.1.2 Regulatory Authorities for the Permitting Processes 115

5.1.3 Regulatory Authorities for Environmental Monitoring 117

5.1.4 Regulatory Authorities for Post-Mining Requirements 117

5.1.5 WaterQuality 117

5.1.5.1 Tailings Plume 117

vi
5.1.5.2 Monitoring 118

5.1.5.3 Plume Modelling 119

Software 119

Predictive Capacity 120

Contributing Factors 120

5.1.6 Water Quality Mitigation 121

5.1.6.1 Discharge Control of the Jig Tailings Effluent 121

5.1.6.2 Operational Procedures 122

5.1.7. Seabed Profile and Mitigation 123

5.1.7.1 Discharge Control of the Trommel Oversize 123

5.1 .7.2 Operational Procedures 123

5.1.8 Costs of Mitigation 124

5.2 Scenario 2: Precious Metals Mining, Alaska - Underwater Miner 125

5.2.1 Background 125

5.2.2 Impact Avoidance 125

5.2.3 Appropriate Technology 125

5.2.4 Water Quality Mitigation 126

5.2.4.1 Discharge Preparation by Cycloning 126

5.2.4.2 Curtain Containment 127

5.2.4.3 Diffusion Launder 127

5.3. Scenario 3: Marine Aggregate Mining, Massachusetts Bay 128

5.3.1 Background 128

5.3.2 Best Available Technology 129

5.3.3 Seabed Change and Mitigation 129

5.3.3.1 Seabed Lithology and Mitigation 130

5.3.3.2 Seabed Profile and Mitigation 131

5.3.4 Water Quality Mitigation 131

5.3.4.1 On-board Processing 131

5.3.4.2 Hopper Overflow Operation 132

5.3.4.3 Overflow Mitigation: Operational Technique 132

5.3.4.4 Overflow Mitigation: Re-cycling 133

5.3.4.5 Overflow Mitigation: Hopper Design 133

5.3.4.6 Overflow Mitigation: Overflow Collection 134

5.3.4.7 Overflow Mitigation: Effluent Discharge 134

5.3.5 Onshore Processing 134

5.3.6 Economics 135

5.3.6.1 Operating Costs 135

5.3.6.2 Capital Costs 135

5.3.6.3 Mitigation Costs 136

5.4 Scenario 4: Heavy Mineral Sands, Virginia 137

5.4.1 Background 137

5.4.2 Impact Avoidance 137

5.4.3 Water Quality Mitigation: Cutter-Generated Turbidity 137

5.4.3.1 Operational Mitigation 138

vii
5.4.3.2 Design Mitigation 138

5.4.4 Water Quality Mitigation: Plant Effluent 139

5.4.4.1 Operational Mitigation 139

5.4.4.2 De-Sliming and Dewatering 140

5.4.4.3 Degassing 140

5.4.4.4 Effluent Discharge Design 141

5.4.4.5 Flocculants 141

5.4.5 Seabed Profile and Mitigation 142

5.4.5.1 Operational Procedures 142

5.4.5.2 Discharge Control of the Trommel Oversize 143

5.4.6 Alternative Enhancement 143

5.4.7 Economics 143

5.5 Scenario 5: Beach Nourishment, Maryland 144

5.5.1 Background 144

5.5.2 Prevention of Coastal Erosion 144

5.5.2.1 Licensing Procedures 145

5.5.2.2 Operating Restrictions 145

6.0 Summary and Recommendations 146

6.1 Summary of Marine Mining Scenarios 146

6.1.1 Precious Metal Mining, Alaska - Bucket Ladder Dredge 146

6.1.2 Precious Metal Mining, Alaska - Underwater Miner 147

6.1.3 Marine Aggregate Mining, Massachusetts Bay 148

6.1.4 Heavy Mineral Sands, Virginia 149

6.1.5 Beach Nourishment, Maryland 150

6.2 General Recommendations 150

REFERENCES 152

APPENDICES

Appendix 1 Excerpts from: Articles on the NOAAINMFS Benthic Database Tab 1

Appendix 2 Excerpts from: Hurme, A.K. and E.J. Pullen. 1988 Tab 2

Appendix 3 Excerpts from: Kenny, A.J. and H.L. Rees. 1993 and 1994 Tab 3

Appendix 4 Excerpts from: Saloman et al. 1982 Tab 4

Appendix 5 Excerpts from: Padan, J.W. 1977 Tab 5

Appendix 6 Excerpts from: Jewett et al. 199 Tab 6

vii'
Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 2.5

Table 2.6

Table 2.7

Table 2.8

Table 2.9a

Table 2.9b

Table 3.1

Table 3.2

Table 3.3

Table 3.4

Table 3.5

Table 3.6

List of Tables

ix

Page

Summary of the scope of environmental and biological data from selected

surveys of gravel and sand deposits 169

Example of variation in number of taxa from year to year 170

Time periods to recovery for benthic biodiversity 171

The Quality Assurance/Quality Control CODIS system (Continental and

Oceanographic Data Information System) 172

Environmental screening information needed for development of mitigation

procedures at offshore dredging sites 173

Likely availability of environmental screening information prior to

implementation of an environmental impact statement 174

Toxin concentration guidelines for sediment, U.S.A. and Canada 175

Example of environmental impacts, mitigation objectives and achievement

criteria for proposed mine sites 177

Weather statistics for the U.S. east coast offshore 178

Weather statistics for the U.S. east coast offshore 179

Typical seasonal ocean current velocities expected for the Virginia Beach

and Ocean City sites 180

Summary of ocean current flow for the Virginia Beach and Ocean City

sites 181

Grain sizes from drillhole program, offshore Nome 182

Silt content of dredged ground showing increase with increasing water

depth, offshore Nome 183

Typical figures of gold grade after application of a suitable cut-off grade,

offshore Nome 184

Typical operating conditions at the Nome site mined by WestGold 185
Table 3.7 Dimensions and specifications of the bucket-ladder dredge BIMA 186

Table 3.8 Support facilities for the WestGold mining operation, Nome 187

Table 3.9 The distribution of solids and water under ideal conditions, WestGold

mining operation, offshore Nome 188

Table 3.10 Dredge products and effluents, WestGold mining operation, offshore

Nome 189

Table 3.11 Dredge feed composition, WestGold mining operation, offshore Nome . . 190

Table 3.12 Environmentally significant operating parameters for the bucket ladder

dredge, BIMA, at the WestGold mining operation, offshore Nome 191

Table 3.13 Wildlife species in the Nome area and their feeding habits 193

Table 3.14 Composition of lag gravel from two different types of glacial moraine,

offshore Nome 195

Table 3.15 Specifications of the Underwater Miner, "Tramrod", used at WestGold' s

dredge site, offshore Nome 196

Table 3.16 Support facilities for WestGold' s "Tramrod" mining operation, offshore

Nome 197

Table 3.17 Operating Capabilities of the "Tramrod" Underwater Miner 198

Table 3.18 The main effluents from WestGold's "Trainrod" mining operation, offshore

Nome 199

Table 3.19 Environmentally significant operating parameters for the "Tramrod"

Underwater Miner operation, offshore Nome 200

Table 3.20 Mining parameters for the "Tramrod Underwater Miner operation,

offshore Nome 201

Table 3.21 Features of a typical ocean-going, trailing suction hopper dredge for the

Massachusetts Bay site 202

Table 3.22 Range of dredge pumping system performance for a trailing suction hopper

dredge 203

x
Estimated and possible minimum total cycle time for travel, dredging and

unloading by a suction hopper dredge, Massachusetts Bay dredge site .

. 204

Environmentally significant operating parameters for marine aggregate

mining, Massachusetts Bay 205

Dredge and dredging parameters for marine aggregate mining,

Massachusetts Bay 206

Virginia Beach current measurement sites for 1973 207

Maximum and mean current vectors, Virginia Beach current measurement

sites for 1973 208

xl

Table 3.28 Wave height exceedence probability offshore Virginia Beach between

1985-1992 209

Table 3.29 Deposit thickness, offshore Virginia Beach 210

Table 3.30 Grain size distribution of the uppermost 4 m of sediments, offshore

Virginia Beach 211

Table 3.31 Environmentally significant operating parameters for dredging heavy

mineral sands, Virginia 212

Table 3.32 Parameters for a cutter suction dredge, heavy mineral sands, Virginia . . 213

Table 3.33 Extreme storm surge and wave height estimates for Ocean City 214

Table 3.34 Average particle size for each vertical 30 cm section interval below the

seabed between drill holes, offshore Virginia Beach 215

Table 3.35 Typical cycle time expected for a suction hopper dredge operating offshore

Ocean City 216

Table 3.36 Dredge and dredging parameters for beach nourishment, Maryland 217

Table 4.1 Variables that affect sedimentation and sediment transport 218

Table 4.2 Jurisdictional guidelines for optimizing mitigation 219

Table 5.1 Legislation and agencies that had jurisdiction over WestGold's mining

operation, offshore Nome 220

Table 3.23

Table 3.24

Table 3.25

Table 3.26

Table 3.27
List of Figures

Figure 3.1(a) and (b)

Figure 3.2 (a) and (b)

Figure 3.3 (a) and (b)

Figure 3.4 (a) and (b)

Figure 3.5 (a) and (b)

Figure 3.6 (a) and (b)

Scalar mean and vector mean wind speed 231

Frequency of high winds 232

Frequency of waves above thresholds 233

Air temperature 234

Sea surface temperature 235

Frequency of precipitation and visibility < 2 nautical

miles 236

Figure 3.7 Slopewater circulation of the U.S. east coast 237

Figure 3.8 The BIMA bucket ladder dredge offshore Nome, Alaska 238

Figure 3.9 Schematic of the bucket ladder dredge BIMA 239

Figure 3.10 Plan view of dredging procedure for a bucket ladder dredge 240

Figure 3.11 Extreme bucket spillage conditions onboard the BIMA 241

Figure 3.12 Offshore bathymetry (in feet) near Nome, Alaska 242

Page

Figure 2.1 Schematic of geochemical zonation in sediments 225

Figure 2.2 Significant cluster groups of benthos in 1991, Island Copper Mine,

Canada 226

Figure 2.3 Cluster diagram for 1991 data showing similarity for stations shown in

Figure 2.2 227

Figure 2.4 Comparative mean annual water column turbidity (at 30 m water depth)

near Island Copper Mine, 1971 to 1992 228

Figure 2.5 Comparative levels of Chlorophyll a near Island Copper Mine in 1992 . . . 229

Figure 2.6 Comparative density of adult zooplankton near Island Copper Mine, 1987

to 1992 230
Figure 3.13

Figure 3.14

Figure 3.15

Figure 3.16

Figure 3.17 (a) and (b) Side view of the underwater mining vehicle "Tramrod" . . . 247

Figure 3.18

Figure 3.19

Figure 3.20

Figure 3.21

Figure 3.22

Figure 3.23

Figure 3.24

Figure 3.25

Figure 3.26

Figure 3.27

Figure 3.28

Figure 3.29

Figure 3.30

Figure 4.1

Dredge course locations for the WestGold operation, offshore Nome,

Alaska 243

Seafloor substrate types at WestGold's 1986 dredge site, offshore Nome,

Alaska 244

Three-dimensional representation of bathymetry of the area dredged in

1986 245

Turbidity monitoring sampling pattern at WestGold's dredge area, offshore

Nome, Alaska 246

Schematic of support vessel for the underwater mining vehicle

"Tramrod" 248

Support vessel for the underwater mining vehicle "Tramrod", offshore

Nome, Alaska 249

Location of the study area and proposed NOMES test mine site 250

Example of a typical trailing suction hopper dredge 251

Trailing suction hopper dredge discharging fines 252

Location of current measurement sites, offshore Virginia Beach 253

Exceedence of significant wave height immediately offshore Virginia

Beach 254

Example of a typical spud-powered cutter suction dredge 255

Bathymetry off Virginia Beach 256

Bathymetry off Ocean City 257

Beach nourishment project in progress 258

Fishing chart for offshore Ocean City 259

Aerial view of the BIMA working offshore Nome, Alaska 260

Buckets on the BIMA's bucket ladder overflowing with fine sediment 261

Xi\
Figure 4.2 Front and side view schematic of the Japanese "refresher" dredge system . 262

Figure 4.3 Marine aggregate unloading facility 263

Figure 4.4 Diagram to illustrate tailings distribution over a dredge course 264

Figure 4.5 Transverse section through a trailing suction hopper dredge showing

conceptual system for waste solids retention 265

Figure 5.1 Diagram to illustrate the physics of the tailings plume formation process 266

Figure 5.2 Above-surface discharge of tailings from the BIMA, offshore Nome,

Alaska 267

Figure 5.3 Tailings discharge configurations used on the BIMA 268

Figure 5.4 Twin tailings launders of the BIMA with surrounding rubber panels 269

Figure 5.5 Details of 1998 tailings discharge configuration for the BIMA 270

Figure 5.6 The BIMA, with tailings discharge system 271

Figure 5.7 Aerial view of the BIMA showing turbulent upwelling experienced with

the larger diameter tailings discharge pipes 272

Figure 5.8 Potential multiport discharge system configuration considered for the

BIMA 273

Figure 5.9 Alternative multiport discharge system configuration considered for the

BIMA 274

Figure 5.10 Controlled surface discharge configuration considered for the BIMA . . . 275

Figure 5.11 Example of a nomogram for predicting allowable production rates under

different operating conditions 276

Figure 5.12 Schematic diagram of the IHC anti-turbidity overflow system 277

Figure 5.13 Schematic diagram of a hopper dredge bin equipped with the Japanesedesigned Anti-Turbidity Overflow System 278

Figure 5.14 Schematic diagram showing front view of a cutter-head showing the cutter

tooth rake angle 279

xv
Figure 5.15 Schematic diagram of the Linatex separator 280

xvi
1.0 Introduction

This report was commissioned by the Minerals Management Service (MMS), U.S.

Department of Interior (DOl). The MMS is responsible for the regulation of mineral exploration and

development on submerged Federal Lands of the U.S. Outer Continental Shelf (OCS).

Environmental protection is an integral part of the mandate of the MMS, and regulatory requirements

exist for the enactment of mitigation measures during the conduct of all offshore mineral exploration

and development activities. In keeping with the requirements of the National Environmental Policy

Act NEPA) and Council of Environmental Quality (CEQ) regulations, the MMS performs

environmental analysis and assessment of proposed offshore mineral developments on a case-bycase basis. This document provides the MMS with practical guidance in prescribing mitigation

measures for future marine mining developments on the U.S. OCS.

1.1 Purpose and Objectives

The purpose of this study is to undertake a detailed analysis of available and proposed marine

mining technologies and mitigation techniques with respect to the mining of specific mineral

commodities on the U.S. Outer Continental Shelf.

The objectives of the study are:

to develop realistic scenarios for marine mining activities that are likely to occur on the U.S.

OCS within a ten-year time span

to make predictions of the environmental impacts associated with the marine mining

scenarios

to establish guidelines for mitigation of the anticipated environmental impacts

to perform cost-benefit analyses of available and proposed techniques for mitigation of the

environmental impacts of marine mining

to formulate recommendations as to the most appropriate mitigation techniques for the

scenarios and similar marine mining developments.

1.2 Project Team

To meet the above stated objectives, a unique multi-disciplinary team was formed consisting

of environmental scientists and marine mining and dredging specialists. The Project Team consisted

of four principal groups, or nodes, which included a Management node, a Mining Engineering node,

a Physical Environmental Effects node and a Biological/Chemical Environmental Effects node.

The Management node was based at the Centre for Cold Ocean Resources Engineering,

Memorial University of Newfoundland, Canada. The Mining Engineering and Mitigation node was

I
led by Richard Garnett, Ph.D., C.Eng., an international mining consultant and President of Valrik

Enterprises of Ontario, Canada. The Physical Environmental Effects node was headed by Laurie

Davidson. physical oceanographer and President of Seaborne Information Technologies Ltd., located

in St. John's, Newfoundland, Canada. The BiologicallChemical Effects node was directed by Derek

Ellis, Ph.D., R.P.Bio., professor of biology at the University of Victoria, Canada, and Principal of

Derek V. Ellis Ltd., marine environmental consultants.

This report was authored primarily by the node leaders. Supporting contributions were made

by Tom Pederson, Ph.D. (environmental chemist; University of British Columbia), Jack Littlepage,

Ph.D. (pelagic biologist; University of Victoria), Robert Jantzen (dredging engineer; Jantzen

Engineering Co., Inc.), Donald Hodgins. Ph.D. (physical oceanographer; Seaconsult Ltd.), and

Laurence Davis, M.Sc. (project manager and marine geoscientist; C-CORE).

1.3 Approach and Methodology

1.3.1 Marine Mining Scenarios

A variety of mineral commodities exists on the U.S. OCS and are potential targets for future

exploitation. These include sand resources suitable for beach nourishment, marine aggregates, heavy

minerals, precious metals, phosphorites, and others. Their exploitation will inevitably result in

environmental impacts that will vary in degree depending on factors such as the scale of operation,

the extraction technology used, and the nature of the geological and environmental setting.

This study deals with two fundamental questions. What are the likely environmental impacts

associated with mining of specific mineral commodities on the U.S. OCS? How might those

impacts be mitigated in a cost-effective manner? These questions are addressed through analysis

of a number of marine mining scenarios. They describe the scale of operation, extraction

technology, and environmental setting of marine mining operations that target a range of mineral

commodities.

The mining scenarios were developed in close consultation with the MMS, and their selection

was based on the following criteria:

Potential for near-term development:

Only mineral commodities likely to be exploited within a ten-year time span were

considered. Other mineral commodities were excluded on the basis of market and technological

constraints.

Commodity resource information:

Much of the available information pertaining to the distribution of mineral commodities on

the U.S. OCS is regional and lacks sufficient resolution to derive reserve or even resource estimates.
Offshore aggregate mining, the subject of the third scenario, is thought to be a likely prospect

in the near-term. The first markets to be served by such an industry will probably be the greaterHowever, there is sufficient detail at least to target general areas of interest (scale of kilometres) and

sometimes more specific areas.

Proximity to markets and market demand:

The proximity to the commodity market, and the market demand, are important

considerations for all potential marine mining developments. The MMS provided valuable advice

with respect to market requirements, which helped considerably in determining the most appropriate

site-selections for the mining scenarios.

Availability of environmental data:

A final but secondary consideration in selecting the marine mining scenarios was the

availability of local environmental data. Where environmental data were sparse or lacking, it was

possible to make appropriate generalizations about distributions of biological assemblages and about

likely ecological responses based on regional information and knowledge of the habitat.

A total of five marine mining scenarios were selected and developed in detail. These include:

Precious metal mining, Alaska - bucket ladder dredge

Precious metal mining, Alaska - underwater miner

Marine aggregate mining, Massachusetts Bay

Heavy mineral mining, offshore Virginia

Beach nourishment, Ocean City, Maryland.

The first scenario is a case history of a large-scale placer gold mining operation offshore

Nome, Alaska. carried out by Western Gold Exploration and Mining Company (WestGold) Limited

Partnership during the late 1980's. While the scale of operation and use of a bucket ladder dredge

are not likely to be repeated in any future offshore mining activities in the region, the scenario is

highly instructive in that it documents real experience with the practice of marine mining

environmental impact mitigation. It is also the only large-scale marine mining development to occur

in U.S. waters to date.

The second scenario, which involves precious metals mining using an underwater mining

vehicle, is based on actual non-production trials carried out towards the end of the WestGold project

life. The two mining methods are at opposite ends of the spectrum in terms of scale of operation and

precision of extraction. They provide a useful comparison of the effects of using different extraction

techniques to mine the same mineral commodity.
metropolitan areas of Boston and New York. Recognizing that marine aggregate resources in the

region are potentially widespread, the choice of a location in Massachusetts Bay for a hypothetical

aggregate mining operation was based mainly on the availability of environmental information. A

comprehensive baseline environmental study was carried out in the early 1970's in preparation for

an experimental offshore aggregate mining project. The experiment did not proceed, but the baseline

environmental information gathered serves as a useful resource for this study.

Although the baseline study also provides what is still perhaps the most complete sitespecific assessment of aggregate resources in the region, it reveals that the geology of the site under

consideration is not particularly conducive to economic dredging operation. The sediments are

poorly sorted and contain, on average, higher proportions of silt and clay than are normally desirable

for an aggregate target. Therefore, the hypothetical marine aggregate mining operation presented

in this report represents a worst case scenario both environmentally and economically. As a

marginal operation the aggregate mining scenario is a sensitive monitor of the economic costs of

mitigation.

The fourth scenario, again hypothetical, involves dredging of heavy mineral sands in Federal

waters off Virginia Beach, Virginia. The site selection for this scenario is based mainly on

commodity resource information. Concentrations of industrial heavy minerals are known to exist

offshore Virginia as well-sorted placer deposits. The geology and environmental setting are thought

to be representative of most areas on the U.S. OCS where heavy minerals could be targeted. Any

proposal for offshore mining development in the area will have to address potential conflicts with

other uses, including commercial fishing, ocean dumping, and military practice.

The fifth scenario considers the use of sand resources in Federal waters (three nautical miles

seaward of the coastline) for beach nourishment. The site selected for the scenario, Ocean City,

Maryland, has undergone repeated coastal restoration projects, the most recent was in the fall of

1994. Borrow sites have moved progressively offshore and the need for sand resources from Federal

waters appears imminent.

1.3.2 Mitigation Analysis

As stated in Section 1.1 above, the ultimate objective of this study is to develop

recommendations as to the most appropriate techniques for mitigation of the environmental impacts

associated with mining of specific mineral commodities on the U.S. OCS. The recommendations

are derived from analyses of a broad range of available and proposed mitigation measures with

consideration given to their relative cost and effectiveness. The selection of mitigation techniques

for analysis was based upon the environmental impacts and mitigation objectives identified for each

of the mining scenarios.

1.4 Structure of the Report

Section 2, Environmental Issues, provides a detailed overview of the environmental impacts

of marine mining with special emphasis on key issues relevant to the marine mining scenarios.

4
Issues relating to modelling, and to environmental monitoring and quality assurance/quality control

(QAIQC) protocols. are also introduced and discussed in the context of environmental impact

mitigation.

The marine mining scenarios are presented in Section 3. Details are provided of their

environmental settings, the commodity resource characteristics, and the extraction and processing

methods. Predictions of the potential environmental impacts of each extraction technology are

provided along with guidelines for mitigation of those impacts.

In Section 4, techniques for mitigation of the environmental impacts of marine mining are

reviewed and in Section 5 these are applied in detail to the marine mining scenarios.

Recommendations are made as to the most appropriate selection of mitigation measures for each

mining scenario. Conclusions and final recommendations are presented in Section 6

D
2.0 Environmental Issues

This section provides an overview of the environmental issues associated with marine

mining. Physical, chemical and biological issues are addressed, with particular emphasis placed on

the ultimate biological impacts of marine mining and how they can be recognized and monitored.

The Mining Scenarios (Section 3) provide examples of the application of the issues to specific sites.

The final section (2.7) is a summary of the environmental information presented in the

context of mitigation. Section 2.7 presents a framework for application of the issues to the mining

scenarios. The framework consists of:

predicting environmental impact

setting mitigation objectives

providing criteria for monitoring achievement of the mitigation objectives.

2.1 Physical Environmental Issues

There are two key physical environmental issues: (1) defining the processes and factors that

are relevant to marine mining methods, potential impacts, and mitigative technologies, and (2)

understanding how these physical processes and factors are connected to potential negative

environmental impacts from mining operations, and how these impacts might be mitigated.

Solutions to these issues are best achieved by (1) establishing a working definition of the

term "physical environment", in contrast with associated terms which at least include "geochemical

environment", "biological environment", and "mining operation"; (2) identifying the two distinct

ways in which physical marine processes affect an offshore mining operation, and (3) considering

the knowledge of the physical environment which may be required to successfully mitigate the

potential environmental impacts of marine mining operations. The following text individually

addresses these three matters.

2.1.1 Physical Environment Definition

For the purposes of this discussion, physical environmental processes are deemed to be those

processes that determine the characteristics and motion of the oceanic water column, and affect its

interaction with its top and bottom boundaries (the atmosphere and the seabed respectively).

Included in this definition are all issues associated with the interaction of the atmosphere and the

ocean, most particularly heat input, precipitation, and energy input from the wind. Floating ice is

considered to be a component of the water column, and hence is incorporated in the spectrum of

physical environmental issues. In addition to spatial and temporal distributions of temperature,

salinity and density, other true water column phenomena of interest include currents (at all length

and time scales), surface waves, internal waves, and changes in water level elevation due to tide,

6
surge, etc. Particularly important is the fact that the turbidity of the water column is considered to

be an aspect of physical environmental considerations.

Excluded from this definition of physical environmental processes and factors are all aspects

of the seabed, including the physical characteristics and particle properties of the mobile sediment

layer (considered as geological issues), all living organisms in the water column or on the seabed

(considered as biological issues), and all issues associated with the man-induced introduction of

materials into the water column from mining activities (considered as mining operation issues).

2.1.2 Physical Environmental Effects on Marine Mining Operations

The physical environmental processes and factors, discussed above, affect marine mining

operations in two ways. Firstly, these processes and factors constrain or control the actual conduct

of the mining operation, and thus are significant in determining the potential environmental impacts

of the operation. Secondly, physical processes govern water column movements which redistribute

particulates returned to the sea by the mining operation, thus also influencing potential impacts.

The first issue is the direct effect of the physical environment on the mining activity. Those

principal factors exercising control over mining operations may vary in importance with the mode

of mining, but in most instances include wind, waves, currents and ice. Temperature, precipitation

and water level considerations are usually of secondary interest, except possibly in locations where

unusually high tides or tidal ranges might impose operating constraints. Seasonal as well as short

time scale changes in these conditions combine to determine both the annual operating season for

a mining activity, and dictate the viability of particular operations or activities on a particular day.

It is generally true that as these physical factors increase in severity (higher winds, higher

waves, stronger currents, more and thicker ice, lower temperatures, etc.), the viability of mining

operations decreases. As with most other industrial activities at sea, there are normally well-defined

thresholds for wind, wave, current, and ice beyond which certain mining activities are required to

be suspended. With respect to the extraction, transport and processing aspects of the mining

operation, these may be considered to represent a self-protecting or self-mitigating situation: as the

environmental factors intensify and increase the risk of disruption to the mining operation, the

operation is forced to respond by reducing its scale or level of activity.

A directly opposite circumstance generally pertains to any physical activities employed at

the mine site to mitigate the zone of impact of particulates transported through the water column.

In this instance, it is generally true that as the severity of the environmental factors increases, the

difficulty in maintaining the mitigative measures also increases. For example, one potential

mitigative technique might be to release slurry from a dredge via pipes extending down into the

water column to introduce the effluent nearer the seabed than would be achieved from a surface

release. The intent of such a technique would be to limit the horizontal zone of influence of

particulates by minimizing the time during descent that the particulates are subjected to the advective

effects of horizontal currents. The structural integrity of such an effluent disposal system would be
increasingly threatened with increasing severity of environmental conditions. Hence, maintaining

the effectiveness of the mitigative technique would similarly be threatened.

The principal sources of potential environmental impact from mining operations are physical

disruption of the seabed, the deposition of disturbed andlor mined materials over undisturbed

portions of the seabed, and the dispersion of particulate material through the water column. Another

issue is the role of physical processes in determining the distribution and motion of this particulate

load through the water column, the timing and location of the deposition of this load, and the

possible resuspension and redeposition of unconsolidated materials after initial deposition on the sea

floor. Horizontal currents in the water column are the critical factors in sediment redistribution. The

magnitude and variability of horizontal currents, and the grain size distribution of particulates (and

particulate cohesive properties), determine both the residence time in the water column, and the

range of horizontal transport of the particles.

Secondary physical issues are stratification (as defined by temperature, salinity, and hence

density distributions) and waves. Strong stratification can inhibit the vertical motion of settling

particulates, and hence can result in such matter being transported greater distances from source than

would be the case in the absence of stratification. This effect is more likely to be important in deep

than in shallow water. Surface waves can be the source of turbulence which increases the dispersion

of suspended sediments. As well, waves interacting with the seabed can cause resuspension of

deposited sediments, and transport them away from their source. Unlike the first issue described

above (impact of the physical environment on the mining operation), it is not possible to generalize

about the positive or negative consequences of increased horizontal dispersion of sediments.

Increased dispersion to minimize the concentration of deposited sediment at any one location may

be most desirable in some scenarios, depending on such factors as chemical, sedimentary and

biological regimes. In other cases, minimizing the zone of influence may be the priority. In yet

others, broad dispersion in some directions may be desirable, if the key issue is the protection of

sensitive resources located in other directions from the sediment source. Clearly, an understanding

of the physical transport mechanisms is not sufficient to determine the nature or severity of

environmental impacts. The key point is that physical processes do not dictate impact. Rather,

physical processes are transport mechanisms. The consequences of the transport of suspended

material determine environmental impacts. Hence, understanding physical processes is only one of

a necessary series of steps required to quantify the environmental impact of mining operations.

2.1.3 Required Physical Environmental Knowledge

The task of predicting environmental impacts from marine mining operations requires

knowledge of physical environmental processes and factors. The specific parameters, time and

length scales of variability of prime interest are dictated by the specific details of that circumstance.

Such specific details are almost always associated with biological, chemical, andlor mining

operations issues. Direct measurements and numerical modelling are the two available methods of

acquiring the requisite physical environmental knowledge.

8
9

2.2 Geochemistry

2.2.1 Diagenetic Zonation and Production of Hydrogen Suiphide

Marine organic matter in seabed deposits fuels a suite of bacterially-mediated postdepositional (diagenetic) reactions in which the organic material is progressively degraded as

oxidants are consumed (Shimmield and Pedersen, 1990; Perry, 1995). The reactions yield a number

of products which may have direct or indirect environmental implications, and these are briefly

reviewed in this section.

Organic matter represents a highly reduced form of carbon which is rich in electrons and is

therefore a primary energy source. Over most of the seabed, marine organic materials are degraded

by aerobic bacteria, wherein oxygen is the terminal electron acceptor in the reaction CH2O + 02 =

CO, + H2 0. Where oxygen is absent, the bacterial community takes advantage of other electron

acceptors, which (in thermodynamic order) include nitrate, manganese and iron oxides and sulphate.

Thus, aerobes are succeeded by nitrate-reducing bacteria, and later by sulphate-reducing bacteria as

long as degradable organic matter remains. This suite of reactions leads to a biogeochemical

zonation in sediments, as shown in Figure 2.1 (modified from Pedersen and Pelletier, 1989, and

Poling, 1995).

Anoxia results where oxygen supply via advection or diffusion is exceeded by demand of

aerobes and by inorganic consumption of oxygen by a reaction with reduced species such as Fe2.

Anoxic conditions at relatively shallow depths (a few centimetres or less) are typical in most coastal

and inner continental shelf sediments. The depth to the oxic-anoxic boundary is highly variable.

It depends on:

The rate of sedimentation, which physically influences the diffusive influx of oxygen into

the deposits

the marine organic matter accumulation rate (i.e. the rate of accumulation of "oxidant

demand")

the extent of bioturbation. which can physically introduce oxygen to the deposits by mixing

in oxygenated seawater

the bottom water oxygen content, which can vary between near saturation and zero,

depending on the residence time of the water at a given site and the length of time since the

water was exposed to the atmosphere.

Of the many products that the oxidation reactions yield, the most important from an environmental

perspective is hydrogen suiphide, which is produced during sulphate reduction. The depth at which

sulphate reduction is first encountered in marine sediments ranges from as much as 2 to 3 m in some

continental shelf deposits to as little as a few millimetres in many nearshore organic-rich sediments.
A review of mine tailings production and their subsequent chemical stability in the marine

environment is given by Poling (1995). Anoxia stabilizes most heavy metal compounds; hence, the

depth at which pore waters become anoxic is an important environmental parameter at proposed

dredging sites.

2.2.2 Potential Consequences of H2S Generation

Two effects of H2S production warrant mention. First, hydrogen suiphide is highly toxic.

In nearshore settings where the rate of deposition of organic matter is high as a result of high primary

productivity or anthropogenic nutrient or organic inputs (e.g. sewage or pulp mill effluent, nutrientrich runoff from agricultural sources), several millimoles per litre of dissolved H2S species can be

present in the interstitial waters within the upper metre. The hydrogen sulphide will quickly de-gas

to the atmosphere if such organic-rich sediments are brought to the surface during dredging

operations, and this could pose a significant safety hazard to humans.

Organic matter has a low density, and organic particles tend to behave hydraulically as if they

were fine-grained. Invariably, relatively coarse-grained dredging targets such as sands and gravels

are impoverished in organic materials and are associated with a low oxidant demand. Sulphate

reduction would not be profound in such settings: hazardous concentrations of H,S would therefore

not be expected.

The second consideration is that dissolved H2S species react readily with a number of metals,

producing sulphide phases which are relatively insoluble as long as the deposits remain anoxic. For

example, the dissolved iron produced in the iron oxide reduction zone (see Fig. 2.1) reacts with

upward-diffusing HS- to produce FeS andlor FeS2 (pyrite). Similarly, molybdenum reacts to produce

MoS2 (molybdenite), arsenic to produce FeAsS2 (arsenopyrite), copper to produce CuS (covellite),

and so on. In extreme cases where sulphate reduction occurs very near the sediment-water interface,

metals such as molybdenum, which is relatively common in seawater, can diffuse into the bottom

to be fixed as the metal suiphide. This can lead to a natural enrichment of molybdenum and other

metals which behave in a geochemically analogous way.

Anoxic sediments are naturally enriched in metal sulphides. If these sediments are exposed

to oxygenated water or the atmosphere, the potential exists for at least partial oxidation of the

sulphides and consequent release to solution of the formerly-reduced bound metals. Molybdenum

is known to be released under such circumstances (Shaw et al. 1990), as are uranium (which is

frequently enriched in sediments as UO2 [uraninite]) and arsenic (Shaw et al. 1990 and 1994).

However, oxidation of the reduced species is not instantaneous, and the concentrations in the solid

phase are usually low (up to several parts per million for uranium, and up to several tens of ppm for

molybdenum and arsenic in extreme cases). The result is that the quantum of metals released to

solution during oxidation is small. Deleterious environmental impacts would not usually be

expected under such circumstances. In addition, both molybdenum and uranium occur at relatively

high concentrations naturally in seawater as the refractory species MoO42 and [UQ (CQ

respectively; coastal waters are therefore not environmentally sensitive to this pair of metals.

10

The discussion above applies only marginally to coarse marine sediments because these are

rarely anoxic to the extent that metals have become naturally enriched. Hence, the potential to

produce negative chemical impacts as a result of dredging aggregate or placer deposits is low to very

low. This conclusion is borne out by evaluation of monitoring data accumulated during the

WestGold placer gold mining operation near Nome, Alaska, (summarized in Section 3.2.3).

Nevertheless it is appropriate to consider the potential for these effects site-specifically by

appropriate measures, and the relevance of applicable sediment quality guidelines (Section 2.6).

2.3 Seabed Biodiversity

Benthic (seabed) organisms are the base feedstock for fishery and wildlife resources such as

seabed feeding fish, diving waterfowl and marine mammals, and also contribute to the larval

biodiversity of the water column. Their populations allow calculations of biodiversity, and

biodiversity can be used as a yardstick for gauging environmental impact.

Biodiversity is measured by: (1) how many different kinds of organisms are present, and (2)

the relative numbers of each (i.e. which are common, which are rare). These are Species diversity

and Numerical diversity respectively.

Organisms should be identified to species where possible, because species as biological units

are the base measure for biodiversity calculations. At most sites there is some uncertainty about the

species identity of many forms present, but separation to species can still be achieved by identifying

to a higher order taxonomic level, say to genus or family. Thus, although the biodiversity unit is not

precisely named, it is measured as one unit for biodiversity calculations.

Detailed lists of species present at any one time are summarised where relevant because,

according to Thorson's (1957) theory of Parallel Level-Bottom Communities, such lists have some

predictive value. It should be possible to predict the kinds of organisms which will recur on similar

deposits. It is very noticeable that identical genera tend to occur on similar habitats, whereas the

actual species within the genus differs from site to site.

At present there is no numerical model which does more than predict biodiversity in a

generalised way. Poiner & Kennedy (1984) show, and develop a model for, increases in total

biomass (weights of organisms) in the far field of a dredging plume. Biomass is related to numerical

abundance in a general way, but not in detail, because the large individuals of a species tend to be

more rare than the small younger individuals.

The benthos of coarse-grained marine placer and aggregate deposits is not well known

because conventional grab samplers (Holme and McIntyre, 1984), used on softer mud and fine sand

to sample the burrowing infauna, do not penetrate well in gravel, or even in coarse sand. Dredges

and trawis sampling the mobile epifauna hang up on rocks, bounce on the surface if towed too fast,

and frighten away the fastest moving fish and shellfish. Such dredges may sample the attached

epifauna, providing they can dislodge the attached organisms, or pick up whole rocks and cobbles

large enough to support growth.

11
There is little information about the biology of gravel and coarse sand deposits; therefore,

there are no up-to-date reviews on which to base a generalised state-of-the-ecosystem statement.

There are, however, some common biological features of marine deposits that apply regardless of

grade (see Section 2.3.4). There are also some recent surveys that demonstrate the biodiversity of

these deposits in a mining context (see Section 2.3.3). A selection of articles is reviewed. These

sections lead to the issue of monitoring sampling design and analysis to document benthic

biodiversity (Section 2.3.6).

Regardless of the grade of deposit extracted, there are two sets of risks arising from dredging

operations: (1) extraction risks (i.e. removal of habitat material with some surrounding disturbance),

and (2) discharge risks (i.e. deposition of discharged tailings). These are introduced in Sections

2.3.1 and 2.3.2 respectively. This review does not consider impacts derived from dredging for

removal of contaminated deposits, nor impacts from the mining of metalliferous muds.

2.3.1 Extraction Risks

2.3.1.1 Resource Losses

Extraction removes sediment habitat and the organisms living within the sediment (infauna),

or on it (epifauna). The burrowing forms are relatively immobile and cannot swim away from the

cutting face of the mining excavation. These benthic organisms include fish and prawn resource

species that maintain burrows, and many species of smaller creatures which can function as

feedstock for resource species.

Epifauna come in two forms. The mobile epifauna generally live on the surface of the

deposits. They may be able to remove themselves from the immediate dredge area. Examples

include flatfish, crabs, sea cucumbers and sea urchins (almost immobile), scallops, and other

resource species. The other epifaunal form is sessile, and is attached to the large cobbles, boulders,

or any rock outcrops. They are generally attached so firmly that they are only removed if their host

substrate is removed. In general, seabed mining will have an impact on resource species, their

feedstock and habitat.

2.3.1.2 Anoxic Sediments

The sediment habitat characteristically is aerated close to the surface. Most organisms live

within the surface aerated layer. However, oxygen reduces with sediment depth depending on

porosity and permeability. Coarse, well-sorted sands and gravels have the thickest well oxygenated

layer, and silts and clays the thinnest. The consequence to bio-diversity of reducing oxygen within

the sediment is that most organisms are competing for space within a shallow aerated zone. Some,

usually the larger forms, can live in deeper protective burrows within the reduced-oxygen (even

anoxic) zone provided they maintain contact with an oxygen and food supply. In general, minerals

dredging is not targeted at low oxygen, fine-grained sediments. However, patches of fine silts and

clays may occur in a dredging area. If they are detectable they should be avoided for both economic

and environmental reasons (such as the release of anoxic or toxic pore water).

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2.3.1.3 Organism Depth Within Deposits

In minerals dredging, coarse sediments are to be expected, hence there is likely to be a deep

aerated layer of probably 1 m or more. Constraints on depth to which organisms can live within

these deposits are related to their abilities to burrow and maintain contact with the surface. Hence

larger organisms tend to live at deeper levels than small organisms. The larger forms tend to be

resource species that may live at depths of 1 to 2 m within the sediment. Conventionally, infaunal

biologists accept 10 cm of seabed penetration as an effective sampling depth, with little

documentation as to whether retrieval to this depth collects all specimens or not. Large deep-living

forms present a further concern in that they may be long-living, slow-growing forms whose biomass

may have taken 100 or more years to form. Once these forms, such as horse clams and geoducks,

are lost it may take decades to reestablish a potentially sustainable fishery. The loss may even be

permanent.

2.3.1.4 Sediment Suspension and Settlement

Extraction involves physical disturbance of the seabed at the dredgehead due to cutting,

and/or suction, and removal. This inevitably involves suspension of sediments, causing raised

turbidity over the seabed with a plume drifting with tidal currents. The resuspended sediments

resettle nearby or at some distance depending on current velocities and particle size. Resettlement

can be in areas of similar or dissimilar deposits (see Section 2.3.2 Discharge Risks, for further

concerns arising from deposition).

2.3.1.5 Benthic Primary Biological Production

In very shallow water, for example 5 to 10 m depth, dredging-induced turbidity may reduce

primary biological production of plant growth downcurrent of the dredged area

2.3.1.6 Organism Exposure

The cutting action of seabed mining will expose immobile organisms to predators, and

exposed injured or dead organisms attract scavengers and predators. A disturbed area can become

temporarily more active biologically, and more attractive for fishing but at unsustainable levels.

2.3.2 Discharge Risks

A discharged tailings slurry falls through the water column, with some dispersion of fines

(also see Section 2.4 Biological Oceanography), and eventually reaches the seabed as a density

current. After initial impact, the tailings spread in some pattern as a seabed dispersal field,

deposition occurring with coarser particles settling first.

13
2.3.2.1 Smothering

Large amounts of tailings may smother and kill organisms over a broad area. Deposition of

the tailings may occur more rapidly than the inhabiting organisms can avoid, or too fast for the

inhabitants to maintain contact with life-giving aerated water, or their feedstock, above or around

them. Smothering kills benthic organisms by preventing them from breathing or feeding. In general

the ability of benthos to avoid smothering, or to burrow up faster than smothering deposits can settle,

is not known. At Island Copper Mine shallow water fiord benthos on silts and clays can tolerate an

accumulation rate of tailings of about 1 cm a year (Ellis and Hoover, 1990). The data do not indicate

the highest deposition rate that can be tolerated (but see Section 2.3.5.2). During tailings discharge

there is an area within the seabed depositional field where benthos cannot tolerate rapid deposition,

and are smothered, or their survival is affected in some other way. This area represents only a part

of the region where deposited tailings can be identified.

2.3.2.2 Crustacean Stocks

Crabs, lobsters and other epifaunal omnivorous forms may be able to maintain themselves

during the tailings discharge phase. Although the reasons for this are not well established, it may

be that damaged feed organisms are available for scavenging. There can be no guarantee that the

maintenance of crab fisheries, as at Island Copper Mine and the WestGold marine placer operation,

will occur at other dredging locations.

2.3.3 Benthos of Gravel and Sand Deposits

This section is a review of selected gravel and sand benthic investigations. The objective is

to draw some generalised conclusions about the biology of such deposits, and changes to be expected

from dredging operations. The selection of articles for review is based on an expected pattern of

biology and impact. The selection includes well-sorted sands and less well-sorted coarse sands and

gravels.

Sands have a well known characteristic fauna (Thorson, 1957), clearly different from muds

and, apparently, from little-investigated gravels. A well-sorted sand (as needed for beach

nourishment) implies high porosity, good aeration, and little anoxic geochemistry to decompose

constituents to toxic components (see Section 2.2). Gravels for aggregate use are more coarse than

sandsper se (although they may be still formally classified as "coarse sands" if the median grain size

is <2mm). Dredged aggregates have to meet market grade requirements, so the best deposits tend

to be well-sorted, and hence well aerated and with little geochemical implications for environmental

mitigation. However, placer deposits may be less well-sorted, in which case anoxia and geochemical

considerations can become important.

Table 2.1 provides summary ecosystem data from four selected investigations of gravel and

sand deposits. These deposits are where dredging operations have been conducted or were planned.

The parameters shown have been selected to indicate important components of the habitat but do not

14
include other relevant and normally recorded oceanographic parameters such as water temperature,

salinity and dissolved oxygen.

2.3.3.1 Formatting Benthic Data

The form of the Summary Table 2.2 needs some explanation. Survey reports are listed by

author (Colunm 1), and a location descriptor is given in Column 2. The environmental data in

Columns 3 to 5 provide descriptor terms and the objective substrate characteristics documented, such

as particle size data and relevant geochemical parameters.

The generalised terms "gravel" and "sand" (Colunm 3) are useful indicators of deposit grade,

but there is considerable variation in their use. Particle size data (Colunm 4) are more explicit. For

particle size it helps to follow convention by including an average statistic such as the median, and

also a sorting coefficient. The latter shows whether the deposit is well- or poorly-sorted (see above).

Poorly-sorted deposits with a geochemically active mud fraction are very different ecologically from

well-sorted deposits with high porosity and good aeration. In addition, if there are cobbles and

boulders present, there will be more attached sessile organisms. These tend to be slower recolonizers

than mobile forms (see Section 2.3.5).

The most important biological data consist of species identifications, counts of organisms,

and calculations of diversity and biomass (Columns 6, 8 and 9). Species identifications are the

fundamental biological unit on which statistics are based. On occasions, identifications may be

undertaken only to Higher Order Taxa (HOT's - Column 7) such as to Genera, Families, Classes,

Orders or even Phyla. This may be appropriate site-specifically (and for comparisons).

Counts of numbers of the identified taxa per replicate sample are the basic data from which

bio-diversity calculations are derived. Counts of organisms give abundance values (numbers of each

species per unit area) conventionally shown as per square metre for global comparability.

Counts of each species can be entered into various formulae to give conventionally used

diversity indices. It should not be overlooked that where a survey has been conducted using a

routine standardised sampling protocol, a simple count of the number of species can also give a

useful diversity index. This will be useful within the survey, or for repeat surveys using the same

procedures, but not between surveys using different sampling procedures.

Biomass values (Column 9) represent calculations of the mass of organic material bound into

the living organisms present. The values are important for measuring biological production over

time, and for measuring the relative importance of large and small organisms (or different age

classes), with their different contributions to ecosystem materials and energy flow. However,

biomass values are inherently less accurate than counts, and several types of corrections are needed

to counter-balance when and how the weights are taken (Ellis, 1987).

Types of impacts are separated into dredging and smothering effects (Columns 11 and 12),

and the sampling paradigm noted; whether Before & After, Test & Reference, or just Baseline

15
16

(Before) or Hindsight (After) data (Column 13). Finally, a record of number of surveys, sampling

sites and replicate samples (Columns 14 and 15) gives an indication of the scale of the surveys.

Comprehensive surveys of the type selected are needed for biodiversity assessment.

2.3.3.2 Scope of Reports Reviewed

Table 2.1 is not intended to be an exhaustive compilation of the relevant literature but shows

the typical types of data collected and methods of analysis for gravel and sand beds, and the format

of data presentation for ease of understanding the biological impacts of dredging. It is apparent how

little reporting uniformity there is in presenting biological statistics. It is difficult to compare the

results of different surveys without some data on benthic and fisheries biodiversity and productivity

and without reformatting the data presentation in a consistent manner.

An exhaustive compilation of data for the areas involved in the mining scenarios has not been

attempted for the reasons above. Two current, relevant projects, plus a recent review of the impact

of beach nourishment dredging, will influence assessment procedures in the future. These projects

are:

The MMS-funded study entitled "Impacts and Direct Effects of Marine Mining Activities

on Benthic Organisms" by the University of South Florida, St. Petersburg. When complete,

this will update and substantially extend prior investigations of dredging impact for beach

nourishment (e.g. Salomon et al. 1982)

The NOAA/NMFS computerised benthic database for the east coast extending from Nova

Scotia to Florida. This database is now being prepared for access by CD-ROM, diskette, etc.

(Theroux, 1994). It will allow electronic access to nearly 200,000 records from over 11,700

sampling sites extending over all types of deposits. The database will allow retrieval of siteand time-specific benthos surveys in the future, but species lists taken from it will have little

quantitative predictive power for what will be present at any one time. A selection of

information about this database is provided in Appendix 1. The distribution of deposit types

for the United States eastern seaboard is mapped, as is the distribution of recorded benthic

samples. The distribution and abundance of major taxonomic groups is also mapped and

tabulated. Historical fisheries information is included.

A comprehensive review article by Hurme and Pullen (1988) of dredging impact surveys by

the U.S. Army Corps of Engineers, including references to relevant European literature.

Selected pages of this report are provided in Appendix 2. One of the survey reports is

summarised below; for the reason that the investigators (Salomon et al. 1982) provide

documentation from post-dredging surveys that were started within 2 days of closure, earlier

than any other known surveys.

Details supporting the comments and issues raised by each of the following reports are

provided in Appendix 1 to 6 by Tables and Figures from these reports, and by summary tables in

which original data have been put into a more consistent format. Taxonomic lists are given where
relevant, as certain types of organisms tend to recur. These lists indicate the levels of identification

that are needed for biodiversity assessment at dredging sites.

2.3.3.3 Kenny and Rees (1994; 1993) - Appendix 3

These two articles report on pre- and post-dredging surveys at Test and Reference sites on

an aggregate source in the North Sea, U.K. After dredging, the median particle size rose from 0.355

to 2 mm due to exposure of underlying gravel. Biological samples were obtained with a 0.2 m2

Hamon grab, supplemented by observations from underwater still and video cameras. There was

substantial change in the benthos with loss of species abundances, diversity, biomass and species

mean biomass. Unfortunately, the actual numbers differ between the two articles, although the

trends are identical. The polychaete worms particularly showed massive reductions in numbers and

species. However there was some, although incomplete, recovery within 7 months in this high

energy, open coastal setting. This was clearly illustrated by an ordination procedure (Appendix 3,

Figure 6).

Two dominant gravel epifaunal species, a barnacle Balanus crenatus and a tunicate

Dendrodoa grossularia, recovered to pre-dredging numbers, but at the same time Reference site

levels had increased from 3- to 7-fold during the interval (the spawning season), but with smallersized organisms.

It was concluded from underwater photography that post-dredging deposition of superficial

sand reduced the area of hard surface available for recruitment of the sessile epifauna, and that loss

of adult stock reduced potential for juvenile recruitment. Natural differences between the Test and

Reference sites (1.6 km apart) were considered possible but unlikely. There was marked similarity

between the fauna of Test and Reference sites, and at the Reference site before and after dredging.

This is presumably a function of the close proximity of the sites, and the short (9-month) period

between the initial and final surveys.

In view of the known seasonality of larval recruitment, natural annual fluctuations in

biodiversity and biomass (Thorson, 1957), and the substantial recovery shown within 7 months at

the Test site, it appears that pre- and post-dredging surveys should be implemented within a time

period over which there is little natural change in the populations, for example about 3 to 6 months

apart.

2.3.3.4 Salomon et al. (1982) - Appendix 4

These investigations are unique in the close timing of their shallow-water (6 - 9 m) surveys.

One set of three pre-dredging surveys was timed for April, June and July 1976. Post-dredging

surveys were started on August 10, just 2 days after dredging ceased, and continued weekly for a

month, twice the next month, and then monthly until November 1976. Divers took replicated cores

each sampling 0.0156 m2 surface area.

17
Sediments uncovered by dredging were of similar grain size to the former seabed, so the

original habitat was maintained. In this high energy, wave-exposed dredging area the excavated pits

were filled and smoothed out with sands of similar grain size after a few weeks. Initially, divers

recorded soft silts but these were covered over the next month. After one year, the Station 1 borrow

pit, initially 3 to 5 m deep, had filled to within 1 m of the surrounding seabed.

Two days after dredging there was a reduced fauna at the Station 1 borrow pit (81 organisms

in the samples, representing 21 species). At 16 replicates of 0.0156 m2, this provides an estimate of

324 organisms per m2. The biodiversity rapidly increased over the next two weeks to 534 organisms

and 60 species, within the range of values collected over the next year. Dominant species tended to

be polychaetes, but bivalve mollusc and amphipod species also became abundant. There were

dominance changes with time. Similarity analysis showed no distinction between dredge and control

samples one year after dredging.

The authors conclude that the loss of biodiversity by removal of organisms was recovered

within 1 year, although biodiversity differences remained between the borrow pits and seabed

outside the pits. Their data show substantial recovery within 3 weeks. They ascribe recovery to the

high energy wave-exposed habitat moving local sand into the pits, and covering the silt-clay deposits

which initially accumulated there. The sand presumably carried many of the local organisms with

it, and was supplemented by regional spawning.

2.3.3.5 Padan (1977) - Appendix 5

This is a baseline report completed in 1973 prior to a planned experimental aggregate mining

project. Several types of habitat were described but only four stations characterized by cobbles are

reviewed here. These stations had from 1640 to 7535 organisms per m2, with number of species

ranging from averages of 31 to 113 depending on whether they were classified as "invertebrates" or

"motile". The deposits were generally quite poorly sorted.

Most dominant organisms on cobble were polychaete worms or amphipods, but occasionally

there were sea urchins or molluscs. Many of the cobble substrate species and dominants were also

found on hard substrate (i.e. were sessile epifaunal forms).

The significance of this report when considered with the recent reports of Kenny and Rees

(1994; 1993) above, is the predominating biodiversity of epifauna attached to the cobbles. For

assessing and mitigating dredging impact on aggregates, it will be necessary to consider the cobble

fraction and its sessile epifauna. The Padan (1977) report is considered in more detail in the

Massachusetts Bay aggregate mining scenario (Section 3.4).

2.3.3.6 ENSR (1991) - Appendix 6

This extensive report is a compilation of environmental data from the WestGold offshore

dredging site at Nome, Alaska, between 1986 and 1990. It reveals the logistical problem, inherent

in placer mining, of determining the pre-dredging monitoring sites. The intended dredging sites may

18
change with new ore delineation information. As a result no true Before & After assessment was

achieved by WestGold. There was one hindsight monitoring station (R6) initiated in 1988 two years

after dredging. One other station (R7), initiated during a year of dredging nearby (1987), was

maintained in 1988 (not 1989) and 1990 with more dredging nearby. Several control stations with

similar cobble or sand were maintained so that a Test & Reference sampling paradigm was achieved.

The 1986 dredge footprint was substantially smoothed by 1990. The 1987 data for the sandy

areas showed biodiversity being substantially lower than for the control station, but by 1990 values

were much higher, and were approaching those of the controls. The Cobble test site between

dredging courses of 1987 to 1990 showed lower values in 1987, with values dropping through 1988

and 1990 as dredging continued. However, the cobble site dredged in 1986 showed higher values

of all biological parameters in 1988 and 1990, but still generally less than control stations S3 and

C3. It appears that biodiversity was increasing but was by no means back to normal four years after

dredging.

Lists of dominant species show a variety of forms spread over many groups, but there was

no distinction made between sessile epifauna attached to cobbles, and infauna burrowing into the

deposits. Survey results support the above-described scenarios, showing that sand deposits can

recover their biodiversity more quickly than gravels. The Nome deposit is less well-sorted than

deposits used for aggregate and beach nourishment, with a correspondingly slow biodiversity

recovery. The reasons for slow recovery cannot be established from the data reviewed here, but may

relate to temperature, as well as to deposit geochemistry and the discharge of tailings. The survey

results are considered in more detail in Section 3.2.

2.3.3.7 Seabed Biodiversity Patterns

In general terms benthic biodiversity can be expected to be sufficiently high on both gravel

and sand beds to allow measurement of the environmental impact. Example counts for burrowing

organisms (the infauna) separated from their deposits by a 0.1 mm2 mesh, would be 50 to 100

different species in and on 1 m2 of seabed, and 1,000 to 10,000 individuals of those species.

Both gravel and sand beds support species ranging over a diverse array of Higher Order Taxa.

There are abundant polychaete worms (both particle feeders and predators), small and large crustacea

(which tend to be omnivores), molluscs (both bivalves [particle feeders] and gastropods [predators])

and many other types. The major distinction between gravel and sand beds is the presence of sessile

species attached to the surface of cobbles and boulders. These include such forms as barnacles and

tunic ates.

The infauna of poorly sorted gravel and sand beds tend to be similar, with species reflecting

the particle size and degree of sorting. For example, there are species that characterize poorly-sorted

silty sands regardless of whether there are cobbles and boulders present. These species include many

of the delicate polychaete worms, that are not able to tolerate high-energy environments with mobile

coarse-grained beds. Small crustacea swim over the still surface, and brittle stars browse on the

organic load. In contrast, there are species that occur abundantly or almost entirely on relatively

19
well-sorted sands (such as cockles and the small clams of genus Tellina). Well-sorted sands thus

have species associations clearly different from those on gravel and poorly sorted sands.

Superimposed on this biodiversity pattern related to substrate, is a pattern related to water

depth. Although the effect of increasing depth on the fauna is in part driven by reducing wave

energy, and hence particle size, other factors such as surface primary production and distance from

land (and river input of nutrients) are also involved. In principle, with increasing depth across a

deposit, changes in biodiversity are to be expected. Relatively slight changes within the surface

euphotic zone (say 0 to 50 m water depth) are most likely to be changes in the actual species present,

rather than changes in species and numerical diversity. In summary, with changing depth the array

of species may change.

2.3.4 Biology of Marine Deposits Relevant to Dredging

Natural annual fluctuations in biodiversity (both number of species, and number of

organisms) are enormous. The classic work of Boysen Jensen (1920) documented the abundance

changes for dominant species from year to year. That study showed the available plaice food was

much larger in 1914 than in 1911 but less than in 1912, with a different array of most abundant

species. Such year to year changes are typical.

A recent data set (Table 2.2) from Test and Reference stations for monitoring STD

(Submarine Tailings Disposal) shows changes in numbers of different species over a 20 year period.

There are no obvious differences in the great range of number of species between sampling stations

from little or no tailings. There are obviously major reductions in number of species under thick

tailings, but even these areas show considerable variability from year to year, often reaching the

lower end of the natural range of the unaffected areas.

Such natural fluctuations cause great difficulty in describing ambient faunas and

environmental impact on benthos. This shortcoming applies to measuring biodiversity before

dredging operations start, assessing changes derived from the dredging operations, and assessing the

progress of recolonisation after dredging.

Spatial differences in biodiversity on uniform deposits means that the fauna of Dredging

(Test) and nearby Reference sites will inevitably differ to some extent from natural causes, and from

year to year. These spatial differences are due to the biology of the species present. Many benthic

species live in clusters (McIntyre et al. 1984). It is quite possible to have a localized herd of sea

urchins crawling on a sand bed over a dense patch of newly burrowed-in clam larvae. These two

species will then dominate the numerical biodiversity at that point. Patchiness of this type seriously

impedes faunal comparisons, because counts of organisms present in samples are not normally

distributed (in a statistical sense). There may be no cost-effective way to obtain accurate and precise

biodiversity estimates of benthos with different and varying patchiness. Crude quantitative estimates

may have to be accepted.

20
The discussion above applies only marginally to coarse marine sediments because these are

rarely anoxic to the extent that metals have become naturally enriched. Hence, the potential to

produce negative chemical impacts as a result of dredging aggregate or placer deposits is low to very

low. This conclusion is borne out by evaluation of monitoring data accumulated during the

WestGold placer gold mining operation near Nome, Alaska, (summarized in Section 3.2.3).

Nevertheless it is appropriate to consider the potential for these effects site-specifically by

appropriate measures, and the relevance of applicable sediment quality guidelines (Section 2.6).

2.3 Seabed Biodiversity

Benthic (seabed) organisms are the base feedstock for fishery and wildlife resources such as

seabed feeding fish, diving waterfowl and marine mammals, and also contribute to the larval

biodiversity of the water column. Their populations allow calculations of biodiversity, and

biodiversity can be used as a yardstick for gauging environmental impact.

Biodiversity is measured by: (1) how many different kinds of organisms are present, and (2)

the relative numbers of each (i.e. which are common, which are rare). These are Species diversity

and Numerical diversity respectively.

Organisms should be identified to species where possible, because species as biological units

are the base measure for biodiversity calculations. At most sites there is some uncertainty about the

species identity of many forms present, but separation to species can still be achieved by identifying

to a higher order taxonomic level, say to genus or family. Thus, although the biodiversity unit is not

precisely named, it is measured as one unit for biodiversity calculations.

Detailed lists of species present at any one time are summarised where relevant because,

according to Thorson's (1957) theory of Parallel Level-Bottom Communities, such lists have some

predictive value. It should be possible to predict the kinds of organisms which will recur on similar

deposits. It is very noticeable that identical genera tend to occur on similar habitats, whereas the

actual species within the genus differs from site to site.

At present there is no numerical model which does more than predict biodiversity in a

generalised way. Poiner & Kennedy (1984) show, and develop a model for, increases in total

biomass (weights of organisms) in the far field of a dredging plume. Biomass is related to numerical

abundance in a general way, but not in detail, because the large individuals of a species tend to be

more rare than the small younger individuals.

The benthos of coarse-grained marine placer and aggregate deposits is not well known

because conventional grab samplers (Holme and McIntyre, 1984), used on softer mud and fine sand

to sample the burrowing infauna, do not penetrate well in gravel, or even in coarse sand. Dredges

and trawis sampling the mobile epifauna hang up on rocks, bounce on the surface if towed too fast,

and frighten away the fastest moving fish and shellfish. Such dredges may sample the attached

epifauna, providing they can dislodge the attached organisms, or pick up whole rocks and cobbles

large enough to support growth.

11
There is little information about the biology of gravel and coarse sand deposits; therefore,

there are no up-to-date reviews on which to base a generalised state-of-the-ecosystem statement.

There are, however, some common biological features of marine deposits that apply regardless of

grade (see Section 2.3.4). There are also some recent surveys that demonstrate the biodiversity of

these deposits in a mining context (see Section 2.3.3). A selection of articles is reviewed. These

sections lead to the issue of monitoring sampling design and analysis to document benthic

biodiversity (Section 2.3.6).

Regardless of the grade of deposit extracted, there are two sets of risks arising from dredging

operations: (1) extraction risks (i.e. removal of habitat material with some surrounding disturbance),

and (2) discharge risks (i.e. deposition of discharged tailings). These are introduced in Sections

2.3.1 and 2.3.2 respectively. This review does not consider impacts derived from dredging for

removal of contaminated deposits, nor impacts from the mining of metalliferous muds.

2.3.1 Extraction Risks

2.3.1.1 Resource Losses

Extraction removes sediment habitat and the organisms living within the sediment (infauna),

or on it (epifauna). The burrowing forms are relatively immobile and cannot swim away from the

cutting face of the mining excavation. These benthic organisms include fish and prawn resource

species that maintain burrows, and many species of smaller creatures which can function as

feedstock for resource species.

Epifauna come in two forms. The mobile epifauna generally live on the surface of the

deposits. They may be able to remove themselves from the immediate dredge area. Examples

include flatfish, crabs, sea cucumbers and sea urchins (almost immobile), scallops, and other

resource species. The other epifaunal form is sessile, and is attached to the large cobbles, boulders,

or any rock outcrops. They are generally attached so firmly that they are only removed if their host

substrate is removed. In general, seabed mining will have an impact on resource species, their

feedstock and habitat.

2.3.1.2 Anoxic Sediments

The sediment habitat characteristically is aerated close to the surface. Most organisms live

within the surface aerated layer. However, oxygen reduces with sediment depth depending on

porosity and permeability. Coarse, well-sorted sands and gravels have the thickest well oxygenated

layer, and silts and clays the thinnest. The consequence to bio-diversity of reducing oxygen within

the sediment is that most organisms are competing for space within a shallow aerated zone. Some,

usually the larger forms, can live in deeper protective burrows within the reduced-oxygen (even

anoxic) zone provided they maintain contact with an oxygen and food supply. In general, minerals

dredging is not targeted at low oxygen, fine-grained sediments. However, patches of fine silts and

clays may occur in a dredging area. If they are detectable they should be avoided for both economic

and environmental reasons (such as the release of anoxic or toxic pore water).

12
2.3.1.3 Organism Depth Within Deposits

In minerals dredging, coarse sediments are to be expected, hence there is likely to be a deep

aerated layer of probably 1 m or more. Constraints on depth to which organisms can live within

these deposits are related to their abilities to burrow and maintain contact with the surface. Hence

larger organisms tend to live at deeper levels than small organisms. The larger forms tend to be

resource species that may live at depths of 1 to 2 m within the sediment. Conventionally, infaunal

biologists accept 10 cm of seabed penetration as an effective sampling depth, with little

documentation as to whether retrieval to this depth collects all specimens or not. Large deep-living

forms present a further concern in that they may be long-living, slow-growing forms whose biomass

may have taken 100 or more years to form. Once these forms, such as horse clams and geoducks,

are lost it may take decades to reestablish a potentially sustainable fishery. The loss may even be

permanent.

2.3.1.4 Sediment Suspension and Settlement

Extraction involves physical disturbance of the seabed at the dredgehead due to cutting,

and/or suction, and removal. This inevitably involves suspension of sediments, causing raised

turbidity over the seabed with a plume drifting with tidal currents. The resuspended sediments

resettle nearby or at some distance depending on current velocities and particle size. Resettlement

can be in areas of similar or dissimilar deposits (see Section 2.3.2 Discharge Risks, for further

concerns arising from deposition).

2.3.1.5 Benthic Primary Biological Production

In very shallow water, for example 5 to 10 m depth, dredging-induced turbidity may reduce

primary biological production of plant growth downcurrent of the dredged area

2.3.1.6 Organism Exposure

The cutting action of seabed mining will expose immobile organisms to predators, and

exposed injured or dead organisms attract scavengers and predators. A disturbed area can become

temporarily more active biologically, and more attractive for fishing but at unsustainable levels.

2.3.2 Discharge Risks

A discharged tailings slurry falls through the water column, with some dispersion of fines

(also see Section 2.4 Biological Oceanography), and eventually reaches the seabed as a density

current. After initial impact, the tailings spread in some pattern as a seabed dispersal field,

deposition occurring with coarser particles settling first.

13
2.3.2.1 Smothering

Large amounts of tailings may smother and kill organisms over a broad area. Deposition of

the tailings may occur more rapidly than the inhabiting organisms can avoid, or too fast for the

inhabitants to maintain contact with life-giving aerated water, or their feedstock, above or around

them. Smothering kills benthic organisms by preventing them from breathing or feeding. In general

the ability of benthos to avoid smothering, or to burrow up faster than smothering deposits can settle,

is not known. At Island Copper Mine shallow water fiord benthos on silts and clays can tolerate an

accumulation rate of tailings of about 1 cm a year (Ellis and Hoover, 1990). The data do not indicate

the highest deposition rate that can be tolerated (but see Section 2.3.5.2). During tailings discharge

there is an area within the seabed depositional field where benthos cannot tolerate rapid deposition,

and are smothered, or their survival is affected in some other way. This area represents only a part

of the region where deposited tailings can be identified.

2.3.2.2 Crustacean Stocks

Crabs, lobsters and other epifaunal omnivorous forms may be able to maintain themselves

during the tailings discharge phase. Although the reasons for this are not well established, it may

be that damaged feed organisms are available for scavenging. There can be no guarantee that the

maintenance of crab fisheries, as at Island Copper Mine and the WestGold marine placer operation,

will occur at other dredging locations.

2.3.3 Benthos of Gravel and Sand Deposits

This section is a review of selected gravel and sand benthic investigations. The objective is

to draw some generalised conclusions about the biology of such deposits, and changes to be expected

from dredging operations. The selection of articles for review is based on an expected pattern of

biology and impact. The selection includes well-sorted sands and less well-sorted coarse sands and

gravels.

Sands have a well known characteristic fauna (Thorson, 1957), clearly different from muds

and, apparently, from little-investigated gravels. A well-sorted sand (as needed for beach

nourishment) implies high porosity, good aeration, and little anoxic geochemistry to decompose

constituents to toxic components (see Section 2.2). Gravels for aggregate use are more coarse than

sandsper se (although they may be still formally classified as "coarse sands" if the median grain size

is <2mm). Dredged aggregates have to meet market grade requirements, so the best deposits tend

to be well-sorted, and hence well aerated and with little geochemical implications for environmental

mitigation. However, placer deposits may be less well-sorted, in which case anoxia and geochemical

considerations can become important.

Table 2.1 provides summary ecosystem data from four selected investigations of gravel and

sand deposits. These deposits are where dredging operations have been conducted or were planned.

The parameters shown have been selected to indicate important components of the habitat but do not

14
include other relevant and normally recorded oceanographic parameters such as water temperature,

salinity and dissolved oxygen.

2.3.3.1 Formatting Benthic Data

The form of the Summary Table 2.2 needs some explanation. Survey reports are listed by

author (Colunm 1), and a location descriptor is given in Column 2. The environmental data in

Columns 3 to 5 provide descriptor terms and the objective substrate characteristics documented, such

as particle size data and relevant geochemical parameters.

The generalised terms "gravel" and "sand" (Colunm 3) are useful indicators of deposit grade,

but there is considerable variation in their use. Particle size data (Colunm 4) are more explicit. For

particle size it helps to follow convention by including an average statistic such as the median, and

also a sorting coefficient. The latter shows whether the deposit is well- or poorly-sorted (see above).

Poorly-sorted deposits with a geochemically active mud fraction are very different ecologically from

well-sorted deposits with high porosity and good aeration. In addition, if there are cobbles and

boulders present, there will be more attached sessile organisms. These tend to be slower recolonizers

than mobile forms (see Section 2.3.5).

The most important biological data consist of species identifications, counts of organisms,

and calculations of diversity and biomass (Columns 6, 8 and 9). Species identifications are the

fundamental biological unit on which statistics are based. On occasions, identifications may be

undertaken only to Higher Order Taxa (HOT's - Column 7) such as to Genera, Families, Classes,

Orders or even Phyla. This may be appropriate site-specifically (and for comparisons).

Counts of numbers of the identified taxa per replicate sample are the basic data from which

bio-diversity calculations are derived. Counts of organisms give abundance values (numbers of each

species per unit area) conventionally shown as per square metre for global comparability.

Counts of each species can be entered into various formulae to give conventionally used

diversity indices. It should not be overlooked that where a survey has been conducted using a

routine standardised sampling protocol, a simple count of the number of species can also give a

useful diversity index. This will be useful within the survey, or for repeat surveys using the same

procedures, but not between surveys using different sampling procedures.

Biomass values (Column 9) represent calculations of the mass of organic material bound into

the living organisms present. The values are important for measuring biological production over

time, and for measuring the relative importance of large and small organisms (or different age

classes), with their different contributions to ecosystem materials and energy flow. However,

biomass values are inherently less accurate than counts, and several types of corrections are needed

to counter-balance when and how the weights are taken (Ellis, 1987).

Types of impacts are separated into dredging and smothering effects (Columns 11 and 12),

and the sampling paradigm noted; whether Before & After, Test & Reference, or just Baseline

15
16

(Before) or Hindsight (After) data (Column 13). Finally, a record of number of surveys, sampling

sites and replicate samples (Columns 14 and 15) gives an indication of the scale of the surveys.

Comprehensive surveys of the type selected are needed for biodiversity assessment.

2.3.3.2 Scope of Reports Reviewed

Table 2.1 is not intended to be an exhaustive compilation of the relevant literature but shows

the typical types of data collected and methods of analysis for gravel and sand beds, and the format

of data presentation for ease of understanding the biological impacts of dredging. It is apparent how

little reporting uniformity there is in presenting biological statistics. It is difficult to compare the

results of different surveys without some data on benthic and fisheries biodiversity and productivity

and without reformatting the data presentation in a consistent manner.

An exhaustive compilation of data for the areas involved in the mining scenarios has not been

attempted for the reasons above. Two current, relevant projects, plus a recent review of the impact

of beach nourishment dredging, will influence assessment procedures in the future. These projects

are:

The MMS-funded study entitled "Impacts and Direct Effects of Marine Mining Activities

on Benthic Organisms" by the University of South Florida, St. Petersburg. When complete,

this will update and substantially extend prior investigations of dredging impact for beach

nourishment (e.g. Salomon et al. 1982)

The NOAA/NMFS computerised benthic database for the east coast extending from Nova

Scotia to Florida. This database is now being prepared for access by CD-ROM, diskette, etc.

(Theroux, 1994). It will allow electronic access to nearly 200,000 records from over 11,700

sampling sites extending over all types of deposits. The database will allow retrieval of siteand time-specific benthos surveys in the future, but species lists taken from it will have little

quantitative predictive power for what will be present at any one time. A selection of

information about this database is provided in Appendix 1. The distribution of deposit types

for the United States eastern seaboard is mapped, as is the distribution of recorded benthic

samples. The distribution and abundance of major taxonomic groups is also mapped and

tabulated. Historical fisheries information is included.

A comprehensive review article by Hurme and Pullen (1988) of dredging impact surveys by

the U.S. Army Corps of Engineers, including references to relevant European literature.

Selected pages of this report are provided in Appendix 2. One of the survey reports is

summarised below; for the reason that the investigators (Salomon et al. 1982) provide

documentation from post-dredging surveys that were started within 2 days of closure, earlier

than any other known surveys.

Details supporting the comments and issues raised by each of the following reports are

provided in Appendix 1 to 6 by Tables and Figures from these reports, and by summary tables in

which original data have been put into a more consistent format. Taxonomic lists are given where
relevant, as certain types of organisms tend to recur. These lists indicate the levels of identification

that are needed for biodiversity assessment at dredging sites.

2.3.3.3 Kenny and Rees (1994; 1993) - Appendix 3

These two articles report on pre- and post-dredging surveys at Test and Reference sites on

an aggregate source in the North Sea, U.K. After dredging, the median particle size rose from 0.355

to 2 mm due to exposure of underlying gravel. Biological samples were obtained with a 0.2 m2

Hamon grab, supplemented by observations from underwater still and video cameras. There was

substantial change in the benthos with loss of species abundances, diversity, biomass and species

mean biomass. Unfortunately, the actual numbers differ between the two articles, although the

trends are identical. The polychaete worms particularly showed massive reductions in numbers and

species. However there was some, although incomplete, recovery within 7 months in this high

energy, open coastal setting. This was clearly illustrated by an ordination procedure (Appendix 3,

Figure 6).

Two dominant gravel epifaunal species, a barnacle Balanus crenatus and a tunicate

Dendrodoa grossularia, recovered to pre-dredging numbers, but at the same time Reference site

levels had increased from 3- to 7-fold during the interval (the spawning season), but with smallersized organisms.

It was concluded from underwater photography that post-dredging deposition of superficial

sand reduced the area of hard surface available for recruitment of the sessile epifauna, and that loss

of adult stock reduced potential for juvenile recruitment. Natural differences between the Test and

Reference sites (1.6 km apart) were considered possible but unlikely. There was marked similarity

between the fauna of Test and Reference sites, and at the Reference site before and after dredging.

This is presumably a function of the close proximity of the sites, and the short (9-month) period

between the initial and final surveys.

In view of the known seasonality of larval recruitment, natural annual fluctuations in

biodiversity and biomass (Thorson, 1957), and the substantial recovery shown within 7 months at

the Test site, it appears that pre- and post-dredging surveys should be implemented within a time

period over which there is little natural change in the populations, for example about 3 to 6 months

apart.

2.3.3.4 Salomon et al. (1982) - Appendix 4

These investigations are unique in the close timing of their shallow-water (6 - 9 m) surveys.

One set of three pre-dredging surveys was timed for April, June and July 1976. Post-dredging

surveys were started on August 10, just 2 days after dredging ceased, and continued weekly for a

month, twice the next month, and then monthly until November 1976. Divers took replicated cores

each sampling 0.0156 m2 surface area.

17
Sediments uncovered by dredging were of similar grain size to the former seabed, so the

original habitat was maintained. In this high energy, wave-exposed dredging area the excavated pits

were filled and smoothed out with sands of similar grain size after a few weeks. Initially, divers

recorded soft silts but these were covered over the next month. After one year, the Station 1 borrow

pit, initially 3 to 5 m deep, had filled to within 1 m of the surrounding seabed.

Two days after dredging there was a reduced fauna at the Station 1 borrow pit (81 organisms

in the samples, representing 21 species). At 16 replicates of 0.0156 m2, this provides an estimate of

324 organisms per m2. The biodiversity rapidly increased over the next two weeks to 534 organisms

and 60 species, within the range of values collected over the next year. Dominant species tended to

be polychaetes, but bivalve mollusc and amphipod species also became abundant. There were

dominance changes with time. Similarity analysis showed no distinction between dredge and control

samples one year after dredging.

The authors conclude that the loss of biodiversity by removal of organisms was recovered

within 1 year, although biodiversity differences remained between the borrow pits and seabed

outside the pits. Their data show substantial recovery within 3 weeks. They ascribe recovery to the

high energy wave-exposed habitat moving local sand into the pits, and covering the silt-clay deposits

which initially accumulated there. The sand presumably carried many of the local organisms with

it, and was supplemented by regional spawning.

2.3.3.5 Padan (1977) - Appendix 5

This is a baseline report completed in 1973 prior to a planned experimental aggregate mining

project. Several types of habitat were described but only four stations characterized by cobbles are

reviewed here. These stations had from 1640 to 7535 organisms per m2, with number of species

ranging from averages of 31 to 113 depending on whether they were classified as "invertebrates" or

"motile". The deposits were generally quite poorly sorted.

Most dominant organisms on cobble were polychaete worms or amphipods, but occasionally

there were sea urchins or molluscs. Many of the cobble substrate species and dominants were also

found on hard substrate (i.e. were sessile epifaunal forms).

The significance of this report when considered with the recent reports of Kenny and Rees

(1994; 1993) above, is the predominating biodiversity of epifauna attached to the cobbles. For

assessing and mitigating dredging impact on aggregates, it will be necessary to consider the cobble

fraction and its sessile epifauna. The Padan (1977) report is considered in more detail in the

Massachusetts Bay aggregate mining scenario (Section 3.4).

2.3.3.6 ENSR (1991) - Appendix 6

This extensive report is a compilation of environmental data from the WestGold offshore

dredging site at Nome, Alaska, between 1986 and 1990. It reveals the logistical problem, inherent

in placer mining, of determining the pre-dredging monitoring sites. The intended dredging sites may

18
change with new ore delineation information. As a result no true Before & After assessment was

achieved by WestGold. There was one hindsight monitoring station (R6) initiated in 1988 two years

after dredging. One other station (R7), initiated during a year of dredging nearby (1987), was

maintained in 1988 (not 1989) and 1990 with more dredging nearby. Several control stations with

similar cobble or sand were maintained so that a Test & Reference sampling paradigm was achieved.

The 1986 dredge footprint was substantially smoothed by 1990. The 1987 data for the sandy

areas showed biodiversity being substantially lower than for the control station, but by 1990 values

were much higher, and were approaching those of the controls. The Cobble test site between

dredging courses of 1987 to 1990 showed lower values in 1987, with values dropping through 1988

and 1990 as dredging continued. However, the cobble site dredged in 1986 showed higher values

of all biological parameters in 1988 and 1990, but still generally less than control stations S3 and

C3. It appears that biodiversity was increasing but was by no means back to normal four years after

dredging.

Lists of dominant species show a variety of forms spread over many groups, but there was

no distinction made between sessile epifauna attached to cobbles, and infauna burrowing into the

deposits. Survey results support the above-described scenarios, showing that sand deposits can

recover their biodiversity more quickly than gravels. The Nome deposit is less well-sorted than

deposits used for aggregate and beach nourishment, with a correspondingly slow biodiversity

recovery. The reasons for slow recovery cannot be established from the data reviewed here, but may

relate to temperature, as well as to deposit geochemistry and the discharge of tailings. The survey

results are considered in more detail in Section 3.2.

2.3.3.7 Seabed Biodiversity Patterns

In general terms benthic biodiversity can be expected to be sufficiently high on both gravel

and sand beds to allow measurement of the environmental impact. Example counts for burrowing

organisms (the infauna) separated from their deposits by a 0.1 mm2 mesh, would be 50 to 100

different species in and on 1 m2 of seabed, and 1,000 to 10,000 individuals of those species.

Both gravel and sand beds support species ranging over a diverse array of Higher Order Taxa.

There are abundant polychaete worms (both particle feeders and predators), small and large crustacea

(which tend to be omnivores), molluscs (both bivalves [particle feeders] and gastropods [predators])

and many other types. The major distinction between gravel and sand beds is the presence of sessile

species attached to the surface of cobbles and boulders. These include such forms as barnacles and

tunic ates.

The infauna of poorly sorted gravel and sand beds tend to be similar, with species reflecting

the particle size and degree of sorting. For example, there are species that characterize poorly-sorted

silty sands regardless of whether there are cobbles and boulders present. These species include many

of the delicate polychaete worms, that are not able to tolerate high-energy environments with mobile

coarse-grained beds. Small crustacea swim over the still surface, and brittle stars browse on the

organic load. In contrast, there are species that occur abundantly or almost entirely on relatively

19
well-sorted sands (such as cockles and the small clams of genus Tellina). Well-sorted sands thus

have species associations clearly different from those on gravel and poorly sorted sands.

Superimposed on this biodiversity pattern related to substrate, is a pattern related to water

depth. Although the effect of increasing depth on the fauna is in part driven by reducing wave

energy, and hence particle size, other factors such as surface primary production and distance from

land (and river input of nutrients) are also involved. In principle, with increasing depth across a

deposit, changes in biodiversity are to be expected. Relatively slight changes within the surface

euphotic zone (say 0 to 50 m water depth) are most likely to be changes in the actual species present,

rather than changes in species and numerical diversity. In summary, with changing depth the array

of species may change.

2.3.4 Biology of Marine Deposits Relevant to Dredging

Natural annual fluctuations in biodiversity (both number of species, and number of

organisms) are enormous. The classic work of Boysen Jensen (1920) documented the abundance

changes for dominant species from year to year. That study showed the available plaice food was

much larger in 1914 than in 1911 but less than in 1912, with a different array of most abundant

species. Such year to year changes are typical.

A recent data set (Table 2.2) from Test and Reference stations for monitoring STD

(Submarine Tailings Disposal) shows changes in numbers of different species over a 20 year period.

There are no obvious differences in the great range of number of species between sampling stations

from little or no tailings. There are obviously major reductions in number of species under thick

tailings, but even these areas show considerable variability from year to year, often reaching the

lower end of the natural range of the unaffected areas.

Such natural fluctuations cause great difficulty in describing ambient faunas and

environmental impact on benthos. This shortcoming applies to measuring biodiversity before

dredging operations start, assessing changes derived from the dredging operations, and assessing the

progress of recolonisation after dredging.

Spatial differences in biodiversity on uniform deposits means that the fauna of Dredging

(Test) and nearby Reference sites will inevitably differ to some extent from natural causes, and from

year to year. These spatial differences are due to the biology of the species present. Many benthic

species live in clusters (McIntyre et al. 1984). It is quite possible to have a localized herd of sea

urchins crawling on a sand bed over a dense patch of newly burrowed-in clam larvae. These two

species will then dominate the numerical biodiversity at that point. Patchiness of this type seriously

impedes faunal comparisons, because counts of organisms present in samples are not normally

distributed (in a statistical sense). There may be no cost-effective way to obtain accurate and precise

biodiversity estimates of benthos with different and varying patchiness. Crude quantitative estimates

may have to be accepted.

20
Apart from sampling variability derived from the biology of the organisms, a further level

of variability arises from the sampling process such as: different sampler types (e.g. Van Veen and

Peterson grabs), different sizes of the same sampler (e.g. 0.1 m2 and 0.2 th Van Veen grabs),

different sampler penetration depths on soft and hard sediments (muds vs. sands), and different sieve

meshes to retain organisms when being extracted (generally 0.5 or 1 mm). The level of taxonomic

identification also affects measures of biodiversity (Wu, 1982; Ellis, 1985).

Various numerical techniques are used to measure similarity between Test and Reference

sites, and between Before and After sampling (Burd et al. 1990). Currently, they tend to be

measures of similarity based on number of species and their abundances, with or without ordination

related to environmental parameters (Burd et al. 1990). Various Diversity Indices may be used

(Washington, 1984), or simple comparisons may be given of basic summary statistics about the total

fauna and the most abundant (dominant) species present. Examples from the mining industry are

by ENSR (1991) and Ellis and Hoover (1990). There are arguments as to whether counts are

sufficient measures, or whether weights should be included (Burd et al. 1990). Weights (biomass)

present further difficulties for obtaining accurate measures (Ellis, 1987).

In contrast to the burrowing infauna, benthic-feeding fishery species (groundfish, cod, crabs,

lobsters, etc) occur in consistent locations designated as "fishing grounds". Therefore the local

patchiness and annual fluctuations of their benthic feedstock within those grounds can be taken as

inconsequential to their continuing presence. Such species are feeding opportunists, capable of

optimizing their foraging strategies and picking the richest food for the least effort. The important

benthic parameter in this context is benthic productivity (biomass produced per time interval), rather

than the biodiversity present at any one time. Measuring this adds a further level of complexity and

inaccuracy in benthic biodiversity assessment.

For measuring dredging impact on the benthos this means:

only gross changes or gross differences in simple measures of biodiversity can be taken to

reflect real population changes or differences.

these measures must be made by site-specific surveys designed for the purpose and cannot

be assembled from old data-bases.

minor changes and differences in biodiversity probably have no significance in monitoring

population changes or differences.

the simplest measures of number of species, and the relative numbers of each, will be the

most cost-effective for monitoring benthic biodiversity.

21
2.3.5 Benthic Recovery After Impact

2.3.5.1 Recolonization

Ninety-five percent of benthos breed by producing pelagic larvae in the tropics, and 95% by

non-pelagic larvae (swimlcrawl-in forms) in poiar regions and the deep sea (Thorson, 1950; 1957).

The proportion changes by latitude, but mid-latitude proportional figures have not been located.

However, Rosenburg (1976) shows how unoccupied seabed recovering from pollution in a Swedish

fjord is colonised first by species with pelagic larvae.

Larvae live for an average of 3 weeks (Thorson, 1950) and larvae of shelf species even longer

(Scheltema, 1986). At a current speed of 1 knot (1.85 km/h), a 3 week lifetime allows larvae to drift

about 900 km before they die off. Whatever the details, we know from the high latitude site of

WestGold's operation in Alaska (ENSR, 1991, and Section 3.2.4) that there were benthos present

within 1 year following termination of dredging, and greatly extended a year later. At lower latitude

sand extraction sites, settlement is within weeks (Salomon et al. 1982, Appendix 4) as has been

found with in vivo experimental boxes in the Baltic Sea (Arntz and Rumohr, 1982). At Island

Copper Mine the number of juveniles is known to be higher in recolonizing tailings than on

unaffected substrates (Burd and Ellis, 1995).

In this context, mitigation procedures facilitating recolonization by pelagic larvae drifting

in from sources up to several hundred kilometres distant, are desirable. Any immigration by

crawl/swim-in forms is a bonus, and does not need to be engineered.

Times to recovery of a reasonable biodiversity for three different deposit grain sizes are given

in Table 2.3. Fine-grained tailings may need only 1 year before achieving a recovery level

biodiversity, medium-grained deposits 1 to 3 years, and coarse-grained deposits 5 or more years.

Recovery is defined as attaining a successional community of opportunistic species providing

evidence of progression towards a community equivalent to that previously present, or at nonimpacted reference sites.

Equivalency in the above definition can be measured in terms of standing crop, biomass,

number of species and numbers of most abundant species, etc. These criteria must be defined during

preparation of an Environmental Impact Statement (or equivalent document) for any site. The

definition depends on the best available monitoring techniques suitable for biodiversity assessment

at that site. In general terms the recovery data from Island Copper Mine (Table 2.2) show that 1 to

2 years after a defaunation event (that may persist for several years) numbers of species return to 10

to 15/0.15 m2. Burd and Ellis (1995) show that numbers of organisms return to 5.000 to 10.000/m2.

2.3.5.2 Tailings Deposition

Tailings have been deposited offshore from Island Copper Mine over 23 years in beds 40 to

50 m thick (Ellis et al. 1995). There is substantial defaunation under such rates of accumulation

22
(approximately 2 rn/yr.). Sampling stations receiving tailings accumulations of less than about 20

cm over 23 years (about 1 cm/yr.) have a biodiversity indistinguishable from sampling stations

without tailings.

There is some indication in the biodiversity data from Island Copper Mine (Table 223) that

an accumulation rate of fine-grained tailings higher than 1 cm/yr. can be tolerated. Stations 5 and

6 do not show defaunation events. Station 5 received approximately 40 cm of tailings over 20 years,

and Station 6 received 32 cm of tailings over 10 years. Station 19, which shows lesser biodiversity,

received 60 cm of tailings over 15 years. Thus 1 cm/yr. is a safe estimate of tolerated accumulation

rate for fine tailings. Two cm/yr. might be tolerable, but 4 cm/yr. appears to affect the biodiversity.

At these slow deposition rates the deposits probably settle in pulses (e.g. 1 cm may accumulate in

a matter of 2 to 3 days but only once a year). This comment applies to silt-sized tailings. There is

no available information relating to sand- and gravel-sized tailings.

2.3.5.3 Benthic Sensitivity to Substrate Change

Benthos are sensitive to substrate change in that predominant species show measurable

variation. Fishery species with dependent, specialist feeding and habitat requirements also change.

In contrast, crabs and some polychaete worms can be very tolerant of such changes. There are no

quantitative models relating benthic species response to altered deposits. Without site- and timespecific Environmental Impact Assessments (EIA's), as at WestGold's Nome dredge site (Section

3.2), collating species lists spanning many years and different sampling instruments, even if they

provide some population numbers, have little predictive value for recolonising species at any site.

For dredging impact assessment two options concerning habitat change should be considered

for any Scenario:

Habitat change not tolerable (for site-specific reasons such as herring spawning ground) -

seabed may be dredged and tailings replaced only in ways which leave similar habitat (leave

a layer of the substrate as with sand or aggregate extraction, and/or replace substrate with

similar grade, e.g. as with placer mining).

Habitat change tolerable (usual scenario), although possibly with specific constraints (such

as no return of large boulders to trawling grounds) - in this case it has to be recognised that

there will be unpredictable differences in the species array between the recolonised area and

elsewhere. Application of appropriate biodiversity criteria can set a required number of

species and organisms to be present, even if they are different from what was there before,

or from the species that have appeared at undredged reference sites nearby.

In summary, in any one year there may be several hundred benthic species present, of which

only 10 or so will be abundant. The 10 abundant species plus all the others can change from year

to year naturally and unpredictably. Fishery species are less variable if the habitat, as opposed to

benthos, remains stable. Bottom fish tend to be opportunistic feeders and take anything that they

see, smell or hear moving.

23
24

For dredging mitigation, the criteria are either to leave deposits as similar as possible to what

was there before, or if a substrate change is accepted to set biodiversity recolonization objectives.

These objectives have to be based on a site-specific pre-permitting ETA.

2.3.6 Design and Analysis of Monitoring Programs

The variability in benthic faunas from place to place, and with time, presents problems in

designing a monitoring program. The need is to demonstrate that biological impacts, and recovery

from dredging, are as predicted. The problem is that Reference site biodiversity is changing

naturally, while the Test sites are changing from impact (but would also have changed naturally).

In spite of this natural variability, multivariate statistical analyses can distinguish between the

biodiversity of grossly impacted Test and Reference stations. For example, the routine similarity

analyses undertaken at Island Copper Mine, Canada, on the type of data shown in Table 2.3, were

used to produce maps of impacted areas as shown in Figure 2.2. These have been derived from an

intermediate step measuring similarities between stations (Figure 2.3) or ordinating similarities in

some way (Kenny and Rees, 1994, Appendix 3). To achieve this kind of biodiversity impact

documentation, the following generalised Before and After, Test and Reference sampling design and

analyses are suggested:

At least 1 year in advance of mining, sample an appropriate grid of stations (single samples)

within and surrounding the proposed mining area (the Before samples).

Undertake an appropriate multivariate statistical analysis to demonstrate clusters of sampling

stations with similar faunas.

Designate 2 sampling sites per cluster of similar sites as "Initially Conserved Areas (ICA's)"

for subsequent annual monitoring thus documenting Reference conditions each year. The

location and area of each ICA must be sufficient to avoid impact from the nearest dredging

operations.

Starting prior to mining, monitor the ICA's annually (same season each year) with at least

3 replicate samples per ICA. Designate sampler(s) and sieving mesh (to separate the

organisms), and use them consistently through the entire life of the mining operation.

Follow required sample processing protocols (Before and After sampling).

Allocate the remainder of mining area for dredging in a pattern to be determined sitespecifically, and adjust according to updated ore delineation information.

As soon as a dredging course with distinctive deposit and fauna is completed (i.e. within 1

month), sample from the course to establish baseline for documentation of recovery at

Dredging Test Areas (DTA's). If timing is off the annual ICA routine monitoring, undertake

additional routine sampling at the appropriate matching ICA's.

Add each DTA to the annual monitoring program.
25

(8) ICA's may be mined after two similar DTA's have achieved recovery as shown by the

adopted multivariate statistical tecimique.

2.4 Biological Oceanography

The water column is a highly dynamic environment where changes occur minute by minute

as a result of sea, meteorological, tidal and biological alterations. A consequence of these alterations

is that environmental change which may take months or years to occur in benthic environments will

take place in minutes or hours in the water column. Conversely, these changes may just as rapidly

disappear by the action of dilution or advection. Mining impact assessment and mining mitigation

procedures directed towards the pelagic environment must take these rapid changes into account.

Most assessment and monitoring programs operate on a time and spatial scale designed to detect

long term environmental changes inappropriate to water column monitoring. However many of

these long term changes are brought about via the water column. Deposition of sediments, sediment

metal accumulation, bio-accumulation of metals, biota alterations, and disruption of migration

patterns are just a few of the long term environmental changes that may be driven by ephemeral

alterations of the water column. Principal water column risk factors associated with marine mining

may be grouped into three general categories:

Production of a suspended sediment turbidity plume resulting from; a) disturbance of the

bottom sediments; b) loss of dredged material during transport to the surface and c) return

of unwanted sediments and process water to the ocean either as initial reject overburden or

as tailings (see Section 3.2.4, and Garnett and Ellis, 1995)

Release of low oxygen pore water or toxic materials (metals, H2S, etc.) either from the

dredged material or from newly exposed mined seabed sediments either directly into the

water column (dissolved) or attached to fine particles (particulate). (see Section 2.2)

A hidden risk factor in all assessment is that the environmental sampling may be faulty and

fail to describe the situation adequately. This is especially true for water colunm monitoring

for the reasons stated above. An inadequate monitoring program increases the likelihood that

changes will go unnoticed until significant environmental impact occurs.

Water column impact assessment must consider all three of these potential risks: suspended

sediments/turbidity; toxic materials and monitoring/analytical procedures.

2.4.1 Suspended Sediments/Turbidity

Suspended sediments within the water column can be divided into two distinct classes: large

particles in the process of settling and deposition, and fine particles that may remain suspended in

the water column for long periods of time and which may be transported great distances,

Particles greater than a few micrometers in diameter rapidly settle out of the water column

and form density currents extending from point sources. Well documented examples occur at Island
Copper Mine (Ellis et al. 1995) and Kitsault Molybdenum Mine (Pedersen et al. 1995). The extent

and direction of these density currents and the subsequent bottom migration of these sediments may

be influenced by water column dynamics such as currents and wave action (see Section 2.1).

Fine particles may remain suspended for long periods of time because of high surface-tovolume ratios. This is especially true of particles suspended in fresh water where surface charges

tend to keep particles from coalescing. In the electrolytic marine environment particles are more

likely to aggregate and settle out. Nevertheless, large turbidity plumes of allochthonous fine

suspended sediments do form both naturally as a result of river input and from industrial discharges

to the sea surface (Pedersen et al. 1995). Plumes of autochthonous fine sediment may occur as a

result of turbidity currents, tidal scouring and other subsurface events. Such turbidity plumes may

persist for long periods of time in sea water. The total amount of suspended material, by weight, is

generally extremely small when compared to deposited sediments but they may reduce light

penetration in the ocean (Island Copper, 1994) and may contain high levels of heavy metals, both

of which may cause adverse biological effects. Allochthonous turbidity plumes resulting from

industrial activities may also contain process water, dissolved heavy metals, process chemicals and

other wastes.

Anderson and Mackas (1986) reported that Kitsault mine tailings (by bioassay) reduced

survival time for zooplankton at 560 mg/l, and respiration rates at 100 mg/l. Tailings concentrations

at low levels (e.g. 40 mg/l) were associated with enhanced survival times and respiration rates. At

the mine site, tailings were less than 15 mg!l 0.5 km from the outfall, and it was concluded that there

was no effect on zooplankton from this and from the field studies (Mackas and Anderson, 1986).

Island Copper Mine (1994) reported that tailings consistently doubled the original ambient

seawater turbidity in Rupert Inlet to approximately 1.00 NTU (Nephelometric Turbidity Unit)

(Figure 2.4). Chlorophyll a and zooplankton levels in Rupert Inlet do not show consistent

differences from elsewhere (Figure 2.5 and Figure 2.6, also Chapter 6 in Ellis, 1989).

Parsons etal. (1986), from experiments in a very large mesocosm (CEPEX), concluded that

Kitsault mine tailings at both 30 and 300 ppm molybdenum caused: (1) a delay in primary

productivity and chlorophyll a maxima; (2) a shift in the balance of phytoplankton species, and (3)

an increased production of zooplankton. In this context, the information supports discharging

tailings below the euphotic zone if the water is deep enough, with the outfall engineered to minimize

dispersion in the water column. However, some dispersion at low concentrations is acceptable.

Other more complex problems that may be associated with turbidity plumes include food

web alterations, such as changes to the microbial loop, and decrease in primary productivity due to

light reduction. Secondary alterations to the zooplankton population might occur also but available

data indicate little or no effect of moderate turbidity plumes on zooplankton. Studies have shown

no reduction in feeding activities. In fact zooplankton may be instrumental in increasing the fall-out

rate of particles by incorporation of fine particles into large, rapidly settling fecal pellets (Anderson

and Mackas, 1986). This activity removes not only suspended sediments from the water colunm but

also associated bound metals, transferring the problems, if any, to the benthos.

26
In some environments, such as glacially fed high runoff inlets, seasonal turbidity plumes

covering hundreds of square kilometres are the norm. Such inlets may have reduced primary

production but otherwise appear to have normal planktonic food webs and often support apparently

normal populations of migrating adult and juvenile salmonids.

2.4.2 Release of Toxic Plumes

Effects of newly exposed sediments on the water column depend on the mining method and

the energetics of the water colunm at that site. Shallow water excavations are likely to be in a highly

energetic environment. Oxygen-rich water would rapidly oxidize exposed sediments without any

noticeable chemical oxygen demand loading of the water. Exposed soluble metals would be rapidly

diluted below detection limits (see Section 2.2). In contrast, deep water dredging may be in a low

energy environment where currents and other advective forces are minimal. Here, deep narrow cuts

could leave relatively stagnant pits. Extraction of sediment salts or advection of seasonally high

salinity water could result in increased water density and lead to stable water masses within the

dredge pits. Long term exposure of this water to reducing sediments could create considerable

chemical oxygen demand and under extreme conditions result in anoxia, and death of organisms.

Occasional (seasonal?) flushing of these "dredge pits" might occur releasing pulses of dense, lowoxygen water. Fortunately such a scenario is very unlikely and could be detected easily by simple

conductivity/temperature/depth (CTD) monitoring.

Although it is unlikely that toxic materials will be released at offshore dredging sites (see

Section 2.2), the potential at any site for a toxic plume to be released to the water column needs

assessment. Both excavational and tailings discharge should be assessed in this context. Prior

anthropogenic disturbance by addition of mercury at WestGold's Nome dredge site (see Section

3.2.3), and the unusual tailings geochemistry at Black Angel Lead-Zinc Mine, Greenland (Poling

and Ellis, 1995), illustrate the nature and consequences of these concerns. At Nome, mercury did

not become a problem. At Black Angel mine there was serious contamination of a fjord by

cadmium, lead, and zinc.

2.4.3 Monitoring

In comparison with benthic monitoring, water column monitoring is of short duration and

high intensity, and is directed at determining whether water quality guidelines, as set by the

appropriate regulatory agency, are being met. Water column turbidity measurements are

conventionally made with a nephelometer. The Environmental Protection Agency (1986) states a

requirement of "no more than 10% reduction of depth for photosynthesis", which means in this

context that tailings should be discharged below the compensation point (which must be measured

for each site). California (1990) gives some numbers for acceptable settleable solids levels in

effluents (e.g. max 3.0 mill) and turbidity (e.g. max 225 NTU). A recent example is provided by

a beach nourishment project off Jacksonville Florida, for which the MMS required daily

nepheiometer readings at the water surface, at the bottom, and at mid-depth. Turbidity was not to

exceed 29 NTU's at any time or dredging operations were to cease.

27
2.4.5 Fishery and Wildlife Resources

In the context of this study, wildlife resources comprise marine mammals, birds and possibly

some reptiles (such as sea turtles). Fishery resources include those of the water column (pelagic

resources) such as tuna, marlin and sardines, and resources of the seabed (shellfish or groundfish)

such as lobsters, crabs, sole, flounder and cod.

There are two sets of risk factors for wildlife and fish: dredge-derived and biologicallyderived. When they occur together, the resource involved may be impacted in a manner which is

difficult to document. A coincidence of biologically-derived risks can be easily underestimated due

to the difficulty of appraising the biological factors (such as documenting that many million fish may

follow a precisely determined route underwater, or that a few hundred individuals of an endangered

marine mammal species may be dependent on well-separated staging habitats). Where wildlife or

fishery resources are present considerable care should be given to assessing the biological risk

factors.

2.5 Dredge-derived Risk Factors

2.5.1 Turbidity

Turbidity can be very high at a dredge site and tailings discharge point. Submersible

observations of tailings at Island Copper Mine have shown such high turbidity that visibility is

effectively nil (Ellis & Heim, 1985). Although some wildlife and fish can live in highly turbid

conditions they rely on sonic or hydraulic sensory systems for information about their environment.

2.5.2 Noise

Noise levels will be high from dredge and mill operations. Marine mammals, particularly

whales and dolphins, use sonar as the main sensory system for feeding and travel. Just as excessive

noise distracts humans, it is likely that high noise levels from dredge operations are a risk to marine

mammals affecting their ability to feed, protect themselves and breed. The topic is little investigated

and needs consideration at any site where marine mammals are a resource.

2.6 Biologically-derived Risk Factors for Wildlife

2.6.1 Territoriality and Homing

Mammals and birds characteristically set up a home site at some stage in their lives. This

may be permanent or temporary. A home site greatly facilitates wildlife survival as the individuals

quickly learn how best to use their home in providing food and protection. Where colonies of a

species become resident in an area, then dislodgement by human action affects many individuals

simultaneously. Although it is unlikely that an offshore dredge site will be functioning as a colony

site for marine birds and mammals, the risk may be significant for groundfish, such as halibut, sole

28
and herring (Oulasvirta & Lehtonen, 1988) that may have aggregated on particularly favourable

gravel beds for feeding or breeding.

Some marine mammals may settle at home sites permanently or temporarily, and then be

resistant to leaving. If they are forced away from home sites, their survival can be reduced due to

other suitable home sites being already occupied.

2.6.2 Mass Migrations

Many marine birds and mammals make long-distance migrations along set routes, such as

the flyways of waterfowl. These routes may cross extensive shallow water marine areas, and the

birds may set down for prolonged periods to feed. Marine mammals appear to have set routes and

seasons for migration in clan aggregates or family pods, keeping in contact with each other and

direction finding by sonar. Marine turtles also migrate along set routes and at set seasons. The same

applies to fish and some crustacea. Where such set routes and seasonal mass movements coincide

with a dredge site, then a large number, possibly the whole stock of a species, will be exposed to

dredge-derived risk factors. Commonly, such mass migrations are not readily visible when

occurring, as the animals concerned pass by a moving boat-based observer in separated groups,

communicating sonically if underwater, or by flock movements if flying.

2.6.3 Noise

The increase in noise by dredging or other industrial operations over the ambient noise level

may be significant. Marine mammals nearshore generally occupy a noisy environment due to waves

and sediment transport, so may be less environmentally sensitive to noise pollution than offshore

species. Coastal waters generally have higher levels of noise when compared to the deep oceans or

high seas. The levels of ambient noise can vary widely and environmental factors such as rain and

wind can greatly increase ambient noise levels. In areas of high ship traffic like the North American

Atlantic coast it is possible that marine mammals have already changed their habits to avoid loud

sources or have become habituated to them. Even remote areas such as WestGold's site at Nome,

Alaska, may have a moderate degree of ship noise due to vessel traffic in and out of the nearby

harbour. Although the issue of vessel-derived noise impacting wildlife has been investigated for

pure research purposes, and for oil drilling and production purposes, there is limited information

available.

The frequency of sound in a particular band range (in Hz to MHz) appears to be important

when examining animal reactions to sound. Studying the types and frequencies of sounds made by

different marine mammals should give insight into what they can hear (Turl, 1982).

In general, mysticete (baleen) whales have good low frequency hearing for communication,

odontocetes (toothed whales) use high frequency sounds (1 to 100 kHz) to echo-locate and

communicate and pinnipeds use mid-range levels with a maximum sensitivity between 10 to 50 kHz.

The high frequency sounds made by the toothed whales tend to be directional and better capable of

discriminating against unwanted background noise (Gales, 1982). This capability to discriminate

29
by directional quality in making noise is referred to as the directivity index (DI) in decibels (Gales,

1982). Numerous auditory sensitivity curves have been created for smaller odontocete whales and

pinnipeds (Cosens and Dueck, 1993). Measurements of the hearing capability of the large mysticete

whales are not available, although there are numerous recordings of mysticete whale vocalizations

(e.g. Thompson et al. 1992).

One study showed beluga whales (Deiphinapterus leucas) and narwhals (Monodon

monoceros) changing their behaviour 40 km or more from ships (Finley et al. 1990). Another study

calculated a detection distance of 30 km for belugas and narwhals for a particular ship and the high

frequency noise component (5,010 Hz) it generates (Cosens and Dueck, 1993). This study, and

others, in the Eastern Canadian Arctic, may be dealing with very naive populations that react to any

unusual sound because of the quiet nature of their environment. Populations of the same species in

the Beaufort Sea (which has much higher levels of ship noise) may react less to unnatural noises

(Cosens and Dueck, 1993). The models of Gales (1982) have shown that marine mammals maybe

able to hear the sounds of offshore oil and gas exploration as far as 190 km away and even farther

under favourable conditions.

The use of underwater acoustical harassment devices (AHD's) in attempts to scare off

harbour seals (Phoca vitulina) from poaching at fish farms on the central British Columbia coast

may have resulted in eliciting avoidance responses in minke whales (Balaenoptera acutorostrata),

humpback whales (Mageptera novaeangliae) and harbour porpoise (Phocoena phocoena) in areas

where they were previously seen (Morton, 1994). Observations of the playback of drillship noise

and airgun (acoustic geophysical device) concussion sounds to gray whales near St. Lawrence Island

in the Bering Sea was published by Malme etal. (1986). They found that drillship noise of 110 dB

was the lowest level that may cause a disturbance of whale feeding activity. At 120 dB, greater than

50% of the local population will avoid a feeding area. 163 dB was considered a level at which

feeding disturbance was possible for an airgun firing. It was found that 173 dB was the level at

which 50% of the local whale population would avoid a feeding area (Malme etal. 1986). Similarly

bowhead whales (Ba!aena mysticetus) reacted adversely to playback of 122 dB dredging noises (see

Morton, 1994). Observations taken by crew members on oil drilling platforms in Cook Inlet in

Alaska noted that beluga whales came very close to the platform when it was in operation but reacted

to noise such as machinery being turned on, and helicopters and boats (Gales, 1982).

Baleen whales may be of greatest concern to marine mining because of their low frequency

communication and endangered status. However, it is difficult to perform controlled studies of

them. Broad band low frequency sounds produced by offshore operations (e.g. dredging, oil

platforms) might be audible to baleen whales over very long distances assuming optimal conditions.

Means of dampening the sounds of machinery may be needed. The human pain threshold is 120 dB

in air, and these levels may also apply to aquatic systems for marine mammals (Turl, 1982 and

Morton, 1994).

In principle it is appropriate to design dredges to be as quiet as possible. Experienced dredge

operators have anecdotal stores of knowledge about the reactions of marine mammals and other

wildlife to their vessels, and means should be found to record and make use of this information.

30
3.2 Scenario 1: Precious Metal Mining, Alaska - Bucket Ladder Dredge

3.2.1 Mining Scenario

Within the U.S. OCS, the precious metals gold and platinum are known to occur

predominantly around the Alaska coast. Potential areas for exploration and eventual marine mining

are indicated by the existence of onshore placers and by glaciation which has carried sediments

offshore. Dredging for gold has been attempted several times this century but only WestGold

achieved a significant production using a bucket ladder dredge from 1986 through 1990. The

exploration efforts and the environmental research of WestGold are fairly well documented in

various publications. Much of the experimental work on reducing the environmental effects of the

WestGold dredging is applicable to other forms of dredging. Therefore the Nome bucket ladder

dredging project constitutes a very useful scenario for study, even though, given the present gold

price, future marine mining efforts are more likely to be directed towards the recovery of aggregates

and heavy minerals by other dredging systems of excavation.

3.2.1.2 Location

In order of priority, the main areas of interest are as follows:

Various sites around the Seward peninsula, especially opposite the City of Nome;

Offshore from Goodnews Bay (platinum) in the Southern Kuskokwim Bay;

The northern part of the Gulf of Alaska, especially between Cordova and Lituya Bay (gold)

and around Kodiak Island.

All of the above areas are in State waters and some, including that of Nome, extend into

Federal waters beyond the 3 mile limit.

3.2.1.3 Marine Conditions

Each Alaskan area is exposed to the variable climatic conditions characteristic of the Bering

Sea. The harshest climate is that prevailing around the coast of the Seward peninsula, including

Nome. Other potential mining areas may experience less harsh winter climates but are more exposed

or offer less infrastructure. The differences all impact on the economics of potential offshore mineral

deposits which is the final arbiter. Nome illustrates all the features of importance and the figures

quoted are actual ones.

Weather in the Nome area is dominated by the circulation of winds about the relatively

stationary low pressure cell that lies over the Gulf of Alaska and the high pressure cell over the

Arctic Ocean. Circulation around the low in the winter brings winds from the east or northeast most

of the time. Expansion of the Arctic high southward in the summer brings winds from the west

during most of the summer season. The wind blows predominantly from the SW to NW in June.

41
It gradually veers south, but exhibits a full range in August. Swinging to the east it blows

predominantly from the east and north by November. The average wind speed gradually increases

from 16 to 18 kmlh in June/July until it reaches 19 to 26 knii'h in November. The maximum 3-hour

wind speed of 56 km/h is attainable at any time from June through November. By September the

wind may be blowing and gusting at more than 48 kmlh for nearly one fifth of the time. A persistent

wind constantly exceeding 32 kmlh may blow in June/July for nearly 20 hours duration, a figure

which increases to over 50 hours in October.

Air temperatures range from a maximum of 30° C observed in July to a minimum of -41°

C in February. Average temperatures are more moderate, being in the range 15 to 24° C in summer

and, except for cold spells, in the -12 to -7° C range in winter. Precipitation is light, averaging about

40 cm/yr., about half as snowfall and the rest as light rains and drizzle.

Water temperatures decrease from around 12° C in early August to -3° C which is attained

in the last week of October or first week of November. By then the average daily air temperature

has declined to between -12 to -7° C. Usually by the second week of November the sea has frozen

over.

Ice develops along the shore of the open sea in early November, but generally is not

shore-fast until mid to late January. Ice cover reaches a maximum thickness of about 1.2 m in March

over 80 to 90% of the sea surface. Wind shifting causes rafting and pressure ridging which result

in much greater ice thickness locally. Break-up in the spring can be at any time from early May to

early June.

Sea salinity ranges from about 2O%o in summer to about 34% in winter. The summer

decrease is caused by river input and mixing. The low salinities and temperatures in summer retard

corrosion of immersed metals to a rate significantly less than that experienced further south along

the west coast of the U.S.A. The sea water is clearest in late winter. By mid-July the suspended

sediment load increases from around 2 to 7 mg/l offshore to around 15 mg/l closer inshore (Hood

et al. 1974). Thereafter it increases steadily with the inflow from local rivers. The natural load

increases with depth. It rises to 100 mg/l or more later in the summer, especially with the incidence

of summer storms.

The Nome coastline is subjected to a significant deep water swell from the southwest with

a wave period which averages between 6 and 8 seconds. One third of the swells exceed a period of

8 seconds. The swells exceed 1.8 m in height in September, October and November when they may

reach 3.6 m in height. They are exaggerated in inshore water depths of less than 18 m. The average

wind-generated sea wave height superimposed on the swells steadily increases from about 0.6 m in

June to 1.2 m in October. A storm is officially defined as the maintenance of 56 kmlh (or greater)

winds for 6 hours or longer. On this basis, an average of three storms occur each year from May

through November; one third in each of October and early November, and the remainder throughout

May to September, inclusive. During the worst of these storms the waves regularly exceed 4.6 m,

with sustained wind speeds of at least 74 km/h (Oceanographic Services, Inc. 1974; 1975). The

wave height is sustained for a day or more. The worst storms, usually in October, combine sustained

42
wind speeds of around 93 km/h with gusts exceeding 185 kmlh. Wave heights of 6.1 m may be

maintained for up to 12 hours.

3.2.1.4 Geological Background

Kaufman and Hopkins (1988) attributed the spacial distribution of particulate gold in the

sediments covering the coastal plain at Nome, including the offshore, to "a complex late Cenozoic

geologic history. The diverse glacial, marine, fluvial, and deformational processes that shaped the

sedimentary sequence on the emergent coastal plain have operated on the submerged continental

shelf south of the Seward Peninsula." Kaufman and Hopkins (1988) believed that "the upper 20 m

of the offshore deposits have been pervasively reworked. Below 20 m, sediments accumulated more

continuously in marine conditions and contain minor amounts of particulate gold. The greatest

(gold) concentrations are found within 4 m of the sea floor; lag gravel deposits formed on till of the

present sea floor are especially enriched." An understanding of the gold distribution "must take into

account the possibility that glaciers advanced on to the coastal plain" including what is now the

offshore "during a period of low relative sea level."

Geological interpretations and all further references to lithologies and grades are derived

from seabed samples, aided by the interpretation of seismic and side-scan sonar records. The seabed

samples were obtained either by drilling with a Becker 180 hammer drill (over 7500 completed) over

a fixed grid, by bulk sampling with a back-hoe in shallow water through the winter ice, or by

sampling the loaded dredge buckets during dredging operations.

Dredged Material

Various writers have attempted to classify the offshore gold-bearing sediments in terms of

either their origin or their constituents. The glaciers extending into the present day ocean eroded not

only bedrock but earlier gold placers of marine and alluvial origin. The transported load was

dumped as glacial moraine and outwash deposits which were subsequently modified by marine

agents to create beaches and lag gravels. All the offshore sediments may be described in generic

terms (till, lag gravels, outwash, beach, etc.), as quantified admixtures of silt/clay, sand, pebble and

cobbles, or in terms of their complete particle size distributions. The deep seated till and all the other

glacial and marine products may be described in these terms. The different sediments have been

redistributed to varying degrees by faulting and over- thrusting with the result that they now exist

as an intermingled, partly stratified accumulation of mud, clay, sand, gravel and till. Changes tend

to occur over 10's or 100's of metres laterally and over less than 1 m vertically. Whatever the

sequence at a particular location it is usually overlain at the seabed by a lag gravel with

discontinuous thin rippled sand. In places, remnants of beaches and drowned sand spits are

discernible. Some drowned fluvial sediments, deposited in erosional channels that cut into the till,

exist offshore. The sediments in the upper 3 m are usually till (often referred to as diamict or clayrich diamict), together with overlying and inter-bedded sand, gravel, sandy mud and mud. The

sequence, topped with a lag gravel, may be repeated at depth. The in situ sediment density varies

from about 2.0 to 2.35 g/cc for till, and is around 2.35 g/cc for silty sand. The water content

immediately below the seabed averages 10 to 15 %.

43
Size Variations in Dredged Material

The percentage representation of each grain-size classification, if containing gold whatsoever,

is illustrated in Table 3.3. During some drilling programs, and in dredging operations, the qualitative

classification of a sample is based on a visual examination rather than on a rigorous, quantitative,

size analysis. In reality, however, each descriptive classification is a mixture of defined particle

sizes. Thus, sand contains some silt and gravel, gravel contains some sand, and till contains between

50 to 75% silt and sand.

The particle size distribution at any given dredging site depends on the extent to which each

lithology is represented in the dredged ground at the site and on the very localized size

characteristics of the lithology. For dredging production purposes there are three main constituents

of importance: (a) silt, because it causes turbidity in the water; (b) clay, because it may cause losses

of clay in the treatment plant, as well as generating more silt when it is successfully treated to

liberate the contained gold, and (c) cobbles and boulders which reduce the throughput rate. Bulk

sampling through the winter ice provides the best samples for measurement purposes. Otherwise

drill samples, if properly collected, may be used to estimate the silt and clay content of ground to

be dredged.

During production the monitoring of trommel oversize generates the cobble/boulder content.

Seabed sampling by divers or the sampling of dredge buckets can give biased results. Depending

on the exact location and the depth of the dredging (below the seabed), the combined silt and clay

content may vary between 10 to 40%. The silt content of the dredged ground increases seawards,

as indicated by drilling in 1 m vertical sections is given in Table 3.4. In general, around 2% of the

dredged ground down to 3 m below the seabed exceeds a particle size of 30.5 cm and this fraction

is sub-angular to rounded and often slab-shaped. Locally, higher concentrations of boulder occur.

Rounded, erratic granite boulders up to 1 m in diameter occur in places.

Distribution of Gold

Gold exists throughout all the ground types to varying degrees. A part of the Nome offshore

area is represented by typical figures in Table 3.5. On average, the gold exists in a very widespread

sense at a low grade, but reserves must be delineated with a proven grade several times that

widespread figure. The highest gold grades generally occur in the uppermost 2 m of the sediments,

and especially in the top 0.5 m where the gold is concentrated in a lag gravel. Such a sequence may

be repeated at a depth of a few metres in localized areas. In places the highest grades tend to occur

where the measured sand content is 50 to 70%; this may take the form of a sandy gravel in which

the silt content may average nearly 10%, or of a washed till with a silt content of around 15%. The

total gold resource is oriented parallel to the coast. It has a length of about 24.8 km and extends

from the foreshore to, in places, beyond the 3 mile limit into Federal waters. The offshore extent

reflects the boundaries of the footprint of glacial deposition. It also mirrors the local intensity and

distribution of placer mining activities onshore.

44
Isolated occurrences of gold, not yet investigated by drilling, have been worked on the

foreshore for several miles to both the east and west. Scout drilling is at a spacing of about 300 m

or more. Possible reserves are delineated by 100 x 100 m or 200 x 200 m drilling. A drillhole

spacing of 50 m, or in places much less, is required to establish proven reserves. The grade, as

measured by drilling, varies very significantly over distances of a few metres. Water depths increase

seaward from the low water mark to about 17 to 20 m at the 3 mile limit. Thereafter the depth

increases gradually offshore to a limit of about 27.4 m. Subtle variations in seabed elevation reflect

the outline of the glacial debris and of possible, more recent beaches and still-stands. No beach is

as well defined as the present one. The evidence is supported by side-scan survey data.

Geotechnical Data

Blowcount observations throughout the resource, coupled with standard penetration tests,

indicate a relative density for the sediments varying from medium dense to very dense (R.& M.

Consultants, 1988). The relative density generally increases with depth and most areas mined are

classified as "dense" within 3 to 5 m below the seabed. Drilling is normally terminated at 10 m or

less if penetration is refused at around 300 to 400 blows per metre. From triaxial tests the surficial

silt is defined as "soft" but as the sand and fine gravel content and depth below seabed increase the

material quickly becomes "stiff', "firm", and eventually "very firm". The sediments show consistent

signs of over-consolidation. A mixture of clay, silt, sand, gravel and till exhibits a swell of 10 to

15% with some daily figures as high as 20%. Although permafrost has not been identified offshore

it exists over large areas immediately above the high water mark. Its possible presence at depth

offshore below the resource and, thus, its influence on the characteristics of the sediments,

determining the ease with which they may be excavated, cannot be discounted.

3.2.2 Dredging System

The dredge typically used for such hard digging conditions as occur offshore Nome is a

bucket ladder dredge. Onshore, Alaska Gold has, until recently, continued to employ two 255 litre

bucket ladder dredges to work ground geologically similar to that existing offshore. The dredges

are capable of achieving their theoretical throughput, however, only if the ground is thoroughly

thawed in preparation. One dredge, No. 6, has been working immediately onshore from the high

water mark.

Dredging offshore was conducted by WestGold using the world's largest ocean-going bucket

ladder dredge, the BIMA, with 850 litre bucket capacity (Garnett, 1996a) (Figures 3.8 and 3.9).

Detailed descriptions and specifications of the BIMA are given in sections 3.2.2.2 to 3.2.2.5. The

dredge is manoeuvred on 4 anchored side lines and an anchored headline. During storms a stern line

is also deployed. The ground dug by the buckets is delivered to two revolving trommels fitted with

9.52 mm and 25.4 mm screens. The undersize is delivered to a treatment plant comprising

multi-stage jigging and final clean-up on shaking tables. Some screen oversize passes through a clay

treatment plant before being returned to the main circuit. The dredge is fitted with a diesel-driven

power plant of more than 5220 kW, generating 440 volts AC. Some components are DC motordriven through thyristors from the main power system. The vessel meets with Bureau Veritas

45
approval and complies with all the rules and regulations of the U.S. Government and the U.S. Coast

Guard.

The dredge maintains a forward pressure by means of an 800 m headline anchor, about which

it swings on an approximately 150 m face. Its lateral motion is achieved through 4 sidelines: 2

forward each of 250 m length, and 2 aft each of 300 m length (Figure 3.10). Except during the very

early part of the dredging season, the dredge ladder is maintained in a position facing the incoming

swells. The dredge effluents, principally the trommel oversize and the combined jig tailings, are

therefore normally discharged on the shoreward side of the dredge.

Where the dredge cut is several metres deep so as to excavate more than one higher grade

horizon, the dredge advances between 3.0 m and 3.5 m at a time with a drop of 0.5 m. The other

extreme is where a single, shallow cut is required to remove the single, surficial lag horizon, in

which case the advance is about 6 m. After a total advance of around 100 m the angles formed by

the side lines relative to the line of advance of the dredge require that all the anchors be moved ahead

accordingly by the same distance. A complete forward move of the anchor sets can be achieved well

within the 12 hour day shift in good windlwave conditions. Under less than good conditions it either

takes longer or becomes impossible. The face width may be as great as 200 m, depending on a

number of features such as the drill spacing and the need for selective extraction occasioned by the

distribution of the reserves and the geology.

The rate of throughput is dictated by the speed of the buckets (buckets/minute), and the

bucket fill factor which, if the correct bucket shape/size is used and the swing speed, bucket speed,

and ladder angle, etc. are optimum, is governed by the characteristics of the ground being dredged,

expressed as its resistance to digging. The performance is also sensitive to the wind/wave

conditions. In very good digging conditions the dredge should operate at 28 buckets/minute with

a maintained bucket fill factor of 70 to 75%.

Typical operating conditions at the Nome site are given in Table 3.6. The dredge operating

season extends from approximately the first week of June to the second week of November, both

limits being determined by the presence of sea ice. During winter the dredge is anchored at the

docking facility, protected from moving ice fronts by an all-surrounding snow berm. On average

the total possible number of operating days is between 155 andl6O.

The gold values do not extend down to bedrock. Enrichment, if any, occurs on a "false

bedrock" clay, which often represents the interface between the base of a lag gravel and the

underlying till. All material dredged is fully treated. Offshore Nome the bucket speed varies

between 14 and 28 buckets per minute, with an average of around 20 buckets per minute. The

maximum dredging depth is rarely as much as 20 m of water and ground combined and the depth

of cut below the seabed does not exceed 10 m. Very frequently it is only 3 m and a minimum cut

of 2 m has been achieved in places. The fill factor varies between 10 to 60% in shallower water of

less than 10 m, with an average of about 40%. In deeper water of more than 10 m the fill factor

varies from 30 to 80% with an average of about 60%. The eventual throughput rate, on a daily basis,

varies between a range of 200 to 600 m3/hour in shallow water of less than 10 m with a cut depth

46
of 2 to 9 m, and a range of 500 to 800 m3/hour in deeper water of more than 10 m with a cut depth

of 2 to 4 m.

3.2.2.1 Design Criteria

The BIMA was designed for dredging to a maximum depth of 45 m below sea level at a

ladder angle of 55° below the horizontal. The minimum water depth for dredging is 10 m, and an

ideal digging face slope is 1:2. Maximum output of the bucket line is 2,043 m3/hr. Under ideal

conditions the maximum capacity of the treatment plant is almost 3,000 dry metric tons/operating

hour. A speed-controlled 1,343 kW drive gives 30 to 36 buckets/minute at maximum horse power

or 0 to 30 buckets/minute at maximum torque. Dredging in waves is possible if the maximum

movement of the bottom tumbler does not exceed 2 m vertically and longitudinally, or 0.4 m

laterally. The dimensions and specifications of the BIMA are given in Table 3.7.

3.2.2.2 Treatment Plant and Products

Two revolving circular screens, each with a perforated length of 17.2 m and a diameter of

3.4 m, with sparge and monitor water, receive dredged ground from the bucket line via the drop

chute. Undersize at 25.4 mm passes to the primary jigs which aim to recover all the contained gold.

Oversize from the screens is discharged into the sea down the screen oversize chute. Ideally, all

materials are slunied so as to liberate fully the gold. The concentrate from the primary jigs is passed

to the secondary jigs which serve to concentrate the gold and thence pass it to the gold room.

Tailings from both the primary and secondary jigs are combined and discharged into the sea down

the twin tailings launders which extend aft of the dredge. The small tailings discharge from the gold

room enters the sea over the side of the pontoon. Barren or unpayable ground may by-pass the

treatment section altogether and be discharged aft via the single overburden stripping chute. No

chemicals, reagents or substances of any kind are added to the dredged material for the purpose of

gold recovery.

3.2.2.3 Support Facilities

The operation is supported by offshore and onshore facilities as shown in Table 3.8

3.2.2.4 Dredge Products and Effluents

The main dredge effluents are the screen oversize and the tailings from the primary and

secondary jigs and from the gold plant. The product is the gold-bearing concentrate which is

transported ashore by helicopter for final clean-up. Both the gold concentrate product and the

discharged effluents can be quantified by observation and deduction. Suspended material from the

excavation process is introduced into the water column underwater at the bottom tumbler and from

off the loaded, rising buckets. Spillage also re-enters the water via the ladder well if it is not

collected by the save-all (Figure 3.11). It is possible to make a guess at the volume of solids entering

the water column in these ways, but there is no practical way to quantify them. Under ideal

conditions treating, for example, 900 m3 of ground per hour the distribution of solids and water is

47
as shown in Table 3.9. All the water intake is discharged with the primary and secondary jig tailings

which are combined in the two tailings launders. At Nome, some of the varying conditions are

indicated by the reconstructed annual average figures for the more important performance parameters

(Table 3.10). The numbers in this table are based on both direct measurements and professional

deduction. The annual variations in performance reflect not only the gradual improvement in

screening efficiency over the years but also the variability of the ground. For example, a typical

monthly average feed (from 1987) as recorded on the dredge might be comprised of an observed

combination of silt, clay, sand, gravel and till. But taking into account the integral composition of

each of the above field classifications the silt content becomes emphasized (Table 3.11). There is,

in fact, twice the amount of silt and 50% more sand than the dredge observation indicates. Both the

material dredged and the throughput rate vary considerably not only from one day to the next but

also throughout a single day. The ease of excavation, the existence of clay, and the presence of

boulders all serve to influence the throughput rate. In shallower water of less than 10 m very rarely

do 3 consecutive days show less than a 20% variation in throughput rate: it is more usually 20 - 50%

on consecutive days. In deeper water the variation is usually 10-30% from one day to the next.

The relative content of silt, sand, and gravel being discharged from the dredge, together with

the rate of discharge, often therefore bear little relationship to the monthly and annual averages.

They may change from one hour to the next, according to the nature of the ground being dredged,

regardless of the efforts of the dredge management to maintain uniformity of feed and rate. The

throughput rate changes accordingly and is affected by the prevailing wind/wave conditions. The

metallurgical efficiency of the treatment plant shows an inverse relationship to the throughput rate

within some ranges. However, the most important attainments in order to maximize the daily gold

production are maximum operating hours per day and maximum throughput rate.

3.2.3 Geochemistry

Environmental information on the WestGold operation is extensive, but much of the discussion on chemical effects associated with dredging by the BIMA is equivocal. This stems from

a number of common problems in sampling methodologies (collection and handling), chemical

analysis, and reporting. There is also no information on the in situ geochemical character of the

deposits that were dredged, other than the bulk chemical analysis of grab samples. In particular, no

data are presented on the presence or absence of dissolved H2S, arsenic, iron or manganese in the

sediment pore waters. It is now known that such data are needed if a reasonably rigorous assessment

of post-dredging chemical behaviour is to be made (see Section 2.2). Nevertheless, if data are

carefully selected from the databank (e.g. dissolved and total metals measurements from splits of

the same bulk sample) it is apparent that the occasionally high metals levels reported stem from

particulate loads only and not from metals in solution. There is no evidence for systematic addition

of dissolved metals to the waters in the vicinity of the dredge. Similarly, there are no unequivocal

data that indicate uptake of metals by organisms in the area.

There are indications that red king crab may accumulate arsenic and nickel, and also that the

crabs might be able to purge themselves of nickel and chromium when moulting their shells. Some

of the crab feed-stock species show erratic elevations of trace metals at the test sites, and in some

48
cases reductions. These variations illustrate the technical difficulty of reaching clear conclusions

with the known variability of trace metal tissue levels. The variability is derived both from

analytical procedures and the biology of the species being tested. Specifically, there is no evidence

for contamination effects particularly from mercury. It is reasonable to conclude that the WestGold

dredging operation did not produce significant deleterious chemical impacts on the local marine

environment. This conclusion is consistent with the principles described earlier in Section 2.2.

Caution is needed when using the WestGold information for determining impacts and mitigation procedures. Providing that potential sites have the same sedimentological and oceanographic

characteristics as those for offshore Nome, the conclusion in the above paragraph may be

extrapolated to such locations. From the broader perspective, only limited generalizations can be

made from the WestGold operation's geochemical information.

Although chemical impacts can be expected realistically to be minor or negligible in the

dredging of coarse sedimentary substrates, different sediment textures, organic contents, and types

of organic material can establish geochemical contrasts from site to site. The adoption of a

precautionary approach requires that these characteristics be taken into account on a site-specific

basis in future projections of environmental impact.

3.2.4 Seabed Biodiversity and Resources

3.2.4.1 Locality and Dredging Characteristics Affecting Benthos

The WestGold site is a shallow area with depths sloping gently to approximately 30 m

(Figure 3.12). Dredging was confined to a range of 7 to 20 m (Fig. 3.13). This depth range is

sufficient that the benthos show depth-related changes. As a high-energy coastline the substrate is

characteristically poorly sorted with a high gravel content (Appendix 6), and clay contents ranging

up to 24% at some localities and up to 97% at the deepest sites (20 m). Environmentally significant

operating parameters for the bucket ladder dredge, BIMA, are shown in Table 3.12.

The distribution of substrate types in 1990 following dredging and sampling include "fine

sand/mud", "gravel/sand", "sand waves", and "cobble". From Section 3.2.3 it is reasonable to

conclude that the redox boundary was probably 0.1 to 0.5 m below the seabed. This is sufficiently

deep for ample small burrowing benthos requiring aerated conditions.

Sampling stations providing the most relevant data are stations C2 and C3 (Controls) ; S2

and S3 (Subsistence) and R6 and R7 (Dredged) (Figure 3.13 and Appendix 6). Subsistence stations

were in areas used for subsistence fishing by the local community. All sampling stations were

characterized by cobble and sand areas. Station R6 was dredged in 1986. Station R7 was dredged

in 1988, but was impacted by nearby dredging in 1987, 1989 and 1990. Following changes in the

original mining pattern, sampling at Stations R6 and R7 was initiated after or during dredging, hence

no before and after comparisons are possible. However the two dredged stations (R6 and R7, both

areas of cobble and sand) can be compared with equivalent areas at both Control and Subsistence

49
stations. Effectively there were two sets of Control stations for all Dredged stations. Reviews are

provided by ENSR (1991) and Garnett and Ellis (1995).

3.2.4.2 Benthos Pre-dredging

ENSR (1991) provides details of biodiversity from 1985 to 1990 (Appendix 6). The number

of taxa (species biodiversity) before dredging at the various stations ranged upward from about 30,

with numerical diversity ranging upward from about 500/m2. These fall within global ranges for

seabeds. On sand deposits the most abundant forms present in 1985 before dredging included

several hundred specimens/rn2 of two species of known sand-bed genera of bivalve molluscs, Tellina

lutea and Macoma lama. The remaining dominant species were polychaete worms, with the

exception of the sand dollar Echinarachnius parma, also a sand-bed genus. In both Reference

stations the five dominant forms alone totalled no more than 1,000 organisms/rn2.

On cobble substrate the pre-dredging benthos in 1986 were less abundant with total numbers

ranging up from about 500/rn2. The dominant species were also different from those on sand predredging a year earlier with no species overlap. Polychaete worms remained abundant but with

different species. The rnollusc present in largest numbers was the cockle Clinocardium

californiense. The echinoderm fauna was represented most abundantly by both sea urchins

Strongylocentrotus droebachiensis and brittle stars Diamphiodia craterodmeta.

3.2.4.3 Benthos Post-dredging

On sand beds, species and numerical diversity were low at the R6 and R7 sampling stations

in 1987 and 1988 one year after dredging. Numbers were lower than were maintained before and

after the dredging years at the various Reference Stations. These reductions also show in the

dominant species list (Appendix 6). In 1987 at R6, four species of small crustacea, one of which was

new, were most abundant. By 1988, Station R6 was recovering with more usual species, and this

trend continued to 1990. At Station R7 there was a similar pattern of reduced species but recovery

was progressing more slowly. However, there were molluscs present in 1988 and 1989, which may

be slower to recolonise than polychaete worms and crustacea.

On cobble beds there was a similar pattern of reduced biodiversity in the year of, and

following, dredging in 1987, with little indication of recovery by 1990. Lists of dominant species

also show reduced numbers. Species present are generally the same as in the Reference stations,

although there are a few novel species in the recovering areas.

In general terms the sand bed biodiversity started to recover within 1 year of dredging. A

few new species entered the fauna and later disappeared, but there is no clear successional pattern.

By 1990, four years after dredging at Station R6, the biodiversity was clearly back into the range of

the four Reference stations.

Cobble bed biodiversity appears to have recovered more slowly. The number of species

remained as high as the Reference Stations, or almost so, even 1 year after dredging. However, the

50
51

number of organisms showed a reduction barely reaching the lower end of the year to year range at

the Reference Stations. By 1990, R6 had done better than R7, dredged a year later. The

corresponding physical changes for the 1986 dredged area (containing Station R6) are shown in

Figure 3.15.

The ENSR (1991) report in summary states:

"The extreme physical disturbance of dredging causes significant alteration of benthic invertebrate communities."

"Recolonization of these disturbed communities appears to begin immediately after dredging

stops."

"Recovery of these communities to their original structure may not occur; instead a

somewhat different assemblage of the original species may result."

"Recolonization of the dredged areas with comparable density, biomass and number of taxa

to control sites may require 3 to 4 years for sand substrate and 5 or more years for cobble

substrate."

"Recolonized dredged areas may have altered community structure but functionally are

similar to the original communities."

3.2.5 Fisheries and Wildlife Resources

The one fishery species monitored in detail was the red king crab (Paralithodes

camtschatica). It was found that crabs move locally over distances which allowed them to transfer

between dredged and undredged areas. Their movements prior to dredging were apparently random,

although crabs often have seasonal shifts with depth. Crabs were fished from both dredged and

undredged areas, with more competition by boats for fishing sites in the dredged than the undredged

areas after mining started. Feeding habits of the crabs at dredged and undredged sites appeared not

to differ. Crabs are opportunistic feeders and take almost whatever is available. Crab monitoring

was scheduled to be discontinued in 1991 due to satisfaction that the feeding studies were adequate,

but also due to the technical impossibility of obtaining clearer results on crab movements relative

to dredged and undredged sites (Garnett & Ellis, 1995). A list of resource species in the area is

provided in Table 3.13. No impact was demonstrated on these species that, in general, are

opportunists not selecting localized areas in conflict with dredging operations.

3.2.6 Biological Oceanography

A bucket ladder operation may produce several distinct types of water column effects

because of the large volume of materials dredged from and returned to the sea floor (500 to 1000

m3/hr). Particles larger than silt (> 0.06 mm) fall rapidly to the bottom creating a turbidity plume

a few metres in diameter extending from the surface discharge point to bottom impact. Due to the
density of this plume, anything not able to avoid it will be destroyed. From the inevitable noise level

of the operation it is doubtful that many large fish, birds and mammals would remain in the

immediate area during dredging operations and therefore there would be little direct impact on this

population. However fish are known to aggregate around tailings plumes (e.g. the Marcopper Mine

in the Philippines) and to support local subsistence fisheries (Ellis et al. 1995). Planktonic

organisms such as algae, copepods, jellyfish etc. are unable to avoid the tailings discharge plume

and some will be entrained and destroyed. Given the low density of biomass within the water

column (mg/rn3) and the restricted area of impact (150 to 300 m2/hr) direct biomass destruction can

be predicted to be only a few grams wet weight per operating hour.

Fines (clay and silt < 0.06 mm) enter into the water column through the dredging process,

dredge bucket spillage, and surface discharge of tailings slurry. The amount of fines in the tailings

slurry depends to a large extent on the clays and silts in the feed composition. At WestGold's Nome

site this reached as much as 25% of the total dredge throughput (125 to 250 m3/hr). Such fine

material settles very slowly and can be expected to remain suspended in the water column for

considerable periods of time and over extended distances.

Sediment turbidity plumes generated by WestGold's Nome operation were modelled by the

DIFCD (Disposal from a Continuous Discharge) numerical model (ENSR, 1991). Model results

indicated that the extent and concentrations of the turbidity plumes were sensitive to operating

conditions (throughput and depth) but depended primarily on the composition of the dredged

materials (i.e. the percentage of silt) and the physical oceanography at the site. Under the most

stringent conditions of high clays and rapid currents, turbidity plumes >25 NTU over background

were predicted at all depths up to 4 km from the source. At WestGold's Nome site turbidity plumes

were easily recognized visually at a distance of 1 km or more from the dredge. Further details

concerning plume modelling at WestGold's dredge site are provided in Section 5.1.5.3.

In Alice Arm, B.C., tailings from the Kitsault Mine (approx. 270 m3 solids/hr) that were

discharged continuously at a depth of 100 m remained in suspension as a near-bottom turbidity

plume at an average distance of 5 km from the source (Pedersen et al. 1995). Natural glacial runoff

turbidity in Alice Arm extended for an average of 8 km down-inlet in surface waters. Therefore the

predicted distance of 4 km for a dredges tailings plume is not unreasonable. There was no evidence

that the plume arising from WestGold operations was environmentally damaging. In Alice Arm an

intensive short duration program to determine the effects of tailings discharge on the inlet was not

able to determine any adverse effects of this turbidity on water column biota (Rambold and Stucchi,

1983).

Bucket ladder dredging did not introduce unusually high trace metal levels to the water

column (ENSR, 1991). Mussels suspended within the turbidity plume for a 1 year period did not

show any bioaccumulation of metals that could be linked to the turbidity plume. Apart from visual

discoloration and minor losses in primary production due to light scatter, there was little direct effect

of turbidity on the biota.

52
3.2.7 Summary of Environmental Impacts, Mitigation Objectives and Assessment Criteria for

Scenario 1

3.2.7.1 Environmental Impacts

Water colunm impact was limited to a turbidity plume extending from the dredge discharge

points. Seabed disturbance was caused by the dredge cuts and deposition of tailings. Dredge

footprints were substantially smoothed by wave and current action within 3 to5 years, even on the

coarsest gravel/cobble habitat. A level of biodiversity encompassing several tens of species and

several hundred organisms/rn2 had recolonized the impacted areas within 3 to 5 years, with recovery

slower on coarser fraction gravel and cobble habitat.

There was no deleterious impact on the local fishery for red king crab, neither in terms of

stock or contamination (from bioactivation of prior mercury settling). There was competition for

fishing sites close to the dredged areas.

3.2.7.2 Mitigation Objectives

Specific mitigation objectives included minimization of turbidity, minimization of release

of toxic materials and bioaccumulation of mercury (residue from prior beach mining). There was

also to be minimal effect on the local red king crab fishery. Benthic disturbance was not constrained,

but a sampling program was required to monitor impact and recovery on both sand and gravel

substrates. Responses included a sub-surface outfall discharge, constraints by the regulatory agency

on discharges and receiving area conditions, and a substantial monitoring program to assess whether

the constraining criteria were met.

Examples of effluent and receiving area constraints set by the regulatory agencies are provided in Tables 7 and 8, Appendix 6. The major receiving area constraint was a requirement for <25

NTU (turbidity) units above background at a 500 m radius from the point of discharge at the dredge's

stern. Maximum trace metal levels were also specified for a 100 m radius mixing-zone boundary.

Sensitive species and habitat were not an issue at this site.

3.2.7.3 Mitigation Achievement Criteria

Turbidity reduction at surface was appraised by appropriate monitoring with the sampling

plan shown in Figure 3.16. Achievement of the permit constraints were appraised by monitoring

effluent and receiving area as frequently as 3 times daily (see Tables 7 and 8, Appendix 6). The red

king crab fishery was considered to be unaffected based on 5 years of monitoring by test fishing.

Test for the release and bioaccumulation of mercury presented considerable analytical problems

initially, but once resolved provided no evidence of impact.

53
3.3 Scenario 2: Precious Metal Mining, Alaska - Underwater Miner

Precious metals are known to exist offshore as surficial concentrations in lag gravels or with

industrial heavy minerals in the upper surface of sand bars, banks and spits. They are also

concentrated in outwash channels on gold-bearing glacial moraines that have since been inundated

by the sea. Examples of such deposits exist around the Alaskan coastline. A Surficial concentration,

especially if no more than 0.25 to 0.5 m in thickness, constitutes a target deposit for excavation by

an underwater miner if:

the deposit is beyond the reach of conventional offshore dredging equipment such as a

trailing suction hopper dredge

the deposit is too close inshore for other equipment to be deployed safely

the deposit is too small to justify the mobilization of larger equipment

the sea conditions prevent the deployment of a suitably small alternative dredging unit

very selective extraction is required, or

the grade is high enough.

An underwater miner (UWM) constitutes the opposite extreme of a large bucket ladder

dredge: it has a comparatively very low throughput, is limited in terms of the depth to which it can

dig below the seabed, and has the ability to be very selective in its extraction, both laterally and

vertically. Dilution is minimal but unit volume costs are high. A bucket ladder dredge, by contrast,

relies for its economic success on its digging power and its high throughput, resulting in a low unit

volume cost. It is very non-selective, laterally and vertically, and a surficial deposit worked by it

is heavily diluted in grade.

WestGold deployed a UWM in the Nome area of Alaska in the summer of 1989 (Garnett,

1991). This was the first recorded occasion when such equipment had been used for an offshore

mineral recovery program other than remotely operated collectors in the earlier deep ocean

manganese nodule projects. The UWM used was the "Tramrod", model 250, developed and

manufactured by Alluvial Mining of the United Kingdom.

Another UWM, the original version of which was designed and built in France, has been

employed since 1990 in the 120 to 140 rn-deep west coast waters of Narnibia in Southern Africa

(Garnett, 1995). It is used for the excavation of diamondiferous gravels approximately 20 km

offshore. The deployment of additional units, both marine and shoreline versions, is presently being

investigated. The equipment also may be used for small-scale beach renourishment projects,

excavating small, localized pockets of potential beach sand.

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55

3.3.1 Mining Scenario

3.3.1.1 Location

Examples of the deposits amenable to exploitation by means of a UWM exist around the

Alaskan coastline, including the Alaskan panhandle. All of the areas are in State waters and some,

including that at Nome, Alaska, extend into Federal waters beyond the 3 mile limit.

3.3.1.2 Marine Conditions

Most areas along the Alaskan coast are exposed, to varying degrees, to the climatic

conditions characteristic of the Bering Sea or Gulf of Alaska. More extreme conditions exist on the

north coast of the Seward peninsula. However, offshore Nome, with its winter freezing of the sea

and variation from calm seas to major storm conditions during the summer, exhibits all the surface

features likely to be encountered to different degrees at all the potential locations.

A full description of the Nome marine conditions is given in Section 3.2.1.3 (Mining scenario

1). In some other areas the winter conditions are more harsh and the summer is of a shorter duration.

In places, no sea ice may form and the winter months are an extension of the least favourable Nome

conditions experienced in October and November. Local variations occur in terms of wind speed

and direction, wave heights and swell, precipitation, water temperatures, and salinity.

3.3.1.3 Geological Background

Gold and platinum occur on large areas of the seabed in Alaska as lag gravel concentrations

in the uppermost metre of the sediments (Nelson & Hopkins, 1972). In places the veneer of lag

gravel is thin, around 0.25 m. Both the thickness of the lag gravels and the degree of concentration

of the precious metals are dependent in part on the past and present water depth. The seabed is, or

has been, within the range of storm effects that serve to winnow out the finer particles. Mineralized

lag gravels may be developed over a range of gold-bearing sediments but are especially prevalent

over glacial till and diamicton. Gold and platinum also are locally concentrated offshore in three

distinct modes of occurrence:

dendritic patterns of narrow and shallow filled channels formed, in now inundated glacial

outwash and related sediments, by former surface-flowing streams

beaches and still-stand areas characterized by well sorted sand and gravel, and

sand spits and sand banks that have been supplied with transported metal particles from

elsewhere.
56

3.3.1.4 Mined Ground

The UWM selectively strips off the surficial deposit from underlying lower grade material.

The vertical cut-off horizon may be dictated either by declining grade or by increasing resistance

offered to excavation, or both. The lag gravel sequence usually comprises an upper surface

armoured with pebbles and cobbles from which all small, lighter particles have been winnowed. In

places the cobbles may attain boulder size. Beneath the stone armouring is sand which possesses

an increasing silt content with greater depth. The loose silt/sand, with some gravel, usually rests on

a clay horizon that may contain cobbles and gravel. The clay is often the upper, slightly softened,

surface of an underlying glacial diamict, but may also represent other lithological components of

glacial moraine and surrounding host sediments. The whole vertical section may be enclosed within

a 1 m interval.

Within a few hundred metres of the shoreline the basal diamict may be loosened to a deeper

extent than further offshore by the mechanical action of sea ice gouging. It may be buried beneath

a few metres of silt, sand, gravel and cobbles. The overall particle size distribution may be similar

to the underlying basal unit - the only difference is the looser structure near surface.

3.3.1.5 Size Variations in Excavated Material

The composition of lag gravels from two different areas representing different parts and

styles of glacial moraine are given in Table 3.14. Seabed channels filled with outwash gravel

contain less silt and sand and in places reveal fairly well-sorted gravel. A typical example contains

40% exceeding 19.05 mm particle size and none exceeding about 0.15 m. Approximately 10%

comprises silt. Beach sand and sand spit deposits consist of 90% extremely well sorted sand with

about 65% of the sediment being -65 + 100 mesh (- 0.212 + 0.106mm). About 10 to 15% comprises

silt. Some beaches may be characterized by larger cobbles and boulders if they have developed on

the flank of a terminal or lateral moraine. For excavation purposes there are five main constituents

and ground types of importance:

silt, because it is easily excavated but causes turbidity in the water

clay, because it may cause losses of gold in the treatment plant, as well as generate more silt

when it is successfully treated to liberate the contained gold

clay/gravel mixtures which can be excavated only at a lower rate

larger cobbles and boulders which cannot be excavated and pumped, eventually bringing

production to a halt, and

till and diamict which contain too little gold and which can be excavated at a low rate only

with difficulty.
In places silt or mud with a high gold content may attain a thickness of about 1 m or more,

but the lateral extent of a very thick and high grade sequence is usually no more than a few metres.

Such an occurrence is insufficient in volume to constitute a discrete mine planning unit.

3.3.1.6 Distribution of Gold

The lag gravels with contained gold are widespread. Their gold content decreases

logarithmically with depth in an undisturbed lag sequence, and is very variable laterally. The exact

location of higher concentrations in the beach and channel deposits depends entirely on local

conditions. For reserve definition purposes a sample spacing of as little as 10 m is necessary.

3.3.2 Mining System

The UWM is normally deployed offshore from an ocean-going barge or self-propelled vessel

which carries a full mineral treatment plant and the UWM's support facilities.

3.3.2.1 Underwater Miner

The "Tramrod" model 250 UWM is a remotely operated, track-mounted machine which is

entirely within the control of an operator at the surface. The machine has undergone several

modifications and design changes since 1985 when it was first deployed in the North Sea. These

changes include general strengthening and the addition of optional cutting devices as shown in

Figure 3.17. The machine consists of a tracked undercarriage on which is mounted an hydraulically

controlled dredging arm. Excavating power is achieved by means of a powerful jet pump assembly

capable of pumping solids of up to 250 mm diameter. High pressure water is supplied to the jet

pump by a 1,035 kPa, surface-mounted pump via two 152 mm diameter hoses. The jet pump system

can be replaced by a centrifugal pump attached to the dredging arm, that increases both the density

of material dredged and the throughput rate. When cutting of the sediments is necessary an optional

cutting wheel, designed to cut clays up to 500 kPa shear strength, may be added to the dredging arm.

This addition improves the throughput where otherwise the suction device alone is ineffective

against resistant soils.

The "Tramrod" UWM is connected to surface by an umbilical cable which carries all control

and other signals as well as electrical power for hydraulic drives. The standard items of equipment

installed on the UWM for operations are given in Table 3.15.

3.3.2.2 Deployment

The UWM usually operates at distances of more than 100 to 200 m from the shore,

depending on the prevailing water depth and other local conditions. The only limitation to its

distance offshore is the water depth which is not exceeded by most of the localities of interest. The

machine is deployed from a 76 m long by 23 m wide and 4.9 m deep ocean-going barge (Figures

3.18 & 3.19). With a draught of only 1.5 m the barge can enter any harbour. The barge is secured

57
58

and manoeuvred by means of a 5-point anchoring system powered by D.C. electric motors. Each

anchor winch is equipped with 610 m of steel cable.

The UWM is placed on, and recovered from, the seabed by a truck-mounted 210 tonnecapacity crane. Mined sediments, to a maximum of 0.25 m in particle size, are suctioned through

a 25 cm-diameter intake and pumped to surface through a 0.305 m internal diameter floating

pipeline. A 0.38 m intake can be fitted, together with an appropriately sized pipeline. Boosted by

a 0.305 x 0.356 m, 11.36 m3/min. pump on deck, the slurried material is delivered to the treatment

plant on the barge.

3.3.2.3 Treatment Plant

The conventional recovery circuit has a nameplate capacity of 250 m3/hour. It commences

with a 22.3 m2 area grizzley with 0.20 m bar spacing. The washed oversize at +0.20 m is discharged

directly into the sea and this size fraction generally represents about 10% of the feed. The grizzley

undersize at -0.20 m is fed, by a vibrating rock chute, to a 20.7 m rotating trommel of 1.83 m

diameter. The trommel receives approximately 200,000 1/hour of sparge water and delivers 3

products:

-12.7 mm which is delivered as a slurry to the jig circuit

-38.1 mm and +12.7 mm which is discharged, via a nugget trap, into the sea, and

+38.1 mm which is discharged directly into the sea.

Products (2) and (3) combined, being +12.7 mm, generally represent about 20 to 30% of the

feed. Product (3), comprising about 60 to 70% of the feed on average, passes to a de-watering

cyclone ahead of the jig plant. The cyclone over-flow is discharged directly to the sea, while the

under-flow passes through a 2-stage jig plant. The combined primary and secondary jig tailings are

discharged as a slurry (5 to 10% solids) to the sea and the gold concentrate is up-graded on a shaking

table.

3.3.2.4 Support Facilities

The support vessel requirements are shown in Table 3.16.

3.3.3 Operations

The barge is deployed on its 5-point anchor spread (one bow and 4 side-lines), bow into the

incoming swells. The UWM is lowered to the seabed from the stern, oriented so that its forward

movement takes it directly away from the barge and on the extension of the barg&s centre line. The

machine is connected by its umbilical cord, the twin water hoses and the floating pipeline. In

addition, the crane hook is kept attached to the UWM lifting strip to ensure a quick machine

recovery from the seabed if necessary.
Mining is effected by the UWM advancing along a pre-determined corridor, excavating a

fairly uniform cut after an initial downward access ramp. The cut is about 1 m deep, but may be

significantly more or less. The depth of the cut is governed by economic considerations such as

grade and geotechnical characteristics, and defines the horizon on which the UWM traverses. The

initial corridor resembles a wide, shallow trench in which the UWM moves forward on the trench

floor, progressively extending the trench. If possible, the forward movement is continuous, but in

places the machine must halt, or reverse, so as to ensure that the base of the cut is being cleaned up

sufficiently.

The barge follows the UWM stern first at the rear of the machine on a discontinuous basis.

When the pipelines, cord and securing line from the barge to the UWM are judged to possess too

little slack for the conditions, the barge is moved on its spread by an appropriate distance to bring

its stern to a point immediately behind the rear of the UWM. The limit of the UWM's advance is

either the boundary of the pre-determined mining unit or the anchor spread limit of movement of the

barge. Where the limit is reached the machine may be hoisted on deck, or may be reversed down

its excavated corridor as the barge returns almost to its original starting position. The barge and

UWM then manoeuvre so as to commence the excavation of a second corridor parallel, and adjacent,

to the first. All effluents are discharged on either port or starboard side of the barge so as to not

dilute virgin ground. The operating capabilities of the UWM are shown in Table 3.17. The daily

throughput rates vary according to the accuracy of digging required and the extent to which cobbles

and boulders may be present. The main objective of future developments of the UWM for precious

metal mining is an increase in the throughput rate. The next plateau to be reached, and which

already has been considered in the design stage, is an average throughput rate of 300 m3/operating

hour. This will involve:

increasing the excavating capability by means of fitting a greater width of cutting wheel

(bucket wheel) and drawing on the experience gained in the use of continuous-wheel miners

in the open-pit mining industry

screening out the larger (e.g. +0.30 m) cobbles and boulders at seabed level on a grizzley

integrated into the UWM

increasing the power supply at the highest possible voltage, and

installing a sluny pump on the UWM, assuming that a pump capacity would be required of

300 m3/hour at 20% solids.

The corridor width depends on whether or not a cutter or other special feature is added.

Without a cutter (suction only) the maximum width is 12.3 m; with a cutter the maximum width is

8.7 m, and with special features (at a rate of 300 m3 solids/hour) a width of 15 to 20 m.

59
3.3.3.1 Mining Products and Effluents

The main effluents from the UWM operation are the grizzley oversize, the trommel mid-size

and oversize, the de-watering hydro-cyclone overflow, and the combined primary and jig tailings

(Table 3.18).

3.3.4 Geochemistry

See Section 3.2.3.

3.3.5 Seabed Biodiversity and Resources

3.3.5.1 Locality and Mining Characteristics Affecting Benthos

Variable types of deposits are expected shoreward of the 3 to 20 m depths monitored for

Scenario 1. Mining is a relatively high precision operation, removing sediments to a depth of about

1 m or less. The UWM can be manipulated to a precision of about 0.2 m vertically. Hence mining

is selective, normally to extract the highest grade deposits. The deposits are localized in lenses,

shaped by wave action, or in strings following irregularly-shaped submerged relic stream courses.

Thus there is extensive, almost identical, benthic habitat left alongside and between dredge courses.

The non-dredged areas may be affected by tailings deposition, thus care is needed in selecting

sampling stations for monitoring purposes. Environmentally significant operating and mining

parameters for the uTramrod UWM are shown in Tables 3.19 and 3.20 respectively.

3.3.5.2 Benthos Pre-mining

Beds of marine grasses and algae may be present between 3 to 20 m, and the associated

benthos are expected to be as abundant and productive as in the deeper water dredged during 1986

to 1990 (ENSR, 1991), with a probable exception at low tide level. In the intertidal zone the benthos

are subjected to an extremely high energy environment, characterized by continuous wave-induced

substrate movement, ice scouring in winter, and melt-water runoff in summer. Considerable

turbidity is expected from continuous wave action on this exposed coast (Figure 3.12), and

intensified during storms in summer. The organisms present are thus exposed to natural conditions

similar to those that are generated temporarily in an extreme form by dredging operations.

3.3.5.3 Benthos Post-mining

Given the natural conditions of periodic wave and turbidity exposure, the small-scale of

operations and known impact from the larger-scale bucket ladder dredge operations offshore, this

form of mining should create only localized effects immediately around the UWM and zone of

tailings discharge. The two main risks are: (1) losses from physical extraction, and (2) smothering

from rapid tailings deposition. The impacts in part overlap depending on discharge location, and

hence probably cannot be distinguished by monitoring. Further from the UWM, raised water column

turbidity from dredging and tailings discharge and settling are not seen as risks to the benthos.

60
Marine grass beds, and to a lesser extent algal beds (if they occur), are at risk. If high grade

gold deposits occur in such beds it is desirable to use the first extractions as an experiment to

determine the extent of loss and the recovery rates relative to the same features for other local beds.

The environmental significance of future extractions can then be appraised.

3.3.5.4 Fisheries and Wildlife Resources

A stock of king crabs supports subsistence and artisanal fisheries (see Scenario 1). Other

resource stocks are present, including marine mammals and birds which support subsistence use.

The more intensive and extensive dredging offshore had no detectable impact on these resources.

The UWM, similarly, has little impact.

3.3.6 Biological Oceanography

Although the operating principles of the UWM are quite different from those of the bucket

ladder dredge, water colunm effects are similar, differing only in degree. Oversized material is either

avoided on the seabed or is pumped to a surface grizzley and trommel from where it is returned to

the sea. Turbidity plumes are produced at the dredge cutter-head of the UWM and at the surface

discharge point. In shallow water the effects are similar to, but less than, those caused by a bucket

ladder dredge (Scenario 1). There is almost no environmental risk associated with sub-surface

discharge of tailings materials and almost none with deep discharge.

3.3.7 Summary of Predicted Environmental Impacts, Resulting Mitigation Objectives, and

Achievement Criteria for Scenario 2

3.3.7.1 Predicted Environmental Impacts

The precision mining technique of the UWM has low impact producing little cutting face

turbidity and benthic impacts are slight from the precision cutting.

3.3.7.2 Mitigation Objectives

Mitigation objectives should include: (1) minimizing turbidity at the cutting face, and (2)

prevention of elevated of surface turbidity, as described in the generalised turbidity objectives

(Section 2.7)

3.3.7.3 Achievement Criteria

Achievement criteria should include:

turbidity from cutting face activity must not rise to surface

surface turbidities should be as stated in the generalised criteria (Table 2.8)

61
(3) dissolved oxygen levels should be as stated in the generalised criteria (Table 2.8).

62
3.4 Scenario 3: Marine Aggregate Mining, Massachusetts Bay

3.4.1 Mining Scenario

This scenario describes a hypothetical marine aggregate mining operation in Massachusetts

Bay, serving the Boston area market. Geological and environmental information relevant to this

scenario are derived from the NOMES (New England Offshore Mining Environmental Study) report

(Padan, 1977), which details the results of a baseline environmental study carried out in preparation

for a proposed marine aggregate dredging experiment.

3.4.1.1 Marine Conditions

The NOMES study was initiated in 1972 by the Commonwealth of Massachusetts and the

U.S. National Oceanic and Atmospheric Administration (NOAA), with the intention of resolving

marine environmental impact uncertainties associated with nearshore marine mining. Multidisciplinary studies were centred at location 40° 21' 41" N by 70° 47' 10" W in Massachusetts Bay,

about 16 km offshore of Boston Harbour in 20 m water depth (Figure 3.20). The project was

terminated in July 1973 as a result of the failure of the Commonwealth to arrange for a suitable site

for the disposal of the three-quarters of a million cubic metres of sand and gravel that was to have

been dredged during a planned spring 1974 test. Although much of the survey work in progress was

interrupted by premature termination of the project, funds were allocated for publication of such

work as had been accomplished. The comprehensive report on the NOMES project (Padan, 1977)

includes a chapter on physical oceanographic processes that summarizes the oceanographic setting

in Massachusetts Bay. The following information is derived from Padan (1977).

Massachusetts Bay lies on the west side of the Gulf of Maine. It is bounded on the north by

Cape Ann, on the west by the Massachusetts coastline centred at Boston, and on the south by Cape

Cod Bay and Cape Cod. Seventy-five percent of Massachusetts Bay and Cape Cod Bay is

surrounded by land. Oceanographic conditions near the eastern mouth of the Bay are influenced by

Stellwagen Bank, a submarine ridge that rises to within 20 m of the surface, and which opens at

either end in channels of about 60 m water depth. The location of the dredging described in this

scenario is inshore of Stellwagen Bank, in 20 m of water.

Historical data were assembled and analyzed to describe the seasonal cycle of temperature

and salinity variation in Massachusetts Bay. The annual temperature cycle in the Bay includes the

development and decay of stratification, but this is felt mostly in waters deeper than at the

hypothetical mine site. Winter temperature and density conditions are vertically mixed with no

stratification. The isothermal temperature minimum occurs in February at 0 to 3° C. The vertical

gradient anywhere in the Bay in February is normally less than 1°C. Warming through April

gradually raises the gradient to about 2° C, at which time the gradient is still diffuse and restricted

to the upper 30 m. In June surface temperatures rise to 13 to 17° C, and peak in August at more than

20° C. Gradients increase to maximum strength during this interval; however, vertical mixing in the

surface layer extends to at least 15 m depth, suggesting that it is only the very bottom waters at the

site of interest which may feel the stratification effects.

63
Salinity at the surface ranges between 3O%o and 33%o, with the minimum occurring in May

and the maximum in March. The mean bottom salinity ranges from 31.6 to 32.5%o. During vertical

mixing in winter the average difference between surface and bottom salinity is 0.2% or less.

Currents in coastal waters are characterized by their time scales as mean currents, tidal

currents, and higher frequency currents associated with turbulence and waves. The most obvious

features are tidal currents that are frequently modified by the wind. The semi-diurnal tidal

oscillation in Massachusetts Bay results in a mean tidal range of about 3 m. The flood tide sets

westward into Massachusetts Bay, and the corresponding ebb is oriented northeastward to eastward.

Tidal currents have been measured 1 m above the bottom in various parts of the Bay. Maximum

tidal current speeds occurring in the deep basin west of Stellwagen Bank, east of the mine site, were

26 to 29 cm!s, and were weaker than those over the Bank (32 to 47 cm!s).

The surface drift west of Stellwagen Bank is cyclonic, tending to follow the coastline

southward south of Cape Ann into Massachusetts Bay, southward past the mouth of Boston Harbour,

and then around Cape Cod and northward past the northern tip of Cape Cod. Bottom residual drifts

are an order of magnitude slower than the surface drifts. In general these bottom drifts are oriented

southward to southwestward over the inner part of the Bay.

As part of the activity within NOMES a survey of current directions, velocities and

dispersion rates was undertaken over two days in late July 1972. Rhodamine dye was introduced

and tracked by vessels and aircraft. Surface and subsurface drifters were also deployed. In total, 18

drogues were tracked. The currents in the area of the proposed mining site possessed a southerly

component of velocity during the early stages of both flood and ebb. Average velocities were 27

cmls (shallow drogue at 1.5 m below surface) and 11 cmls (deep drogue at 9.0 m below surface).

Experiments were conducted at the NOMES site (Hess and Nelson, 1975; Nelson et al.

1977; Mayer, 1975) to predict the fate of a discharge plume from the proposed mining site. These

investigations were reviewed by Padan (1977). To simulate the dispersing plume 2700 kg of fine

particles (0.5 to 50 tm diameter) were released to the water surface in the form of a slurry simulating

the overflow of a hopper dredge. The movement of the resultant plume was traced by drogues for

ten days, and the plume was sampled for temporal and spatial distributions of particles.

During the month of the particle dispersion experiment, a strong north-south current shear

zone was observed. The mean motions within 10 km of shore were predominantly northward,

beyond that, the motions were exactly southward. The investigators concluded that in this

experiment east-west tidal motion transported the tracers through the sharp current shear zone, into

the southerly flow. They note that it is quite conceivable that the whole experiment could have

evolved differently if the tracers had been deployed at different times, and on different phases of the

tide. Conclusions are that very site-specific, local conditions, with short time and length scales, had

significant effects on the large-scale results of the experiment. Basic conclusions of the work were

that dispersion of the particle plume was contingent on (1) the tidal phase at the time of introduction;

(2) the seasonal stratification of the water column; and (3) transient storm effects. The NOMES

dispersion experiment was a unique attempt to deal empirically with marine mining dispersion

64
issues, and offers important guidance for any such similar investigations that might be undertaken

at other sites in the future.

3.4.1.2 Market-based Determination of Annual Output

Sand and gravel production is a market-driven industry. The production scale is determined

by the total market size and the expected market share. Product quality is all important. Given that:

the population of greater Boston is about 2.0 million

the U.S. national average aggregate consumption is about 3.2 metric tons per person per year

the per capita aggregate consumption in a U.S. urban concentration is probably twice that

overall nationwide, and

the management of a new operation would probably seek to achieve a 25% market share

therefore the possible annual production from Massachusetts Bay is 3.2 (2.0 x 3.2 x 2 x 0.25) million

tonnes per year of aggregate (mixed sand and gravel). Assuming a factor of 0.75 m3 of aggregate

to 1.36 metric tons, the annual production is (3.2/1.3 6) x 0.75, equivalent to approximately 1.8

million m3/year.

3.4.1.3 Deposit Details

Details of the dredging area are provided on pages 55 to 62, and 64 to 65 [Table 22],

inclusive, of Padan (1977) (see Appendix 5). The location within the 3 mile limit offshore from

Boston is shown in Figure 3.20.

The deposit is part of a complex mixture of glacial products very similar to that offshore

Nome, Alaska. It is described on page 60 of Padan (1977). Bedrock is overlain by ".... glacial till,

a heterogeneous mixture of boulders, gravel, sand, silt, and clay ranging in thickness from a thin

veneer to nearly 30 m. Two marine clays, separated by an erosional unconformity, overlie the

glacial till" and "the NOMES deposit appears to be a gradational feature resting on, and yet

geologically part of, the upper marine clay. " The surficial veneer of "sand and gravel" has been

investigated by vibrocore at 32 sites. The lithology changes rapidly over short horizontal distances

and with each vertical metre below the seabed, as shown by the 3 figures on page 59 of Padan (1977)

(Appendix 5).

The dimensions of the square area illustrated on page 59 of Padan (1977) are 4.25 x 4.24 km.

A mixture of"gravelly sand", "sand", "mud", and "muddy sand" underlies the 1.5 rn-thick main body

of surficial "sandy gravel" (surface dimensions approximately 3.5 x 1.0 km). The volume of sandy

gravel is therefore 5.25 million m3 (3,500 x 1,000 x 1.5). Similarly, Padan (1977) states on page 58

that there are probably "...over 5 million m3 of sand and gravel..."

65
It may be assumed that in reality the variation and distribution of the different lithologies is

more complicated than is shown on page 59 of Padan (1977). The complexity is a function of the

sampling density (approx. 0.10 km2 per vibrocore sample) and the line spacing of the side-scan

sonar survey (approx. 185 m). Some of the complexity visible on a 3 x 3 km scale may become

visible during production on a 0.5 x 0.5 km scale. All the lithologies listed in Table 21 on page 57

of Padan (1977) will be dredged because the dredging operation cannot be too selective if costs are

to be contained.

It should be noted that the proportions of "sandy gravel", "gravelly sand", "muddy sand"

"sand" and "mud" that will be dredged during production can only be grossly estimated at this stage.

Whatever the blend of"soils" brought on board the dredge there will be a mud content of 0 to 33.5%.

This statement remains true even if selective dredging is 100% successful and only "sandy gravel"

and "gravelly sand" are excavated. No precise definitions are given for the different lithologies but

presumably they conform to standard definitions. The area will require more exploration before it

could may a production site

3.4.1.4 Daily Production Rate

Assuming 7 months of operation per year (because of the frequency of adverse sea conditions

in the winter), and 25 days of operation per month (i.e. no Sunday working) then the daily

production rate is 10,286 m3/day [1.8 x 106/(7 x 25)1, equivalent to 18,000 tonnes/day.

3.4.1.5 Project Life

The potential dredge area reportedly comprises about 5.25 million m3 of sand and gravel in

water depths of less than 18 m. Assuming that 100% extraction is achieved, then the productive life

of the deposit is 2.9 years. The minimum required project life is 10 years, and preferably 20 years

or more. Therefore, in practice, additional or (more likely) alternative dredging sites will have to

be found before the project is started.

3.4.1.6 Dredging and Production Methods

The type of dredge to be used will be an ocean-going trailing suction hopper dredge (Figure

3.21). The methods of excavation, shipboard treatment, and onshore processing of the sands/gravels

are described on pages 112 to 116 of Padan (1977). The dredging method adopted will be the predetermined movement of the dredge along prescribed lines, backwards and forwards across the area

designated. The dredged furrow remains as a shallow trench, the dimensions of which depend on

the size and specifications of the single or twin drag-head. An area may be dredged several times

creating overlapping, adjacent, and/or coincident furrows.

The dredge product is optimized according to market requirements by selective dredging, if

possible, on-board sorting of the dredged sediment and onshore processing of the landed sand/gravel.

Selective dredging may take the form of either returning all the dredged material intermittently to

the seabed if it is unsuitable, or attempting to define sufficiently large areas of consistent sediment.

66
On-board sorting takes the form of allowing the fines (disintegrated, dredged "mud') to overflow

the dredge hopper into the sea as dredge effluent. Alternatively, if there is too much fine sand

(determined by marketing requirements), screening the dredged sediment will immediately return

both fine sands and fines to the sea (Figure 3.22). Onshore processing usually comprises washing

to remove salt and any remnant fines, and screening for products of different size characteristics.

Another approach might be to use, instead of a self-propelled hopper dredge, a barge

propelled by a tug in the notch. The "Long Island" (formerly the "Ezra Sensibar") is an example of

such a design and is shown in Figure 4 of Padan (1977) (Appendix 5). It is possible to use one 3,000

h.p. (2,237 kW) tug and two barges. The tug may be leased because it is not required during the

winter.

3.4.2 Dredging System

The basic system will be a self-digging barge (integrated digging and transport) similar in

capacity and dimensions to a Great Lakes Dredge and Dock Co. barge. It will be an ocean-going,

trailing suction hopper dredge. Features of the dredge are given in Table 3.21.

3.4.2.1 Dredge Pumping System

A range of performance is possible with different sediments as shown in Table 3.22

3.4.2.2 Effluent

As determined above, the rate of effluent discharge is 170,300 1/mm. when dredging fine

gravel and sand, and 257,400 1/mm. when dredging coarse gravel. Generally, one would expect the

discharge to vary within these limits. The fines effluent contains solid particles which are -200 mesh

(-80 microns) in size. Normally, less than 10% (3.8 to 6.2%) of the soils dredged and delivered to

surface are discharged at sea as effluent overflow. For the purposes of this scenario it is assumed

that no screening is undertaken at sea. However, the high reported mud content of the NOMES

deposit may result in a range between 0 to 33.5% of the solids being in the reject category.

3.4.3 Dredging Cycle Times

3.4.3.1 Loading

For each 1 m of dredge advance the drag-heads each excavate a furrow 2.7 m wide and 60

cm deep, i.e. 1.6 m3 in volume. Two such drag-heads excavate a total of 3.2 m3 per 1 m of dredge

advance. With a dredging speed of 6.5 kmlh the hourly advance is 6,500 m, resulting in excavation

rate of 20,800 m3/hour (6500 x 3.2). Therefore, the time required to load 5,100 m3 (trip capacity) is

15 minutes (5,100/20,800 hours). If the cut depth is reduced from 155 to 77.5 cm then the loading

time is increased from 15 minutes to 30 minutes. Allowance for other factors and activities increases

the loading time to approximately 75 minutes.

67
3.4.3.2 Unloading

Assuming that discharge is effected with an 11.4 m3 capacity clamshell with a cycle time of

1 minute then the number of cycles needed to unload the vessel per trip is 447 loads (trip capacity

of 5,100/11.4). The unloading time is 7 hours and 27 minutes.

3.4.3.3 Travelling Time

Assuming that the travelling speed from the digging area to the discharge point is 11 kmlh

and the distance from the digging area to the discharge point is 28 km then the travel time in each

direction is 2.5 hours (28/11) each way.

3.4.3.4 Total Time

The total cycle time, estimated from the above assumptions, is 13 hours 42 minutes. This

time could be reduced to 8 hours 54 minutes by increasing the travelling speed and reducing the

unloading time by using a larger, 20 m3, clamshell for unloading (Table 3.23).

3.4.4 Number of Dredging Units Required

Using the figures given in Table 3.23 the number of dredging units required is 1 unit (with

a night shift worked) or 2 units (with no night shift worked).

3.4.5 Costs

The landed sales price of washed and screened aggregate in Boston is reported to be $5.50

per tonne (equal to $9.87/rn3) and from two other independent sources that the price is $9.21 and $

1 0.52/m3. Assuming a capital cost of $25 million, a project life of 10 years, and a production rate

of 1.8 million m3 per year, equating to a cost of $1.41/rn3, then a required profit of $1.97/m3 leaves

$6.49 per m3 [$9.87 - (L41 + 1.97)] for cash working costs, including overheads and fixed costs.

3.4.6 Possible Problems

Possible problems include:

the selected dredging site is inside the 3 mile limit

the ground is of glacial sands/gravels with a high mud content

the deposit has a limited productive life

no obvious aggregate storage area exists if the product has to be stored during the winter.

68
3.4.7 Seabed Biodiversity and Resources

3.4.7.1 Locality and dredging characteristics affecting benthos

The Massachusetts Bay scenario is an extensive and low selectivity dredging operation,

therefore all the very different lithologies present from mud to gravel (see Appendix 5, Fig. 5,

Tables 3 and 4) are liable to be dredged. Trailer dredging creates paired, 3 rn-wide furrows of at

least 1,000 to 2,000 m length, with the pairs of furrows 8 m apart. Multiple series of these lines are

produced, some of which overlap. Environmentally significant operating and mining parameters for

the dredge operation are given in Tables 3.24 and 3.25.

According to Padan (1977), "for each cubic metre of sand and gravel extracted,

approximately 10 m3 of bottom also would have been withdrawn during the planned operation.

During each of the planned 2-hour mining cycles, approximately 370 m3 of fine material would have

been discharged from the dredge Roughly 250 km2 of the sea floor was expected to be covered

by fine sediment to a depth greater than 0.01 rn.' The proposed mining scenario will last longer, and

spread impacts over a greater area.

3.4.7.2 Benthos Pre-dredging

The fauna of the Massachusetts Bay area were documented by Padan (1977) during the

summer of 1973 (see Appendix 5, Tables 5 to 7, and 13). The biodiversity was rich and variable

with differences between epifauna on hard rock (including on exposed cobble surfaces) and the

burrowing infauna. Padan (1977) described habitats of hard, cobble, sand and mud substrates. Over

650 species were sampled. The number of species per station ranged dramatically, from 31 to 146

per station, as did the number of organisms/rn2, from 305 to over 11,000. Sedentary polychaete

worms (filter feeders) dominated on most substrate types, but rankings for specific stations showed

errant polychaete worms (carnivores) and amphipod crustacea (onmivores) dominating at some sites.

Ranking by feeding guilds showed some dominance of harder substrates by suspension feeders and

of softer substrates by deposit feeders.

The species complexes found at different substrate types differed but statistical correlation

with deposit type was not possible. The temporal variation was enormous from month to month,

making the use of dominant species only possible at specific site locations that cannot be generalised

to a substrate level. Padan (1977) recommended that if further pre-mining sampling is carried out

then fewer, less frequently sampled stations are needed, but that more replicates per station should

be taken to better understand the patchy distribution of the benthos. It also noted that the different

benthic communities might be better described if both numbers of organisms and biomasses were

recorded. Another possibility is to characterize differences by monitoring feeding guilds as in Table

8, Appendix 5.

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3.4.7.3 Benthos Post-dredging

Benthic communities directly in the path of the dredge are destroyed. The benthos in

undredged areas between the furrow lines (8 m apart) are affected by the increased turbidity and

sedimentation. The impacts depend on the depth of sediment that accumulates and depth of the

furrows. Some benthic communities can survive tailings sedimentation rates of at least 1 cm per

year (Island Copper Mine, 1994 and see Section 2.3).

Unlike those scenarios in which most of the extracted sediments are returned to the seabed,

during the dredging of aggregates a substantial percentage is removed. This changes the particle size

characteristics of the remaining deposits. Larvae that recolonize the altered environment reflect

these changes. It is probable that the silt/clay fractions will increase in the dredged area because the

coarser sediments are retained by the dredging process.

Soft-bottom communities should recover faster than communities found on coarse sediments,

or than the epifauna on cobbles and boulders. Expected times to recovery can be based on data in

Kenny and Rees (1993; 1994), and ENSR (1991) with summary data in Appendices 3 and 6

respectively. These data suggest times to recovery of 1+ years on deposits with least cobbles, with

slower recovery where substantial cobbles are removed. The observed recovery time of 4 to 5 years

at the Nome, Alaska site may be indicative of recovery of the coarsest areas and their associated

epifauna. It is to be expected that infaunal benthos reproducing by pelagic larvae will start to

recolonise during the next larval settlement stage after closure, and that there will be substantial

recovery of some of these forms within a year. However, based on experience gained in the U.K.

aggregate dredging industry in the North Sea (Kenny and Rees, 1993; 1994), it is to be expected that

the epifauna of hard rock and cobble will recolonise more slowly than the infauna of soft deposits.

This appears to be a function of reduced surface area for attachment of hard rock epifauna

attachment related to the depletion of cobbles and boulders. However, such "hard rock organisms"

generally are less available to feeding fish and shellfish, due to their ability to protect themselves by

secreting cemented-down and non-removable shells.

The pattern of dredging is partially dictated by the pattern of recovery in the benthic

community. The area between dredged furrows, not heavily impacted by the tailings disposal or

affected by unstable sediment, may allow some degree of localized recolonisation of species that do

not have a planktonic stage.

There may be a release of nutrients and/or toxins from the disturbed deposits. The toxins

may be derived from sulphur-reducing deposit conditions (see Section 2.2), or directly from human

waste disposal systems close to the area. In view of the proximity of the Boston urbanised area the

latter is quite likely. The potential for toxin release needs to be determined by appropriate

geochemical risk assessment in advance of mining.

70
3.4.8 Biological Oceanography

During aggregate extraction the sand and gravel fraction is retained but silts and clays are

returned as tailings to the sea. Discharges include overflow washing from the dredge hopper, and

also unsuitable sediments (high in fines) may be discharged to the sea. These may contribute to

substantial surface and water column turbidity downcurrent from the dredge.

Turbidity at the seabed produced by the trailing drag-head suction dredge is expected to be

low, and will not significantly affect the water column. The narrow dredge furrow area and shallow

highly oxygenated waters make water column changes unlikely as a result of oxidation of exposed

sediments.

Losses in primary production due to light scatter (see Section 2.4) are a possible result of

increased surface turbidity. However there is little likelihood of impact on planktonic organisms

because of ephemeral exposure to a dispersing turbidity plume (Section 2.4). In some cases plankton

may actually benefit from increased feeding because of organic materials bound to the particles.

Species shifts have been correlated with high turbidity levels (Section 2.4), although it is not clear

that this would be deleterious.

3.4.9 Fisheries and Wildlife Resources

The NOMES report mentions only briefly the resource species of the area. In a proposed

mining scenario the pre-mining stock and life cycles of resource species of commercial and

recreational importance must be investigated. The resource species are obviously diverse and

support substantial local commercial and recreational fishing.

Experience elsewhere suggests that on a regional scale, overall fish stocks are not likely to

be significantly altered. However site specific stocks of gravel-spawning fish like herring could be

affected over the long term (Driiman and Bliss, 1986; Oulasvirta and Lehtonen, 1988). During

dredging operations commercial fishing presumably is not able to proceed in these areas. Some

species may be attracted to the dredging sites and, during temporary (seasonal?) dredging-related

closure, conimercia1 fishing may restart. The seasonal nature of some local fishing and eco-tourism

(marine mammal/bird watching and sports fishing) industries may conflict directly with dredging,

effectively eliminating them around the site.

Rock scallops, like the blue mussel, may be sensitive to elevated levels of siltation. The

eastern lobster (Homarus americanus) seems capable of tolerating a fair degree of siltation. But in

laboratory experiments their larval stages have been shown to be sensitive to unnaturally high levels

of kaolin clay and finely ground quartz in the water column and may not survive in their presence

(Cobb, 1972). This may also be true for other commercial groundfish. Some of the commercially

important groundfish like cod and winter flounder are known to be attracted to sediment plumes

caused by mechanical clam digging, possibly attracted by the large numbers of benthic organisms

stirred up into the water column (Padan, 1977).

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A lack of resource information for both the aggregate dredging and fishing industries can

limit the planning process (Drinnan and Bliss, 1986). It is important to map the breeding grounds

and migratory routes of all potentially commercial resource species before extraction takes place

(Oulasvirta and Lehtonen, 1988).

3.4.10 Prediction of Environmental Impacts, Setting of Mitigation Objectives and

Achievement Criteria for Scenario 3

3.4.10.1 Prediction of Environmental Impacts

Apart from the expected impacts (described in Section 2.7) there is the site-specific

consideration that the surficial geology is variable. Thus a main impact will be from surface

turbidity plumes from the dredge due to the separation of fines from the aggregate, from hopper

overflow and from discharge of unwanted sediment. Dredging will destroy benthos on the seabed,

and the overflow discharge of silt and clay particles may alter the habitat. Consequently a different

benthos can be expected to recolonise than was previously present, and the substrate change may

affect the variety of fin-fish and shellfish species present. An Environmental Impact Statement will

be needed.

3.4.10.2 Mitigation Objectives

Mitigation objectives should include:

reduction of dredge-derived turbidity through improved outfall design and vessel operations

development of alternative enhancement uses for under-sized and over-sized fractions

return of fines to the dredge footprint.

3.4.10.3 Achievement Criteria

The following criteria should be satisfied during dredging:

EPA turbidity criteria for minimizing impact on primary biological production, as described

in Section 2.7

development of benthos recovery in the dredge footprint, as described in Section 2.7.

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3.5 Scenario 4: Heavy Mineral Sands, Virginia

3.5.1 Mining Scenario

This scenario describes a hypothetical heavy mineral dredging operation in Federal waters

offshore of Virginia Beach, Virginia. The complex subjects of market share and of product

suitability and pricing to determine the scale of an operation are not considered in this scenario.

3.5.1.1 Marine Conditions

Quantitative measures of surface and subsurface currents offshore from Virginia Beach, just

south of Cape Henry, are contained in a report by Boicourt (1981). The two studies in this report

are instructive in assessing the potential for transport of sediment produced from marine mining

activity in this location. In defining circulation in the Chesapeake Bay entrance region Boicourt

(1981) analyzed the results of eleven vertical arrays of current meters and temperature/salinity

recorders deployed in the Bay entrance and on the adjacent inner shelf during the early months of

1979. These measurements were later supplemented with data from an additional twenty moorings

deployed in summer 1980. Of relevance to the mining site is Boicourt's winter 1979 site, MF6

(36° 52' N by 75° 57' W), in about 10 m water depth, about 3 km directly off the northern end of the

city of Virginia Beach. Key findings of earlier work were that (1) as the Chesapeake Bay low

salinity outflow rounds Cape Henry, it turns to the south and flows as a quasi-geostrophic jet along

the Virginia and North Carolina coasts (Boicourt, 1973), and (2) the inner shelf of the Middle

Atlantic Bight, away from the mouths of estuaries, is dominated by wind forcing (Boicourt and

Hacker, 1976). The 1979 and 1980 measurements corroborate this earlier directional evidence, and

provide representative speed values. Specifically at mooring MF6, the mean velocity for a 240 hour

interval in February 1979, at 5.8 m depth, was 10 cmls oriented exactly parallel to the coast, and

flowing consistently southward under the influence of both the outflow coastal jet and of the winter

winds. Boicourt provides no detail as to extreme near-bottom current speeds.

In the summer 1980 measurement program, the nearest relevant mooring (MF7) was located

further offshore at the same latitude as the winter mooring (MF6). Located 9 km offshore (at 36°

52' N by 75° 53' W) in 16 m water depth, mooring MF7 had current meters at 3.7 m and 13.1 m

depth. Over a 38-day interval beginning in late June 1980, the near surface mean flow was parallel

to the coast, flowing southward at 6 cmls. The deeper, near bottom mean flow at 13.1 m was of

slightly greater magnitude (8 cm/s), but was oriented towards the north northwest, flowing into

Chesapeake Bay. These flow velocities suggest that there is insignificant energy in the nearshore,

near bottom currents, and such currents would probably restrict the horizontal advection of

particulates entering the water column as a result of marine mining activity. However, the

directional consistency of the nearshore winter mooring (MF6) suggests that any sediment plume

evolving from mining activity off Virginia Beach might be expected to be asymmetrically elongated

in the downstream (southerly) direction.

Current measurements analyzed and reported by Ludwick (1978) provide more of a

quantitative understanding of flow conditions on the nearshore shelf directly offshore from the city

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of Virginia Beach. Data were collected between July and October 1973 along a band paralleling

Virginia Beach 3.5 km offshore, just seaward of the littoral zone. The original purpose of the

measurements was to support environmental studies for a proposed ocean outfall at Sandbridge,

Virginia. The analysis described by Ludwick (1978) makes use of these early data to consider

sediment transport potential in the circulation off Virginia Beach. Locations of the six measurement

sites are displayed in Figure 3.23. Descriptions of water depth, distance offshore, and vertical depth

for each instrument are presented in Table 3.26. Key results of the analysis, including both vector

mean and measured maximum current values for each instrument record, are given in Table 3.27.

Figure 3.23 shows that Site 1, at the mouth of Chesapeake Bay northeast of Cape Henry, is

possibly situated in a different flow regime from the remaining five sites. Statistics of most interest

are those from Sites 2 through 6. At all but one of these sites the surface maximum values exceed

51 cmls, ranging from 52.1 cm/s at Site 5 in October to 79.6 cm/s at Site 6 in October. The only

maximum value less than 51 cmls is 42.3 cmls at Site 4 in July/August. Near bottom, the measured

current maxima generally fall between 25 to 51 cm/s. Extreme values reach 66.8 cm/s at Site 6 in

October. The directions associated with most maximum speeds are oriented towards the southeast

or south. There is greater directional variability in the occurrence of extreme flows near bottom than

near surface. Vector mean current speeds are as much as an order of magnitude less than the extreme

speeds and are evidence of the rotary nature of tidal currents in this shelf region. Ludwick (1978)

noted that "... tidal currents are rotary, with long ellipse axes approximately parallel to the shoreline

Maximum tidal current speed decreases with distance to the south away from the entrance to

Chesapeake Bay".

Applying temporal filtering techniques to the very thorough set of current measurements

described by Tables 3.26 and 3.27, Ludwick (1978) assessed the implications for sediment transport.

He concluded that "... north-directed sediment transport in the littoral zone off Virginia Beach exists

along with a south-directed intermittent sand stream on the sea floor farther offshore in a zone

approximately 5 km in width extending from, say, 8 - 15 m depths". The implications of these

measurements and analyses are significant to the present work. It is clear that any mining operation

in 10 to 20 m water depth off Virginia Beach will be subject to considerable longshore sediment

transport due the strength and orientation of local tidal currents.

Dredge planning must take into account both the possibility of sediment transport due to

coastal currents and the local wave climate. Regional data from Meserve (1974) (Figure 3.3) provide

estimates of the frequency of occurrence of waves above certain height thresholds, applicable to the

entire shelf region offshore Virginia and Maryland. These statistics are expected to over-estimate

the severity of the nearshore wave climate. Data presented by Man (1993) describe wave conditions

at Site CHLV2, located on a shoal (approximately 36° 55' N by 75° 43' W), some 20 km directly

offshore Virginia Beach, in 12 m water depth. Surrounding water depths are typically 20 m.

Figure 3.24 displays the wave height exceedance curve for this location over the interval February

1985 to December 1992. The curve was developed from a joint distribution of significant wave

height versus peak wave energy period. The log-normal plot allows easy interpretation of the

probability of occurrence of extreme wave heights. Exceedance values for lesser wave heights,

scaled from a linear version of this plot (not displayed) are given in Table 3.28. These values,

74
derived from almost eight years of wave measurements provide an excellent representation of the

severity of the wave climate on the nearshore shelf. It is immediately clear that the statistics quoted

from Meserve (1974) are significant over-estimates of the wave climate severity very near to the

coast.

3.5.1.2 Deposit Details

Details of the deposit are taken from Berquist and Hobbs (1988). Grab sample data only are

available for the area immediately offshore Virginia Beach, but the results of 60 vibrocore holes (25

by the Minerals Management Service and 35 by the U.S. Geological Survey) are available from the

general offshore vicinity.

3.5.1.3 Deposit Thickness

Thickness of the deposit is defined from core data reported by Berquist and Hobbs (1988)

(Table 3.29).

3.5.1.4 Grade

Berquist and Hobbs (1988) stated that "The total heavy mineral concentration for all samples

averaged 3.5% and the highest value was 14%. Offshore sediments average about 30 feet [9

m] in thickness" and" economic mineral occurrences exist on the inner continental shelf' (page

iv, abstract). However, the data presented in the report do not appear to support the above statements

without major qualifications. The area immediately offshore Virginia Beach has been subject to grab

sampling only, a technique which does not indicate the grade over any significant depth. The abovementioned vibrocoring results provide the best basis for measuring the realistic deposit parameters.

Weighted for core length, vibracore results reveal an average total heavy mineral (THM) content of

2.91%. However, an incremental analysis (1 m vertical intervals) reveals that the THM content is

3.65% in the uppermost metre. The THM content decreases steadily with increasing depth below

the seabed, and the incremental THM content, on average, falls to slightly less than 3% (the grade

most probably required in such an offshore operation) at a depth of 4 m.

3.5.1.5 Grain-Size Distribution

Characteristics of the uppermost 4 m of sediments are given in Table 3.30. There is no

calcification of the sands, compared with cemented layers often encountered during on-land dredging

for heavy minerals (personal communication, Carl Hobbs, Virginia Institute of Marine Sciences).

On-land dredging of cemented sand layers ("hard-pan") results in a swell factor of 20%. Two

different drilling contractors were used, each achieving the same results; the clay recovered in the

cores occurs in thin beds about 2 cm thick and is very sticky and sometimes very stiff

3.5.2 Dredging and Production Methods

There are 3 components to the excavation and treatment of the heavy mineral bearing sands:

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76

the excavation of the sands and their delivery to surface

the first treatment, at sea (completed in the "wet mill"), whereby oversize is screened off and

a heavy mineral concentrate is separated from the undersize

the final treatment ashore, in the "dry mill", to produce the saleable products, determined by

mineral constituent and grade.

3.5.2.1 Dredge

The dredge is a 68.6 to 76.2 cm-diameter cutter suction type, with a throughput of 115,000

to 150,000 1 of slurry per minute (Figure 3.25). Two possibilities exist for mounting the cutter in

the prevailing water depths of 12 to 15 m, beyond the 3-mile limit: (1) a floating pontoon with a

cutter suction arm in which the dredge is secured by a 5-anchor spread, with the headline providing

forward pull and the sidelines dictating the arc through which the cutter moves, or (2) a jack-up

platform, capable of remaining on-site through all storms, and from which the cutter arm is

deployed. The choice is open to discussion, but is decided by the comparative availabilities achieved

by each system, depending on the relative performance under inclement weather conditions and the

relative time lost in moving forward.

The minimum practical cut that may be taken by the specified cutter suction dredge is 3 m.

For this scenario the average cut is assumed to be 4 m, with isolated areas extending to 5 - 6 m. Any

ground with a payable total heavy mineral content extending for less than 3 m below the seabed

could be unpayable.

3.5.2.2 Wet Mill

The wet mill is installed on a separate pontoon about 300 m astern of the dredge (or the jackup platform). The wet mill pontoon is approximately 10 times the size of the dredge pontoon. The

flowsheet commences with screening, with closed circuit crushing if any cemented sands exist.

Screen undersize of < 6 mm is fed to rougher spirals. The resulting concentrate is passed over

cleaner and finisher spirals, the rejects from which are re-cycled. The rejects from the rougher

spirals are rejected.

The wet mill has a feed capacity of 2,032 tonnes per hour of THM-bearing sands, being all

the cutter suction product conveyed by pipe line, and yields three "products":

a heavy mineral concentrate (-75% THM) from the finisher spirals, amounting to 4 to 5%

of the feed, which is transported ashore for processing in the dry mill

a trommel oversize, usually> 1.25 cm (and amounting to about 5% of the feed if a cemented

sand "hard-pan" exists), discharged into the sea
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(3) the balance of the feed as an effluent, being all sediments < 0.6 cm and amounting to about

90 - 91% of the total solids feed (95 - 96% if there is no trommel oversize), discharged into

the sea.

It should be noted that excess water, perhaps half the wet mill feed, possibly requires removal

from the flowsheet by means of cycloning ahead of the rougher spirals. Most of the -200-mesh

particles accompany the excess water. The heavy mineral concentrate is transported ashore at

regular intervals by a separate vessel which is also used for crew changes.

3.5.2.3 Dry Mill

The heavy mineral concentrate is treated ashore in two stages: (1) scrubbing with NaOH in

scrub tanks to remove surficial mineral coatings, and (2) dry magnetic separation to yield separate

products of ilmenite, leucoxene/rutile, staurolite and zircon.

3.5.2.4 Tailings Disposal

Tailings could be disposed of as follows: (1) from the wet mill at sea, tailings could be

pumped ashore for beach rejuvenation, provided that the material is within the required size and

quality specifications; (2) from the dry mill on land, tailings could be trucked to the beach for

rejuvenation purposes.

3.5.3 Seabed Biodiversity and Resources

3.5.3.1 Locality and Dredging Characteristics Affecting Benthos

The proposed dredging is a fairly large scale operation, with a high throughput of well-sorted

sands (2,000 tonnes/operating hour). Only 4 to 5% of the sediment dredged is kept as concentrate,

with the balance returned to the seabed as tailings. Most of the excavated sediments could be

replaced at or near the site of dredging. Environmentally significant operating and mining

parameters for the dredge operation are given in Tables 3.31 and 3.32.

3.5.3.2 Benthos Pre-dredging

It is expected that these sands will have a high biodiversity, as is usual at this latitude and

water depth. In the old terminology of Thorson (1957) the area would be expected to have a sandbottom Venus community. At this site prior human disturbance has probably erratically introduced

contaminants or otherwise rendered the ground unsuitable or even dangerous for dredging. There

are dump-sites and areas of other uses indicated on hydrographic charts (Figure 3.26), and the

benthos will probably show localized impact from these uses. These effects must be determined by

site-specific baseline surveys.

On this exposed coast the benthos may have been subject to frequent natural as well as

anthropogenic disturbance. The area is close to a Navy firing range and a dump site for dredged
material is located nearby. A disturbed area such as this may show even more than normal variation

between sites. Extensive pre-dredging sampling is needed to show if there are areas of recent

disturbance with little benthos, stable areas with a normal variability of the fauna, and areas in which

elevated levels of benthos may occur due to enhancement effects of the nearby disturbance (Poiner

and Kennedy, 1984). The site's close proximity to the City of Virginia Beach may pose further

stresses to the environment from sewage, storm drain and industrial runoff. Existing status of fishery

and fish-nursery must be documented by an appropriate literature search.

3.5.3.3 Benthos Post-dredging

Impact will come from a combination of the removal of substrate and then smothering from

the large amounts of tailings. Impacts will be extensive, as experienced at the WestGold bucket

ladder dredge operation (Section 3.2.4.3.), with time to recovery determined by the nature of the

deposit and energy level of the site. In general terms recovery on sand beds can be expected within

3 to 5 years.

Benthos directly in the path of the dredges will be destroyed, and surrounding benthos will

be affected by sedimentation and turbidity. Trenches will develop as will elevated areas but these

should be smoothed from wave action and storms. Recolonisation should begin immediately. As

studies in Alaska have shown, sandy bottom communities should recover in 3 to 5 years but the

exact composition of the fauna will vary from place to place, and may not resemble the original

fauna, or whatever is then present at the reference monitoring sites. Monitoring the nature of the

recovery due to mining may be difficult because the area already may be affected by other

anthropogenic influences, and these may continue to affect the site.

3.5.4 Fisheries and Wildlife Resources

Loggerhead turtles breed on beaches as far north as Virginia and breeding records should be

obtained in case they are near the proposed dredge site. Groundfish should be able to move out of

the way of the dredging operation. Fish may be attracted to the sediment plume to some degree.

Fish which lay eggs in the sediment could have spawning grounds destroyed if they are present

during the mining and those nearby may show a reduction in productivity. Invertebrate resource

species such as crustacea and bivalve molluscs may show an initial drop in numbers locally, but

should not be affected in the long term, with recruitment from surrounding areas.

3.5.5 Biological Oceanography

This project is very similar in concept and scale to the WestGold operation in Alaska

(Scenario 1). As such the comments in Section 3.2.6 apply here. However, the shallow water ( 15

m), action of longshore currents and the presence of humans in coastal communities complicate the

issue. The visual effect of the mining operation will be obvious to residents, fishers and tourists.

Over 95% of the dredged materials will be returned to the sea after treatment. If discharged to the

sea surface this will result in an extensive surface turbidity plume which may extend down-current

along the coastline for several kilometres.

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3.5.6 Prediction of Environmental Impacts, Mitigation Objectives and Achievement Criteria

3.5.6.1 Prediction of Environmental Impacts

There will be a substantial turbidity plume produced by discharge of tailings (95% of

extracted materials). This may have only slight effects on primary productivity (see Section 2.4),

but will be highly visible if at surface. The return of large amounts of tailings to the seabed will

smother benthos within and beyond the dredge footprint. Recolonization within a dredge course

should have progressed to a biodiverse successional community within 3 to 4 years after final

tailings deposition from adjacent dredge courses.

3.5.6.2 Mitigation Objectives

Objectives should include:

minimization of surface turbidity (see Table 2.8)

reduction of benthic impact by constraining tailings discharge to the dredge footprint and

nearby

development of an alternative enhancement project to remove previously-dumped material

may be possible.

3.5.6.3 Achievement Criteria

Achievement can be measured by applying the water column, seabed and fisheries and

wildlife criteria as given in Table 2.8.
3.6 Scenario 5: Beach Nourishment, Maryland

3.6.1 Mining Scenario

Ocean City's beach is eroded at a rate of about 1 m per year and requires replenishment, by

agreement with the authorities, every 2 to 3 years. The program, one of flood protection, is funded

30% by the State, 65% Federally, and 5% by Ocean City. The length of beach subject to

renourishment is from the Ocean City harbour northwards to the Maryland/Delaware line, a distance

of about 9.7 km. The process consists of three stages:

excavation of sand from an offshore site

transport of sand to the beach which requires renourishment

disposal of sand on the beach in an approved manner.

Ocean City has a harbour (Figure 3.27) with a very difficult entrance, even in good sea

conditions, and water depths against the jetty are approximately 3 m. Weather is critical to the

mining operation. Storms develop from the southeast but the waves approach the beach from the

northeast.

3.6.1.1 Marine Conditions

As one component of a major 1989 hurricane protection project for the coast of Maryland

(U.S. Army Corps of Engineers, 1989), nearshore wave conditions and natural sediment transport

volumes for Ocean City were analyzed. This study provided the best available site specific

information for characterising marine environmental conditions in this vicinity. Attention is focused

on the nature of storms which generate maximum wave and surge, thereby affecting beach erosion.

Sediment transport volumes are estimated, but no quantitative detail regarding the vertical

distribution of currents is provided. Relevant observations from this work are summarized in the

following paragraphs.

The Maryland coastline is subject to two distinct types of severe storms, hurricanes and

"Northeasters" which cause high waves and which raise coastal water levels due to storm surge. The

term hurricane refers to a tropical cyclone in which the winds spiral inwards toward a low pressure

"eye", and in which maximum surface winds exceed 130 km/h. Hurricanes originate principally

during the months of August through October. In most instances hurricane intensity has diminished

considerably by the time such a storm reaches as far north as Maryland. The most severe storm type

affecting the Maryland coastline is locally known as a "Northeaster". These storms occur frequently

during the interval of September through March, and are characterized by strong onshore winds,

predominantly from the northeast. Storms of this nature can stall in the region, or move slowly

eastward, resulting in long persistence intervals for onshore winds, and consequently for severe

waves. Storm population analyses undertaken as part of the 1989 work confirmed that due to the

mid-latitude location of the Maryland coast, and to the particular orientation and offshore bathymetry

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The choice of dewatering system depends on the characteristics of the material being

handled, the volume to be treated, and the practicality of incorporation of the system on board the

dredge with its inherent spacial and layout limitations.

No system provides a perfect separation between solids and water. In each case solid

particles of less than a certain size will separate with the water. Hydro-cycloning generally extracts

particles coarser than about 75 microns; finer particles remain with the waste water effluent. This

is a significant limitation where the waste solids are predominantly clay-size. Hydro-cyclones have

been used for dewatering on land-based dredges for thirty years but their employment has been

directed towards the improvement of treatment plant efficiency. They have, however, proved to be

useful in de-watering the tailings effluent from bucket ladder dredges.

Where sediments are predominantly silts the potential benefits to be gained from dewatering

are minimal. Another problem is that the finer sediment becomes bulked in the process of separation

resulting in some loss of the efficiency gained by use of the hydrocyclone (Maintenance Dredging,

1987). Solids collection sumps, like hydro-cyclones, benefit from their simplicity of design and

have an application in some forms of dredging, such as the treatment of overflow water from trailer

suction hopper dredges (Figure 4.5).

Having separated the solids from the water, various options remain for the dredge operator,

for example:

to discharge the solids and waste water simultaneously but independently from each other

retain the solids for disposal or alternate use elsewhere

attempt to recirculate either the water or the solids, depending on the volumes involved and

the consequent, perhaps prohibitive, build-up in the system.

4.4.3.2 Optimization of Effluent Discharge System

A single dredge effluent may consist of a wide range of particle sizes from silt to small

boulders, or the reject from the dredge may have been sized before discharge. The settlement of

coarse material through the water column is dictated primarily by gravity. The movement of finer

material is more complex. To design appropriate engineering responses with the objective of

minimizing the effect of significant dredge effluent discharge, it is first necessary to understand the

physical processes that dictate the fate of effluents discharged into the sea.

These processes are summarized here. Two basically independent processes dictate the fate

of discharged particulates from an effluent stream: (1) sedimentation of the effluent from the

discharge point to the seabed, and the three-dimensional dispersion of this material through the water

column, and (2) the susceptibility of the deposited materials to be transported along or near the sea

bottom in response to wave and current forces.

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Effective mitigation of effluent dispersal is best achieved if prior knowledge is obtainable

concerning the probable behaviour of the effluent stream in a mining operation prior to the design

and implementation of mitigative techniques. Such knowledge is most readily gained through the

application of numerical models. Hence the modelling of sedimentation and of potential sediment

transport constitutes a practical first step in the mitigation process. Variables that affect

sedimentation and sediment transport are given in Table 4.1.

Discharge Design

Significant decrease of the initial velocity of effluent discharge and associated turbulence can

be achieved through design modification of the effluent conduit. Success, however, requires an

interactive process of full-scale experimentation at sea, monitoring, and model calibration for a

particular site. One of the most important variables to be considered in the design of the discharge

is its position relative to sea level.

Sub-surface Discharge

In any marine mining operation the most convenient and economical practice is to discharge

effluents directly at the sea surface. However, this practice leads typically to high elevations of

surface water turbidity and to broad dispersion of suspended particulates. Reduction of sea water

turbidity and of the areal extent of solids deposition can be achieved by discharging effluents below

the sea surface.

The depth of discharge below surface is critical. There are two basic design considerations:

(1) sea state and the related potential for shearing of the effluent conduit, and (2) proximity to the

seabed: where discharged effluents have a high initial velocity, sediment scouring and reflection of

the effluent stream can occur if energy is imparted to the seabed, thereby generating excessive

turbidity.

Discharge of dredge effluent below the water surface may be achieved in association with

extraction technology, and yields significant environmental benefit when properly implemented.

Discharge Location

In some marine mining operations the on-board treatment process requires early segregation

of the excavated sediments into different grain sizes, usually as a result of screening. The separated

waste products are discharged as effluents into the sea at different locations depending on their point

of generation in the treatment flowsheet.

If a large dredge is being used the discharge points may be separated by tens of metres. In

shallow water the redeposition of the falling sediments on the seabed may reflect that separation.

The forward and sideways movement of the dredge during its continuous operation therefore results

in sediments, deposited by the most rear-mounted discharge, covering and obscuring the sediments

discharged from more forward locations.

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Thus, a seabed with an original lithology characterized by a mixture of fine and very coarse

sediments may be converted by dredging into one of sand if the fine tailings are discharged at the

stern. Cobbles and boulders may become completely obscured by sand tailings. Conversely,

reversing the discharge locations may result in a modified seabed of cobbles and boulders.

The effectiveness of this technique is dependent on the grain-size distribution of the different

effluents. Where there is a particularly high proportion of silt and clay any attempt at an ordered

discharge is rendered ineffective. The residence time of fines suspended in the water column will

far exceed the practical retention time of tailings in the processing stream, with the result that the

entire seabed may finally consist of a thin cover of sand and silt.

4.4.3.3 Control of Discharged Effluent

Polymers

The residence time of solids suspended in the water column is mainly dependent upon

particle size. Silt- and clay-size particles can remain suspended for time scales ranging from hours

to days, and therefore have potentially wide-reaching effects. The settling time of fine-grained

particulates can be accelerated through the use of organic polymers which come in two forms:

organic flocculants and various coagulants.

Both organic and inorganic flocculants and coagulants work on the principle that charged

ions attract and cause flocculation of particles with the opposite charge (Littler, 1990). Their

addition improves the settling velocities of finer particles (Spargo, 1986). Groups of loosely-bonded

particles, or flocs, behave in a manner equivalent to larger particles. Floes thus have greater mass

and faster settling rates. Colloidal matter smaller than 1 micron in diameter never settles but can be

flocculated out of suspension (Littler, 1990).

Organic flocculants are relatively long chain molecules that have either positive or negative

charge sites (Personal Communication, Mr. C. Pfleegor, Nalco Chemical Co.). They may be

categorized into three groups: cationic (positively charged), anionic (negatively charged) and nonionic (zero net charge). The charge sites attract the opposite charge sites on the suspended particles,

pulling them together to form large floes.

Flocculants are oil-based and are not ready for use in their delivered form: they must be

activated by rapid mixing with clean water prior to use. A small amount of fast mixing of the

polymer solution and the water is required to force the solid particles and the polymer together.

Excessive mixing of the water and polymer solution can cause the floe to break up, resulting in less

than optimum settling.

Coagulants are chemicals which, when properly used, make cloudy water look crystal clear.

These chemicals are used by municipal water plants to clarify drinking water. Coagulants are waterbased and are supplied "ready to use". Coagulants can be entirely organic, blends of organic and

inorganic, or inorganic (e.g. aluminum sulphate, ferric chloride, and hydrated ferrous sulphate). The

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inorganic coagulants typically are acid-based and the contained, treated water usually requires a

second chemical to raise the pH after the chemicals are applied (Littler, 1990).

The addition of coagulants causes destabilization of the electrical charges that generally

allow particles to repel one another to form a stable system. Following destabilization, further

mixing with flocculants is frequently necessary in order to encourage aggregation into larger

particles (Spargo, 1986).

Flocculants cause suspended solids to settle faster than when conditioned with a coagulant.

But, unlike coagulants, flocculants do not clarify the water. They are an important means for

managing the silt- and clay-size fraction of dredge effluents, with the potential to reduce both the

degree of turbidity and the size of the mixing zone in the sea.

Flocculating agents are currently used in some civil dredging applications and in certain

forms of industrial waste-water treatment. Organic polymers have been employed for many years

to accelerate the settling of silts dredged from the bottoms of lakes. They have also been used to

clarify the slimes that collect in the ponds of land-based dredges. Although their application has

been questioned on environmental grounds they are biodegradable and a number of flocculants have

been approved for potable use. However, despite their use on land, it is likely that further research

concerning the environmental implications of flocculants and their practical and effective use at sea

will be needed before they are accepted as a viable mitigation option.

Silt Curtains

The use of silt curtains, vertically-suspended, weighted material used to contain fine-grained

suspended sediments within a prescribed area, is a mitigation technique that has received wide use

in connection with small-scale dredging and construction projects in inland waterways. The

turbidity mixing zone created by a dredging operation can be reduced by using silt curtains to

contain the plume within a specified area. However, the limitations and disadvantages of this system

are considerable (Continental Shelf Associates, Inc. 1993):

it cannot be used where the currents are more than 2 km/h

noticeable waves and swell render it inoperative so that it can be used in only the most calm

and sheltered sea conditions such as harbours and estuaries

it is applicable only for small-scale, anchored operations

it limits the maneuverability of the dredge

being susceptible to rapid destruction in the open sea it constitutes not only a hazard to the

seaworthiness of the dredge but also a potential danger to other shipping in the area.

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Though it is frequently proposed as a mitigation technique for offshore marine mining, its

potential application is extremely limited. With the exception of a modified form for Scenario 2,

the use of a silt curtain is not considered as a mitigation option for any of the mining scenarios.

4.5 Remediation

4.5.1 Re-Dredging

Of the various effects of marine dredging only the remediation of changed seabed profile and

seabed lithology are technically feasible. At the extreme a dredged area could be re-dredged to

remove erratic topography caused by the previous excavation. Such an attempt probably would

require the application of a different type of dredge from that used in the original mining and could

never return the seabed profile and lithology to exactly the same original state. This method would

be prohibitively expensive, raising the costs by about 100%. However, according to location, views

differ considerably regarding the need to rehabilitate the seabed profile. For example, in Japan,

where coastal trawling is not a popular fishing technique an irregular, undulating, seabed is not

considered to be a problem. Marine aggregate miners are therefore not required to reclaim the sea

bottom (Narumi, 1989).

4.5.2 Bed Leveller

Bed-levellers are seafloor-smoothing devices towed over the seabed by tugs or other smaller

craft following the passage of a dredge which has either been working fine-grained sediments or has

left behind a seafloor covered with mud and/or sand. Tolerances in the region of 10 to 20 cm are

claimed in some situations but the operator may not always consider it economical to attempt such

accuracy (Maintenance Dredging, 1987). Bed-levellers may have a wider application. For example,

it has been reported that the system has been used successfully in the United Kingdom to flatten

isolated outcrops of boulder clay by as much as 2.5 m (Van de Graaf, 1987). Another example was

reported when a contractor successfully used a bed-leveller permanently fitted to a trailer dredge

without experiencing any problems with propeller damage. Provided that a bed-leveller can be

towed behind a dredge and operated simultaneously with the excavation process it could probably

be applied usefully, perhaps mechanically combined with each individual drag shoe on a trailing

suction hopper dredge.

4.6 Compensation: Alternative Enhancement

An advantage of the offshore mining industry is that dredges can be made available, when

not otherwise occupied on mining projects, to undertake one-off proj ects such as reclamation, beach

nourishment and coastal or offshore substrate stabilisation. In the United Kingdom such projects

play an important role in establishing the present day economic viability of aggregate dredging

operations (Nunny and Chillingworth, 1986).

A dredge engaged in recovering heavy minerals from an offshore sand produces huge

quantities of fine and coarse sand as a by-product - the very same products which may be in short

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supply on an eroded beach nearby. Such a by-product thus may become a resource either for beach

renourishment, removing the necessity for dedicated beach renourishment dredging or for use in the

construction aggregate industry (Industrial Minerals, 1990).

Some examples of such imaginative applications which could benefit both the marine mining

industry and address major environmental concerns already exist. One U.S. local authority is already

pursuing a program to maximize the beneficial use of dredged material by creating a 9 hectare

wetland habitat in West Galveston Bay. The scheduled dredging, through the Aransas National

Wildlife Refuge, will also include the beneficial use of the dredged material to create and protect

vital habitat of the endangered whooping crane (U.S. Army Corps of Engineers, 1992). There is,

therefore, potential under certain circumstances for alternative enhancement projects which could

be incorporated into a marine mining operation to compensate for environmental impacts associated

with such an operation. Examples of such projects include:

use of tailings for beach nourishment or the creation, restoration or protection and

maintenance of coastal wetlands

deployment of artificial reef materials and seabed landscaping for habitat enhancement

remedial dredging of contaminated sediments.

use of dredged-out pits for burial and capping of contaminated wastes or dredge spoils.

Ideally the by-product sediments, usually tailings, should be capable of simultaneous

transport by barge or pipeline to the enhancement site. Flexibility in terms of ship maneuverability

and discharge capabilities are obviously a distinct advantage (Nunny and Chillingworth, 1986).

However, the dredging operation and requirement for reclamation material are not necessarily

coincident in time, even if the relative locations are ideal. There may be a need to stockpile perhaps

hundreds of thousands of tonnes of material produced by dredging until it is required for the

alternative enhancement project. For stockpiling to be economic the temporary storage site usually

must be in shallow accessible water where later recovery is facilitated and no irreversible

environmental impact is envisaged (Maintenance Dredging, 1987).

4.7 Regulatory Mechanisms

Although the procedure for licensing of offshore mining is a subject in itself and beyond the

scope of this report, the subject is of considerable importance when considering mitigation. The

licence should stipulate what equipment may be used, how it may be used, and what measures must

be applied in order to mitigate against the consequent environmental effects. Regulatory

mechanisms constitute a form of mitigative measure by not allowing mining to start if such

operations are capable of inflicting environmental damage.

The United Kingdom and Japan together account for about 85 % of the world's output of

marine aggregates by offshore dredging. In the United Kingdom the production in 1990 amounted

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to nearly 25 million tonnes annually, equivalent to 16 to 17% of the country's total from all sources.

In Japan, the leading producer, the equivalent figure is almost 40 million tonnes (Benbow, 1990).

4.7.1 International Examples

4.7.1.1 United Kingdom

In the United Kingdom the marine dredging activity is almost entirely for sand and gravel

aggregates (> 5 mm in size). The industry is dominated by half a dozen major producers operating

a total of about 50 dredges. Almost all of the dredges are of the trailing suction hopper design and

the most modern additions to the fleets represent the state of the art in western European dredge

design and construction.

Two types of licences are required: a prospecting licence, valid for 2 or 4 years, and a

production licence. There is also a provision for the collection of a 1,000 tonne bulk sample (Webb,

1988). For a new licence to be granted the following factors must be taken into account (Fox, 1989):

the likelihood, nature, and extent of conflicts with other users of the seabed (fishing, defence,

and navigation interests)

whether the residual seabed after exploitation will retain a suitable surface compatible with

the original configuration

the possible effects on the coastline and coastline stability

the existence of an acceptable resource management scheme ensuring optimum use of the

available materials.

The lead agency in the United Kingdom, the Crown Estate Commissioners, consults five

government ministries plus about five agencies. Local authorities are also involved. There is a

special procedure for minimizing the effect on ocean fisheries set out in the ministry's "Code of

Practice for the Extraction of Marine Aggregates" (Nunny and Chillingworth, 1986) which requires

the dredge operator to:

maintain close communication between the dredge and the local fishermen

restrict dredge operations to specific areas on a short term basis, thus allowing unhindered

fishing over the majority of the licensed area (called the "strip grazing" principle)

dredge the ground in such a way that, on cessation of dredging, previous fishing areas

recover ecologically and become available again to the local fleet.
4.7.1.2 Japan

In contrast to that of the United Kingdom, the Japanese marine aggregates industry is highly

fragmented, with, in 1988, about 250 companies operating approximately 450 dredges. Dredging

is by clamshell, suction pump and booster pump with all the product being sand (Industrial Minerals,

1990; Narumi, 1989).

Registered Japanese companies may apply for either a provincial or a federal extraction

licence: in both cases for one year and renewable. The provincial licence, however, covers only 1

km2 in area and is non-exclusive. The federal licence requires the submission of a dredging plan and

the completion of an environmental impact study.

Approval for the issue of a licence must also be given by relevant Japanese fisher's

cooperatives, and an agreement is generally reached between each fishing organization and the

mining company. Under such an agreement, which is not standardized, the cooperative receives a

negotiated financial compensation for the dredging activities based on a charge per m3 of sand

extracted. The fishery organization then applies some of this compensation money to the

modernization of the fishery (Narumi, 1989).

Of the several problems that need to be resolved in the Japanese marine aggregate mining

business, the most important is the regulatory mechanisms. In particular, the required standards for

application and approval have not been consistently established throughout the country. This

deficiency, especially with regard to the various arrangements to be made with the fishing

cooperatives, has destabilized the Japanese marine aggregate business and needs to be resolved

(Narumi, 1989).

4.7.2 Requirements

4.7.2.1 Clear Jurisdiction

Some important guidelines, critical to the start-up and success of an offshore mining industry

emerge from the experience of existing and previous marine operators in the United Kingdom, Japan

and North America. The same guidelines, if followed, are more likely to ensure that the optimum

mitigation techniques are researched and applied by the mining company. Their application is, in

effect, one of the most important mitigation methods available (Table 4.2).

The U.S. Bureau of Mines' post-mortem on the WestGold marine gold placer project in

Alaska constitutes an invaluable record of the agency-company relationship from both viewpoints

(Gardner, 1992).

Permits issued to an operating company will impose monitoring and mitigation requirements.

Ideally these should not involve, nor be perceived to involve, the application of double standards,

especially in the forms and extent of mitigation required. One apparent standard for existing marine

industries and another that is considerably more harsh in its standards for a contemplated or

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developing marine mining industry, would do little to encourage the further development of marine

mining. Any suspicion that such a situation exists will discourage and deter potential founders of,

or entrants into, the marine mining industry.

4.7.2.2 Data Availability

Application Review

The lead regulatory agency in reality has a double role: to determine the ability of the

dredging company to develop appropriate operational plans, and to ensure that the company adheres

to its plans, especially those for mitigation.

The forms of offshore mining described in this report are examples of marine placer mining.

Experience of on-land placer mining has demonstrated repeatedly that the technical/financial risk

lies in two areas: the reliable estimation of the ore reserves, and the correct selection of the most

appropriate mining equipment (Garnett, 1 996b.

Offshore exploration is more expensive and is more susceptible to error and misjudgment

than on land, thus an exploration program designed to estimate the reserves in support of any

application is vital (Fox, 1989). An inexperienced mining company, as a result of lack of appropriate

experience, may tend not to award the necessary importance and funds to the exploration phase. As

a part of the exploration program it is necessary to accumulate geotechnical information to assist in

dredge selection.

The final result may be reduced operational profitability and a consequent lack of enthusiasm

or inability to undertake the mitigative measures necessary, especially if the operating company is

a small one. The extreme case is premature termination of the project, leaving some of the

environmental mitigation programs incomplete. Such a scenario is particularly important when

considering precious metal mining ventures offshore. Smaller organisations cannot easily compete

in the marketing-dominated heavy minerals and aggregate industries, but face no such impediment

in considering a small offshore gold mining venture. The role of the appropriately experienced

consultant in the review of applications cannot be over-emphasized.

Data Collection

In the United Kingdom production licences are usually granted within 2 years of the

application date. Denial or delay of a licence is often attributable to the inadequacy of the data

supplied in connection with fishery and coastal protection issues rather than because of any official

conviction that harmful effects will occur (Nunny and Chillingworth, 1986).

During the exploration and planning stage the baseline environmental data should be

collected in support of a permit application. This stage is an ideal opportunity in which to gather

data that may be directly relevant to both reserve management and eventual protection of the

environment. Data collected for one is invariably of importance for the other: a further example of

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good mitigation techniques being synonymous with good engineering practice. For example,

underwater photography should not only provide the necessary data concerning biota but may

unexpectedly reveal the presence of problematic cobbles and boulders to be considered during

equipment selection and design (Nunny and Chillingworth, 1986).

Information regarding ocean current direction and speed may allow the dredge course to be

designed so as to minimize over-sanding (that is, the disposal of tailings on virgin ground).

However, this is not an opportunity to impose unrealistic data gathering demands on the aspiring

company. In the United Kingdom, for example, it is recognized that "With environmental data

collection, the scale of any program needs to be closely tied to the expected level of return from the

project" (Nunny and Chillingworth, 1986).

At all stages from permit application to post-mining, data measurement and its ensured

reliability constitutes a weak link in the whole process of dredging (Maintenance Dredging, 1987).

The deficiency manifests itself in three important and inter-connected subjects, for each of which

a solution to the problem is necessary for the eventual successful application of mitigative

techniques:

baseline studies during the exploration phase

environmental monitoring around the dredge operations

dredge operation management.

A technical advance in measurement techniques applicable to one of the above is

immediately and directly of benefit to the others. In recent years enormous improvements have

become possible through the use of sophisticated instrumentation and electronics, and the ability of

dredgers to monitor their production operations has been greatly improved. For example,

continuous-reading in-pipe flow and density meters are now available (Bide, 1989). Several systems

are now available which can give an accurate particle size distribution, within minutes, of both intake

and effluent (Ports and Dredging, 1993a). For example, the visual size identification (VSI) system

was developed to meet the frequent demand for a means of measuring oversize in a slurry flow. It

does not provide definitive statements of oversize or product size distribution, but "produces regular

feedback of trends as each sample is analyzed" (Quarry Management, 1990). Ultrasonic techniques

are now available to measure water flow rates, and radioactive probes can determine concentrations

of solids. The product of the two parameters yields the mass flow rate (Maintenance Dredging,

1987). Another major improvement of recent years is in navigational equipment. For the dredging

industry continuous positioning is now possible through the Global Positioning System (GPS) (Ports

and Dredging, 1992) the accuracy of which is at times remarkable but which varies from place to

place.

Accelerating advances in computer technology have been accepted rather slowly by the

generally conservative dredging industry. Computers allow more advanced data collection and

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monitoring (Taggart, 1992; Ports and Dredging, 1990c; 1989). A high degree of automation is now

possible (Maintenance Dredging, 1987; Ports and Dredging, 1991a).

There are obvious advantages in the improved monitoring of slurry characteristics and

aggregate quality in conjunction with knowledge of a dredge's accurate position whilst dredging

(Bide, 1989). A continuous record of the quantity and nature of dredged material with the exact

location from which it was dredged would be an essential management tool on board any dredge.

It would provide invaluable information on the resource being excavated as well as providing data

to compare with any simultaneous environmental measurements (Parrish, F.G. 1989). There is a

danger, however, that such improved monitoring could be regarded by the dredge operator as

policing of the dredging operations (McDonnell, T.M. and R.K. Tiliman. 1992). Nevertheless, the

on-board provision of such instrumentation and "black boxes for dredges" will become an essential

part of the successful application of mitigation techniques.

However, "... sophistication of instrumentation does not necessarily mean greater accuracy

of measurement" (Maintenance Dredging, 1987). The reliability of the data is as important as the

quantity. A voluminous amount of dubious data is of no advantage to either the operator or the

regulator. But the extent to which snap-shot measurements and analyses are representative must also

be questioned. For example, the large variability in measured turbidity may be greatly reduced and

be better understood by analyzing continuously recorded data. Such data may be averaged over

chosen periods to give a more realistic picture of the water quality, and short term averaging of

continuously recorded data should be the adopted procedure.

4.8 Economics

4.8.1 Marine Mining Costs

Dredging costs are divisible into capital costs (i.e. dredge and auxiliary equipment) and

operating costs (i.e. salaries, fuel, maintenance, and insurance). The fixed cost element of operating

costs can be surprisingly high for precious metal dredging operations in inhospitable marine

environments. While revenue is not being earned during downtime most costs continue to be

incurred. Some of the variable costs, such as fuel, may be saved.

It is generally not recognized that marine mining is considerably more expensive, on a $/m3

basis, in terms of capital investment and operating costs, than its onshore equivalent. The difference,

of course, is attributable to the generally greater downtime experienced, the larger and more

sophisticated equipment, and the greater back-up facilities required. In the United Kingdom the cost

of producing seabed aggregates is higher than from equivalent land-based deposits (Fox, 1989). As

a result the sales price (ex-wharf) of marine material is generally higher than land-won aggregates

of the same quality (Nunny and Chillingworth, 1986). An example from Alaska is a direct

comparison between two bucket ladder dredges. Both produced gold from almost identical glacial

sediments. One, WestGold's "Bima", was working 1.5 km offshore and the other, Alaska Gold's

"D6", immediately onshore. WestGold's costs were approximately three times that of Alaska Gold

per m3. Similar evidence from offshore tin dredging in South East Asia and the marine mining of

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diamonds on the western coast of southern Africa (Garnett, 1995) confirms that the unit cost of

marine production can be between two and five times higher than that of equivalent on-land

production.

Dredge capital and operating costs are summarized for each of the mining scenarios in

proportional terms. Actual dollar values are not presented for reasons of confidentiality and because

proportional values are more meaningful in the long-term as costs increase due to inflation.

Methods of capital financing may be different for any new offshore mine development:

leasing, public flotation, retained equity, etc. For uniformity, the capital cost of a dredge with

auxiliary equipment is in each case based on a 'tax shelter" lease arrangement with the following

terms: a ten-year lease with a ten-year internal revenue life for depreciation; a mid-year start date for

depreciation; a 30% salvage value after ten years; and payments based on the seven-year treasury

note interest rate at the start of the operation. A minimum ten-year project life would be expected

for most marine mining developments. In the case of a projected twenty-year life, the lease could

be renegotiated after ten years such that payments are then less than half the original rate.

The question of profit is not dealt with in detail but warrants mention as profit expectation

is the determining factor in proceeding with any marine mining development and operation. Simply

stated, profit is the remaining balance of commodity sales revenue once all operating and capitalderived (depreciation, etc.) expenses are paid. Profit requirements vary from company to company

and commodity values can fluctuate considerably over time, so it is neither appropriate nor

meaningful to include quantified profit in economic considerations of the hypothetical mining

scenarios.

Profit objectives can only be met if the expected excess of commodity sales revenue over

operating costs is realized at the conclusion of each operating season. The balance can be seriously

altered by unscheduled downtime associated with extreme storm events, emergency maintenance

and certain forms of mitigative action that previously were not stipulated. Unscheduled downtime

translates into a loss of commodity sales while fixed and some variable operational costs are still

incurred, with a corresponding loss of profit.

4.8.2 Investment

The use of the correct equipment is of major importance in marine mining not only in terms

of type but also the scale or capacity (Garnett, 1996a,b,c). With some important exceptions where

selectivity of extraction is more important than throughput, the larger dredges will operate at lower

cash costs. In Japan productivity is low from the regional, small-scale, operations (Narumi, 1989).

The necessary capital investment, however, is greater for higher capacity dredging systems,

a situation which requires more careful assessment of the risk involved. If a company considers that

offshore dredging licences will not be granted it will postpone investment in the appropriate new

vessels. Conversely, without suitable vessels the company is unable to apply for a licence when the

opportunity arises. For example, it is generally recognized in the United Kingdom that "... the

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uncertainties brought about by the lack of new licences has generally delayed further investment by

many of the companies involved" (Nunny and Chillingworth, 1986).

In a marine aggregate industry the onshore processing plant, with its attendant capital

investment, must match the offshore capacity. The absence of certain onshore facilities such as drag

scrapers and belt-conveyor systems prevents the Japanese industry from using larger-sized ships

(Narumi,1989).

The cost of exploration, with no guarantee of success, represents a considerable investment

for most companies, and one which is lost if a production licence is refused. In the United Kingdom

this burden has been recognized and it has been suggested that the lead agency, the CEC, "... might

encourage more detailed prospecting by offering partial recompense of prospecting costs for failed

licence applications" (Nunny and Chillingworth, 1986).

4.8.3 Mitigation Costs

Aside from their success or otherwise in minimizing or preventing environmental impact,

all mitigation techniques must be judged not only in terms of their practicality of use at sea but also

in terms of their effect on costs. The cost of implementing various mitigation techniques can be

similarly categorized into capital and operating (fixed and variable). If the techniques are

substantially operational in approach they may lead to increased operating expenses. If technologybased they will involve capital outlay with some negative effect on the operating costs such as fuel

and labour.

An additional economic cost may be involved in the implementation of mitigation

techniques: an opportunity cost which represents the loss of revenue, and thus of potential profit.

It may be incurred by either the curtailment of productive capacity or downtime caused by the

installation and maintenance of special mitigative equipment. Some suggested forms of mitigation,

while possessing technical merit, are capable of erasing profits by overwhelming erosion of revenue

through lost operating time,

The above considers the cost of application of mitigation methods from the viewpoint only

of the operating company. The whole cost of marine mining, including the mitigation, should be

examined in the context of the benefits to the host country of an on-going marine mining industry.

The benefits of the mining activities must be viewed in the light of the losses that might occur as a

result of a decrease in the size or total catch of an existing fishing industry which may be disrupted

by the mining activities.

To undertake such a cost-benefit analysis requires a quantification of the relative value of

inshore fishing compared with the mining. In the United Kingdom it has been suggested that this

major study be undertaken in order to assist the government in developing its view on dredging

licence applications (Bide, 1989).

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5.0 Application of Mitigation Techniques

The mitigation objectives and techniques outlined above do not apply equally, if at all, to

each of the five selected scenarios. Rather than using a repetitive, approach the following scenarios

are used to illustrate a particular technique in detail. Descriptions are given of the most appropriate

choices of mitigation techniques for the scenarios and similar marine mining developments.

Invariably the choice is dictated by practicality. Mitigation techniques which clearly should not be

taken under consideration are also identified.

5.1 Scenario 1: Precious Metal Mining, Alaska - Bucket Ladder Dredge

5.1.1 Background

The placer gold mining operation carried out by Western Gold Exploration and Mining

Company (WestGold) Limited Partnership from 1986 to 1990, inclusive, in state waters off Nome,

Alaska, was the subject of comprehensive permitting and an intensive environmental monitoring

program overseen by both state and federal regulators. The permits awarded by the state and federal

authorities determined the extent and nature of various programs designed to monitor the following:

dredge effluents

water turbidity and trace metal content

impact on, and biological recovery of, sand and gravel substrates.

Constraints were applied by the permitting agencies involved and the monitoring programs

were designed to:

assess whether the constraining criteria were being met

aid in the design of mitigation techniques intended to: a) meet the criteria on a continuous

basis, and b) reduce the effects of the dredging even further.

Sample collection was undertaken in such a manner that the same procedures and samples

could be used for a variety of purposes. Specific mitigation objectives were neither set nor itemized

at the start-up of the operation but in principle included minimization of water turbidity and

minimization of release of toxic materials. The objectives were quantified once dredging was

underway.

5.1.2 Regulatory Authorities for the Permitting Processes

As related by Gardner (1992) "the project activities under regulatory scrutiny included the

dredging of gold-bearing sediments, the discharge of process water, and the discharge of dredged

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material back on to the seabed, approximately along the dredged path and to the rear of the

advancing dredge."

Several features resulted from the unique nature of the project and from the offshore location

within Alaskan state waters and the federally-mandated coastal zone. The project did not conform

to the existing regulatory mould and numerous agencies were involved, issuing permits at federal,

state and local levels (Table 5.1).

The Alaska project consistency review commences with the identification by the Alaskan

Division of Governmental Coordination (ADGC) of the requirements in the form of information and

permits required by the following three state resource agencies in accordance with Alaska Statutes

and Alaska Administrative Code:

Alaska Department of Natural Resources (ADNR)

Alaska Department of Environmental Conservation (ADEC)

Alaska Department of Fish and Game (ADF&G).

In addition, the USEPA must determine whether unreasonable degradation of the ocean will

result from the discharge of any or all of the effluents from the dredge, based on a consideration of

the following:

quantities, composition, and potential for bio-accumulation or persistence of the pollutants

discharged

potential transport of such pollutants

the composition and vulnerability of biological communities exposed to such pollutants

the importance of receiving water area to the surrounding biological community

the existence of special aquatic sites

potential impacts on human health and on recreational and commercial fishing

the applicable requirements of approved Coastal Zone Management Plans, and marine water

quality criteria developed pursuant to the Clean Water Act.

The various reviews, by both the public and the agencies involved, are subject to 60-day and

180-day time lines which are frequently exceeded.
5.1.3 Regulatory Authorities for Environmental Monitoring

Both the state and federal permitting processes require environmental monitoring programs.

The Environmental Protection Agency, in compliance with the Clean Water Act, requires the dredge

operator to undertake a monitoring program in order to determine whether the effluents comply with

the limitations imposed by the permits and to assist in the development of effluent limitations. The

Alaska Department of Governmental Coordination requires that environmental monitoring

requirements are met in order that the dredging operation is consistent with the Alaska Coastal

Management Program.

5.1.4 Regulatory Authorities for Post-Mining Requirements

Alaska may stipulate any post-mining requirements, such as continued monitoring or

rehabilitation, through the Alaska Coastal Management Program, in an Approved Plan of Operations

and the various permits. Federally, the Environmental Protection Agency may also dictate postmining requirements through the NPDES permit.

5.1.5 Water Quality

5.1.5.1 Tailings Plume

The NPDES permit No. AK-0043 19-2 originally issued to WestGold required the company,

as operator, to comply with both federal and state water quality criteria at the edge of the mixing

zone at a 500 m radius from the point of discharge of the dredge effluent. State water quality criteria

required that turbidity levels should not exceed 25 NTU's above background. There were three main

sources of turbidity within the mixing zone, each creating a distinct plume visible at surface, in the

following order of decreasing importance:

the jig tailings carried along two stern launders and discharged into the sea

the excavating action of the dredge stirring up fines into the water column through turbulence

around the bottom tumbler and from being washed off the rising, loaded dredge buckets

the trommel oversize discharged immediately astern of the vessel.

The behaviour of tailings effluent disposed of as a single stream into open water may be

described as comprising three phases (Figure 5.1):

(1) Convective Descent

The material behaves as a jet descending towards the sea floor. Jet movement is governed

by its initial momentum as it leaves the disposal pipe, by gravity forces due to the density

difference between the jet (consisting of a water/sediment mixture) and the ambient receiving

water and by viscous shear at the jet's periphery. Viscous shear action causes turbidity in the

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water colunm as the jet mixes with the ambient water. Sediment particles in the mixing zone

begin to act independently, rather than as a part of the jet.

Dynamic Collapse

When the jet impacts on the seabed it collapses. It becomes a major source of turbidity as

it scours and re-suspends material, including re-deposited tailings, from the seabed. The

momentum of the jet may be partially conserved and be re-directed both upward and

laterally, suspending material in the water column. In shallow water a "boiling" effect is

created.

Long-term Diffusion

Once the momentum of the jet is dissipated during dynamic collapse, material that has been

suspended in the water column is convected by ambient currents and diffused by turbulence.

Individual sediment particles fall through the water column and are deposited on the seabed.

For trace metal concentrations, federal marine water quality criteria led to the establishment

of a 100 rn-radius mixing zone from the point of effluent discharge with the requirement that trace

metal levels for 8 elements should not be exceeded at its periphery for 4-day averages and 1-hour

maxima.

5.1.5.2 Monitoring

Monitoring involved the collection of samples of the following intake and effluents, it being

impossible to sample the trommel oversize or the excavation discharges:

tailings launder effluent

gold room effluent

sea water both up-current and down-current of the dredge.

Turbidity and water quality monitoring stations, with some changes from year to year, were

sited at the following distances from the dredge (Figure 3.16) and at different water depths:

Up-current: 0, 500 and 2,000 m

Down-current from the tailings discharge: 0, 50, 100, 480, 500, 520, 800 and 1,000 m

(3) Down-current from the bottom tumbler: 50, 100 and 500 m
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5.1.5.3 Plume Modelling

Software

WestGold and its consultants employed the DIFCD (Disposal from a Continuous Discharge)

3-dimensional water quality and sediment plume computer model developed by the U.S. Army

Corps of Engineers Waterways Experiment Station. It was used in conjunction with a Wave Current

Sediment Resuspension Prediction (WCSRP) model. The DIFCD and WCSRP computer programs

proved to be invaluable in predicting the plume creation behaviour of the tailings discharge.

The DIFCD model "simulates the movement of disposed material as it falls through the water

column, is transported and diffused as suspended sediment by the ambient current, and is deposited

on the bottom. It computes the movement of disposed material in a continuous fashion at a constant

discharge rate. It takes into account the ambient environment at the disposal site, the characteristics

of the material, the disposal system, and the general physics involved" (R.& M. Consultants, 1988).

To overcome a major limitation of the DIFCD model, namely, the basic assumption that once

solid particles are deposited on the bottom they remain there, the model was used in conjunction

with the WCSRP model. "The underlying assumption of the WCSRP model is that naturally

occurring sediment resuspension in the near-bottom waters is controlled by the intensity of the

bottom boundary shear stress associated with wave oscillatory flow and tidal currents, storm surges,

or currents associated with other mechanisms. In general, there exists a threshold boundary shear

stress below which no apparent resuspension could occur. Once bottom boundary shear stress

exceeds the threshold value, sediment resuspension occurs and will increase or decrease according

to the magnitude of the boundary shear stress. The model uses the flow conditions and bottom

sediment grain size as input for computing the bottom boundary shear stress" (R.& M. Consultants,

1988).

The DIFCD model was calibrated and used, with the WCSRP model, to:

predict (for each set of operating conditions, such as water depth) tailings effluent discharge

depth and discharge rate and the distance from the dredge where water quality standards

would be met

predict the tailings effluent discharge pipe configuration, and pipe depth below the water

surface, resulting in the lowest turbidity and metal concentrations at the edge of the mixing

zone

simulate the range of conditions, including dredge operating water depths, expected to be

encountered during the life of the offshore dredging project, using both an average range of

conditions and a worst case scenario.
Predictive Capacity

Actual conditions observed through the monitoring program were used to determine the

predictive accuracy of the computer models. The process was interactive, allowing a gradual upgrading of the mitigating techniques, and resulting in a reduction in turbidity and an improvement

in the validity of the model. The monitoring was conducted in conjunction with bathymetric and

side-scan sonar surveys of the seabed both before and after dredging.

The predictive precision of the models was good. At 500 m downstream, on the edge of the

mixing zone where the permit called for no more than 25 NTU above background, the precision was

+1- 7.3 NTU with a standard deviation of 9.9 NTU. At other distal locations a similar precision was

achieved, but it decreased at proximal, near-dredge, locations. The WCSRP model predicted

reasonably well the background turbidity during storms when, with dredging halted, the turbidity

would rise to between 40 and 60 NTU.

Contributing Factors

The results indicated, as expected, that at a given distance (such as the edge of the mixing

zone) turbidity increased as silt content of the discharge increased and as the rate of discharge

increased. Turbidity increased from a minimum at the water surface to a maximum at the sea floor,

most probably due to settling of the turbid discharge. Turbidity decreased logarithmically with

increasing distance from the dredge. As expected the plume became diluted during movement away

from the point of discharge. However, the discharge plume was not homogeneous in character and

clouds or eddies of turbid water persisted amid the surrounding, much clearer water which consisted

of either unaffected seawater or rapidly diluted portions of the plume.

The most significant increase in turbidity was associated with the change from average to

maximum silt content of the dredged material and by the effect of increasing ambient currents from

average to maximum values. The turbidity was least sensitive to changes in the dredge throughput

rate and thus to the rate of discharge of the jig tailings. Within the normal range of dredge

production rates the effects of changes in the production rate on turbidity were small when compared

with the impact of changing environmental conditions.

Contrary to first expectations, the water turbidity increased with the dredge operating depth,

despite the greater column of water available for dispersion. The phenomenon was apparently a

result of the increasing silt content of the ground dredged in deeper waters. The trebling of the

percentage silt content between 7 m to 20 m water depth was detectable only from the drill logs and

not from any other seabed sampling techniques. The turbidity also increased in very shallow water,

although the ground contained less silt, because of the smaller water colunm available for dispersion.

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5.1.6 Water Quality Mitigation

5.1.6.1 Discharge Control of the Jig Tailings Effluent

A large variety of mitigation techniques were considered and many were tried over the 1986

to 1990 operation period of the BIMA dredge. The most important was the control of the tailings

effluent discharge so as to minimize the degree and extent of sea water turbidity. The relative

contribution of each of the two processes of convective descent and dynamic collapse as a source

of sea water turbidity is important in determining which is the preferable discharge alternative. The

long-term diffusion phase is not a source of turbidity but a continuing result of the previous two

phases. In practice, if the major contributor to turbidity is mixing at the tailings jet periphery during

convective descent, then turbidity is reduced by minimizing the opportunity for mixing.

Alternatively, if the major contributor to turbidity is dynamic collapse, then turbidity is reduced by

minimizing tailings jet velocity at the seabed.

Experimentation on the optimum jig tailings discharge configuration continued from 1986

to 1990 (Rusanowski, 1990). Several designs were tested (Table 5.2) in varying depths of water

from about 5 to 21 m for each of the two identical tailings launders (Figures 5.2 to 5.6). The various

discharge systems that were tried suffered from several disadvantages (Rusanowski, 1990). The sea

state required that their construction be rugged and rigid to prevent immediate damage or

destruction. Their repair or replacement at sea, even in relatively calm seas, was not only very

difficult and dangerous but also involved the cessation of dredging, usually when the potential for

a high rate of production was at its greatest.

Direct surface discharge produced the largest turbidity plume visible at surface. Flexible

pipes, especially the initial panels, were severely loosened, damaged, or destroyed by both the

abrasive action of the tailings slurry and the waves in the first of the frequent summer storms. Their

life span was sometimes no more than a few days. The large surface area of vertical flexible pipe

exposed to horizontal currents also tended to result in the pipe becoming seriously bent.

Larger diameter pipe caused the tailings effluent to form a large vortex within the pipe

entrance. Air became trapped in the tailings slurry and was carried down the pipe into the sea where

it caused a portion of the plume to rise causing, at the water surface, a massive "boiling" action and

a consequent increase in the extent of the turbidity (Figure 5.7). Flexible pipe also tended to float

due to buoyancy of the entrained air.

These problems were overcome by using a smaller (0.50 rn-diameter) rigid steel pipe, the

constriction of which inhibited the formation of a vortex. The deflector plate (Figure 5.3) proved

to be a slight advantage, causing diversion of the slurry jet, but abrasion by the fast-moving slurry

resulted in its destruction before the end of the summer season (Rusanowski, 1990).

Experience over a number of years showed that the different systems each possessed

advantages in particular conditions of water depth and current speed. However, none was the

optimum design under all conditions to be expected during a single dredging season. But once the

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dredging season was started the system installed prior to start-up could not be changed. Emphasis,

therefore, was directed towards the installation of a discharge system that would survive the marine

state, did not require maintenance, and provided some reasonable mitigation under all the operating

conditions expected and planned for the dredging season. Ease and rapidity of repair were the

decisive factors in the adopted design.

The optimal design developed from four years of dredge-operating experience comprised

fitting the existing 1.8 rn-diameter header pipes with cones that terminated in a flanged collar of 0.50

m diameter. A straight circular steel pipe was attached to the flange and extended vertically down

to within about 0.5 m of the water surface. A flexible hose of the same diameter was affixed to the

steel pipe and extended 2.7 rn below the water surface (Rusanowski, 1990; Ellis and Garnett, 1996).

Several other configurations were considered but were not employed. They were discarded

in the design stage after operating experience quickly demonstrated their impracticality under the

prevailing marine and dredging conditions. They included multiport systems (Figures 5.8 and 5.9),

a form of controlled surface discharge (Figure 5.10) and flexible stinger pipe designed to lie on the

seabed.

The multiport system, a cluster of small pipes, proved to be no different from a single pipe.

To be effective, its construction necessitated such large spacing between the individual smaller pipes

that the tailings were dispersed over too wide an area. A flexible stinger pipe could have interfered

with the operations and safety of the dredge. The controlled surface discharge was considered to

possess a too short potential operating life in the face of the obvious engineering difficulties in the

local marine conditions.

Exhaustive experimentation and the use of the two computer models allowed a series of

nomograms to be produced (Figure 5.11). The nomograms indicate, for future planned operations,

whether or not the permit stipulations regarding turbidity at the edge of the mixing zone will be

complied with under different conditions of silt content, ambient current and the dredge operating

depth and production (discharge) rate.

5.1.6.2 Operational Procedures

A feature of the Nome offshore deposit was the occurrence of glacial kettle holes and paleolagoons behind the storm water marks of previous beaches. Both features are characterized by

concentrations of mud and clay which in places border or cover payable gold-bearing ground. Such

areas, easily identified by prior seismic surveys, were avoided and excluded from the dredge course

plans so as to avoid undue generation of fines and excessive water turbidity.

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5.1.7. Seabed Profile and Mitigation

At the outset of WestGold's operation there was an important agency stipulation regarding

dredging in Alaskan state waters. The stipulation was that tailings from the dredge of any form

should not cause change in seabed topography such that the water depth after dredging should be less

than 1 8 m deep, and thus tailings should not approach to within less than 1.8 m of the sea surface.

In deeper water, adherence to this regulation was not difficult but in shallow water problems arose

due to the swell factor of discharged tailings and the method of dredge operation.

5.1.7.1 Discharge Control of the Trommel Oversize

The Nome precious metal deposit was created by a high energy environment, resulting in an

average gravel content of 20 to 50%. The treatment plant of the BIMA therefore produced a

trommel oversize effluent that represented a high proportion of the excavated sediments. The fine

sediment matrix of the gravel was discharged separately as the jig tailings. After return to the seabed

the gravel and fines were not recombined and consolidated. As a result an original 1 m3 volume of

in situ sediment occupied a greater volume after excavation and treatment. The new volume of 1.1

to 1.3 m3 was expressed as a swell factor of 10 to 30%.

The trommel oversize discharge rate varied between 88 and 236 m3/hour (Table 3.10). The

lower effluent rate was the result of efficient disaggregation of clay in the trommel so that the

discharged effluent consisted of gravel only, with cobbles and boulders up to 1 m in diameter. At

higher discharge rates the gravels were mixed with large lumps of clay in which case a swell factor

was always evident.

Discharged trommel oversize was not easily redistributed by normal wave action. Isolated

mounds of oversize, therefore, remained on the seabed that were covered by a blanket ofjig tailings

as the dredge passed. Major storms or ice gouging were the agents that smoothed this artificiallycreated topography. In practice a partial solution to this swell factor problem may be achieved by

mechanically recombining, where feasible, the fine and coarse effluents during discharge. By siting

the discharge points as close together as possible maximum re-mixing may be achieved with a

subsequent reduction in swell factor (though never to 0%).

5.1.7.2 Operational Procedures

Figure 4.4 illustrates how the effluents from a bucket ladder dredge are deposited on the

seabed relative to the excavation. At the start of the dredge course the jig tailings and trommel

oversize are discharged more than 100 m to the rear of the dredge. The exact distance behind the

start-up face is given by the horizontal separation between the bottom tumbler and the various

discharge points. After the dredge has advanced from the start-up face by a distance greater than the

defined separation the effluents are discharged into the excavation.

A problem may be created when the dredging conditions involve a combination of shallow

water and deep digging (Figure 4.4a). The solution is to gain digging depth slowly, but this may be

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achieved at an economic cost. Either unpayable ground must be dug ahead of the deposit block, or

some of the deeper reserves must be sacrificed by remaining undug and then covered by tailings.

The operational difficulties can be decreased in shallow water by selecting an appropriate

face width (Figure 3.10). The problems described above can be minimized by starting with a small

face width which is gradually increased as digging depth is gained. However, a narrow face width

may create another problem. With the bow away from the centre line an experienced dredge

operator will angle the dredge into the two corners of the face to achieve complete excavation at the

base of the cut. This practice results in the trommel oversize being discharged only in the centre of

the dredge course, accentuating the swell effect and causing the tailings to reduce water depth.

5.1.8 Costs of Mitigation

The total operating costs during a representative 24 month period between 1987 and 1990

are given in Table 5.3. Environmental monitoring alone added exactly $ 0.50 per m3 of sediment

excavated to the dredge's operating costs. Other expenditures involved in the construction of effluent

discharge devices, etc., were absorbed in the dredge maintenance costs and probably added another

$ 0.50 per m3, approximately, of dredged sediment.

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5.2 Scenario 2: Precious Metals Mining, Alaska - Underwater Miner

5.2.1 Background

In the summer of 1989 Western Gold Exploration and Mining Company (WestGold) Limited

Partnership conducted the trial operation of an underwater miner (UWM) for a period of four months off

Nome, Alaska. Like the operation of the BIMA, the test project was the subject of comprehensive permittir

and an intensive environmental monitoring program overseen by both state and federal regulators.

With respect to mitigation, trials with the UWM "Tramrod", focused oncontrolling the discharge of

effluents to the sea with the aim of minimizing surface water turbidity and the extent of sedimentation.

5.2.2 Impact Avoidance

A number of location-related restrictions applied to the operation that were designed mainly to

protect migrating salmon. Dredging could not take place:

within 30 m of the low water mark

within 1.6 km of the mouth of a river used by migrating salmon

in water depths of less than 5 m.

The Environmental Protection Agency concluded that the pimary determinants of the turbid plume

were sea current velocity and the percentage of fines contained in the dredge effluent. The Agency found

that discharges of solids from dredging operations at depths of 5 m or greater should comply with theAlaska

state water quality standards under the various expected conditions of currents and silt content, except for

high current velocity and high silt content. In addition, solids discharge in water depths of 7 m or greater

should comply with state standards under all conditions of current velocity and silt content. The operating

constraints therefore prohibited dredging in water depths of less than 5 m and required judgement and

mitigation procedures, such as decrease of the throughput rate, to be applied between 5 and 7 m.

5.2.3 Appropriate Technology

As noted previously, the highest grades of gold are concentrated within theuppermost I m of seabed,

and frequently within 0.30 m of the seabed. To avoid dilution in both the vertical and horizontal sense an

excavating unit is required which is capable of digging to an accuracy of about 0.10 m vertically and to withii

about 3 m horizontally. No floating system of excavation in the open sea offshore Alaska allows such

accuracy because of continuous ocean swell. The secret to economic success is the almost surgical extracticn

of ground versus the bulk mining approach of a dredge which seriously dilutes the grade by its lack of

selectivity.

The use, on such a resource, of any marine mining system other than an UWM during the summer,

ice-free months, results in excessive ground being excavated from the seabed. There is a consequent, and

unnecessarily greater, environmental impact because of the larger volumes of tailings being generated per

ounce of gold recovered. The use of a selective, small scale, operation allows greater flexibility in mitigatirg

the deleterious effects of the effluent discharges.

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The use of an UWM also serves to mitigate the important impact of changing seabed profile caused

by dredging. A frequent feature of mining by bucket ladder dredge is the subsequently elevated seabed

profile which can be caused by the swelling of sediments during processing and disposal of tailings. The

effluent discharge arrangement may be designed in such a way that the trommel overze and the undersized

jig tailings may be discharged close together because the mining operation is smaller. By this means the

coarse and fine material may be allowed to remix naturally on the seabed, thereby preventing both the

development of a high swell factor and reduction of the water depth.

The flowsheet arrangement on the barge deck is a modular one and it can be changed without any

significant structural modifications to the barge assembly. The physical arrangement of the flowsheet and

the points of discharge of the effluents therefore may be modified so as to optimize discharge mixing. The

lengthy downtime, requiring significant dock facilities, needed by a conventional dredge is thusavoided, with

considerable economic benefit.

5.2.4 Water Quality Mitigation

Several approaches may be taken to mitigating the effluent discharges. The incentive for minimizir

the mixing zone is also economic. Unless the tailings are discharged down-current from the excavation theie

is the possibility of over-sanding (the disposal of tailings on virgin ground). The most simple disposal

arrangement is a single discharge pipe extending to approximately 1.5 m below the water surface, equal to

the draft of the barge.

5.2.4.1 Discharge Preparation by Cycloning

Just as the jig feed is de-watered by means of a hydro-cyclone, thei are advantages in similarly dewatering the jig tailings before discharge into the sea. The two cyclone products, the sand underfiow andthe

water/fines overflow may then be discharged separately. The sands may be dcharged from a single outfall

where they may enter the water and settle under the force of gravity. The overflow waste water and fines

may be discharged separately. The intent of this method is to minimize the kinetic energy (and thus

turbulence) of the sands at the point of discharge, thereby reducing particle dispersion. Cycloning does not

concentrate solids finer than 75 microns, so all clays remain with the waste waters. The advantageof this

grain-size sorting is that the sand sinks quickly without being caught up in the higher volume and more

turbulent slurry containing the fines. The sand is therefore distributed over a smaller area, lessening the

chance of over-sanding and increasing the extent to which it naturally re-mixes with the trommel oversize.
5.2.4.2 Curtain Containment

A technique which justifies further evaluation is the deployment of a small diameter, circularcurtain

containment structure surrounding the fines slurry discharge pipe, the curtain effectively containing the

mixing zone. Such a system may be either floating and tethered to the barge or may be suspended from an

outrigger and cable system. However it is deployed it should be on the lee side of the vessel if possible and

should be capable of being recovered fairly quickly in the event of a storm.

The curtain may provide a pool of about 5 m maximum diameter within which the most severe

turbulence may be contained. It may have a full depth of at least 3 m below the sea surface which in the

shallowest water will place it within 2 m of the seabed. It is a device which is applicable only to asmall scale

operation such as the one described here for an UWM. Any mining system with a greater throughput and

effluent discharge requires a larger silt curtain to be deployed. The size quickly becomesunmanageable and

the financial investment increase in equipment whith will be destroyed and lost altogether in the first rough

seas.

5.2.4.3 Diffusion Launder

Another novel outfall design, consisting of two large troughs fixed alongboth sides of the treatment

plant barge, was used by WestGold to collect and discharge the jig tailings effluent. The trough was

perforated along its base to provide discharge ports so that the slurry was released as a series of small point

discharges. The slurry may be discharged in the sea either from above or may be fed down a series of

flexible hoses, with quick-coupling disconnect fittings, capable of discharging at various depths above the

seabed. Diffusion of the effluent around the barge perimeter, coupled with the low gradient of the flexible

subsea hoses, minimized the turbulent kinetic energy of the discharge. This approach was made especially

practical by the comparatively low throughput rate of the UWM.

Constructed of steel and approximately 30 m long, each trough was 0.3 m wide and 0.4 - 0.6m deep

and was affixed to the port and starboard sides of the barge. The two were inclined at an appropriate angle

to ensure free and easy flow of the slurry away from the point of entry into the trough. A total of 10 to 15

discharge ports were located along each trough. Depending on required length and location, each port was

between 7.6 and 15.0 cm in diameter and was originally designed to be controlled by a manually-operated

on/off valve.

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-

128

5.3. Scenario 3: Marine Aggregate Mining, Massachusetts Bay

5.3.1 Background

Dredging of marine aggregates by means of trailing suction hopper dredges is a well established

industry in the North Sea of western Europe. The experience gained there has led to a good understanding

of the optimum method of dredge operation and of the effects of their use

In terms of the development of hull design and sea-handling characteristics, a trailing suction hoppudredge designed for aggregate recovery at sea is generally indistinguishable from any meicantile fleet vessel

(Nunny and Chillingworth, 1986). Most modern European hopper dredges have one or two drag-arms. If

there are two drag-arms then one is mounted on each side of the dredge. Each drag-arm is fitted with an inpipe suction pump which gives the dredge a depth pumping capability of 45 to 50 m (Nunny and

Chillingworth, 1986). During trailing-dredging the ship lowers the drag-head(s) to the seabed. The ship then

steams in a straight line, raising and lowering the drag-head as appropriate with reference to an echo-sounduplot on the bridge. When fully lowered, the drag-head is pulled across the seabed by the drag-arm. Steamirg

is usually either with, or directly against, the run of the tide (Littler, 1990).

The seabed sediments are hydraulically lifted by the drag-head which may be custom designed for

the area. Sediments are transported up the drag-arm and, after screening out of the undesired oversize

fractions, are stored in hopper bins in the dredge's hull. Low density turbid water at the surface of the filled

hoppers contains silt and fine sand which reduces the value of the cargo.

The average aggregate storage capacity of dredges employed in western Europe has increased to

around 4,000 to 4,500 tonnes (approaching 10,000 m3). Each vessel is purpose-built and incorporates all the

latest navigational and dredging technology (Webb, 1988). Each is around 100 m in length with a draft, whi

loaded, of 6 to 6.5 m, with service speeds of 12.5 to 13.0 knots. The vessels meet all the current regulations

of the marine classification societies for construction and safety at sea. A cargo of about 4,S00tonnes of sand

and gravel can be dredged in approximately 2 hours (Littler, 1990).

A typical smaller dredge vessel has an overall length of 65 to 70 m, a bieadth 13.0 m, a loaded draft

of 4.0 m. With a cargo capacity 1,300 tonnes (approximately 750 m3) and a speed of about 10 knots it can

operate at distances of more than 25 km from the coast. Typically, the operations are automated with the

engine room unmanned in the open sea. Such vessels are classified by Bureau Veritas as "Dredger Coastal

Waters".

During the filling operation turbid water carrying fines continually overflows. The higher density

gravel and coarser sand remain in the hopper and constitute the desired product. The water containing silt

and fine sand is usually discharged through ports located on both sides of the dredge. Each dredge master

is concerned with maximizing the load of marketable product per trip. Therefore, pumping of the seabed

sediments and the discharge of the didged material as a slurry into the hoppers is often continued after the

hoppers have been filled.

During the dredging operation re-suspension of fine-grained sediments is caused in four ways:

(I) the drag-heads stir up clouds of fine material as they are pulled through the sediment
courses are controlled by hydrographic constraints rather than by the geology of a deposit (Nunny and

Chillingworth, 1986). Any effects such as significant changes to the seabed lithology and profile are

therefore accelerated. Dredging carried out in stages in diflèrent areas avoids concentrating undue amounts

of change to the seabed within the total region to be dredged. An "unacceptable degree of adverse effect on

fishing interests' is also avoided (Webb, 1988).

5.3.3.1 Seabed Lithology and Mitigation

Repeated excavation of aggregates within a dredge course eventually may reveal different lithologies

As the workable deposit thins through repeated removal of sediments, high-spots in the underlying sequenc

are eventually exposed. These may be hard bedrock or glacial clay. To the dredge operator such exposure

raises the possibility of unwanted cargo contamination. When this contamination exceeds a certain critical

level dictated by the marketing restraints the aggregate reserves within the dredge course are considered to

be worked out and a new dredge course or dredging location is sought (Nunny and Chillingworth, 1986).

Over-dredging has other effects. Local fishing may be affected if boulders become exposed since

the potential for damaging trawling gear is increased (Continental Shelf Associates, Inc. 1993). A layer of

the original substrate should be left on the seabed to inclease the ability of the original benthic communities

to recolonize (Nunny and Chillingworth, op. cit.). The problem of over-dredging to too great a depth,

thereby revealing a different lithology, is especially common when dredging glacial till and other sediment

similar to that proposed as the aggregate source in Massachusetts Bay. The content ofsuch glacial sediment

concentrations is unpredictable in terms of their three dimensional distribution if sufficient exploration has

not been carried out.

The mitigative solution is two-fold. Either avoid suchconcentrations of glacial sediments, or apply

restrictions to the extent that aggregate may be removed only from the top of the vertical sedment sequence.

Alternatively, the dredging company could be required to undertake side-scan sonar surveys at intervals

before and during dredging to reveal the extent. if any, to which the seabed lithology is changing as a result

of mining activity.

A significant change in the characteristics of the seabed sediments also may result from the

preferential non-removal of oversize material which is too large to pass through the drag-head trash bars.

After dredging is complete the oversize is left in situ as a residual deposit on the seabed. This effect is

particularly common when a deposit is dredged, especially a glacial one, that contains substantial quantities

of cobbles and boulders (Nunny and Chillingworth, 1986). The only mitigative process available may be a

decision not to licence such areas for aggregate dredging.

In western Europe it is now common practice to screen the pumped aggregate at sea before it is

discharged into the dredge hopper. A percentage of the sand content of the pumped aggregate is removed

so as to provide a saleable cargo with the optimum ratio of sand to gravel. The screened out sand is

immediately returned to the sea within the dredging licence area. The long term effect is a gradual increase

in the percentage of sand within the upper sediment layers of the sea floor (Nunny and Chillingworh, 1986).

To the disadvantage of the operator, reserves of gravel may become lost to dredging beneath a cap of sand

that has been reworked many times. As discussed in a subsequent section, one recourse is to prohibit onboard screening of aggregate at sea.

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On average, depending on its particle size, sand settles at rates of between 3 and 20 mm/second.

Assuming an average rate of 5 mm/second, then in 25 m of water the sand is transported 40 m from the

dredge for each relative 1.75 km/h of tide (Nunny and Chillingworth, 1986). To some extent this drifting

effect of the sand can reduce over-sanding. To harness this effect different areas can be dredged on the ebb

and flood tides, working inwards from the parallel boundaries of the deposit, and steaming at a slight angle

to the tide. The rejected sand is carried to one side of the vessel and back-fills the dredged area. If the tide

is strong enough to disperse rejected sand on the seabed then it may be necessary to dredge from only one

side of the vessel during only half of the tide.

5.3.3.2 Seabed Profile and Mitigation

Ideally, the drag-head furrows left by a trailing suction hopper dredger in a single pass are only 20

to 30 cm deep, so there should be minimal danger of creating significant change to the seabed profile.

However, there is always the temptation for the dredge master to excavate repeatedly in a favourite location.

The intensive working of such a dredge course can produce a large scale shallow trench in the seabed (Nunny

and Chillingworth, 1986).

Even if some care is taken by the avoidance of an excessively repetitive dredge course and directior

trailer suction hopper dredging does not always yield a sufficiently flat seabed profile. After final excavatim

the seabed surface frequently contains too many peaks and valleys because the drag-heads have a tendency

to track the previous excavation. Contrary to theory, the dredging results in several deep tracks separated

by unworked seabed representing remnant reserves of aggregate (Maintenance Dredging, 1987). Such

dredge furrows may persist for years (Nunny and Chillingworth, 1986). The objective of dredging should

be to plane, rather than to trench, the seabed. Operators should attempt to dredge courses adapted to the

seabed geology, even if the courses are oriented across tidal flow. Ideally, the courses should be adapted to

the contours of equal sediment thickness (isopachytes) (Nunny and Chillingworth, 1986). Courses

intersecting at shallow angles can be superimposed over a period of time, while still being generally aligned

to the direction of strongest tidal flow (Nunny and Chillingworth, op. cit.). The use of GPS (Global

Positioning System) navigation removes any possible excuse that such necessary precision is unobtainable

Another pre-requisite is accurate data from initial prospecting of the deposit. Both the operator and

the environment benefit from the advantages of achieving such accuracy. For the operator there is less

chance that areas of unworked aggregate reserves are left on the seabed, and the possibility of contamination

by other lithologies is also reduced. Environmentally, a relatively smoother sea floor of uniform substrate

type remains.

To counter the formation of trenches, pits, mounds and the exposure of loulders on the seabed, 'the

French have experimented with abutting dredging tracks to obtain an unobstructed seabed. In other cases,

rejected oversize materials can be guided back into the collector tracks immediately behind the mining

device" (Continental Shelf Associates, Inc. 1993).

5.3.4 Water Quality Mitigation

5.3.4.1 On-board Processing

The on-board screening of slurry and the rejection of unwanted particle sizes results in an effluent

which is discharged continuously and usually from above the water surface. On-board processing presents

131
considerable practical problems. The cost and operational difficulties experienced have caused the Europeai

marine aggregates industry to limit its processing equipment at sea to single deck screens (Nunny and

Chillingworth, 1986).

Efforts continue to be made by the industry to improve the operational performance of on-board

screens. The objective is to clean up hopper feed so as to achieve particle size characteristics that

approximate the market-dictated requirements (Nunny and Chillingworth, 1986). Someoperators claim that

there are no major advantages offered by processing at sea and that reening and sorting is best undertaken

in a shore-based plant that has improved quality control. Although on-bcard screening continues, the extent

of turbid water created by effluent discharge can be minimized by the design and installation of appropriate

discharge arrangements as described in connection with Scenario 1.

5.3.4.2 Hopper Overflow Operation

A major cost in aggregate dredging is that of transport at sea between the dredging zone and the

unloading area. To offset these costs by enhancing productivity the maximum loading of marketable solids

must be attained in the hopper (Ports and Dredging, 1991 b; 1991 c). Ideally, the vessel must return to its

shore-based unloading station with a hold full of saleable product. Forcoarse sand and gravel deposits with

high settling velocities a maximum load can be obtained by overflowing relatively clear water from the

hopper. If the sediments being dredged comprise clean sand the overflow contains a low percentage of

solids. Economic loading is generally achieved by pumping slurry into the hold until all the feed is

overflowing (Raymond, 1984).

The rate at which the level of slurry rises in the hopper during loading ranges from about 5 mm to

well over 10 mm/second (Scott, 1992). This method of operation ensures that where dirtier sediments are

being dredged most of the suspended clay and silt pumped into the vessel's hold returns to the sea with the

overflowing hold water. The silt (<75 microns) content in the hopper is thereby maintained at the low level

required for concreting aggregates (Littler, 1990). Once the hopper loading exceeds a certah percentage the

overflow increases dramatically, together with the loss of marketable coarse sand. At 65% of capacity the

loss of coarse sand as overflow exceeds 10%. When the loading reaches 80% of capacity, the loss of coarse

sand reaches 40%. If fine sand is being dredged the hopper cannot be filled completely because the overflow

losses become 100% when the hopper loading reaches about 85% (see graph on p. 256 of Herbich, 1975).

The optimization of loading can be assisted by instrumentation whith is now available (Ports and Dredging,

1991 b; 1991 c). The turbidity caused by overflowing high concentrations of suspended fine sediments is the

most important cause of the turbidity plume associated with trailing suction hopper dredges (Scott, 1992).

5.3.4.3 Overflow Mitigation: Operational Technique

A practical mitigative technique is to delay the commencementof hopper overflow. This is achieved

by emptying all water out of the hopper before starting to pump any sediments on board. Water is dischargI

directly overboard as the dredge pump is primed. A switching mechanism is then used to direct the slurry

to the hopper once sediments are entrained in the pumping system. As a result overflow does notoccur until

the hopper is filled to within 60 to 70% of its dredged material capacity. The time during which sedimentladen waters are released to the sea is thereby substantially reduced.

Aggregate quality can be judged visually. However, the installation of a drag-head sensor

automatically coupled to a voiding valve immediately down-flow from the pump would result in major

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133

advantages (Nunny and Chullingworth, 1986). Poor quality dredge feed could be recognized and diverted

more rapidly. Both contamination of the loaded aggregate and high silt overflowwould thereby be reduced.

An additional benefit would be that a continuous record of pumped aggregate quality and impurity

concentration would be provided. In conjunction with position fixing and data logging systems the

information would substantially supplement the prospecting data.

5.3.4.4 Overflow Mitigation: Re-cycling

The drag-heads of some trailing suction hopper dredges are equipped with water jets that are

designed to assist in the liberation of compacted sediments when dredging hardground. A small percentage

of the hopper overflow water can be pumped to the drag-head jets instead of using clean seawater, reducing

slightly the total volume of sediment-laden overflow (Shepsis and Hartman, 1992).

In Japan a system has been developed whereby all the turbid overflow water from the hopper is recycled to the drag-head intakes. This system is said to improve efficiency by increasing the density of the

sand slurry (Ofuji and Ishimatsu, 1976). The concentration of suspended particles is reduced and the

transparency of hopper overflow water is increased after the installation of such an anti-turbiditysystem. The

results have reportedly been reviewed favourably by the local fishing industry representatives (Nunny and

Chillingworth, 1986). An inherent problem with this technique is that the overflow water continually

recirculates in a closed loop. The slurry becomes increasingly laden with silts and clays until it attainsa

density beyond the capabilities of the dredge pump. The benefits are therefore short-term since the only

solution is to purge the system at intervals. Short duration dcharges of high density effluent are produced,

as opposed to a more continuous dilute overflow, with a minimal overall net benefit.

5.3.4.5 Overflow Mitigation: Hopper Design

The level of the aggregate surface in a loaded hopper always must be above the load line in order to

prevent surplus water from being transported to shore in addition to the solids. This can be achieved by

modifying the shape of the hopper to keep much of the contents above the load line. Alternatively, special

compartments may be employed on either side of the hopper (Figure 4.5) (Maintenance Dredging, 1987).

Recent developments include deeploading and semi-deeploading systems (Maintenance Dredging,

1987). The purpose of these systems is to reduce the amount of energy required to load the hopper and to

limit the volume of air in the supernatant water passing to the overflow. A reduction in the turbidity of the

overflow water, and thus that surrounding the vessel, also can be achieved by means of an anti-turbidity

valve which can be installed in the IHC telescopic, cylindrical overflow weir (Figure 5.12) (Maintenance

Dredging, 1987).

Several methods for increasing solids conceniration in the hopper are in the experimental stage and

represent examples of how environmental mitigation simultaneously improves the economics of the dredgirg

operation (Scott, 1992). They include the use of:

slurry feed diffusers to reduce turbulence in the hopper and to enhance the settling of suspended

sediments

centrifugal separation to de-water the solids
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(3) inclined plates in the hopper to increase the settling rate of the slurry suspension.

5.3.4.6 Overflow Mitigation: Overflow Collection

An anti-turbidity measure proposed for hopper dredging is to install compartments on both sides of

the dredge (Figure 4.5). These are designed to intercept and temporarily retain the overflow, allowing partii

settling of suspended silts within them. However, the following disadvantages of this approach are all

potentially serious, practical, limitations:

the retention time needed for substantial settling to occur

the need, at intervals, to dispose of the accumulated silts

the significantly reduced hopper capacity caused from the loss of vessel space by incorporating

settling compartments.

5.3.4.7 Overflow Mitigation: Effluent Discharge

A relatively simple technique for handling hopper overflow, called an anti-turbidity overflow systTI

(ATOS), has been developed in Japan (Nunny and Chillingworth, 1986) (Figure 5.12). The overflow

collection system is streamlined to minimize tk entrapment of air bubbles in the overflow water. Removal

of the air bubbles, which otherwise make the particles buoyant and prolongs settling, allows thefines to settle

at a faster rate. This system has been successfully applied to trailing aiction hopper dredges off the coast of

Japan (Ofuji and Ishimatsu, 1976). In addition, the overflow discharge ports are moved from the sides of he

vessel to the bottom of the dredg&s hull (Raymond, 1984) (Figure 5.13). Discharged particles descend

rapidly in the water column with a minimum amount of dispersion. The improvement has been incffporated

in existing dredges through modifications of their overflow systems. Japanese dredges with hopper capacities

of between 2,000 and 4,000 m3 have been converted with the result that the turbidity in the uppermost 1 m

of the surface waters has been reduced from around 110 to 120 gIl to 6-9 g/l at 30 m aft of a dredge. The

system does not, however, reduce overall resuspension, only surface turbidity (Raymond op. cit.).

5.3.5 Onshore Processing

Discharge at a shore plant can be by crane, self-discharge by scraper and conveyor, or self-discharge

by pumping. The sediments are washed to remove salt, the content of which may exceed construction

standards, and screened into marketable product(s). During this process waste wash water containing salt

is produced. The silt can be settled out (Quarry Management, 1986). Occasionally, flocculant is added to

the slurry stream in order to settle the solids quickly and more evenly (Littler, 1990). The rate of settling

generally increases with the amount of flocculant used, but only up toa certain dosage (Quarry Management,

1986).

Onshore desalting is popular with operators but difficulties often arise in the supply and the

environmentally acceptable disposal of the flushing water. Re-circulation is only a partial answer.

Sometimes the problem may be overcome by blending the salt-contaminated marine aggregate with a salt-fr

one from a land source to obtain a product that conforms to the required quality standard. Alternatively, an

on-board desalting process can be used while the vessel is still at sea, with a beneficial impact on the
economics. Vessels may be fitted with an appropriate freshwater tank and the contaminated wash water is

discharged at sea (Narumi, 1989).

5.3.6 Economics

5.3.6.1 Operating Costs

The operating costs of a trailing suction hopper dredge include royalties, fuel, wages, maintenance,

port dues, pilotage, unloading and insurance (Nunny and Chillingworth, 1986). Fuel is the most important

item contributing to costs. Fuel consumption, and therefore cost, increaseswith the distance to the dredging

area and thus with the steaming time. Crew costs are next in orderof importance. The ratio of cargo capacity

to number of crew is related to the vessel size which may vary within the limitations imposed by features sudi

as harbour entrance dimensions (Nunny and Chillingworth, op. cit.) Maintenance costs are affected by the

type of aggregate being dredged. Sand and gravel is more costly to produce than is sand because of the

higher abrasive effect of the contained gravel (Nunny and Chillingworth, op. cit.).

Dredging costs are heavily dependent on the cycle time from loading, steaming to port, unloading,

and steaming back to the loading site. The extent to which the cycle time meshes with the tides on which

a vessel can land its cargo affects the overall economics (Littler, 1990). The most efficient operations are

those for which the cycle time equates to one tidal cycle. Larger vessels dredging more distant grounds

operate on a 2-tide cycle, and 3-tide cycle dredging is not unknown in Europe (Nunny and Chillingworth,

op. cit.).

In western Europe the target for most dredges is to be fully operational for 24 hours/day for about

350 days/year. However, downtime averages about 20% due to annual refits, operational delays, breakdowr

and poor weather. Weather conditions rarely prevent steaming but may seriously reduce the rate at which

loading is accomplished. It may also affect the ability of a dredge toenter its harbour for unloading (Nunny

and Chillingworth, op. cit.).

5.3.6.2 Capital Costs

In western Europe a modern trailing suction hopper dredge capable of supplying 1.0 to 1.25 million

tonnes of aggregates annually cost around U.S. $12-is million in the late 1980's and has since increased.

That capital cost is inflated by an additional 50% for the provision of a system capable of handling material

being discharged directly from the vessel at a rate of about 2,000 tonnes/hr, including receiving, washing,

grading and stocking facilities (Webb, 1988). A few years later a new 5,000 tonne-capacity vessel cost about

U.S. $16 million (Hollinsworth, 1994).

Although the capital cost of a new dredge is normally amortised over iS to 20 years, its productive

life is usually a minimum of 20 years. If a mjor refurbishment is implemented after 20 years its active use

can be extended for a further iO years (Nunny and Chillingworth, op. cit.). The best return on the capital

investment is achieved by maximizing the life and the availability of the vessel (Nunny and Chillingworth,

op. cit.).

A major decision facing a company producing matine aggregates is when to invest in a new vessel.

Most operators understandably are not prepared to commit to the investment unless they have possession of

delineated reserves fully licenced for immediate mining (Nunny and Chillingworth, op. cit.). An existing,

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or reasonably anticipated, market share for the planned production is also essential. The delivered price of

the marine aggregate is the major factor influencing the extent to which the product can penetrate a local

market. Onshore haulage cost and user prejudice are both of critical importance (Nunny and Chillingworth,

op. cit.).

The after tax rate of return for a marine operation is considerably less than that normally available

from a conventional onshore operation of the same scale of output because of higher capitl costs (Industrial

Minerals, 1990). Onshore aggregate operations normally involve less onerous capital investment in

machinery for the extraction and transport of sand and gravel material to a processing plant (Nunny and

Chillingworth, op. cit.).

5.3.6.3 Mitigation Costs

The most visible effect of trailer suction hopper dredging, and consequently of public environrrental

concern, is the turbidity caused by on-board screening and by hopper overflow. Complete disallowance of

overflow clearly limits the efficiency of the dredging operation. An alternative is to classify the sediments

and allow the overflow of none or only some of the classifications (Maintenance Dredging, 1987). Ideally,

the overflow and screening restrictions should be clearly stated and imposed at the outset. Such a procedure

allows potential operators to consider the cost of converting any existing vessels or to allow for overflow

mitigation methods in the design of new vessels. Restrictions should not be imposed after the commencement

of operations when a more relaxed initial approach may be considered to have been inappropriate.

If imposed without any forewarning, the prohibition of overflow as an operating technique

significantly raises the cost of production of marine aggregates. Trailer suction hopper dreds are designed

with a certain hopper capacity intended to hold sand and gravel with a density of I .8to 2.0 tonnes/m3. If the

operator cannot discharge at sea any contained silt, which has a density of 1.1 to 1.2 tonnes/ni, then the load

capacity of the vessel cannot be fully utilized. It therefore operates uneconomically (Maintenance Dodging,

1987). Excessive enforcement of mitigation measures can prevent an infant industry from securing and

maintaining its market share. The cost to the operator may be an operating loss for a period, followed by

closure.

The environmental arguments in favour of marine mining of aggregates have be well stated in the

informed media. "Adverse environmental effects (of onshore aggregate mining) are visited upon the

surrounding community by conventional sand and gravel open pit operations." They include "... the

diseconomics of noise, fugitive dust and added traffic burden associated with extraction, processing and

distribution" (Industrial Minerals, 1990). Offshore aggregate mining "... does not involve valuable land

which is more cost effective for urban development than for the mining of construction aggregate. In

addition, people do not want (onshore) mining activity, with its attendant dust, noise, traffic and

environmental concerns, in their neighbourhoods" and "... increasingly tough zoning, environmental and

reclamation laws (are) restricting the industry's expansion by dry land methods" (Industrial Minerak, 1990).

There are four specific advantages of marine aggregate mining: "First, marine transportation costs

of this low value, high bulk commodity are much lower than onthore transportation, so that sand and gravel

can be mined farther from the market area. Second, marine aggregate can be delivered directly to urban

water fronts close to the centre of demand... Third, marine dredging, when properly conducted, does not

result in the environmental problem commonly associated with on-land operations Finally, marine deposits

are very large, so depletion of the source is unlikely" (Industrial Minerals, op. cit.).

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5.4 Scenario 4: Heavy Mineral Sands, Virginia

5.4.1 Background

The dredging of heavy mineral sands for their titanium content is an established onshore

industry in the south-eastern U.S.A. In other countries the onshore reserves have been mined out

and the on-going dredging activity has moved to the coastline (e.g. Australia, New Zealand and

South Africa). Environmental conflicts have developed as a result of proposed and operational

mining of coastal, mineral-bearing dunes, beaches, and offshore sand bars. Several regulatory

authorities have enforced location-based and time-based constraints on the dredging operations.

Concerns over environmental effects during permitted dredging have taken several forms related to

the creation of water turbidity and to changes in the seabed lithology and profile.

5.4.2 Impact Avoidance

Mitigation through voluntary downtime can take two forms; one being anticipated downtime

during an environmentally sensitive period, such as an annual or seasonal spawning event, and the

other involving unscheduled downtime in the event that members of a particular protected species

are detected in the vicinity of the mining operation, as with walruses off Nome, Alaska (Scenario

1).

5.4.3 Water Quality Mitigation: Cutter-Generated Turbidity

During the operation of a cutter suction dredge seabed sediments become suspended in the

water close to the cutter-head as the dredge swings back and forth across the dredge face. A properly

designed cutter-head excavates the sediments and guides them efficiently towards the hydraulic

suction intake. However, the cutting action and the water turbulence created by the rotation of the

cutter cause some of the sediments to be become suspended. The degree of suspension is directly

related to the type and quantity of sediments disturbed by the cutter-head but which remain on the

seafloor. The suction's inability to gather up and entrain these loosened sediments determines the

quantity that remains on the seabed or that is lifted into suspension in the surrounding water

(Raymond, 1984).

The amount of sediments suspended by the suction dredge's cutter-head is reported to

increase exponentially (Barnard, 1978) as the following increase

thickness of cut

rate of cutter swing (usually about 0.1 rn/second)

rate of cutter rotation

rate of production.

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Within about 3 m of the cutter-head the concentrations of suspended solids vary

considerably, but may be as high as a 1,000 mg/l. Immediately above the seabed concentrations may

remain elevated at a few hundred milligrams per litre for distances of up to 300 m from the cutterhead. Above the cutter-head concentrations decrease exponentially with decreasing water depth.

The amount of sediment lifted into suspension increases with increased current speeds (Raymond,

1984).

5.4.3.1 Operational Mitigation

Several operational controls are recommended for the reduction of sediment resuspension

(Raymond, op. cit.). Most involve the manner of excavation. Very thick single-pass cuts should

be avoided, because the cutter-head tends to become buried. As a result more sediment is excavated

and dislodged than the suction can successfully entrain, leading to high levels of suspension. A thick

deposit is best excavated by means of several horizontal slices, each layer of excavation being a

single thin cut depth. Excessive layer cutting, however, also may be responsible for unacceptable

levels of turbidity. Loose sediments remaining on the newly exposed seabed after each cut are easily

resuspended by the returning cutter-head.

In other places, depending on the depth below the seabed to which payable grades of heavy

minerals extend, it may be more economical to excavate and remove most of the sediments very

roughly in a single cut. Final clean-up to the base of the pay zone may be delayed until the final

stage. However, the loosened sediments, while awaiting clean-up, may be at risk of suspension in

a variety of ways.

Where the mining plan involves dredging to a few metres below the seabed within a defined

block the operation should proceed without excavating vertical walls in each cut. The sides should

be inclined sufficiently so as to inhibit the slumping of loose sediments into the cut. The final walls

of a cut should be terraced by reducing the area of cut with each successively deeper slice. On a

smaller scale the formation of distinct cuts should be minimized. The dredge should be swung and

advanced so as to cover equally all the seabed within the block. The cutter-head should be advanced

by increments that are sufficiently small to ensure that very close concentric arcs are described on

the seafloor with each swing of the cutter-head.

5.4.3.2 Design Mitigation

The cutter-head's suction should be sufficiently powerful to collect all the sediments it

disturbs. To enhance the pick-up capability and to decrease the amount of suspended solids several

features should be incorporated in the dredge design. These include water jet booster systems or

ladder-mounted submerged pumps. Another factor that may influence the amount of suspended

sediment is the shape of the cutter-head (Raymond, 1984). The cutter-head may be designed in such

a way that the suction is brought closer to the sediments (see Figure 8 of Raymond, op. cit.), thereby

improving the chances of entrainment. Differences in shape are particularly important if the cutterhead is not completely buried in the sediments,

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139

The acute angle between the base of the cutter tooth and the upper surface of the sediments

is referred to as the rake angle (Figure 5.14). Proper design of the cutter-head with an appropriate

rake angle is an important mitigation factor. If the rake angle is too large, a gouging action throws

soft, fine-grained sediments outward. If the rake angle is too small, however, resuspension occurs

through impact.

5.4.4 Water Quality Mitigation: Plant Effluent

The most obvious water turbidity is created by the surface discharge of slurry effluents as

tailings from the treatment plant. Approximately 95% of the sediments dredged are returned to the

sea as tailings. Of this amount more than 85%, on average, is sand. Mitigation may be attempted

by:

minimizing the possibilities of re-digging of the settled tailings through operational

procedures

improving the nature of the effluent by de-sliming, dewatering or degassing

optimizing the manner in which effluent is discharged by employing sub-surface discharge

and/or discharge diffusion

modifying effluent behaviour in the sea by the use of flocculants.

5.4.4.1 Operational Mitigation

An important stage in the production of marine heavy minerals is the estimation of reserves

and the delineation of an approved dredge course. Not only must the planned dredging program

allow for optimum mineral recovery but it must also take into account the environmental effects of

possible re-dredging of previous tailings (Garnett, 1 996c).

Re-dredging of previously excavated areas, even on their boundaries, may result in two

negative effects. Previously deposited tailings may be more easily suspended than undisturbed

sediments. Secondly, previously dredged areas, at some stage of recovery from the previous

incursion of the dredge, will be disturbed a second time due to overflow, or over-sanding, by tailings

returned by the dredge and plant.

The mine plan should be designed so as to avoid, if possible, significant encroachment of the

dredge on the boundaries of a mining block which previously has been dredged and the surface of

which is now covered with tailings. The deliberate exclusion of areas of low-grade ground from a

first dredge course, on the assumption that they may be dredged as part of a subsequent program,

should be avoided (Ellis and Garnett, 1996). Mining in an area should be continued until it is

unlikely that operations will return close to, and disturb, a recovered site.
5.4.4.2 Dc-Sliming and Dewatcring

Separation of the effluent flow into its component parts of solids and water allows the

materials to be discharged into the sea with less impact. The solids are not held suspended in the

water by the boiling action which otherwise is evident. The water, although containing some fines,

is more rapidly diluted by the surrounding sea water. The objective in de-sliming and dewatering

is the same: to separate the water and slimes from the coarser solids.

De-sliming and dewatering can be achieved in a number of ways with different equipment.

Various proven systems are in standard use onshore in a variety of industries, including those

involved in the production of aggregates and heavy minerals. Such systems include hydrocyclones,

hindered settling classifiers, bucket-wheels, and screws and dewatering screens (Littler, 1990).

The hydrocyclone is the most commonly used tool for classification of coarse sand and silt

slurries. It is an extremely simple mechanical device with no moving parts. Cylindro-conical in

shape, the cyclone is mounted with the long axis aligned vertically, and may be up to nearly 1 m in

width for the treatment of the largest particle sizes. Slurry is fed continuously by either a centrifugal

pump or a gravity feed system. The feed head is converted to both angular and linear acceleration,

creating a cyclone effect whereby the particles in the slurry are separated by size and/or specific

gravity (Schilt et al. 1992). An acceleration by about 50 times the natural free-settling process is

achieved.

Dewatering by cycloning is conventionally employed at the beginning of an on-board

flowsheet for the recovery of heavy minerals. The cyclone removes excess water from the undersize

feed, after separation of the oversize, before the sands are fed to the spirals. A dewatering cyclone

should also be incorporated at the end of the process stream to remove excess water from the sand

constituent of the tailings. The solids comprise about 20% by volume of the total effluent, and if

discharged separately they are not subjected to a turbulent dispersion in the sea. The water, generally

containing particles finer than 200 microns, is discharged separately.

The cyclone is the most suitable dewatering system for installation on board a dredge or wet

mill pontoon because it does not require deck space and is easy to install. It has been used in the

onshore mineral dredging industry for over 30 years, especially for dewatering jig tailings before

their discharge astern of a bucket ladder dredge. Other variations of the same cyclone principle exist,

such as the Linatex Separator (Figure 5.15) and the Linafuge (Littler, 1990).

5.4.4.3 Dcgassing

It is not known whether methane or hydrogen sulphide are present in seabed sediments

offshore Virginia in the vicinity of the mining site, though it appears that the geology and benthic

environment may not be conducive to natural gas generation. If either of these gases does occur

degassing equipment should be employed downstream of the dredge pump to prevent de-priming.

Both the pump and degasser must be installed underwater because the water depth is greater than

10 m. To limit water colunm turbulence associated with released gases the installation of a conduit

from the degasser to the surface is necessary to allow gases to be vented to the atmosphere.

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5.4.4.4 Effluent Discharge Design

Surface water turbidity and the deposition of tailings on the seabed around the wet mill

pontoon can be controlled to a significant extent by using a sub-surface discharge. Such an

arrangement consists of a reinforced rubber pipe, probably with a nominal inside diameter of

approximately 1 m, attached to the pontoon by a beam and/or steel cables. There may be many

variations in the configuration of the discharge pipe. It may be deployed in any attitude ranging

from vertical to near horizontal. A near vertical orientation may be preferable but it may also be

desirable to redirect the outlet so that the effluent stream can be released at depth with minimal direct

impact of the bottom. The optimal configuration and depth below surface of the discharge will have

to be determined for each mine site through a combination of numerical modelling and field

experimentation. It is likely that all effluents could be discharged through dedicated, single, subsurface pipes. A reduced effluent discharge velocity could also be achieved, within practical limits,

by fitting the end of a discharge pipe with a flared opening. This modification may require some

experimentation to achieve the optimum arrangement.

5.4.4.5 Flocculants

It is possible to inject a continuous, metered dosage of chemical flocculants into the effluent

stream prior to its discharge from the wet mill and introduction to the sea. The use of flocculants

comprises the only means available of speeding the settlement of entrained silts. They have been

widely used for many years to great effect in the onshore mineral dredging industry. However, there

is little experience of their use in the open sea. Their effectiveness depends on many factors and site

specific experimentation will be necessary before flocculants may be used with confidence.

Flocculants have two effects. Firstly, because they are flocculated, the fine sand and silt are

deposited together, thus preventing the natural separation of fine sand and silt that occurs during

settling with increasing distance from the effluent discharge pipe. Secondly, the deposited silt is

increased in volume by the entrapment of water. However, volumetric increase should be minimal

if the correct amount of flocculant is used (Littler, 1990).

A flocculant is usually purchased in powder form and is diluted to a 0.1% solution before

introduction into the effluent. Dosage rates are between 0.5 to 5 ppm. The lowest dosage typically

creates flocs of around 1 mm in diameter which possess an increased settling rate to 10 to 15

cm/mm. At higher dosage rates larger flocs up to 3 to 4 mm across settle out rapidly at

approximately 560 cm/mm (Littler, 1990).

The feed equipment is typically a 500 gallon tank with a propeller mixer to invert or mix the

polymers with clean water for a period of 20 to 30 minutes. The resulting solution can be injected

as a continuous, metered dosage of chemical flocculants to the effluent stream prior to its

introduction to the sea. Under some circumstances undiluted polymer and effluent can be mixed

rapidly but with less quality control. For short-term operations a flocculant in soluble brick form

can be suspended in wire baskets within the effluent flow (Littler, 1990).

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142

5.4.5 Seabed Profile and Mitigation

During the marine mining of precious metals and heavy minerals, almost all of the excavated

sediments are returned to the sea after dredging. Contrary to a simplified view of the digging

process, the screen oversize is not returned as tailings to exactly their point of origin on the seabed.

They are discharged astern and fall quickly to the seabed at a distance equal to that of the discharge

point in plan from the excavation head.

The cutter suction unit employed for heavy mineral dredging rotates about the long headline.

Because of the deployment of sidelines the dredge describes an arc (in plan view) whereby the travel

distance of the cutter-head is greater than that of the vessel's stern and of the wet mill pontoon astern.

Material that is excavated, treated, and then discarded during the digging process is therefore

deposited, as the dredge moves forward, over a smaller area than that from which it was dug. If the

ground contains significant amounts of screen oversize and coarse sand these accumulate on the

seabed as a ridge that forms along the track of the dredge.

The accumulation of tailings on the seafloor astern is exaggerated when the dredge

commences its cut in a mining block. At the start of the dredge course excavated sediments are

dumped on to the original seabed instead of into a cavity created immediately beforehand by passage

of the dredge. The seabed elevation may be raised as a result by an amount at least equal to the

depth of cut.

Where gravels and very coarse sand occurs the volume of the deposited sediments may be

increased over the in situ volume by an amount indicated by the swell factor. A high swell factor

is created when the discharged fines are not re-mixed with the coarse screen oversize on the seabed.

The pore spaces between the larger particles remain unfilled by fines, unlike those in undisturbed

sediments.

5.4.5.1 Operational Procedures

The creation of elevated ridges of tailings on the seabed along the dredge course, especially

at its start, may be minimized during operations in several ways:

a sufficiently wide cut face must be dredged

the dredge must not be restrained by sidelines which are so short as to cause it to angle

excessively

the dredge must not be angled deliberately and unnecessarily into the corners of the cut face

the gaining of depth at the start of a deep cut must be undertaken at a slow rate.
5.4.5.2 Discharge Control of the Trommel Oversize

Every effort should be made at sea to re-mix the oversize and the undersize. This involves

discharging the screen oversize as closely as possible to the solids produced as the cyclone product,

without causing unnecessary turbulence.

5.4.6 Alternative Enhancement

Considerable potential exists for alternative enhancement initiatives when dredging for heavy

minerals. In particular, the undersize tailings can be pumped ashore, if needed, for beach

renourishment, the remediation of contaminated areas, and the creation of wetlands.

5.4.7 Economics

The annual costs of the heavy mineral mining scenario offshore Virginia is summarized in

Table 5.4. With the exception of voluntary downtime and the use of flocculants as an ongoing

operational mitigation measure, all of the techniques given in Table 5.4 are technology-based. They

therefore entail an initial capital outlay in the first year of the project. Subsequent costs would only

be for maintenance and supplies unless replacements or upgrades are required.

Maximum excavation efficiency and throughput rates are attained by digging deeper, rather

than shallower: cuts such as a 450 ladder depression versus a 20' ladder depression for the same

depth. Any restrictions in the operational procedures on the incremental depth of cut and the rate

at which the full depth of cut is attained at start-up seriously impact the efficiency and economics

of the operation.

The cost of unscheduled downtime is very high. An annual operating window is seasonally

defined and must be achieved in order to meet production objectives. The resulting loss of

commodity sales and, consequently, of profits as a result of unplanned shutdowns of long duration

could render an operation uneconomic.

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5.5 Scenario 5: Beach Nourishment, Maryland

5.5.1 Background

Of prime concern in all marine mining, even if conducted well out to sea, is the possibility

of coastal erosion. This scenario offers an opportunity to explore the mitigation techniques which

are available.

Seabed excavation, for whatever reason, invariably produces some changes in bathymetry.

Extreme examples are provided by the dredging of aggregates and the removal of sand for beach

renourishment, both of which involve the removal of almost all of the excavated sediments. The

criteria for judgement of likely coastal impact are several (Brampton, 1985). Critical questions

include whether or not:

the dredging area is within the zone of seasonal beach sediment change

the sediments at the dredge site are mobile

the dredging area includes sand banks that, if removed, would increase wave activity at the

shoreline

the area is sufficiently distant from the coastline.

All such dredging that results in bathymetric changes may cause a change in the local wave

climate. Erosion, especially of beaches, may occur with the sediments being transported and

accumulated in another locality. The original formation of a beach is primarily attributable to the

action of waves against a shoreline. The minimum grain-size of the beach particles is approximately

180 microns. Below this figure normal wave action effectively winnows out the finer particles and

transports them out to sea (Nunny and Chillingworth, 1986). The irony of this situation is that if

sufficient technical research is not undertaken beforehand then the very act of removal of sand from

offshore banks to re-nourish nearby beaches may lead eventually to erosion of the same beaches.

5.5.2 Prevention of Coastal Erosion

Coastal erosion may result if the wave- and sediment movement patterns are sufficiently

modified in the nearshore by an offshore excavation. To avoid the possibility of erosion both the

local hydrodynamic conditions and the sediment characteristics of the deposit being dredged must

be considered. Wave refraction analyses and wave transformation modelling are a necessary input

to a study (Byrnes et al. 1991). Prevention of erosion may be achieved by adopting a correct

licensing procedure and by restricting the operation of a dredge to specified water depths and/or

areas.

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5.5.2.1 Licensing Procedures

In western Europe the role of marine dredging in previous cases of coastal erosion around

the North Sea has been closely studied. In Britain, for example, experienced scientific advice is

available to the licensing authorities. The national Hydraulics Research Station provides a reasoned

view as to whether or not an individual application for a dredging licence is likely, if granted, to

place the nearest coastline at risk (Spreull and Uren, 1986). Each licence application is judged on

its merits, and the authorities take into account details of the proposed dredging site (Table 5.5). A

licence is issued only if all the criteria are met. However, even if the Hydraulics Research Station

approves an application other involved authorities may object, possibly delaying an application for

several years (Spreull and Uren, 1986).

5.5.2.2 Operating Restrictions

The regulatory authorities may specify a minimum working distance from the coastline or

a minimum water depth based on the local wave climate. For example, in Holland, dredging for

sand and gravel is not permitted within 20 km of the coast. In Britain the minimum permitted

distance from the coast is 5 km (Cronan, 1986). Regarding acceptable water depths, an interim guide

to safe practice is obtainable from British experience. In coastal waters of the North Sea dredging

for aggregates "... reportedly causes no problems in water depths greater than half the normal wave

length or more than one fifth the length of extreme waves" (Drinnan and Bliss, 1986; Continental

Shelf Associates, Inc. 1993).

European authorities have attempted to translate the technical advice into quantified water

depths. As a general guideline the British Crown Estates Commission, the regulatory authority, has

adopted a water depth of 18 m as the minimum for offshore dredging (Nunny and Chillingworth,

1986). The actual depth limit identified for potential damage was 13 m but an additional 5 m was

added as a precautionary measure" (Continental Shelf Associates, Inc. 1993). In practice, dredging

in waters deeper than 20 m is usually approved by the authorities. A licence application for dredging

in 10 to 20 m of water receives a more detailed review and requires specific information regarding

the site. A proposal to dredge in waters shallower than 10 m requires substantial study (Drinnan and

Bliss, 1986; Continental Shelf Associates, Inc. 1993).

Unlimited dredging of offshore deposits of sand can be detrimental by altering the wave

climate in relatively shallow waters. Restrictions are therefore placed on the permitted extent of

removal. On the coast of New Jersey, for example, where a dredge operator planned to excavate

sand from an offshore sand bar, the authorities insisted that part of the bar be left undredged. The

remnant bar served as an additional protective barrier between the borrow area and the sand beach

(World Dredging, Mining and Construction, 1 992a) Under such circumstances a dustpan dredge has

some advantage because it can be operated in such a manner that thin, precise, layers of sand can be

skimmed from the seabed (World Dredging, Mining and Construction, 1993). Fine silt is an

unwanted size fraction for beach renourishment so the operator of a trailing suction hopper dredge

will allow the unwanted fines to flow overboard during hopper loading at sea. A generally coarser

and more stable material is placed on the beach (World Dredging Mining and Construction, 1 992b).

145
6.0 Summary and Recommendations

As outlined in Section 1, the purpose of this study is to undertake a detailed analysis of

available and proposed marine mining technologies and mitigation techniques with respect to the

mining of specific mineral commodities on the U.S. OCS.

The approach has been to develop a number of marine mining scenarios that target mineral

commodities which have the potential for exploitation within a ten-year time span. The scenarios

are presented in Section 3, and provide details of the geological and environmental settings, the

extraction and processing methods, the scales of operation and the observed or anticipated

environmental impacts.

An overview of the environmental issues related to marine mining is presented in Section

2. It provides a framework for the discussion of environmental impacts associated with each mining

scenario. A general discussion of marine mining mitigation techniques is presented in Section 4, and

the application of mitigation techniques to each of the scenarios is discussed in Section 5.

Sections 6.1 .1 to 6.1.5 below provide brief summaries of the mining scenarios with particular

emphasis on the predicted environmental impacts and relevant mitigation techniques. The most

appropriate mitigation techniques for the scenarios and for similar marine mining operations, based

on relative cost, effectiveness and practicality are rcommended. A summary of the conclusions of

this study is provided in Section 6.2.

6.1 Summary of Marine Mining Scenarios

6.1.1 Precious Metal Mining, Alaska - Bucket Ladder Dredge

Scenario 1 documents a bucket ladder dredging operation carried out by WestGold offshore

of Nome, Alaska. The scenario provides a comprehensive case history of the only large-scale marine

mining operation that has been conducted to date in U.S. waters. This scenario provides practical

experience with marine mining mitigation.

The mined ground comprised mainly gold-bearing glacigenic sediments that were inundated

and reworked by the transgressing sea. Gold concentrations were greatest in lag gravels forming the

uppermost 0.5 m of seabed. Mining was by means of a large ocean-going bucket ladder dredge (the

BIMA) with an 850 litre bucket capacity and an onboard treatment plant. The choice of a large

bucket ladder dredge was influenced by the difficult ground conditions which characterized the

deposit. The depth of cut was generally 3 m or more which led to some dilution of gold grades.

During mining, the dredge was secured by 5 winch-controlled anchor lines (one head line) which

moved the dredge up to 200 m laterally while slowly advancing along its course. The average rate

of throughput was 300 to 600 m3 per hour with more than 95% of mined material returned to the sea

as effluents.

146
Environmental monitoring and mitigation were directed mainly at the observation and control

of tailings discharges to the sea, with the objective of minimizing water column turbidity and the

areal extent of re-sedimentation. Of particular interest was the response of the king crab population,

which supported a local fishery, to the effects of seabed disturbance and tailings deposition.

This scenario is particularly valuable in terms of the practical experience gained in tailings

management and disposal. A successful, iterative approach to the design of a sub-surface tailings

discharge system was adopted that coupled numerical plume modelling with water column

monitoring and successive discharge design modifications. A similar approach is advocated for

future marine mining operations that involve the disposal of the majority of mined materials at sea

as waste product. The incorporation of a sub-surface tailings discharge designed for the local

operating conditions represents a small portion of the capital cost of a dredge, and yields significant

environmental benefits by reducing far-field turbidity and the areal extent of sedimentation.

Seabed gold concentrations offshore Nome are localized and variable, and required intensive

sampling to obtain reserve estimates and to direct the dredging program. Location-based constraints

were imposed on the WestGold project from the outset. Such constraints would likely be imposed

on the mining of any precious metal or heavy mineral deposit, and highlights the need to identify

potential restricted areas (e.g. sensitive habitats) well in advance of project initiation. This enables

the economics of the mining operation to be realistically assessed.

A time-based constraint placed on the WestGold operation involved the interruption of

mining whenever one or more sea mammals came in sight of the dredge. Unpredictable constraints

of this nature can have significant economic implications, particularly when mining in a harsh

environment with a limited operating season. From an economic perspective, similar constraints on

future mining operations are not generally advisable and should be considered cautiously.

Another mitigation method proposed for the WestGold operation was the reduction of

throughput rate when anomalously fine-grained sediments were encountered. This technique is not

particularly effective because it serves to prolong the period when finer-grained sediments are

discharged, and does not substantially affect the ultimate disposition of fines on the seabed. It also

has very significant negative economic implications because the profitability of an operation is based

on the maximum possible rate of throughput.

6.1.2 Precious Metal Mining, Alaska - Underwater Miner

Scenario 2 documents production trials carried out by WestGold using the underwater mining

vehicle "TRAMROD". The UWM experience provides a useful comparison with the BIMA

operation, because a more precise, selective and smaller-scale extraction technique was applied to

the same mineral deposit. The UWM is a track-mounted vehicle that supports a hydraulically

controlled dredging arm. The vehicle was deployed by crane from a barge and was remotelycontrolled from the surface. The barge also contained the treatment plant. The solids discharge rate

averaged 150 m3 per hour. As discussed in Section 5.2, the low throughput rate associated with the

underwater miner also facilitated the development and use of relatively simple but effective systems

147
for dewatering, diffusing and controlling tailings. The depth of cut was as little as 10 cm, so that the

UWM selectively extracted higher grade, surficial lag deposits. The environmental impacts of such

an operation are of lesser magnitude than those associated with a bucket ladder operation by virtue

of the low throughput rate and high degree of selectivity of mined seabed.

An UWM can only be used to mine high value commodities such as placer gold or diamonds,

because the low throughput rate will not support economic production of lower value ones, such as

heavy minerals. When the combined deposit characteristics and economic considerations indicate

a viable operation based on an UWM then the use of such a system constitutes a mitigative measure.

6.1.3 Marine Aggregate Mining, Massachusetts Bay

Scenario 3 describes a hypothetical aggregate mining operation in Massachusetts bay serving

the Boston area market. The site selection and details of the environmental and geological settings

were based on studies conducted in the 1970's roject NOMES) in preparation for an experimental

aggregate mining operation that was designed to assess potential environmental effects of marine

mining. The experimental mining project did not proceed, but the baseline study results have

provided a useful foundation for the development of this scenario.

The aggregate resource was investigated by coring, sidescan sonar and other methods and

was found to be of glacial origin and, in general, more poorly sorted than is desirable for an

aggregate target. Extraction would be by means of a large ocean-going trailing suction hopper

dredge. The operation would produce about 18,000 metric tonnes of aggregate daily for 25 days

each month, and would run for approximately 7 months of the year (winter sea conditions prevent

year-round operation). Cycle times would be in the range of 8 to 13 hours depending on vessel

speed and the capabilities of the offloading facilities.

During dredging, muds are discharged to the sea with the hopper overflow. While

excavating, the twin trailing drag-heads create parallel furrows in the seabed each about 2.7 m wide

and 0.5 m deep. Potential environmental impacts include the destruction of benthic fauna entrained

by the dredge, changes in substrate grain size through exposure of different lithologies or settlement

of fine sediments (possibly leading to recolonization by a different benthos), habitat alteration

through modification of the seabed topography, and transient water column effects on primary

production related to elevated turbidity levels.

A number of practical and cost-effective mitigation techniques can be implemented to reduce

the extent of deposition of fines and water column turbidity (see Section 5.3). An example of such

a technique is hopper overflow that may delayed by emptying the hopper of all water before

pumping sediments on-board. Overflow does not occur until the hopper achieves 60% to 70% of

fill capacity, thereby significantly reducing the total time of overflow discharge.

For drag-heads equipped with water jets to dislodge compacted sediments, it is possible to

recycle a portion of the hopper waste water to the jets to reduce the total surface discharge.

Preventing overflow by recirculating all process water to the drag-heads is not of particular benefit

148
because the slurry quickly reaches a density beyond the pump capabilities, so that the system needs

to be purged at intervals. The adoption of sub-surface discharge systems, such as the IHC and

Japanese anti-turbidity overflow systems, may reduce significantly surface water turbidity and

sediment dispersion and should be considered as a mitigation option.

Changes to seabed lithology and topography can be minimized through careful planning of

dredge course locations and frequencies. The depth of seabed dredged can be limited to avoid

revealing different underlying lithologies, as guided by detailed geological survey data and on-board

feed monitoring systems. Dredge courses should be planned to avoid overdredging (overdeepening)

and oversanding, to limit repeated disturbance of areas of seabed undergoing benthic recolonization,

and to take into account as much as possible the areal and sub-surface distribution of the aggregate

resource, within the constraints of dominant current directions.

6.1.4 Heavy Mineral Sands, Virginia

Scenario 4 describes a hypothetical heavy mineral mining operation offshore Virginia Beach,

Virginia. The resource is comprised of well-sorted sands with heavy mineral concentrations

averaging about 3 to 4% by weight, occurring mainly in the uppermost 3.5 m of seabed. Potential

mining is by cutter suction dredging from a floating or jack-up platform. The dredge advances with

the aid of winch-controlled anchor lines and the cutter-head arm moves through a broad arc with an

average cut depth of 4 m. The feed is screened and heavy minerals are concentrated in the onboard

wet mill. Approximately 95% of the sediments dredged are returned to the sea as tailings.

The primary potential environmental impacts of such an operation include the destruction

of biota entrained by the dredge, modification of benthic habitat through changes in bottom

topography, smothering of benthos by deposited tailings, and transient effects on primary

productivity due to elevated water column turbidity.

As discussed in Section 5.4.4.4, the site specific design and implementation of a (subsurface)

tailings discharge system is a cost-effective and recommended mitigation technique. Other costeffective approaches to limiting the turbulent dispersion of sediments introduced to the sea include

the use of de-gassing and de-sliming equipment, and end-of-stream cycloning to dewater as much

as possible the solids effluent.

The use of chemical flocculants to enhance settling of discharged fines holds some potential

but requires further investigation of its practical application at sea and of potential environmental

implications. Flocculants are costly and would be expected to increase annual mining costs by more

than 10%. For these reasons, the use of chemical flocculants is not unconditionally recommended.

Turbidity generated at the cut-face can be reduced practically by appropriate cutter-head

design and by the dredging technique (i.e. controlled depth of cut, sloped vs. vertical cut face, etc.).

Dredging technique can also affect the degree of seabed modification (e.g. extent of lateral cut face

vs. dredge advance) and can be modified as appropriate within reasonable economic bounds.

149
150

As in the case of Scenario 1, the heterogenous nature of heavy mineral deposits means that

location-based constraints placed on the mining operation can significantly affect its economic

viability. Such constraints should be prescribed prior to commencement of the project.

6.1.5 Beach Nourishment, Maryland

Scenario 5 details a beach nourishment project at Ocean City, Maryland, utilizing a sand

resource (borrow site) located in Federal waters seaward of the State boundary. The dredge is

expected to be either a cutter suction dredge with transport of sand to shore by pipeline, or a trailing

suction hopper dredge that delivers sand to a shore-connected pumping station. In each case there

is virtually no solids effluent discharged at sea, and resuspended materials are localized to the

vicinity of the excavation tool.

Potential environmental impacts include (but are not necessarily limited to) the destruction

of benthos, transient water column effects related to increased turbidity, and alteration of bottom

topography leading to either the development of anoxic dredge pits or the risk of shoreline erosion.

The potential risk of inducing shoreline erosion warrants consideration with any beach nourishment

project and can be minimized through numerical modelling and informed, appropriate selection of

borrow sites.

6.2 General Recommendations

General recommendations derived from the analysis presented in this report include:

The use of Best Available Technology should be practiced with any marine mining

operation. The use of Best Available Technology is not only beneficial environmentally but

in most cases improves operational efficiency and is therefore of value to the operator

Numerical modelling is a useful and cost-effective tool in the development of effluent

discharge systems, the design of environmental monitoring programs, and the assessment of

the potential risk of coastal erosion. Modelling should be implemented as part of any mining

project that involves substantial discharge of effluents at sea, or of projects that may

potentially affect coastal stability

Well-designed environmental monitoring programs with appropriate QA!QC are a necessary

component of any marine mining project. Monitoring ensures that regulatory constraints are

observed, and also provides the operator with ongoing guidance as to the level of effort

needed to meet those constraints

Operational constraints of location, time and scale need to be determined before the initiation

of any mining project, so that the economic viability of the project can be realistically

assessed. From an operational and economic perspective, constraints on operations that are

based on random occurrences such as sightings of particular species in the mining area are
151

detrimental and potentially very costly. The possible implications of such constraints should

be carefully considered

In some cases mitigation can be achieved cost-effectively through changes in operational

procedures, such as the planned order and distribution of dredge courses or the depth and

width of cut. The planning of any new marine mining venture should take into consideration

the range of operational mitigation techniques presented in this report

Many technology-based mitigation techniques, such as sub-surface tailings discharge

systems, dewatering and degassing systems, and onboard feed monitoring systems,

measurably reduce the impacts associated with a mining operation and can be implemented

at reasonable cost. The implementation of these techniques should be considered for any

future offshore mining development

The potential for Alternative Enhancement projects should be explored during the planning

stages of future marine mining developments. For example, consideration should be given

to the possibility of using tailings produced by a heavy mineral mining operation for

beneficial purposes, such as beach nourishment or the creation or restoration of coastal

wetlands.
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168
Table 2.1 Summary of the scope of environmental and biological data from selected surveys of gravel and sand deposits

Higher Order Taxa

2 One or more calculated Diversity Indices

B/A = Before and After Impact, T/R = Test and Reference, B = Baseline

V = present

169

Kenny &

Rees, 1994

and

North Sea Gravel Particle

size

V'(no raw

data)

#ofsp. V sp. V B/A T/R 4(2) 2(2-5)

Kenny &

Rees, 1993

North Sea Gravel Particle

size

V(no raw

data)

V #ofsp. V sp.+

HOTs

V ? B/A T/R 4(2) 2(2-5)

Saloman et

a! 1982

Gulf Coast

of Florida

Sand Grain

size,

Phi, O/

Temp,

Salinity,

Carbon

V V,#of

sp.

sp.+

HOTs

V B/A T/R 4 + 3 2(4) + 1(32,35,36)

+6(90)

Padan.

1977

Atlantic

Ocean

(Massachu

setts Bay)

Cobble,

gravel +

sand

None Water

Chemistry

V V

feeding

guilds

- B T/R 8 8 (varies) mud

stations excluded

ENSR,

1991

Bering Sea

Sand +

Gravel

Grain

size, %

Water

Chemistry

V VI V V V V T/R 7(8-6)

Environmental Data Biological Data Other Data

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Reference Location Gravel Particle Sediment Species HOTS' Diversity Biomass Noted Dredging Smothering Sampling No. of Cruises No. of Stations! site

or Sand Size & Water

Chemistry

(sp)

Counts

Counts Indices2 Taxa Impact Impacts Paradigm3 (& sites/

cruises)

(reps! station)
Table 2.2 Example of variation in number of taxa from year to year

3 cm mixed tailing in 1993

Tailing discharge started October 1971

nd = no data

Samples taken during September from 1977 onwards. Diversity values not obtained 1973-1976

Data taken from 3x0.05m2 sampler at each of a set of Test and Reference sampling stations around a tailings outfall. (Data courtesy of Island Copper Mine, Canada).

170

No visible tailings Tailings 1-20 cms Tailings > 20 cms

Survey 24 1 8 11 21 25 2 3 4 7 10 12 20 22 23 5 6 9 13 14 15 16 17 19

1970 Jan 16 8 9 10 15 20 10 17 23 5 7 16 15 7 10 21

1971 Jun 60 34 25 28 7 14 11 29 29 31 26 42 42 18 23 42 4 28 26 13 29 18

1971 Dec2 41 15 20 18 35 24 52 25 13 63 26

1972 Jun 24 41 31 26 11 10 10 20 32 31 30 39 17 11 15 13 10 34 1 nd3 10 15

1972 Dec 34 23 43 28 8 10 16 29 32 34 26 40 49 18 21 19 4 14 3 nd 10 13

1977 Sep4 20 24 36 28 20 18 15 17 23 29 18 18 25 43 13 19 17 12 14 16 4 4 4 17

1978 20 30 28 22 25 13 16 13 26 19 23 21 31 27 20 17 24 12 7 16 5 0 1 13

1979 19 31 31 26 27 29 13 15 19 29 27 25 23 41 46 17 23 15 13 8 23 4 3 9 10

1980 18 31 38 40 31 15 21 17 15 33 35 32 27 40 38 18 17 18 22 1 28 9 8 13 13

1981 36 22 37 42 60 57 17 9 12 40 25 35 29 44 74 nd 24 23 10 9 17 1 3 16 13

1982 30 21 45 46 52 57 25 16 19 42 38 33 32 63 47 11 23 25 5 4 19 0 5 11 20

1983 45 41 43 54 50 54 27 10 23 55 37 32 37 64 83 12 24 23 13 4 30 2 2 6 15

1984 40 43 37 54 61 52 28 29 31 54 61 41 35 69 84 11 37 34 29 14 40 2 4 4 19

1985 43 41 54 40 49 49 24 23 28 41 46 39 28 55 70 10 25 27 12 8 27 11 4 6 11 1986 40 76 63 92 102 63 19 26 25 71 52 45 31 93 111 20 39 37 32 22 43 8 2 4 14

1987 48 78 77 84 103 59 25 31 28 88 99 66 50 83 121 8 42 20 22 11 49 3 9 13 25

1988 64 85 75 78 83 77 28 34 25 89 76 48 54 120 142 13 32 18 26 18 49 6 10 15 30

1989 53 89 52 93 87 72 26 19 25 61 48 39 52 116 139 15 23 27 18 5 35 7 10 15 19

1990 58 124 73 77 101 100 34 24 32 85 70 64 49 120 154 27 28 28 32 14 35 5 8 15 15

1991 54 88 73 95 77 126 27 31 28 69 64 59 68 100 132 8 39 27 8 8 32 3 15 14 22

1992 55 79 59 78 76 121 31 42 30 81 71 71 46 107 123 10 26 25 20 10 37 8 10 9 6 1993 54 81 57 76 67 115 32 24 31 62 55 51 42 118 123 nd 30 22 16 14 36 4 8 9 6
Recovery times are based on no further impact from dredging at adjacent sites

(e.g. no settling of a drifting tailings plume).

Recovery is defined as attaining a successional community of opportunistic species

providing evidence of progression towards a community equivalent to that previously

present, or at non-impacted reference sites.

Equivalency can be measured in terms of standing crop, biomass, number of

species and numbers of most abundant species, etc. These criteria will need definition

during preparation of an Environmental Impact Statement (or equivalent document) at any

site. The definition will depend on the Best Available Monitoring techniques suitable for

biodiversity assessment at that site.

See Ellis et al. 1995 (Island Copper Mine tailings disposal)

2 Sands - See 3.2 Scenario 1: Precious Metal Mining, Alaska - bucket ladder dredge

and Appendix 4, Salomon et al. 1982. See also Garnett and Ellis (1995)

Gravels - See 3.2 Scenario 1: Precious Metal Mining, Alaska - bucket ladder dredge

and Appendix 3, Kenny and Rees 1993;1994. See also Garnett and Ellis (1995)

171

Table 2.3 Time periods to recovery for benthic biodiversity

(1) Fine-grained deposits'

muds/silts/clays

can contain some rocks and boulders

1 year

(2) Medium-grained deposits2

sands

can contain some silts/clays and gravel

1-3 years

(3) Coarse-grained deposits3

gravels

can contain some finer fraction and

some rocks and boulders

5 years

gravels with many rocks and boulders 5+ years
Table 2.4 The Quality Assurance/Quality Control CODIS system (Continental and

Oceanographic Data Information System) of Blyth et al. (1993)

0 Data are found to be wrong. The data source contains obvious errors.

1 = Data are suspect because of suspected internal inconsistencies; patterns or

trends within the data are probably not real.

2 Insufficient information is provided to assess the quality of the dataset; trends

in the data may, or may not, be real.

3 Data are internally consistent; patterns of trends within the data are probably

real; comparisons with other datasets may be difficult or impossible.

4 Data are internally consistent and are sufficiently standardized to permit

comparison with other datasets of this quality rating.

9 Data not yet rated.

172
Table 2.5 Environmental screening information needed for development of mitigation

procedures at offshore dredging sites

173

Fisheries

Stocks present, and current use

Includes both bottom and open water stocks

Includes fin-fish and shell-fish

Feeding habits, and level of habitat dependency

Sensitive Species and Habitats

Wildlife (mammals), waterfowl (birds) and others such as reptiles

Stocks present, numbers, timing and patterns of seasonal changes

Spawning grounds (fish), migration routes and migration stopover sites (mammals and

birds), and timing of their use. Importance of the site to the species, i.e. is the site

unique or one of how many others?

Benthic Habitat and Biodiversity

Particle size analysis and geochemistry of deposit

Number of species and their numbers per unit area

Identifications of most abundant (characterising) species

Physical/Chemical Oceanography

Wind and Seastate records

Current records - patterns of velocity and direction

Water clarity, and turbidity plume dispersion modelling

Basic seawater characteristics - temperature, salinity, dissolved oxygen, etc, and seasonal

variations

Biological Oceanography

Characteristic levels of primary production

Characteristic phytoplankton, zooplankton and nekton

Geochemistry

Natural variables determining benthic biodiversity - depth within deposit of reduced

oxygen layer and associated geochemistries

Anthropogenic variables - presence of site-specific toxins determined from prior site use,

or proximity to anthropogenic input, i.e. cities, major shipping routes, dump-sites, etc.

Screening information is to be assembled during preparation of a preliminary Environmental Impact

Statement (EIS). Some field surveys may be needed to update or extend archived information.
Table 2.6 Likely availability of environmental screening information prior to

implementation of an environmental impact statement (based on data

searches, 1994-5)

174

Fisheries - probably non-technical fishing maps

Much archived information in Federal files

Sensitive species/habitats

Much archived information in Federal files

Non-technical documents may be readily available

Benthic biodiversity

Much archived species information in government and university records, but

probably old, non-quantitative and not site-specific

Physical/Chemical Oceanography

Much archived information in government and university records. Probably

accessible electronically.

Biological Oceanography

Much archived information in government and university records, but probably

old, may be quantitative, but not site-specific

Geochemistry

Probably some archived information, but may not be site specific. If there are

prior pollution studies, there may be a recent compilation.
Table 2.7 Toxin concentration guidelines for sediment, U.S.A. and Canada (Table Cont'd)

175

Chemical

Low Values High Values EP Values

U.S.A.

Effects Range Low1

Canada

Threshold Effects Levels2

U.S.A.

Effects Range Median3

Canada

Probable Effects Levels4

1% Carbon Sediment

METALS (ppm):

Antimony 2 - 25 - - Arsenic 33(8.2) 7.24 85(70) 41.6 8.2

Cadmium 5(1.2) 0.676 9.0(9.6) 4.21 7.7

Chromium 80(81) 52.3 145(370) 160

Copper 70(34) 18.7 390(270) 108 34

Lead 35(46.7) 30.2 110(218) 112 33

Mercury 0.15 0.13 1.3(0.71) 0.70 0.008

Nickel 30(20.9) 15.9 50(51.6) 42.8 - Silver 1 0.73 2.2(3.7) 1.77 - Tin - - - - - Zinc 120(150) 124 270(410) 271 190

POLYCYCLIC AROMATIC

HYDROCARBONS (ppb):

Acenaphthene 150(16) 6.71 650(500) 88.9 - Acenaphthylene (44) 5.87 (640) 128 - Anthracene 85(85.3) 46.9 960(1100) 245 - Fluorene 35(19) 21.2 640(540) 144 - 2-methylnaphthalene 65(70) 20.2 670 201 - Naphthalene 340(160) 34.6 2100 391 - Phenanthrene 225(240) 86.7 1380(1500) 543.5 - Benz(a)anthracene (261) 74.8 (1600) 693 - Benzo(a)anthracene 230 - 1600 - - Benzo(a)pyrene 400(430) 88.8 2500(1600) 763 - Benzo(e)pyrene - - - - - Biphenyl - - - - - Chrysene 400(384) 107.8 2800 846.0 - Dibenzo(a,h)anthracene (63.4) 6.22 (260) 135 - Dibenz(a,h)anthracene 60 - 260 - - 2,6-dimethylnaphthylene - - - - - Fluoranthene 600 113 3600(5100) 1494 3600

1-methylnaphthalene - - - - - 1 -methylphenanthrene - - - - - Peiylene - - - - - Pyrene 350(665) 153 2200(2600) 1398 - 2,3,5-trimethylnaphthalene - - - - - Total Polycyclic Aromatic Hydrocarbons 4000(4022) - 35000(44792) - - Low molecular weight PAH (552) - (3160) - - High molecular weight PAR (1700) - (9600) - -
Table 2.7 Toxin concentration guidelines for sediment, U.S.A. and Canada (Cont'd)

'concentrations above which adverse effects may begin' (Long and Morgan, (1991)

'concentrations below which effects rarely occurred (Anon, 1994)

"concentrations above which effects were frequently or always observed' (Long and Morgan, 1991)

'concentrations above which effects frequently occurred" (Anon. 1994)

Numbers in brackets are from Long ef al. (1995)

176

Chemical

Low Values High Values EP Values

U.S.A.

Effects Range Low1

Canada

Threshold Effects Levels2

U.S.A.

Effects Range Median3

Canada

Probable Effects Levels4

1% Carbon Sediment

PESTICIDES (ppb):

Lindane - - - - - Chlordane 0.5 2.26 6 4.79 - Heptachior - - - - - p'p'-DDD 2 1.22 20 7.81 3240

p'p'-DDE 2(2.2) 2.07 15(27) 374 6930

p'p'-DDT 1 1.19 7 4.77 1.6

DDT, total 3(1.58) 3.89 350(46.1) 51.7 - Aldrin - - - - - Dieldrin 0.02 0.715 8 4.3 - Endrin 0.02 - 45 - - Lindane - 0.32 - 0.99 - Mirex - - - - - MISCELLANEOUS ORGANICS (ppb):

PCBs, total 50(22.7) 21.5 400(180) 189 29

Bis(2ethylexyl)phthalate - 182 - 2647 -
Table 2.8 Example of environmental impacts, mitigation objectives and achievement

criteria for proposed mine sites

Water column Impacts (turbidity)

Impact (Primary):

reduced primary biological production and changed species

Impact (Secondary-derived):

modified secondary production of zooplankton, hence possibly fish.

Objective:

minimize elevations of surface water turbidity

Criteria:

surface turbidity does not reduce the compensation depth by more than 10%. Increase of surface turbidity

is less than a site-specific preset value (a dilution zone may be set within which these objectives need not

be reached)

Seabed Tm pacts

Impact (Primary):

benthic kills, toxin releases, and altered substrate (habitat)

Impact (Secondary):

changed benthic biodiversity, fisheries changes (losses or enhancement both possible)

Objectives:

minimize dredge course overlaps and gaps

minimize return of tailings outside dredge footprint

leave surface at same grade

no release of contaminants

no anoxic dredge furrows

recovery to a successional community within time scale of: silts - 2 years, sands - 3 years, gravels - 5 years

avoid sensitive areas - endangered species, isolated breeding grounds, etc.

Criteria:

to be set on basis of dredging plan, and site-specific EIA

dissolved oxygen level >3 mg/I

apply EPA or NOAA guidelines for specific contaminants

set recovery criteria site-specifically in terms of biodiversity (number of species, number of organisms and,

possibly, selected species)

3. Impacts on Fisheries and Wildlife Resources

Impacts:

little impact expected usually, as wildlife and fish are able to avoid vessels

fisheries may be impacted by presence of mining vessels

seabed spawning grounds may be seriously impacted

local reactions by wildlife are known to occur at noisy oil rigs, generalised avoidance may occur

Objectives:

avoid sensitive habitats, such as finfish and shellfish spawning grounds, and wildlife migration staging

grounds or intense migration routes

design quiet vessels to Best Practical Technology

Criteria:

changes in migration, spawning and other behaviour of wildlife as set site-specifically

noise measures to be set site-specifically if needed to protect sensitive species

dredge plan implementation monitored

177
Table 2.9a Weather statistics for the U.S. east coast offshore (from Meserve, 1974) Site 20: represents offshore Massachusetts

178

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

WIND

Scalar mean wind speed [kts]

Wind directions with >20% frequency [from]

Vector mean wind speed [kts]

Vector mean wind direction [from]

Percent frequency winds > 28 kts [%]

Percent frequency winds > 34 kts [%]

18.9 19.6 18.5 15.7 12.9 11.2 10.5 10.8 12.7 15.2 17.5 18.7

NW W W NW W NW W NONE SW SW SW W NONE NONE NW W NW W

8.4 9.0 7.5 4.9 2.7 3.5 4.4 3.1 2.4 4.3 6.0 8.4

NW NW NW NW W SW SW SW NW NW NW NW

19 22 18 11 4 2 1 3 4 10 16 18

9 11 8 4 1 <1 <1 1 1 4 8 9

AIR TEMPERATURE

90% upper limit air temp [deg. C]

Mean air temperature [deg. C]

10% lower limit air temp [deg. C]

13 12 12 15 19 22 25 25 24 21 18 14

6 4 6 8 13 17 21 21 19 16 11 8

0 -1 1 4 7 12 16 17 15 11 6 2

SEA SURFACE TEMPERATURE

90% upper limit sea surface temp [deg. C]

Mean sea surface temperature [deg. C]

10% lower limit sea surface temp [deg. C]

16 16 16 17 20 23 25 25 25 23 20 17

9 9 8 10 13 17 21 22 21 18 14 12

3 3 4 6 10 14 17 16 13 10 7

PRECIPITATION AND VISIBILITY

Percent frequency precipitation [1%]

Percent freq. restricted visibility < 2 n mi [%]

23 24 18 11 9 7 8 7 8 10 15 21

5 6 5 6 11 14 12 9 4 4 3 3

SIGNIFICANT WAVE HEIGHT

Percent frequency wave height > 3 m [%]

Percent frequency wave height > 4 m [%]

Percent frequency wave height > 6 m [%]

Percent frequency wave height > 8 m [%]

28 29 22 16 9 3 3 3 7 15 23 27

13 15 9 6 4 1 1 1 2 5 10 14

5 3 2 1 <1 <1 <1 1 2 4

1 <1 1 1 <1 <1 2
Table 2.9b Weather statistics for the U.S. east coast offshore (from Meserve, 1974) Site H: Represents Offshore Virginia/Maryland

179

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

WINO

Scalar mean wind speed [kts]

Wind directions with >20% frequency [from]

Vector mean wind speed [kts]

Vector mean wind direction [from]

Percent frequency winds > 28 kts [%]

Percent frequency winds > 34 kts [%]

20.3 21.9 20.9 18.9 16.0 12.9 12.4 13.2 13.8 15.2 17.9 20.5

NWSW NW W NW SW S SW SW SW SW NONE NE NW NW W

9.1 9.9 8.5 7.1 6.0 4.4 8.1 4.4 0.9 2.9 4.9 9.3

W W W SW SW SW SW SW E NE W W

24 30 25 18 8 3 2 3 5 7 19 25

12 16 13 8 2 <1 <1 1 1 2 8 12

AIR TEMPERATURE

90% upper limit air temp [deg. C]

Mean air temperature [deg. C]

10% lower limit air temp [deg. C]

19 18 18 21 23 26 27 27 27 25 22 20

15 13 13 19 21 23 26 26 25 22 18 15

8 6 6 13 18 21 24 24 22 18 11 8

SEA SURFACE TEMPERATURE

90% upper limit sea surface temp [deg. C]

Mean sea surface temperature [deg. C]

10% lower limit sea surface temp [deg. C]

22 22 21 21 24 26 27 27 28 26 24 22

19 19 19 19 22 24 26 26 27 24 22 20

13 13 12 18 21 22 25 25 25 21 17 15

PRECIPITATION AND VISIBILITY

Percent frequency precipitation [1%]

Percent freq. restricted visibility < 2 n mi [%]

21 20 18 13 11 9 10 8 8 11 15 18

4 1 <1 <1 <1 <1 <1 <1 <1 <1 2

SICNIFIC4NT WAVE HEIGHT

Percent frequency wave height> 3 m [%]

Percent frequency wave height> 4 m [%]

Percent frequency wave height> 6 m 11%]

Percent frequency wave height> 8 m [%1

36 40 35 27 11 6 6 7 10 14 28 33

17 21 17 10 2 2 1 2 2 6 12 15

s 5 4 2 <1 <1 <1 1 2 4

2 <1 1 <1 <1 1
180

Nearshore Virginia Nearshore Maryland

Spring

Summer

Fall

Winter

<40 cmls to SE

<40 cm/s to SE

<40 cm/s to S

<40 cmls to SE

<40 cmls to SW

<10 cm/s to SW

<10 cmls to SSE

<40 cm/s to SW

Table 3.1 Typical seasonal ocean current velocities expected for the Virginia Beach and

Ocean City sites (based on data from U.S. Department of the Interior,

Minerals Management Service, 1985)
Table 3.2 Summary of ocean current flow for the Virginia Beach and Ocean City sites

181

Nearshore Virginia Nearshore Maryland

Average surface current speed

Typical bidirectional tidal

surface current speed,

oriented along the coastline

Extreme bidirectional tidal

surface current speed, oriented

along the coastline

Extreme magnitude of transient

flow (variable directions)

Average bottom current speed

(variable directions)

10 cm/s toward south

20 - 30 cm/s

50 cm/s

50 cm/s

2 cm/s

10 cm/s toward

southwest

20 - 30 cmls

50 cm/s

50 cm/s

2 cm/s
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Table 2.1 Summary of the scope of environmental and biological data from selected surveys of gravel and sand deposits

Higher Order Taxa

2 One or more calculated Diversity Indices

B/A = Before and After Impact, T/R = Test and Reference, B = Baseline

V = present

169

Kenny &

Rees, 1994

and

North Sea Gravel Particle

size

V'(no raw

data)

#ofsp. V sp. V B/A T/R 4(2) 2(2-5)

Kenny &

Rees, 1993

North Sea Gravel Particle

size

V(no raw

data)

V #ofsp. V sp.+

HOTs

V ? B/A T/R 4(2) 2(2-5)

Saloman et

a! 1982

Gulf Coast

of Florida

Sand Grain

size,

Phi, O/

Temp,

Salinity,

Carbon

V V,#of

sp.

sp.+

HOTs

V B/A T/R 4 + 3 2(4) + 1(32,35,36)

+6(90)

Padan.

1977

Atlantic

Ocean

(Massachu

setts Bay)

Cobble,

gravel +

sand

None Water

Chemistry

V V

feeding

guilds

- B T/R 8 8 (varies) mud

stations excluded

ENSR,

1991

Bering Sea

Sand +

Gravel

Grain

size, %

Water

Chemistry

V VI V V V V T/R 7(8-6)

Environmental Data Biological Data Other Data

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Reference Location Gravel Particle Sediment Species HOTS' Diversity Biomass Noted Dredging Smothering Sampling No. of Cruises No. of Stations! site

or Sand Size & Water

Chemistry

(sp)

Counts

Counts Indices2 Taxa Impact Impacts Paradigm3 (& sites/

cruises)

(reps! station)
Table 2.2 Example of variation in number of taxa from year to year

3 cm mixed tailing in 1993

Tailing discharge started October 1971

nd = no data

Samples taken during September from 1977 onwards. Diversity values not obtained 1973-1976

Data taken from 3x0.05m2 sampler at each of a set of Test and Reference sampling stations around a tailings outfall. (Data courtesy of Island Copper Mine, Canada).

170

No visible tailings Tailings 1-20 cms Tailings > 20 cms

Survey 24 1 8 11 21 25 2 3 4 7 10 12 20 22 23 5 6 9 13 14 15 16 17 19

1970 Jan 16 8 9 10 15 20 10 17 23 5 7 16 15 7 10 21

1971 Jun 60 34 25 28 7 14 11 29 29 31 26 42 42 18 23 42 4 28 26 13 29 18

1971 Dec2 41 15 20 18 35 24 52 25 13 63 26

1972 Jun 24 41 31 26 11 10 10 20 32 31 30 39 17 11 15 13 10 34 1 nd3 10 15

1972 Dec 34 23 43 28 8 10 16 29 32 34 26 40 49 18 21 19 4 14 3 nd 10 13

1977 Sep4 20 24 36 28 20 18 15 17 23 29 18 18 25 43 13 19 17 12 14 16 4 4 4 17

1978 20 30 28 22 25 13 16 13 26 19 23 21 31 27 20 17 24 12 7 16 5 0 1 13

1979 19 31 31 26 27 29 13 15 19 29 27 25 23 41 46 17 23 15 13 8 23 4 3 9 10

1980 18 31 38 40 31 15 21 17 15 33 35 32 27 40 38 18 17 18 22 1 28 9 8 13 13

1981 36 22 37 42 60 57 17 9 12 40 25 35 29 44 74 nd 24 23 10 9 17 1 3 16 13

1982 30 21 45 46 52 57 25 16 19 42 38 33 32 63 47 11 23 25 5 4 19 0 5 11 20

1983 45 41 43 54 50 54 27 10 23 55 37 32 37 64 83 12 24 23 13 4 30 2 2 6 15

1984 40 43 37 54 61 52 28 29 31 54 61 41 35 69 84 11 37 34 29 14 40 2 4 4 19

1985 43 41 54 40 49 49 24 23 28 41 46 39 28 55 70 10 25 27 12 8 27 11 4 6 11 1986 40 76 63 92 102 63 19 26 25 71 52 45 31 93 111 20 39 37 32 22 43 8 2 4 14

1987 48 78 77 84 103 59 25 31 28 88 99 66 50 83 121 8 42 20 22 11 49 3 9 13 25

1988 64 85 75 78 83 77 28 34 25 89 76 48 54 120 142 13 32 18 26 18 49 6 10 15 30

1989 53 89 52 93 87 72 26 19 25 61 48 39 52 116 139 15 23 27 18 5 35 7 10 15 19

1990 58 124 73 77 101 100 34 24 32 85 70 64 49 120 154 27 28 28 32 14 35 5 8 15 15

1991 54 88 73 95 77 126 27 31 28 69 64 59 68 100 132 8 39 27 8 8 32 3 15 14 22

1992 55 79 59 78 76 121 31 42 30 81 71 71 46 107 123 10 26 25 20 10 37 8 10 9 6 1993 54 81 57 76 67 115 32 24 31 62 55 51 42 118 123 nd 30 22 16 14 36 4 8 9 6
Recovery times are based on no further impact from dredging at adjacent sites

(e.g. no settling of a drifting tailings plume).

Recovery is defined as attaining a successional community of opportunistic species

providing evidence of progression towards a community equivalent to that previously

present, or at non-impacted reference sites.

Equivalency can be measured in terms of standing crop, biomass, number of

species and numbers of most abundant species, etc. These criteria will need definition

during preparation of an Environmental Impact Statement (or equivalent document) at any

site. The definition will depend on the Best Available Monitoring techniques suitable for

biodiversity assessment at that site.

See Ellis et al. 1995 (Island Copper Mine tailings disposal)

2 Sands - See 3.2 Scenario 1: Precious Metal Mining, Alaska - bucket ladder dredge

and Appendix 4, Salomon et al. 1982. See also Garnett and Ellis (1995)

Gravels - See 3.2 Scenario 1: Precious Metal Mining, Alaska - bucket ladder dredge

and Appendix 3, Kenny and Rees 1993;1994. See also Garnett and Ellis (1995)

171

Table 2.3 Time periods to recovery for benthic biodiversity

(1) Fine-grained deposits'

muds/silts/clays

can contain some rocks and boulders

1 year

(2) Medium-grained deposits2

sands

can contain some silts/clays and gravel

1-3 years

(3) Coarse-grained deposits3

gravels

can contain some finer fraction and

some rocks and boulders

5 years

gravels with many rocks and boulders 5+ years
Table 3.3 Grain sizes from drillhole program, offshore Nome (American Smelting and

Refining Company, 1974)

182

Number of Driliholes 568 500

Grain Size Grain Size

% %

Clay (<0.0039 mm): 20 24

Silt (and mud) (0.0039 - 0.0625 mm): 4 1 Sand (0.0625 - 2.0 mm): 5 24

Gravels (2.0 - 64 mm): 35 37

Till: 28 0

Other (including rock): 9 15
Table 3.4 Silt content of dredged ground showing increase with increasing water depth,

offshore Nome (ENSR Consulting & Engineering, 1991)

183

Water Depth

(m)

Average Silt Content

%

Maximum Silt Content

%

2.3

7 26

6.5

10 22

6.5

15 33

7.5

20 97
Table 3.5 Typical figures of gold grade after application of a suitable cut-off grade,

offshore Nome

184

Ground Type

Total Ground

(%)

Average Gold

Grade

(average =100)

Total Contained

Gold

(%)

Clay 5 144 1 Silt 8 166 13

Sand 28 56 17

Gravel 52 117 65

Till 5 55 3

Broken rock 2 47 1
Table 3.6 Typical operating conditions at the Nome site mined by WestGold

Expected throughput rates when dredging loose mud, sand and gravel to about 15 m

below seabed in about 10 m of water:

Operating time per year:

Throughput rate:

Average area dredged:

Average depth of cut:

Average grade recovered:

Average dredge performance from 1987 through 1990:

2,700 hours

457 m3 per operating hour

37.2 halyear

3.2 m

775 mg gold/rn3

185

- stripping: 1,200 m3 per operating hour

- treating: 800 - 900 m3 per operating hour, and

- average: 1,000 - 1,100 m3 per operating hour.

Average downtime:

- Mechanical: 12%

- Electrical: 3%

- Weather: 8%

- Operational: 7%

- Total 30%
Table 3.7 Dimensions and specifications of the bucket-ladder dredge BIMA

Length of pontoon: 110 m

Breadth of pontoon: 30 m

Depth of pontoon (at side): 6.5 m

Length of ladder: 88 m

Average draft when working with 100% dead weight of 1,800 toimes: 3.73 m.

Maximum displacement: ± 12,000 m3

Bucket capacity: 850 litres at a dredging depth of 26 m

Number of buckets installed: 137

Total installed diesel power: 5,667 kW

Total output of bucket bank drive motors: 1,342 kW

3 fuel-oil storage tanks, each with a 318 m3 capacity

Total lubricating and sludge oil tanks capacity: 22 m3

Outreach of overburden stripping chute: 42 m 1:6.6 slope.

Outreach of twin tailing launders: 22.7 m 1:18 slope

Outreach of screen oversize chute: 11 m 1:4 slope

Discharge height of overburden chute above sea level: 6.5 m

Discharge height of tailings launders above sea level: 4.0 m

186
Table 3.8 Support facilities for the WestGold mining operation, Nome

187

Offshore:

- ocean-going tug for anchor moves and dredge towing

- supplies transport vessel

- personnel carrying helicopter

- part-time drilling vessel

- part-time geophysical survey vessel

Onshore:

- workshop

- office, laboratory and warehouse

- docking facility

- camp accommodation

Personnel levels, including both supervisory and hourly paid, total about 70 during the

dredging season, falling to 30 - 50 during the winter months.
Table 3.9 The distribution of solids and water under ideal conditions, WestGold mining

operation, offshore Nome

188

Feed: 900 m3 per operating hour

Water intake: 6.15 m3/m3 of feed

Total water requirements: 5535 m3 per operating hour

Screen oversize: 90 m3 per operating hour (10%)

Primary and secondary tailings: 808 m3 per operating hour contained in 5,535 m3 of

water
Table 3.10 Dredge products and effluents, WestGold mining operation, offshore Nome

189

1987 1988 1989 1990

Operating hours

per day

15.8 18.2 17.0 16.1

Feed

throughput rate

(m3/hour)

423 590 353 429

Water intake

rate

(m3/m3 feed) 11.3

-- -- -- Total water

intake

(m3/hour)

4780 4780 4780 -- Trommel

oversize

(m3/hour)

105-210

[25-50%]

236

[40%]

88

[25%]

86-107

[20-25%]

Primary jig

tailings

(m3/hour)

85-155 295 220 -- Secondary jig

tailings

(m3lhour)

58-233 58 45 -- Total Jig

tailings

(m3/hour)

213-318 353 265 322-343
Table 3.11 Dredge feed composition, WestGold mining operation, offshore Nome

190

Observed composition of the

dredge feed (%)

Deduced composition

of the dredge feed (%)

Silt -- 30

Silt and clay 14 -- Sand 31 45

Gravel 18 -- Till 37 -- Gravel and Cobbles -- 25
Table 3.12 Environmentally significant operating parameters for the bucket ladder dredge, BIMA, at the WestGold mining

operation, offshore Nome (Cont'd next page)

191

Annual Period of Operations: Average 155-160 days per year, during summer

Specifications of the bucket ladder dredge BIMA:

Length (pontoon): 110 m

Width (pontoon): 30 m

Height: 22.7m above main deck to top tumbler

Draft (average): 3.7 m

Cutting system: buckets (8501 capacity)

Delivery system: buckets, line-mounted on ladder

Scale of Operations:

Throughput:

approx. 500 - 800 m3/hr in water> 10 m depth

approx. <600 m2/hr. in water < lOm depth

approx. average 457 m3/hr

Maximum: approx. 1000 m3/hr

Dredge Course Parameters:

Face width: 100 - 150 m

Cut thickness (average): 3.2 m

Maximum depth: 9.0 m in practice, but designed to be capable of approx. 35 m

Course continuation (length, area, timing) ie. amount of advance before moving dredge: approx. 100 m

Average area dredged/year: 37.2 ha

Course length: 100's of metres

Area: average 37.2 ha/yr

Water feed: 4,780 m3 per operating hour (average)

Tailings discharge rate (= total onboard feed, throughput and water):

average 457 m3/operating hour

1 1.3m3 + water/ m3 feed/operating hour

5,964 m3 water/operating hour

Selectivity: vertically: ± 0.50 m

horizontally: minimum face width of approx. 150 m with positioning of ± 2 - 3 m

Dredging Conditions:

Water Depths: 7 - 20 m

Size Fractions:

silt/clay: 28%

sand: 1%

gravel: 28%

cobbles: 43%

Total Area of of WestGold's Nome lease: 8,802 ha
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Table 3.13 Wildlife species in the Nome area and their feeding habits (Cont'd next page)

SEABIRDS -

Numerous other Seabirds breed here including the rare Aleutian Tern.

WATERFOWL AND SHOREBIRDS -

Large nesting concentrations of waterfowl and shorebirds: Yukon River Delta

Other important areas: South shore of the Seward Peninsula

St. Lawrence Island, Golovin Lagoon

Moses Point lagoon and delta

Species comprising majority of Alaskan and/or North American population:

Eider

Red Knot,

Rock Sandpiper,

Endangered Bird Species: Peregrine Falcon - Feeds and lives

near seabird colonies.

SEABED-FEEDING MAMMALS -

Ringed Seal (Phoca hispida):

primarily a seabed feeder: cod, sculpin, shrimp, mysids and amphipods

- most common and abundant. 1-1.5 million

- breeds in Norton Sound during winter on land-fast ice?

Bearded Seal (Erignathus barbatus):

a bottom feeder: crabs, clams, shrimp and some fish (cod and sculpins)

-commonly found throughout Norton Sound

- live on moving ice overlying shallow water.

Pacific Walrus (Odobenus rosmarus):

a bottom feeder: primarily clams, specifically the Greenland cockle

(Serripes groenlandicus)

- less common but populations on increase

- calving in deltas in summer, reported in Norton Sound.

Other seabed-feeding mammals reported in Norton Sound:

Gray Whales, Steller's Sea Lion and Sea Otter.

Feeding Guild Habitat/Status

Family Alcidae

Least Aukiets Planktivore Critical Breeding Habitat

Crested Auklets

Common Murres Piscivore Breeding Habitat

Thick Billed Murres

Family Laridae(?)

Black Legged Kittiwake Piscivore Breeding Habitat

Emperor goose, Cackling Canada

goose,

Black Brant, Spectacled Eider, Steller's

American Golden Plover, Whimbrel,

Black Turnstone, Western Sandpiper,

Dunlin and Bristle-thighed Curlew

193
Table 3.13 Wildlife species in the Nome area and their feeding habits (Cont'd)

MARINE MAMMALS (not feeding from seabed)

Spotted Seal (Phoca vitulina largha):

- primarily feed on pelagic fish

- common throughout Norton Sound in summer to fall

- breeding?

Beluga Whales (Deiphinapterus leucas):

- feeds on pelagic and benthic organisms particularly Herring in early summer

- common throughout Norton Sound in summer

- calving reported in Norton Sound, estuarine deltas preferred.

Polar Bear (Ursus maritimus):

- feeds on seal and walrus

- enters Norton Sound in fall with the advancing ice following seals

- breeding?

Other marine mammals found in Norton Sound:

Ribbon Seal, Killer Whale, Minke Whale, Bowhead Whale and other large whales.

194
Table 3.14 Composition of lag gravel from two different types of glacial moraine,

offshore Nome

195

Area 1

%

Area 2

%

Silt (and clay) 6-20 8-46

Sand 38-74 30-47

Gravel 20-30 8-24

Cobbles and Boulders 0-7 0-30
Table 3.15 Specifications of the Underwater Miner, "Tramrod", used at WestGold's

dredge site, offshore Nome

Equipment installed on "Tramrod"

- Submersible hydraulic power pack: 37 - 56 kW

- Reverse flush valve

- Hydraulic control valves in one atmosphere chamber

- Hydraulic compensators for pressure and volumetric displacement

- Marinized hydraulic rams

- Optional centrifugal slurry pump

Sealed and lubricated track chains

S.I.T. video cameras

- Scanning sonar

- Directional gyroscope

- Bathymetric suites

Dimensions of "Tramrod' with associated equipment

196

Item Quantity Dimensions Weight

(m) (metric tons)

Tramrod without cutter: 1 12.65 x 4.2 x 4.0 38.2

Tramrod with cutter: 1 10.5 x 4.2 x 4.0 42.5

Control cabin: 1 6.0 x 2.3 x 2.5 4.0

Water pump (+ 1 spare): 2 4.0 x 1.6 x 2.0 4.5

Umbilical spooler: 1 3.0 x 2.5 x 2.0 4.0

Workshop: 1 6.0 x 2.4 x 2.5 4.0

Spares container: 1 6.0 x 2.4 x 1.0 4.0

Motive hose spooler: 1 3.0 x 2.5 x 2.0 4.5

Dimensions of "Tramrod"

Ground length of tracks: 4.0 m

Overall length of tracked undercarriage: 5.8 m

Overall width of tracks: 4.2 m

Diameter of cuffing wheel (if attached): 1.25 m

Maximum possible reach of ladder assembly: 9.7 m

Slewing angles from centre-line of machine: 500

Deepest digging depth below seabed level: 2.0 m

Greatest digging height above seabed level: 2.8 m

Operating depth (maximum to date): 310 m

Design depth: 800 m

Minimum depth to date: 3 m
Table 3.16 Support facilities for WestGold's "Tramrod" mining operation, offshore

Nome

The support vessel requirements:

- Power supply without cutter

- Power supply with cutter

- High pressure water supply to suction intake

- Compressed air supply:

- Crane capacity:

- Deck space:

- Deck loading:

150 kVA 440v 60 Hz 3 phase;

250 kVA 440v 60 Hz 3 phase;

9093 1/minute @ 1034 kPa;

724 kPa 2.83 m3/minute

(minimum requirement);

SWL 50 tonnes @ 6 mlminute

from vessel's side with single

fall preferred;

250 - 300 m2

6-7 tonnes/m2

Offshore support facilities:

- ocean-going tug for anchor moves and barge towing

- vessel for transporting supplies and personnel

- part-time geophysical and sampling vessel(s)

Onshore support facilities:

- workshop, office, laboratory, and warehouse

- availability of docking or harbour facilities

197
Table 3.17 Operating Capabilities of the "Tramrod" Underwater Miner

Maintained throughput rate capabilities of the Underwater Miner:

- Design stage completed and partly tested.

2 - Design stage completed, with a target throughput of 300 m3/operating hour.

198

Material being Dug Without Cutter

(m3/hr)

With Cutter With Special Features

(m3lhr) (m3lhr)

Silt (only) depending on

thickness: 100-500

Soft mud: 80 100 --150 2 Sand and gravel: 80 --150 2 Small gravels: 80 - --150 2 Coarse gravels: 25-50 150 2 Clay (150 kPa shear strength): - 50

Soft rock (e.g. chalk): 50

Maximum, non-digging, traverse speed on level ground:

Maximum gradient negotiable:

Maximum width of digging corridor:

400 mlhour

450

- without cutter: 12.3 m

- with cutter: 8.7 m

Attainable dredging accuracy: 0.10 m

Selective extraction capability:

- variable thickness mud layers: 0.20 m

- thickness of dense fine sand: 0.15 m

Maximum solids content of dredged soil: 60%
Table 3.18 The main effluents from WestGold's "Tramrod" mining operation, offshore

Nome

* - accompanied by a little water ** - water only

The above conditions are true for a solids feed rate of 150 m3/operating hour; an accompanying

water feed rate of approximately 700,300 1/hour, or 11,700 1/minute; for the jigging circuit and

other purposes an extra water feed rate of approximately 341,000 1/hour or 5,700 1/minute

199

Size Range Rate of Solids

Discharge

(m3/hr)

Solids

(%)

Rate of Water

Discharge

(1/hr)

Grizzley oversize: +0.20 m 15 100,000 - Trommel oversize: -0.20 m 34 * Very low

+38 1 mm

Trommel mid-size: -38.1 mm 3

* Very low

+12.7 mm

Hydro-cyclone overflow: - * * Very variable

Primary & secondary

jig tailings

-12 7 mm 98 10 1,000 very

variable

Total solids discharged per hour = 150 m3
Table 3.19 Environmentally significant operating parameters for the "Tramrod" Underwater Miner operation, offshore

Nome

200

Annual Period of Operations: Early June to early November (summer operations); possibly January - March (winter

operations under ice)

Scale of Operations:

Throughput: approx. 100-3 00 m3/operating hour; average 150 m3/operating hour

Cut thickness: < 1.0 m

Face width: 8.7 - 15.0 m

Course length: 100 to several 100 m

Area: average 150 - 300 m2/operating hour

in total summer: 400,000 - 800,000 m2 (40-80 ha.)

Water feed: for average throughput: 700 m3 + 340 m3 = 1040 m3/operating hour

Tailings discharge rate (= total onboard feed, throughput and water): average 150 m3/operating hour +

water/operating hour

1040 m3

Selectivity: vertically (approx. 0.20 m)

horizontally - minimum face width approx. 5.0 m

Mining Conditions:

Water depths: 3 - 20 m (but "Tramrod" technically capable of working in considerably deeper water)

Size fractions: silt/clay: 6-46%

sand: 30-74%

gravel: 8-30%

cobbles: 0-30%
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Table 3.21 Features of a typical ocean-going, trailing suction hopper dredge for the

Massachusetts Bay site

trip capacity (i.e. hopper capacity) of 5,100 m3

digging depth capability of 2 1 m

twin trailing drag-heads

- cut depth of 40 - 60 cm below the seabed

- furrow width of 2.7 m (i.e. a 2.4 m drag-head width)

drag-head separation of 25 m between the centre lines

minimum furrow length of 300 - 600 m

dredging speed of 6.5 km/h

- travelling time of 11 km/h

202
Table 3.22 Range of dredge pumping system performance for a trailing suction hopper

dredge

203

Fine gravel and

sand

Coarse gravel With mud

Number of pumps 2 2 -- Size of pumps (cm) 76.2 81.3 -- Throughput of slurry

pumped

(1/minlpump)

113,562 151,416

-- Total throughput of

slurry (1/mm)

227,124 302,832 -- Volume of solids in

the slurry (%)

25 -26 15 - 16 -- Volume of solids

retained (%)

25 15 -- Volume of solids

discarded (%)

1 l -- Sediments dredged

(%)

3.8 6.2 0 - 33.5

Rate of effluent

discharge (1/mm)

227,124x 0.75

170,343

302,832x 0.85=

257,407

--
Table 3.23 Estimated and possible minimum total cycle time for travel, dredging and

unloading by a suction hopper dredge, Massachusetts Bay dredge site

204

Operation Estimated Possible Minimum

Travel to dredging site

Dredging (loading)

Travel to unloading site

Unloading

Total

2 hr 30 mm

1 hr 15 mm

2 hr 30 mm

7 hr 27 mm

1 hr 42 mm 1 1 hr 15 mm

i hr 42 mm

4 hr 15 mm 2 13 hr 42 mm 8 hr 54 mm

Assuming a travelling speed of 16.5 kmlh (optimistic)

2 Unloading time if using a 20 m3 clamshell
Table 3.24 Environmentally significant operating parameters for marine aggregate mining, Massachusetts Bay

205

Annual Period of Operations: 7 months/year (25 days/month, 24 hour/day)

Scale of operations:

Throughput: approx. 1.8 million m3/year produced

Cut thickness: 0.4 - 0.6 m

Face width: 2.75 m

Course length: minimum of 300 - 600 m

Area: approx. 360 ha excavated (minimum)

Water feed: approx. 225-300 m3/operating minute

Tailing discharge rate (= total onboard feed, throughput and water):

170 m3/operating minute @ 3.8% solids

to 257 m3/operating minute @ 6.2% solids

Selectivity: only by dredge location

Dredging Conditions:

Water depths: 30 - 40 m

Size fractionsof surficial units (from Appendix 5 [Padan, 1977]):

sand units: 0 - 25% gravel: 75 - 100 % sand: 0 - 25% mud

sandy gravel units: 33.5 - 75% gravel: 12.5 - 50% sand: 0 - 33.5% mud

gravelly sand units: 12.5 - 50% gravel: 33.5 - 75% sand: 0 - 33.5% mud

Total Area of Prospect: 18 km2
Table 3.25 Dredge and dredging parameters for marine aggregate mining, Massachusetts Bay

Trailing Suction Hopper Dredge Characteristics:

Length: 106 m

Width: 21 m

Height: 9 m

Draft: 8m

Volume: 4,600 m3

Cutting system: drag heads (twin)

Cut width: 2.75 m

Delivery system: hydraulic

Timing (refit needs, weather): 7 months/year (24 hr./day, 25 days/month)

Dredge Course Parameters:

Face width: 24.4 m between centre lines of drag heads

Cut thickness (usual depth): 0.4 - 0.6 m

Mminimum length of furrow: 300 - 600 m

Effluents:

Type 1- Overflows:

Discharge rate:

Particle size:

Position of discharge point on vessel:

Type 2 - Jig Tailings:

170 - 257 m3/minute

silt and fine sand (3.8 6.2 ?% solids)

port and starboard

N/A

206
Table 3.26 Virginia Beach current measurement sites for 1973 (reported by Ludwick,

1978)

207

Position of

current

meter

Distance Water above Record

Latitude! offshore depth bottom Start length Site Longitude (km) (m) (m) date (days)

1 36° 58' N 4.1 23.8 14.9 upper Jul 22 27 upper

75° 59' W 1.8 lower 29 lower

2 36° 53' N 3.5 8.2 3.7 upper Jul 22 27 upper

75° 57' W 1.8 lower 5 lower

3 36° 50' N 3.6 9.1 4.9 upper Jul 21 29 upper

75° 56' W 1.8 lower 29 lower

4 36° 47' N 3.6 9.8 5.5 upper Jul 21 34 upper

75° 55' W 1.8 lower 34 lower

5 36° 47' N 7.1 15.2 11.0 upper Jul21 29 upper

75° 53' W 1.8 lower 29 upper

6 36° 44' N 3.9 12.5 8.2 upper Jul 21 29 upper

75° 54' W 1.8 lower 10 lower

4 36° 47' N 3.6 9.8 5.5 upper Sep 02 28 upper

75° 55' W 1.8 lower 28 lower

1 36° 58' N 4.1 23.8 16.1 upper Sep 30 5 upper

75° 59' W 31 lower

3 36° 50' N 3.6 9.1 4.3 upper Sep 30 30 upper

75° 56' W 1.2 lower 30 lower

4 36° 47' N 3.6 9.8 5.2 upper Sep 30 30 upper

75° 55' W 1.2 lower 30 lower

5 36° 47' N 7.1 15.2 10.4 upper Sep 30 30 upper

75° 53' W 1.2 lower 30 lower

6 36° 47' N 3.9 12.8 7.6 upper Sep 30 30 upper

75° 53' W 1.2 lower 30 lower
Table 3.27 Maximum and mean current vectors, Virginia Beach current measurement

sites for 1973 (reported by Ludwick, 1978)

208

Site Month

Position of

current

meter above

bottom

Maximum

current speed

Current

direction

at maximum

speed

Vector

mean

current

speed

Vector

mean

current

direction

(m) (cm/s) (toward °T) (cm/s) (toward °T)

Near

Bottom

1 Jul/Aug 52.5 282° 8.8 296°

2 Jul/Aug 1.8 37.9 004° 2.1 156°

3 Jul/Aug 1.8 32.1 164° 0.4 344°

4 Jul/Aug 1.8 30.4 158° 0.7 128°

5 Jul/Aug 1.8 28.4 012° 1.8 272°

6 Jul/Aug 1.8 21.9 340° 1.1 000°

1.8

4 Sep 42.4 167° 0.5 143°

1.8

3 Oct 50.6 169° 0.6 144°

4 Oct 1.2 46.2 172° 2.6 192°

5 Oct 1.2 31.2 107° 1.0 328°

6 Oct 1.2 66.8 165° 0.8 335°

1.2

Near

Surface

1 Jul/Aug 86.5 301° 7.2 270°

2 Jul/Aug 14.9 72.1 330° 2.8 214°

3 Jul/Aug 3.7 58.4 166° 7.9 217°

4 Jul/Aug 4.9 42.3 170° 5.6 184°

5 Jul/Aug 5.5 60.7 155° 9.1 171°

6 Jul/Aug 11.0 66.2 159° 10.6 178°

8.2

4 Sep 66.0 172° 5.0 185°

5.5

1 Oct 75.5 097° 12.2 080°

3 Oct 16.1 64.2 165° 3.4 197°

4 Oct 4.3 61.1 170° 4.3 152°

5 Oct 5.2 52.1 183° 5.6 163°

6 Oct 10.4 79.6 175° 6.5 158°

7.6
Table 3.28 Wave height exceedence probability offshore Virginia Beach between 1985-

1992

209

Exceedance Probability (%) Wave Height (m)

50% 2.00 m

75% 1.63 m

90% 1.18 m

95% 0.84 m
Table 3.29 Deposit thickness, offshore Virginia Beach (from core data reported by

Berquist and Hobbs, 1988)

210

Minimum

core length

(m)

Maximum

core length

(m)

Average

core length

(m)

Minerals Management Service cores

U.S. Geological Survey cores

1.30

1.45

6.40

8.20

3.45

3.50
Table 3.30 Grain size distribution of the uppermost 4 m of sediments, offshore Virginia

Beach

average sand content (>0.063 mm and <2 mm) of 86.56%

average mud content (<0.063 mm) of 11.95%

average gravel (>2 mm) content of 1.46%

Very fine sand (0.063 - 0.125 mm) 0.7%

TOTAL 96.1%

Ranges in grain size recoverable during drilling over short time periods (several

minutes) and long periods (several days)

Short Periods Long Periods

(%) (%)

211

Mud 0 - 41% 8.0 - 12.5%

Sand 54 - 98% 85.5 - 86.0%

Gravel 0 - 19% 0.7 - 2.5%6

Grain size distributions of samples from one typical drill hole

Granules (>2 mm) 3.9%

Very coarse sand (1 - 2 mm) 6.6%

Coarse sand (0.5 - 1 mm) 20.6%

Medium sand (0.25 - 0.5 mm) 57.8%

Fine sand (0.125 - 0.25 mm) 6.5%
Table 3.31 Environmentally significant operating parameters for dredging heavy mineral sands, Virginia

212

Scale of Operations:

Throughput: approx. 6,900 - 9,000 m3/operating hour

Cut thickness:

Face width:

Course length:

average 4 m

120 m

60 m

Dredging Conditions:

Water depth: 12 - 15 m

Size fractions:

silt/clay:

sand:

gravel:

11.95%

86.56%

1.46%
Table 3.32 Parameters for a cutter suction dredge, heavy mineral sands, Virginia

213

Cutter Suction Dredge Characteristics:

Cutting system: cutter

Delivery system: hydraulic (69 - 76 cm diameter)

sel type: cutter suction dredge with separate floating wet mill

Dredge Parameters: Wet Mill Parameters:

Specifications:

Hull length: 180 m, hinged Hull length: 180 m

Hull width: 24 m Hull width: 120 m

Hull depth: 6 m Hull depth: 9 m

Draft: 3 m Draft: 6 m

Parameters of Cutting/Delivery System:

Operating water depth: 12 - 15 m

Cut width: 120 m

Dredge Course Parameters:

Face width: 120m

Cut thickness:

Usual depth: 1.3 - 6.4 m (average 4 m)

Maximum depth: 55 m

Course continuation (length, area, timing) i.e. amount of advance before moving dredge: 600 m

Effluents:

Type 1 - Trommel oversize:

Discharge rate: 100 tonnes/operating hour (about 5% of feed)

Particle size: > 12.7mm

Position of discharge point on wet mill: port or starboard

Type 2 - Spirals tailings:

Discharge rate: 1800 tonnes/operating hour (about 90% of feed): Note, some may be

discharged separately by cycloning
Table 3.33 Extreme storm surge and wave height estimates for Ocean City

214

Return

period

Storm surge height

(m)

Extreme wave height

(m)

5 years 1.31 4.33

10 years 1.43 4.79

20 years 1.59 5.37

50 years 1.71 5.73

100 years 1.83 6.19

250 years 1.98 nla
215

Vertical Interval Grain Size

0-0.3 m 0.32- 07mm

0.3 - 0.6 m 0.35 - 0 7 mm

0.6 - 0.9 m 0.35 - 0.74 nmi

0.9 - 1.2 m 0.35 - 0.78 mm

Table 3.34 Average particle size for each vertical 30 cm section interval below the seabed

between drill holes, offshore Virginia Beach (drill log data from U.S. Army

Corps of Engineers, 1994)
Table 3.35 Typical cycle time expected for a suction hopper dredge operating offshore

Ocean City

These figures assume a working distance of 4.2 km from the discharge

station and the dredge operating 24 hours/day.

216

hours/day

Loading 1.00

Turning and docking 0.75

Discharging 2.50

Weather 0.50 - 0.75

Total cycle time

(excluding repairs) 4.75 - 5.00
Table 3.36 Dredge and dredging parameters for beach nourishment, Maryland

217

Suction Cutter Dredge

General Characteristics:

Water depth:

Cutting system:

Delivery system:

Length:

Width:

Height:

Draft:

Trailing Suction Hopper Dredge

General Characteristics:

> 15 m > 15 m

Cutter Drag heads

Pipeline to shore & hydraulic Hydraulic

90 m 100 m

15m 20m

5 m 9 m

3m 8m

Parameters of Cutting!

Delivery System:

Operating depth:

Cut width:

15 m

90 m

15 m

24 m

Dredge Course Parameters:

Average cut thickness:

Maximum cut thickness:

Course length:

1.22 m

1.52 m

600 m 600 m

Throughput:

Average: 1,500m3/hr 2,000 m3/hr

Total area of source sand: 2.5 million m2
Table 4.1 Variables that affect sedimentation and sediment transport

218

Factors outside the operator's control: naturally varying oceanographic

parameters such as:

water depth

ambient horizontal current velocity

water colunm stratification

wave- or current-generated water colunm turbulence.

Factors partly within the operator's control:

rate and velocity of effluent discharge

particle size distribution of solids in the effluent.

Factors wholly within the operator's control:

design, orientation and depth/height, relative to sea level, of the effluent discharge.

Note:

Several of the above are only partly independent. With the exception of

oceanographic features, they can be influenced to some degree by the selection and

design of the dredge and related on-board technologies in use.
Table 4.2 Jurisdictional guidelines for optimizing mitigation

The minimum possible number of agencies should be involved and the jurisdictional

responsibilities of each should be clearly defined

One lead agency clearly appointed and through which communication with the

mining company is maintained

Recognize that all forms of placer mining, but especially marine systems, are

extremely cost-sensitive. Such operations cannot withstand imposed expenditures on

generic environmental research programs geographically well outside the zone of

influence of the dredging operation

If individuals with the necessary industrial experience to interface with the mining

companies are not available within the agencies then appropriate industry consultants

should be retained as is the case in the United Kingdom (E.G. Hydraulics Research

Ltd.)

219
Table 5.1 Legislation and agencies that had jurisdiction over WestGold's mining

operation, offshore Nome

220

National Environmental Policy Act (NEPA) of 1969 which required the production

of an Environmental Assessment (EA) and an Environmental Impact Statement

(EIS)

Rivers and Harbors Act of 3 March 1899: A U.S. Corps of Engineers (USCOE)

Department of the Army Permit required pursuant to Section 10

Clean Water Act of 1977:

A U.S. Corps of Engineers Department of the Army authorization to

discharge material into the water required pursuant to Section 404

A National Pollutant Discharge Elimination System (NPDES) permit required

to be issued by the U.S. Environmental Protection Agency (USEPA) for

ocean discharges in compliance with the Ocean Discharge Criteria pursuant

to Section 403

State of Alaska Water Quality Standards: A Certificate of Reasonable Assurance,

issued by the Alaska Department of Environmental Conservation (ADEC), together

with a Water Quality Certification, required under the Clean Water Act and in

compliance with the requirements of (5) immediately below

Standards of the Alaska Coastal Management Act of 1977 through the Federal

Coastal Zone Management Act of 1972: a state project consistency review and

approval required through the Alaska Coastal Management Program
Table 5.2 Jig tailings discharge designs tested from the BIMA between 1986 and 1990

in water depths of 5 to 21 m, offshore Nome

221

Direct surface discharge from the launder 6 m above the water surface (Figure 5.2)

Steel square-section, pipe, 0.60 x 0.60 m, extending 1.0 m below the water surface

1.8 rn-diameter steel pipe, 2.5 m below the water surface, around the circumference

of which were attached six, overlapping, 6 rn-long, flexible, heavy rubber panels

held together at the bottom by a weighted ring collar (Figure 5.4)

1.8 rn-diameter steel pipe, 2.5 m below the water surface, to the end of which was

attached a flexible, weighted, rubber pipe of the same diameter extending

approximately to 3.5 m below the water surface (Figure 5.5)

1.8 rn-diameter steel pipe, beneath and over which was suspended a 2.1 m telescopic

sleeve, made of rubber or a lightweight membrane, with its base at a variable depth

below the water surface (Figure 5.5)

Flexible pipe of 0.60 m diameter either held at the surface or 0.50 m below the

water surface

Steel pipe, reducing from 1.8 m to 0.50 m in diameter, either held at 1.5 m or 4.5 m

below the water surface (Figure 5.6)

Steel pipe, reducing from 1.8 m to 0.50 m in diameter, fitted with a steel deflector

plate (to break the jet action), either held at 1.5 m or 7.0 m below the water surface

(Figure 5.3)
Table 5.3 Total operating costs (in percent) during a representative 24 month period

between 1987 and 1990 for the WestGold dredging operation, offshore Nome

Dredge Maintenance 26

Dredge Operations 19

Admininstration & Overheads 13

Support Vessels 4

Winter Berthing of Dredge 4

Environmental Monitoring 4

sub-total 70

Depreciation,Depletion

& Amortization 13

sub-total 13

Drilling 11

Other 1 Sales Deductions 5

sub-total 17

TOTAL 100

222
Table 5.4 Estimated annual costs of the heavy mineral mining scenario, offshore

Virginia Beach

Estimated additional costs of implementing mitigation techniques.

223

%

Capital lease 61

Personnel 26

Fuel 5

Insurance 4

Maintenance 4

TOTAL

Additional Costs

%

Use of flocculants 10.5

Scheduled downtime (per 10 days) 3.3

Desliming/Dewatering 0.4

Sub-surface discharge pipe

(including modifications) 0.2

Degassing equipment 0.2
Table 5.5 Typical details of a proposed dredging site taken into account by licensing

authorities

Depth of water and distance offshore

Existing bathymetric variations and the arrangement of any sediment banks

Degree of exposure, and the direction of prevailing winds

Direction and velocity of tides

Frequency, direction and severity of storms

Reflection and refraction of waves

Characteristics and degree of mobility of the seabed sediments

224
/

CONCENTRATION BIOGEOCHEMICAL ZONE

4,

9

4

2

SO

4

NO3

F

+

Mn2

02 Aerobic

Oxidation

Dentrification

Manganese and Iron

Oxyhydroxide Reduction

Sulphate

Reduction

Methanogenesis

Schematic distribution of biogeochemically important molecules and ions in

interstitial waters in sediments showing the zonation typically observed in

marine and lacustrine sediments. This distribution will stretch or compress

in response to changing physical, chemical and biological conditions.

Figure 2.1 Schematic of geochemical zonation in sediments (after Pederson and Pelletier,

1989)

225

+

CH4
Figure 2.2 Significant cluster groups of benthos in 1991, Island Copper Mine, Canada (after

Burd and Ellis, 1995). Similar groups can be identified for an area using statistical

techniques (Figure 2.3) and mapped. Similar deep and shallow benthic faunas are

shown here by similarly shaded areas. Submarine tailings are discharged into deep

water from the mine in Rupert Inlet (Figures 2.4 to 2.6)

226
Figure 2.3 Cluster diagram for 1991 data showing similarity for stations shown in Figure 2.2.

Stations B, 9, 13 and 15 represent a group of similar benthos and are not shown in

Figure 2.2 227

Cluster dendrograiu and significances for linkages: 1991

********************************************************9101

**

** **************

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* ** * *

* * * * * ** * * *** * * * *********

* *

* *

* ******************************************************9121

** *** * * *

* * * * * * ****

* * * *

* * *

* * *

* * ***********

* * * *

* * * * * * ************************9106

* * * * * * * * ************

* * * * * ****** * * * * * * *************9117

* * *********

* * ************9119

* *

* *******************************************************9124

*

*

* * ************* **********************9113

******************

*

I I I I 0 .2 .4 .6 .8 1

SIMILARITY Scale Factor = 0.1E+Ol

/
(.n..LN) 'p!q,L

LL) .- L 0

0

\

I-C. LI aD

11:111:: (N 0

0

=

aD0

QD

0) a) N.

0)0

0

[1 NLI iN

CoN

Figure 2.4 Comparative mean annual water column turbidity (at 30 m water depth) near

Island Copper Mine, 1971 to 1992. The mine discharges tailings into Rupert Inlet

where turbidity increased to approximately 1 NTU (Nephelometric Turbidity Unit)

from pre-discharge levels of 0.5 NTU in 1971 (after Island Copper Mine, 1994).

Note that sampling stations differ for turbidity, chlorophyll (Figure 2.5),

zooplankton (Figure 2.6) and benthos (Figures 2.2 and 2.3)

228

(N
Figure 2.5 Comparative levels of Chlorophyll a near Island Copper Mine in 1992. The levels do not show tailings-related differences, unlike turbidity (Figure 2.4) (afterIsland Copper Mine, 1994)

229
N

(w/#) suu

C

C

C

V

C')

230

Figure 2.6 Comparative density of adult zooplankton near Island Copper Mine, 1987 to 1992.

The densities show no consistent differences between the fiords. Station A is

closest to the mine in Rupert Inlet
Scalar Mean and Vector Mean Wind Speed

Site 20: Represents Massachusetts

.2O

0C

25

5

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Scalar Mean Vector Mean

0C

Scalar Mean and Vector Mean Wind Speed

Site H: Represents Virginia/Maryland

25

2O

'15

a)

ÜY1O

C

4

a

a p-.

a

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Scalar Mean .- Vector Mean

Figure 3.1 (a) and (b) Scalar mean and vector mean wind speed

231

vv

.
Figure 3.2 (a) and (b) Frequency of high winds (>28 knots [52 kin/h]: >34 knots [64 km/h])

232

30

25

Frequency of High Winds

Site 20: Represents Massachusetts

2O

Jan Feb Mar ---- Apr May

Speed >

Jun

28 kts

Jul ---- Aug

Speed>

Sep Oct

34 kts

Nov Dec

!

Frequency of High Winds

Site H: Represents Virginia/Maryland

30

25

20-

a /

/

S

. ___

Jan Feb Mar --

Apr May

Speed >

Jun

28 kts

Jul Aug

-- Speed>

Sep Oct

34

Nov

kts

Dec
Figure 3.3 (a) and (b) Frequency of waves above thresholds

233

Frequency of Waves Above Thresholds

Site 20: Represents Massachusetts

40

:.:,i1.II

Jan Feb Mar Apr May Jun

>4m--'-->6m--'-->8m

Jul Aug Sep Oct Nov Dec

Frequency of Waves Above Thresholds

Site H: Represents Virginia/Maryland

Jan Feb

---->3m

Mar Apr May Jun Jul Aug

>4m--'->6m--->8m

Sep Oct Nov Dec
Air Temperature

Site 20: Represents Massachusetts

1?::LjiIi'IIIj1' 5 IIi*iiiiii'i'

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

90% Upper --- Mean -- 10% Lower

Air Temperature

Site H: Represents VirginiaJMa,ylafl

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

- 90% Upper -'-- Mean -- 10% Lower

Figure 3.4 (a) and (b) Airtemperature

234
Figure 3.5 (a) and (b) Sea surface temperature

235

Sea Surface Temperature

Site 20: Represents Massachusetts

30

.25

I :141 : ::

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

' 90% Upper Mean ---- 10% Lower

Sea Surface Temperature

Site H: Represents Virginia/Maryland

30

15 /

0) /

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

90% Upper Mean --- 10% Lower
Figure 3.6 (a) and (b) Frequency of precipitation and visibility <2 nautical miles (3.7 km)

236

Precipitation and Reduced Visibility

Site 20: Represents Massachusetts

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Precipitation Visibility < 2 n ml

Precipitation and Reduced Visibility

Site H: Represents Virginia/Maryland

25

20

5iu_ I.

5 lull

= - - = = - = =....

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

- Precipitation Visibility < 2 n ml
/

Figure 3.7 Slopewater circulation of the U.S. east coast (from SAIC, 1987)

237

5O

45

4QO

,MM$Joe,ea

7O 6O

PA

VA..

CANADA

ME

ME

/

SLOPE SEA GYRE

. a Newfoundland

CLSW

5O

SHELF EDGE _s.

4a

I /

1

I

70 60 50°
Figure 3.8 The BIMA bucket ladder dredge offshore Nome, Alaska (photograph courtesy of

WestGold staff)

238
WATER SETTLED MINE TAILINGS
Face width 100 -150m (may be 50 to 200 m wide) Figure 3.10 Plan view of dredging procedure for a bucket ladder dredge (after Bray, 1979). 240
Figure 3.11 Extreme bucket spillage conditions onboard the BIMA (photograph courtesy of R.

Garnett)

241
64°30' 64° 0 '- 90 AREA OF ENLARGEMENT IN FIGURE 3 13 Qe Nome 1 7/ It 1 66°30' 30 60\SIeciga Island 166° 0 "S \ 1 65°30'I 0 km 165° 20 5' ou 0 164° 30'
eD

a

Ca

CD

C

aC

C0 CD

Ca

N

CC

CuN

aC

CaN

Iv

6rn,SOBA

V

XII

INSERT A

BLOCK 8

165000. 167000. 169000.

R61956

AREA OF ENLARGEMENT

IN FIGURE 3.14

C3

D BLOCK

XI

171000.

NOME PLACER PROJECT

OREDGE COURSE LOCRTIONS

VI

MLOCK R

SEE INSERT A"- :.::''.' VII ATHER/

173000.

Ii,.

3 KILOMETER MORAIj

x

175000.

D-8

KILOMETER

MORAINE

177000.

YEARS

NE

A

fl:I

.

179000.
Figure 3.14 Seafloor substrate types at WestGold's 1986 dredge site, offshore Nome, Alaska

(after Jewett etal. 1991)

244
Figure 3.15 Threedjmensjonaj representation of bathymetry of the area showing condition dredged in 1986 in (a) October 1987; (b) October 1987; (c) September (d) July 1990. 1989; The vertical axis scale (in metres) is 10 times the exaggerate relief. The horizontal to two horizontal axes (in metres) coordinates and Correspond to the axes in Figure 3.4. (after Jewett etal.1991)

245
N_IS.

U2000

x Control Sites

x Sampling Location

I 100-200ueters

r

i Dredge

I Pattern r influen

,1

Prevailing Current Direction

Gold Room Etliuent

0100

01000

"0500 x0800

Edge of State

Defined Mixing

Zone 1985 - 1990 Edge of State

Defined M xing

Zone 199

General sampling pattern for 1990 NPOES water

quality monitoring program in relation to the BIMA

mining vessel dredging pattern.

N.T.S.

U2000

and U500

'C Control Sites

Drge Foorint

x- Sampling Location

100 - 200 Meters

Dredge

Pattern "-r-- Influent 2 BIMA

Prevailing Current Direction

.1 B.0 Bl.,00 BL00

50 0100

'C 'C X Effluent

X *200500

x 0500

1

-200500

01000

Edge ot State

Defined Mixing

Zone 1985 - 1990

Edge of State

Defined Mixing

Zone 1991

General sampling pattern for NPDES water quality

monitoring program in relation to the BIMA mining

vessel dredging pattern. Turbidity plume will disperse in direction of prevailing current. U2000 and

01000 were added in 1989; BL5O, 100, and 500 and

U500 were sampled in 1988.

Figure 3.16 Turbidity monitoring sampling pattern at WestGold's dredge area, offshore Nome,

Alaska (after Jewett et al. 1991)

246

Dredge Footprint/' Tailings Launder Effluent 'C
Figure 3.17 (a) and (b) Side view of the underwater mining vehicle "Tramrod"

(photographs courtesy of Alluvial Mining, U.K.)

247
eD UTILITY LIGHT POLE GRIZZLY AND WATER SPRAYROCK TAILI NGS WINCHES LIGHT POLE TROMMEL AND SCREEN TAILINGS PUMP SUCTION INLET SCREENS TAILINGS PLUME CURTAIN 0 10 20 30 metres LABORATORY AND OFFICES GOLD ROOM PLACER JIGSGENERATORS LIGHT POLE STRUCTURAL CANOPY TOOL AND SUPPLY STORES SEA LEVEL SEABED
Figure 3.19 Support vessel for the underwater mining vehicle "Tramrod", offshore Nome,

Alaska (photograph courtesy of R. Garnett)

249
42° 22

42°21

4 2°20N

SOS rONd

H4RBOR

AREA OF

ENLARGEMENT

26

Aggregate Deposit

Test Site

/ 35

32

I

Figure 3.20 Location of the study area and proposed NOMES test mine site (shaded) offshore

Boston Harbour. Isobaths in metres (from Padan, 1977)

250

I 0° 48 10041' 1O°46'W

0 1 2 3

kilometres
Figure 3.21 Example ofa typical trailing suction hopper dredge (provided by Jan de Nut n.y., Holland)

251
Figure 3.22 Trailing suction hopper dredge discharging fines (photograph from Ports and

Dredging magazine)

252
Figure 3.23 Location of current measurement sites, offshore Virginia Beach (1973) used by Ludwick (1978)

253

30 \ -ç. '-. -:

CHARLES_.

CAPE -,

. ' CHESAPEAKE

BAY BRIDGE /

3700N /

30

CAPE

NRY

1

..-

,.

4

'.2

OCEAN

60- 2

LIGHT

T9WER

36 ON

.,

CHESAPEAKE

BAY

ENTRANCE

STATION LOCATION

76OOW

5

/

4 75

/ 50W

-

d

. 0

-

5 KM
100

10

0.01

0.001

% Exceedance of Hs

Figure 3.24 Exceedence of significant wave height immediately offshore Virginia Beach

254

0 1 2 3 4 5 6 7

Significant Wave Height, Hs (m)
Figure 3.25 Example of a typical spud-powered cutter suction dredge

255
.4' 4, , PS / PROHIBITED

AREA

334.400

44 (seenoteA)

PS 4 0

4. 0'

I

4' :l;S). e4' 4'C.)O k'.

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tr

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(dredged material)

(see noteS) N Depths from

surveys dated

'1980 - 1989

43

N

kegs

/954

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334.380 (sO,' note

29 \ 29

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SN 47 Figure 3.26 Bathymetry off Virginia Beach (depths in feet)

256
7

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\ 1eedyJ

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Figure 3.27 Bathymetry off Ocean City (depths in feet)

257

44

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N 41 47 3?

45 4Zo

46

47

48

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ôrS

LYS

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67 /\/ /72

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3

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Figure 3.28 Beach nourishment project in progress; note the toe dyke to the left (photograph

taken from World Dredging, Mining and Construction magazine, April, 1992)

258
_',ait

rb4v41iL

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t for offshoreOceafl City

29 I1Sb1' char

259
Figure 3.30 Aerial view of the BIIMA working offshore Nome, Alaska. Note the two turbidity

plumes (photograph courtesy of R. Garnett)

260
261

Figure 4.1 Buckets on the BIMA's bucket ladder overflowingwith fine sediment (photograph

courtesy of R.Garnett)
GAS COLLECTOR

HYDRAULiC

CYLINDER

RL/SBER PLATE

TUR8IOISENSOR

* SHUTTER

UNDER WA TEA

TV CAMERA

SWING D;RECTION

a. FRONT VIEW

UNDER WA TEA TV CAMERA

b. SIDE VIEW

MONITOR PLATE

COVER

SHUTTER

SUCT/ON CONTROL PLATE

GATHER HEAD

GATHER HEAD

Figure 4.2 Front and side view schematic of the Japanese "refresher" Raymond, 1984) dredge system (after

262
Figure 4.3 Marine aggregate unloading facility (photograph taken from World Dredging, Mining and Construction magazine, April, 1992)

263
Tailings Berm
bottom doors

Special silt compartments

265

siltoverflow

valve

sea level sand level

siltspace hopper space

4-

0

x --- valve

E

Figure 4.5 Transverse section through a trailing suction hopper dredge showing conceptual

system for waste solids retention (after Van Drimmelen and Van Zutphen, 1987)
4a)

U) c0

d

I j

N

w

I

266

Figure 5.1 Diagram to illustrate the physics of the tailings plume formation process (after

Jewett etal. 1991)
Figure 5.2 Above-surface discharge of tailings from the BIMA, offshore Nome, Alaska

(photograph courtesy of R. Garnett)

267
J- 7- .;. ::' 4.6 500mm 0 PIPE 4.6m (15ET) VERTiCAL PIPE BIMA - A" s':.. A: O.6m (2p VER11CAJ PIPE 41JL TAIUNG LAUNDER" V B: t5m (SET) VERT1p, PIPE l.Sm 0 t5m (5FT) VERTgj PIPE W/DEpLEc- 500mm 0 PIPE DEFLECTOW;. PLATE 81M4
Figure 5.4 Twin tailings launders of the BIMA with surrounding rubber panels (photograph

courtesy of R. Garnett)

269
scole /4: It_OI

1

0

Cross section of

& discharge ptpe seee

CO

e

0.

8IM

Thns chute

Tailings discharge system ixofile sca'a: 3/32": .Q"

6ft. dia. discharge pipe polypropylere rope

top collar

7ft dia. flexible sleeve

-sea surface

support collar

grommeted strip

weighted bottom collar

polyp*opy1ere

rope

support

collar

Gft dia.

dischorqe pipe

r,'

.rommte

.- grommeted strip rl 7ft dia.

flexible s'eeve ''

mat cl

Longitudinal section of sLeeve

scole f/4:

Figure5.5 Details of1998tailingsdischarge configurationfor theBIMA 270
Figure 5.6 The BIMA, with tailings discharge system (twin, reduced steel pipes hanging

vertically on right), raised from the water by dry-tow barge (photograph courtesy

of R. Garnett) 271
Figure 5.7 Aerial view of the BIMA showing turbulent upwelling experienced with the larger

diameter tailings discharge pipes (photograph courtesy of R. Garnett)

272
NL#

--

/

-I,

-

Figure 5.8 Potential multiport discharge system configuration considered for the BIMA (after

Jewett etal. 1991)

273

\I

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PLAN

12la. RUBBER HOSE WITH SYNTHETIC

FIBER REIFORC1HG

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DISCHARGE MANIFOLD EXTENSION

MANIFOLD EXTENSION SAME AS OTHER SIDE

-.---- I, <r I -.

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t

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CNERBUROEN CHUTE MOORING ROPES WATER LINE HIGH DENSITY POLYETHELENE PIPE (DIAMETER TO BE DETERMINED) TALING LAUNDER SOLID C*JRTMN OR STREAMERS SOLID CuRTAiN (TO BE DETERMINED) CHAiN BALLAST IN EftCH CURTAIN ELEVATIONHELCOPTER PAD PRIMARY JIGS
1w

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0

Figure 5.11 Example of a nomogram for predicting allowable production rates under different

operating conditions (after Jewett etal. 1991)

276

liii! III If I I I I liii. 0 0 0 0

0 0 0 4N

CO (0 4 N4NI.
II

overflow

wei r

telescope and

cylindrical

overflow system

anti-turbidity valve

Figure 5.12 Schematic diagram of the IHC anti-turbidity overflow system (after Van

Drimmelen and Van Zutphen, 1987)

277
/NCL INED

BAFLE

A T

OVERFLOW WEIR

,., I i 4

0 VERFLOW414,,j

TROUGH 'S I'

/ a -

OVERFLOW CHUTE

HOPPER

HYDRAULIC

CYLINDER

7

r

FLOW / REGULA TING

ATE

/

Figure 5.13 Schematic diagram of a hopper dredge bin equipped with the Japanese-designed

Anti-Turbidity Overflow System (ATOS) (after Raymond, 1984)

278

a - a -

W. L.
V iv c, wy,' 4ff

RAKE ANGLE a

TOOTH

CUTTERHEAD

Figure 5.14 Schematic diagram showing front view of a cutter-head showing the cutter tooth

rake angle (after Raymond, 1984)

279
Appendix 1

Excerpts from. Articles on the NOAA/NMFS Benthic Database.
Gulf of Maine NEWS

V41 J4)o/

Benthic Data Base Update Available in 1994

Roger B. The rous Northeast Fisheries Science Center

The Woods Hole Laboratory of the NOAA/NMFS

Northeast Fisheries Science Center has, through the years,

accumulated an extensive data base of qualitative and quantilative (wet weight and number per square meter) data on the

composition, distribution and abundance (including a variety

of environmental measurements) of the wacrobenthic invenebratc fauna of the U.S. cast coast continental shelt, slope and

upper rise ranging from the mouth of the Bay of Fundy to Key

West, Florida.

This data base is cunently being updated with results of

numerous taxonoinic studies, and assembled into a (cnn which

is expected to be made available to the scientific community

within a year. At this time the intent is to make the data

available on a variety of media (e.g. networks, CD-ROM,

diskette, etc.) and to reside in a variety of localities (i.e.

NEFSC master database, NOAA database, NODC, USGS,

USNMJSI, etc.).

The data base consists of nearly 200,000 individual

records (nan somewhat over 11,700 sampling sites. The

'mnples were obtained by a wide variety of means including

hand collecting in the intertidal zone to the use of research

vessels (17 different ones) on 208 separate cruises using 49

different types of sampling gear (ranging from dipnets to

trawis, dredges, grab type devices etc.) at sea out to 4,000

meters depth. The split between qualitative and quantitative

samples is 9,580 vs. 2,210. Slightly over 8,000 benthic

sampling stations are represented from the Gulf of Maine

region alone, including the Bay of Fundy, Georges Bank and

as far west and south as Nantucket Shoals; more than 3,000 of

these samples are from the principal basins of the GOM. The

time (tame involved extends from 1881 to 1975; however, the

majority of records are from 1953 to 1975. Supporting data

for each sampling Site or station include date, Iocaiión, depth,

sediment type, gear type, vessel, bottom temperature, laxonotate code and name, weight, number, etc.

The vast majority of the quantitative grab samples in this

biological data base have an exact match (on a station basis)

with a detailed geological data base (including physical and

chemical characteristics) maintained by the U.S. Geological

Survey which was generated by the U.S. Continental Margin

Program during the 1960's and early 1970's. Information

relating to this and other databases maintained by USGS may

be obtained from Marilyn ten Brink at the USGS in Woods

Hole.

The entire data base is backed up with a comprehensive

Specimen Reference Collection which, since June of 1993, ii

housed at the U.S. National Museum of Natmsral History at the

Smithsonian Institution in Washington, DC. The phylogenetic

representation in this collection (and in the data base) ranges

from Protozoa (Foraminifera) to Choradata (Urocliordata);

including sonic lesser protosomes: (Pogonophoi-a,

Sipunculida, Echiurida, Piiapulida); and (laetognatha. Also

included are collections of animal and plant remains and a

variety of manne fossil materials, chiefly mollusk shells.

Systematic diversity includes over 11,000 taxonotnic names

maintained in a data bas" laxonomic code file associated with

the waster data base and the Specimen Reference Collection.

Information relating to accessibility and use of the Specimen

Reference Collection at the Smithsonian may be obtained

from the Invertebrate Collection Manager Dr. Michael

Sweeney at IJSNM.

Interest in and studies relating to the benthic invertebrate

fauna of the marine environment off the coasts of the United

States formally began in a systematically organized manner

with the establishment, by Spencer F. Baird, of the U.S. Fish

Commission in 1871. Woods Hole, Mass thusrfts was

chosen as the site of the first permanent laboratory in the U.S.

solely devoted to the scientific study of all aspects of the

marine environment. Collections and data gathered on the

benthic fauna from those early days, for the most part, were

distributed to many other institutions and museums through

the intervening years. However, some material (specimens

and data) were preserved at the Woods Hole Laboratory and

have been incorporated into the data base and Specimen

Reference Collection under discncsion herein. The vast

majority of the data and specimens, however, are the result of

activities conducted at the Woods Hole Laboratoiy, beginning

in the mid 1950's, as pan of studies relating to the feeding

habits of commercially important demersal fishes. The

perceived lack of ecologically oriented data on the composition, distribution, abundance, and ecological and environmental relationships of the benthic fauna making up the diet of

these fishes led to the establishment of studies designed to

provide the necessary information on those topics. When the

USGS Continental Margin Program was initiated hi the early

1960's in cooperation with the Woods Hole Oceanographic

Institution, the Woods Hole Fisheries Lab's Beñthic Dynamics

Investigation, then conducting similar studies, was invited to

join the effort to provide biological expertise. The data bases

created as a result of these joint efforts remain to this day

without peer in the scientific world.

The benthic data base and the invertebrate collection

continue to be used by ecologists, systematists, and taxonomists, to great advantage in preparing environmental impact

statements, characterizing the benthic fauna of selected sites

for a host of specific purposes (e.g. mining, dumping, drilling,

marine sanctuaries, etc.). Through the yeats many (well over

40 at last count) specialists in a variety of disciplines have

made use of these data and specimens for a multitude of

purposes. In addition to numerous unpublished reports in the

gray literature, these boldings have resulted, to date, in more

than 100 formal, peer reviewed, scientific papers and reports

on a variety of subjects, as well as yielding eleven new species

to science.

5
IAM!:S

NEW

6,o

\

4

4:

4

Figure 1.Chart of U.S. east coast showing sampling locations for bivalve

collection.

68

BOTTOM SEDIMENTS

GRAVEL SAND

SHELL SLI-CLAY

Figure 2.Distribution of predominant bottom sediments.
NEW 

YORK 

. - 

S S 

S S S . 

- S S 

iS.' 

S 

I- S . S . 

.S _..S' i ). ,, S :S 

S. S 

S 5/ S S ./. 5 - ,S S S 

: S S S 

I. S S 

S S --- I . 

.1 S S S 

S. * 

I. IS . 

. , S S 

-5 5) 

S . 

. 

S S .1':. S 

\.-. S . 

5 . .1 

S S S 

S S S S 

S 5 S 

IS 5_.... 

I \ S 

Sf..... 5 S S 

/ S -: - S . . 

5 S 

'j. tS % S S S S S 

5 5 

5 S - S .. 

:100KM 

loca- 1.Station Figure 

Atlantic Middle the in tion 

quantitative where Bight 

collect- were samples grab 

and Wigley (After ed. 

1979.) Theroux 
BOST

o :

I

0

('J

Gravel

Sand- Gravel

She/I

Sand- Shell

Sand

Silty Sand

Silt

Clay

Figure 3.Geographic distribution of bottom sediment types in the Middle

Atlantic Bight. (After Wig.

ley and Theroux 1979.)
discharge

pressure gauges

overflow

overflow

pipe

regulator

water

transfer box

L

extension

tank

overflow

tank

feed *

L]

overflow box

feed box

vortex finder

conical

section

diaphragm

housing

di aph ram

operated

lever

swivel chute

fixed chute

Figure 5.15 Schematic diagram of the Linatex separator (after Littler, 1990)

280
Appendix 2

Excerpts from: Hurme, A.K. and E.J. Pullen . 1988.

Biological effects of sand mining and fill placement for beach

replenishment: Lessons for other uses. Marine Mining 7: 123-

136.
Biological Effects of Offshore Borrowing

Offshore borrowing for beach replenishment is an attractive alternative when

suitable material from channel dredging is unavailable. Borrowing from nearshore

bays and estuaries is often unacceptable because valuable bottom habitat may be

destroyed or altered, and sediments are usually unsuitable from a coastal engineering perspective (U.S. Army Corps of Engineers 1981). Upland sources of

sand are becoming scarce as demand for onshore construction sites, trucking

costs, and hauling distances increase. Numerous offshore sites have been identified in many regions as containing suitable replenishment materials (Duane and

Stubblefield 1986; Williams 1986).

The literature on the effects of offshore borrowing is sparse; study techniques

and findings vary widely. A literature review by Thompson (1973) found few

studies addressing directly the potential impact of offshore mining. Extrapolating

from estuarine research, a more thoroughly studied coastal ecosystem, Thompson

predicted that the effect of offshore dredging depended on the magnitude of the

disturbance created and the site selected. Shifting sands, rather than stable beds,

would provide the ecological advantages of sparse, transitory plant and animal

communities of lesser commercial value. As a practical matter, shifting beds provide a source of course, pure sand without mixtures of mud, silt, or detritus.
130 Arthur K. Hurme and Edward J. Pu/len

Borrow sites in active areas would be quickly refilled, erasing the physical effects

of dredging. He concluded that even extensive dredging operations on shifting

beds would have little long-term environmental effect in contrast to dredging of

more productive stable bottoms. Shallow dredging over an extensive area rather

than deep pit dredging, he predicted, would also cause less environmental effect.

He encouraged further studies on fill-in rates and biota repopulation under a wide

range of conditions and bottom types.

The New England Offshore Mining Environmental Study (NOMES) (Padan

1977) was designed to address the impact of commercial-scale mining. Baseline

conditions for benthos and phytoplankton were measured for a full year in Massachusetts Bay. High biological variability in the natural environment confirmed

the difficulty in adequately sampling this environment. Unfortunately, the study

was terminated prior to the actual test mining. However, the NOMES Final Report

provides a good projection of the potential direct and indirect environmental effects of sand and gravel mining.

The NOMES study identified a number of potential consequences of offshore

hydraulic mining of sand and gravel. For example, the excavation process, while

broadening the market base and lessening the impacts associated with onshore

mining, might have negative impacts by causing stagnant water to form in the

pits, causing beach slump or coastal erosion if located too close to the shore, or

impacting the water table if the freshwater aquifer were penetrated. Further, the

fishery could be either harmed or improved depending on the nature of the bottom

after mining.

The potential impacts from the discharge plume and the resultant blanket of

fines were generally felt to be deleterious unless needed nutrients andlor new

substratum for colonization by desirable species resulted. Potential negative impacts include the introduction of pollutants and unwanted nutrients; the interference with filtering, feeding and respiratory mechanisms in ocean species; the

smothering of benthic species and attendant loss of food or habitat; reduced photosynthesis and oxygen levels; or an unpleasant appearance to the water. Also

changes in bathymetry and bottom type could alter populations, migration patterns

or result in unwanted deposition of materials in navigation channels or in coastal

areas in general.

Padan (1977) discusses each of these concerns more fully and specifically

applies them to the NOMES case when possible. However, the actual likelihood

of the effects occurring at all, or in sufficient magnitude to be of concern, is not

addressed because of insufficient information caused by the termination of the

study.

Brinkhuis (1980) assesses the potential biological effects of sand and gravel

mining in the Lower Bay of New York Harbor, based on the literature. He concludes, "The probable effects of sand mining operations on biota per se appear

to be minimal." However, he notes that, although there is little quantitative data

on the seasonal abundance and distribution of species, much of the Lower Bay

may be characterized as impoverished and perturbated by pollution; therefore,

impacts attributed to marine mining might be difficult to detect.

De Groot (1986) presents an overview of the information published by the

International Council for the Exploration of the Sea (ICES) and other work, by

country, in the North Atlantic, North Sea, and Baltic Sea. Most marine mining

activity for sand and gravel production occurs in the southern part of the North
Biological EffectsMarine Sand Mining 131

Sea. He concludes that where". . . the nature and structure of the substrate do

not differ substantially before and after dredging, the bottom communities will

recover from the effects." However, where conditions and bottom substrate are

changed, the potential negative effects on fisheries, although never clearly, demonstrated, might occur. He discusses the proposed ICES (1975) code of practice

for marine aggregate dredging as protecting fisheries interests. The code addresses

the physical and geological aspects of regulation rather than the biological considerations themselves. It was proposed by the Netherlands and has been adopted

in part by France and the United Kingdom. Canada is currently considering adopting portions of the code as well to regulate marine mining. De Groot (1981) updated

a bibliography of pertinent literature, including many European publications not

commonly identified in the United States.

Studies of nearshore borrow areas in the United States show that the persistence of the borrow area as an identifiable bathymetric feature may vary greatly.

Repopulation of the borrow area's benthic community varies depending on the

size and configuration of the borrow area, its exposure to waves and currents,

the similarity of the sediments surrounding the area, the new sediment-water

interface, and possible changes in water quality. Given the wide range of possible

biological responses to offshore borrowing, the selection and benthic survey of

the borrow sites may be more important than the survey of the beach to be replenished. Although the physical environment was altered and benthic communities destroyed, recovery was almost immediate for some motile species, and

almost complete within one year for dominant groups including annelids, mollusks, and crustaceans (Saloman et al. 1982). However, recovery of coral in Hawaii was long-term (5 years or more) or did not occur in the length of the study

(Maragos 1974; 1979). Increased suspended sediments, changed bathymetry, and

destruction of the benthic population were most evident during and immediately

following dredging. Increases in suspended sediments were usually localized and

of short duration and are in direct relationship to the grain size of the material

dredged (Courtenay et al. 1974; Maragos et al. 1977).

Inshore borrow pits in areas of low wave energy isolated from currents and

located in sheltered bays and estuaries have created problems in the past. If in

an area of fine bottom sediments with high levels of organic materials, the pits

are apt to fill slowly and develop anaerobic conditions (Saloman 1974).

Fortunately, many of the borrow pits studied in conjunction with beach nourishment were in the nearshore zone, exposed to wave action and nearshore currents, and surrounded by similar sediments. Benthic recolonization is fairly rapid

under such circumstances (Applied Biology, Inc. 1979; Saloman et al. 1982; Culter

and Mahadevan 1982), even if the pits do not refill rapidly (Turbeville and Marsh

1982). Fish and other motile animals are able to avoid unfavorable conditions and

usually return after dredging ceases.

Sand and gravel have been mined in the Lower Bay of New York Harbor

since the early 1950s, in addition to regular channel maintenance dredging. However, no controlled before and after studies have been conducted. The pits are

relatively deep, some more than 15 meters below adjacent bottom, and generally

in areas with restricted circulation and low wave energy. They have accumulated

fine organic sediments from adjacent areas. Studies by Cerrato and Scheier (1984)

showed differences in the number of species, species composition, species diversity and abundance between the pits and the surrounding bottom.
132 Arthur K. Hurme and Edward I. Pullen

Current Concerns About Impacts of Offshore Sand and Gravel Mining on

Marine Benthos

The National Oceanic and Atmospheric Administration (NOAA) prepares a "Federal Plan for Ocean Pollution Research, Development and Monitoring" every

three years. The most recent plan for fiscal years 1985-1989 concludes, "The

effects of sand, gravel, and shell mining in coastal areas (excluding Alaska) are

largely known from previous studies conducted or supported by the U.S. Army

Corps of Engineers and others. . . . There are currently no environmental studies

being supported by the Federal Government which are directly related to the

effects of marine sand, gravel, and shell mining." (U.S. Department of Commerce

1985).

Although no active research programs addressing marine sand and gravel mining activities are presently being conducted in the United States, analysis of the

15 years of research on beach replenishment by the U.S. Army Corps of Engineers

and other studies permit the projection of certain key elements for forecasting

the biological effects of marine sand and gravel mining. Several statements can

be made which should lessen concerns of the potential impact on the marine

environment. First, the technology for marine mining of sand and gravel is not

new, and will likely be similar to that of marine mining for beach nourishment.

Excavation of clean, offshore deposits will not release toxic pollutants through

resuspension of the sediments (Cruickshank and Hess 1975). Second, the preferred mining sites will have surficial deposits where the amount of fine material

is small, and the removal of large amounts of overburden will not be required.

Duane and Stubblefield (1986) note that, with certain exceptions, surficial sand

and gravel deposits on the U.S. Atlantic continental shelf wifi contain less than

five percent clay and silt. Third, appropriate navigation and positioning systems

will assist in avoiding known valuable biological resources. Fourth, appropriate

marine mining equipment will assure that the desired final bathymetric form of

the borrow area will be created and that large areas of dissimilar surface sediments

will not be exposed unless considered desirable. Fifth, mining will occur in areas

where surficial sediments are being reworked by waves and currents. And finally,

the [governmental] permit process for leasing of offshore mining sites may provide

some degree of review and monitoring of impacts.

The value of shoals and undersea ridges as desirable habitat for many fish

species is well known. Although much work has been done on the effects of

building artificial reefs to create new fish habitat, little is known about the potential

effects of modifying the general offshore bathymetry on fisheries. Since offshore

mining could be conducted to either eliminate or modify ridges, or it could create

ridges and troughs, the final effect could vary. Therefore, any mining which would

substantially change the form of the existing bathymetry should be undertaken

with caution.

Guidelines for Planning Ecologically Sound Offshore Mining Projects

Applying the knowledge learned from beach nourishment and replenishment studies as they relate to nearshore borrowing, a number of guidelines can be proposed

to assure ecologically sound offshore mining of sand and gravel (Naqvi & Pullen

1982). These suggestions are: 1. Conduct a pre-project baseline survey in the
Biological EffectsMarine Sand Mining 133

source areas to avoid important benthic resources such as clam beds or active

spawning areas, and to determine the need for further sampling. 2. Time the

borrowing period to avoid the peak spawning seasons of marine species using the

site. 3. Leave a sufficient layer of sediments that match as closely as possible the

original surface layer to avoid exposing a dissimilar sediment unless exposing a

new substratum is desired. 4. Use equipment and techniques which will minimize

high suspended solids and siltation when sensitive species such as hard corals are

nearby. Dredge only when currents do not carry suspended sediments to nearby

sensitive resources. 5. Take borrow materials from broad, shallow pits in deeper

waters with an actively shifting bottom.

Conclusion

The U.S. Army Corps of Engineers and others have monitored and evaluated the

disruption and recovery of the physical and biological environment during beach

replenishment operations. While primary focus has been on beach areas where

the material is deposited, borrow areas from which the material has been taken

have also been examined. Currently, replenishment material is obtained from new

channel construction, dredging operations, nearshore and inshore borrowing.

Based on what is known about the biological impacts of beach nourishment

and nearshore borrow areas, nearshore and offshore mining operations should

not create long-term adverse impacts on organisms and physical characteristics

in the area if appropriate site selection, timing, techniques, and monitoring are

carefully performed. Operations should be planned to avoid important bethnic

resources and peak spawning seasons. Material should be taken from broad, shallow pits in areas with an actively shifting bottom, leaving a sufficient surficial

layer of similar sediments for recolonization by benthic species. State-of-the-art

technology and equipment for navigation and dredging can reduce the risk of

indiscriminate impacts on marine resources. Monitoring may be required as part

of the (governmental) permit process to assure wise use of valuable marine

resources.

Acknowledgments

Permission from the Corps of Engineers to publish this paper is appreciated.

However, the views and conclusions are the responsibility of the authors and do

not represent official Corps position unless accompanied by other authorizing

documents.

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Appendix 3

Excerpts from: Kenny, A.J. and H.L. Rees. 1994. The

effects of marine gravel extraction on the macrobenthos: early

post-dredging recolonization. Marine Pollution Bulletin, 28 (7):

442-447.

and

Kenny, A.J. and H.L. Rees. 1993. Preliminary results on

the effects of marine gravel extraction on benthos: post-dredging

recolonization. ICES Report, Annex VIII.
Summary of: Kenny, A.J. and H.L. Rees. 1994. The effects of marine gravel extraction on the

macrobenthos: early post-dredging recolonization. Marine Pollution Bulletin 28 (7): 442-447. (See

also Kenny & Rees, 1993).

Location: North Sea. English East Coast (see Figure 1).

Equipment: 0.2 m2 Hamon grab, plus underwater still and television cameras.

Sieve: 5 + 1 mm Sediment volume 10-20 litres.

Biomass: Wet and dry calculation (Elefthuriou and Basford, 1989 accuracy ±0.0001.

Collection Dates: Before dredging March 1992; After dredging May, December

1992

Deposit description: Gravel

General Site Characteristics and Changes:

Deposit No. of No. of Biomass

Median organisms! species Dry WtJm2

(mm) m2

Biological Characteristics:

Taxa noted:

Balanus crenatus (CB)1

Dendrodoa grossularia (Tu)

Flustra foliacea (Br)

Modiolus modiolus (MB)

Polychaete worms

Sabellaria spinulosa (PS)

Amphipods

Quantitative details - Table 1.

1CB = Crustacea, Barnacle

Tu = Tunicate

Br = Bryozoa

MB = Mollusc, Bivalve

PS = Polychaete, Sedentary

Before 0.355 5,000- 30-40 20-50 g

10,000

After 2.0 100-5,000 13-26 2-7 g

Fig. 3 Figs 4 and 5.

- epifauna 44% total organisms

- epifauna 45% total organisms

- epifauna

- epifauna heaviest.

- epi or infauna

- epifauna

- epi or infauna
MIB (Mean Individual Biomass) in gm = No.

organisms/biomass

B. crenatus D. grossularia

Before 4.1-5.6 0.7-1.7

After 1.4 0.1

Statistical analyses -see Figure 6.
Pergamon

The Effects of Marine Gravel

Extraction on the Macrobenthos: Early

Post-dredging Recolonization

A. J. KENNY and H. L. REES

MAFF Directorate of Fisheries Research, Fisheries Laboratory, Burn ham-on-Cro uch, Essex CMO 81-IA, UK

A small area of sea bed off the English east coast was

experimentally dredged by a commercial suction-trailer

dredger. Some 50000 t of mixed aggregate were

removed, representing about 70% of the sea bed area

down to an average depth of 0.3 m. Results from

benthic surveys undertaken at the experimental site

and at a nearby reference site, Indicate that significant

reductions had occurred in the variety, abundance and

biomass of benthic organisms as a consequence of

dredging. Subsequent recolonization of dunuded

substrates by the dominant taxa proceeded relatively

rapidly, although the dredged site had clearly not fully

recovered some 7 months later. Differences in the

recruitment success of the dominant taxa, notably

Dendrodoa grossularia and Balanus crenalus, between

the reference and treatment sites pre- and postdredging were observed. }ssible explanations for these

differences in relation to the observed physical

alterations to the sea bed are discussed.

During the 1980s the demand for aggregates in the

United Kingdom (from both land-based and marine

deposits) steadily increased. The presence of large

quantities of 'high quality' aggregates in coastal areas

adjacent to the major markets in the south and east of

England provided an opportunity to meet the increased

demand by increasing the production of sand and

gravel from marine resources. Consequently the

dredging industry invested in new ships which allowed

existing licensed deposits to be exploited more

efficiently and new deeper water resources to be

dredged.

Accompanying these developments has been

growing concern over the effects of marine aggregate

extraction on the marine environment, in particular, the

damage caused to benthic communities and their

dependent commercial fisheries. This concern was

recognized by the International Council for the

Exploration of the Sea (ICES) which, in 1975,

established a scientific working group on the 'Effects on

Fisheries of Marine Sand and Gravel Extraction'. This

0025-326X(93)E0003-Q Pruned in Great Bnam

0025-326X/94 S7.00+O.00

C 1994 Crown Copyright

provided a forum for member states, including the UK,

to present their research findings and to recommend

appropriate management actions (ICES, 1975, 1992).

In the United Kingdom, during the 1970s, research

into the environmental effects of aggregate dredging

was undertaken by the Ministry of Agriculture,

Fisheries and Food (MAFF). For example, Shelton &

Rolfe (1972) and Dickson & Lee (1972) examined the

impacts of suction-anchor dredging on a shingle bank in

the English Channel, while Millner & Dickson (1977)

examined the impacts of suction-trailer dredging off

Southwold in the southern North Sea. More recently,

Lees et al. (1990) studied the impacts of suction-trailer

dredging at a licensed extraction area off the Isle of

Wight in the English Channel. However, due to

difficulties of sampling coarse sediment, the above

studies were not able to accurately quantify the impacts

on the benthos or describe the processes of recolonization on cessation of dredging.

This paper presents some of the results from a

continuing study jointly sponsored by MAFF and the

Crown Estate Commissioners (CEC). The research

aims to combine an assessment of the initial impacts of

marine aggregate extraction on the benthic macrofauna

and the subsequent processes of recolonization

following dredging.

Methods

Selection of an offshore experimental site

A wide-scale survey of gravel communities off the

English east and south coasts (Kenny et aL, 1991)

indicated potential sites for an offshore field experiment. The chosen location was off North Norfolk,

England (Fig. la), where the gravel deposits were of

commercial quality and which supported a relatively

'rich' and stable epifauna. Among the most common

species present, were the barnacle Ba/anus crenatus, the

sea-squirt Dendrodoa grosularia, the 'horn wrack'

Flustra foliacea and the 'horse mussel' Modiolus

modiolus. A 'reference' site (about 1 mile distant) was

also sampled to provide information on the natural

physical and biological changes during the 'recovery' of

the experimental 'treatment' site (Fig. ib).
Volume .b umDC1 / iuis 1994

FIg. 1 (a) Location of the experimental area off North Norfolk. (b)

Location of the experimental 'treatment and 'reference' sites

showing the sample positions for each survey.

Experimental dredging

During 5 days in April 1992, the MV Sand Harrier, a

modern commercial suction-trailer dredger, extracted a

total of 52 000 t of mixed aggregate from an area

measuring 500 X 270 m. In order to ensure that an

impact could subsequently be detected using conventional remote sampling techniques, it was necessary

to systematically dredge the entire site. The dredging

operation was closely monitored using a Sercel 'NR53'

differential Global Positioning System (GPS) which was

previously calibrated against a 'range-range' system

which showed that the GPS was accurate to ± 10 m.

Field sampling procedures

Ship-board sampling of the benthos was achieved

using a Hamon grab (see Holme & McIntyre, 1984)

which proved to be effective in the quantitative

sampling of coarse sediments. The Hamon grab

operates by taking a scoop out of the sea bed with a

relatively constant surface area of 0.2 m2. Pictures of

the sea bed post-dredging were obtained using an

underwater camera sledge fitted with a single-lens reflex

camera and an underwater television camera which

provided images covering an area of about 0.75 m2.

Grab stations were located at random within the

'treatment' and 'reference' sites (Fig. ib). On retrieval,

an estimate of the sample volume was made, which

typically ranged between 10 and 20 1. Following the

removal of a 11 sub-sample for particle size analysis,

the samples were washed with seawater over 5 mm and

1 mm square mesh sieves to remove the excess

sediment. The retained macrofauna were fixed in 4-6%

formaldehyde solution (diluted with seawater) for

laboratory identification and enumeration.

Laboratory procedures

Samples were first washed with freshwater over a 1

mm square mesh sieve in a fume cupboard to remove

the excess formaldehyde solution. Samples were then

sorted and the specimens placed in jars or petri-dishes

containing a preservative mixture of 70% methanol,

10% glycerol and 20% tap-water. Specimens were

identified to species level, as far as possible, using the

standard taxonomic keys. For each positive identification a representative specimen was retained in order to

establish a reference collection.

Partial wet weights for each species were determined

by placing specimens on blotting paper for 12 h

before measuring their weights to an accuracy of

± 0.0001 g. Biomass estimates in g ash free dry wt m2

were then calculated from partial wet weights using

conversion factors given in Eleftheriou & Basford

(1989).

Sediment sub-samples were analysed for their

particle size distributions according to the UddenWentworth Phi classification, where Phi--log2d and d

is the particle diameter in millimetres. Each sample was

first wet sieved on a 63 m mesh sieve to provide an

estimate of the fine fraction (<63 sm). The remaining

sample was then oven-dried for approximately 12 h at

100°C, and allowed to cool to room temperature before

being sieved through a stack of geological test-sieves

ranging from -6 Phi (64 mm) to +4 Phi (64 sm). A

weight for each size fraction was measured using a toppan balance to an accuracy of ± 0.01 g.

Non-metric Multi-dimensional Scaling ordination

(M1)S; Kruskal & Wish, 1978) was performed on a

(dis)similarity matrix derived from log(x+1) transformed species abundance data using the Bray-Curtis

dissimilarity index [B- (I I Xj;Xu I )(I(X11+X))1

(Bray & Curtis, 1957). The MDS plot provides a twodimensional summary of inter-sample affinities, i.e. the

distance measured between any two samples, or sample

groups, provides a measure of biological (dis)siniilarity.

Results

Phical obserwuions

On examination of the sea bed using side-scan sonar

and underwater cameras it was apparent that the draghead had created well-defined furrows measuring 1-2

m wide and 0.3-0.5 m deep. It was estimated that the

sea bed had been lowered by nearly 2 m in areas where

the dredge tracks had crossed each other several times.

The dredge tracks invariably had a layer of sand ripples

along their base which, upon inspection by SCUBA

divers, was found to be 1-2 cm thick (Fig. 2a,b).

Despite these superficial accumulations of sand,

particle size analysis of samples taken at the treatment

site (Fig. 3a) indicated that the gravel content of the

sediment (>2 mm; <-1 Phi) had actually increased

from 30% in March (pre-dredging) to 50% in May

(post-dredging), although this had fallen slightly to 43%

in August (post-dredging). By contrast, the proportion

of gravel (>2 mm) at the reference site remained

relatively constant at 36% (Fig. 3b). A possible

explanation for these differences is given below.

Biological observations

The average number of species recorded from five
It

Hamon grab samples taken at the treatment and

reference sites pre- and post-dredging is shown in Fig.

4. The number of species remained almost constant at

the reference site (about 35) during the 8-month

sampling period, whereas at the treatment site the

numbers fell from 38 to 13 immediately following

A

Fig. 2a Photograph of the sea bed taken at the 'reference' site during the post-dredging survey in May.

:

Fig. 2b Photograph of the sea bed taken at the 'treatment' site during the same survey showing the sand

ripples associated with a dredge track.

dredging. A limited increase in the number of species

during the 7-month post-dredging period from 13 to 26

species suggests that some recolonization had occurred.

The most abundant animals, both before and after

dredging, were Balanus crenatus and Dendrodoa

grossularia; these accounted for, respectively, 44 and
1 90

70

40

.9 in In in in in '.i in - in 0 in .- In i' in 4'., ('Cd'.9 d c.i Particle ze (Phi units)

ic

go

70

#60

40

d2°

10C

Reference Site

in in .* in i', in c in .- in 0 in - in c'i in c', in

Icl'.# ()C 0 4 ('

Particle Size (Phi units)

FIg. 3 (a) Cumulative curves of particle sizes at the 'treatment' site preand post-dredging. (b) Cumulative curves of particle sizes at the

reference' site pie- and post-dredging. Each curve represents

the average of five samples.

FIg. 4 Average number of species from five Hainon grab samples

taken at the 'treatment' and 'reference' sites pre- and postdredging with 95% confidence 1imits

45% of the total number of individuals found during the

sampling period. There was a marked reduction in the

abundance of D. grossularia and B. crnatus

immediately post-dredging, but substantial recolonization was evident in later samples (Table 1B). This was

also the case for amphipod crustaceans and the 'Ross'

worm Sabellaria spinulosa. However, it can be seen that

the 'starting' densities of these two taxa were relatively

low (Table 1). Densities of all taxa at the treatment and

reference sites are shown in Fig. 5a. An average of 2769

1 U Ttealmeid

ic

10

Abundance

Bloantis

0.1 Fig. 5 (a) Derived mean 'treatment' and 'reference' site abundances

m2 from five Hamon grab samples pre- and post-dredging with

95% confidence limits. (b) Derived mean 'treatment' and

'reference' site biomass in g ash free dry WI m from five

Hamon grab samples pie- and post-dredging with 95%

confidence limits.

TABLE I

Average 'reference' (A) and 'treatment' (B) site abundance and biomass values (g ash free dry wt) for D. grossularia, B. crenatus, amphipods and

S. spinulosa from five Haxnon grab samples (0.2 m2 per sample) taken pie-and post-dredging with 95% confidence limits (CL).

Amphipods £ spinulosa

CL Abun. CL Bio. CL Abuts. CL Bio. CL

2.8 114 55 <0.1 <0.1 32 16 <0.1 <0.1

2.3 170 43 <0.1 <0.1 57 32 0.1 <0.1

9.2 245 165 <0.1 <0.1 14 10 0.1 0.1

15.9 333 126 <0.1 <0.1 104 54 0.2 0.1

3.6 134 60 <0.1 <0.1 45 26 <0.1 <0.1

0.2 24 8 <0.1 <0.1 2 2 <0.1 <0.1

0.3 458 482 <0.1 <0.1 3 5 <0.1 <0.1

2.6 151 131 0.1 0.1 123 80 <0.1 <0.1

D. grossularia B. crenatus

Abuts. CL Bio. CL Abun. CL Rio.

(A) Reference site

March 1010 493 0.7 0.4 1743 549 7.2

May 1096 245 0.8 0.2 1683 690 7.3

August 1703 1180 0.3 0.2 2468 1518 11.1

December 7740 2401 2.3 0.8 7020 2591 33.2

(B) Treatment site

March 757 337 1.3 0.5 1508 631 8.5

May 48 46 <0.1 <0.1 41 59 0.1

August 304 100 <0.1 <0.1 122 120 0.2

December 3065 1648 0.3 0.4 1067 1759 1.5

Mardi May Augu Dar

Mardi May Augu Dn

ic

10
individuals m2 were present at the treatment site in

March1 whereas in May (post-dredging) this had fallen

to 129 individuals m2. At the reference site the mean

densities n March and May remained similar at 3299

and 3347 individuals m2, respectively.

Biornass data for each site pre- and post-dredging

support the abundance data (Fig. 5b). The presence of

Modiolus modiolus in two of the samples taken in

March (pre-dredging) from the treatment site accounted

for the high mean biomass at this time. However,

immediately post-dredging the biomass was reduced

from 23 to 1 g (ash free dry wt) m2, whereas at the

reference site the biomass remained relatively constant

at 13 g (ash free dry wt) m2. The difference between

the two sites pre- and post-dredging is again highlighted

by comparing the biomass data for the numerically

dominant B. crenatus and D. grossularia in Table 1.

It is apparent from the output of non-metric MDS

ordination (Fig. 6) that the reference site samples have

been grouped with the treatment site samples taken in

March (pre-dredging), indicating that both sites were

biologically similar before dredging. In addition, there

was very little variation between the samples at the

reference site throughout the 8-month sampling period.

However, at the treatment site, the samples taken

immediately after dredging were clearly separated to

the left of the main cluster and an increase in the

derived variance (S2) of the data between sampling

times was apparent, especially for May and August.

446

-

August (post-dredging)

S2 = 0.23

\ (

March (pre-dredging)

S2 = 0.03

Fig. 6 MDS ordination on log(x + 1) transformed species abundance

data from both 'treatment' and 'reference' site samples pre- and

post-dredging. Estimates of the variation of log-transformed

counts between sampling times are super-imposed.

This would support the contention of Warwick &

Clarke (1993) that increased variance may be

interpreted as a symptom of 'stress'. Figure 6 also

indicates that there is a shift in the fauna with time

towards the pre-dredging state, that is towards

'recovery although this is clearly not yet complete.

Discussion

The apparent increase in the proportion of gravel

(>2 mm) in the sediment at the treatment site following

dredging was unexpected. However, examination of a

vibrocore sample taken at the treatment site before

dredging indicated that a greater proportion of gravel

was present in a layer between 0.05 and 0.7 m deep.

The action of suction-trailer dredging would therefore

have resulted in exposure of this layer. Nevertheless, a

proportion of fine sediment would have been redistributed on the sea bed by tidal currents and wave

action following disturbance by the drag-head and an

amount of sediment would also have returned to sea via

the spillways on the dredger itself. These two sources of

suspended sediment may account for the presence of

the thin superficial layer of sand within dredge tracks

which was observed by SCUBA divers and underwater

cameras.

Samples taken at the treatment site in March (predredging) had large numbers of species which were

generally abundant, whereas in May (post-dredging) the

May (post-dredging)

S2 = 0.16 Reference site samples

Treatment site samples

December (post-dredging)

s2 = o.o9

'

--! o oI---. /-o.' ' :::-o QY0 0
olume 2t Number 7 July 199.4

number of species was greatly reduced as were their

counts. However, comparing May anples with those

taken in December, it was apparent that certain taxa

present immediately post-dredging had increased in

abundance. Notable among these were B. crenatus, D.

grossularia, amphipod crustaceans and S. spinulosa

(Fable 1). It is known that S. spinulosa may be predated

upon by the pink shrimp Panda/u.s (Warren, 1973),

although the densities of this worm observed in the

present study were too low to provide a significant food

source.

A comparison of the ratios of abundance and

biomass (i.e. mean weights per individual; see Pearson

& Rosenberg, 1978) for the dominant taxa provides a

useful insight into the early processes of recolonization.

The Mean Individual Biomass (MIB) of B. crenatics at

the reference site in March and December was 4.1 and

4.7 (g ash free dry wt X 1000), respectively. However, at

the treatment site the MIB of B. crenatus had fallen

from 5.6 in March to 1.4 (g ash free dry wtX 1000) in

December. This indicates that a high proportion of the

individuals present at the treatment site, 7 months after

dredging, were new recruits. For D. grossularia the MIB

at the reference site was 0.7 in March and 0.3 (g ash

free dry wt)( 1000) in December, whereas at the

treatment site it was 1.7 in March and 0.1 (g ash free dry

wt X 1000) in December. The reduction in the MIB for

D. grossularia at both sites, between March and

December, indicates that the reference site, like the

treatment site, had a high proportion of juveniles. This

is perhaps not surprising since D. grossularia is known

to live for 1 or 2 years only (Svane & Young, 1989).

Between March and December, populations of the

'opportunists' D. grossularia and B. crenatus at the

reference site showed an eight-fold and four-fold

increase in abundance, respectively (Fable 1). However,

during the same period at the treatment site (Fable 1),

D. grossularia showed a four-fold increase while the

density of B. crenatus remained about the same.

The above differences in the mean individual weights

and densities of B. crenatus and D. grossularia between

the treatment and reference sites may be explained by a

combination of the following:

At the treatment site, deposits of superficial sand

present within the dredge tracks reduced the area

available for recruitment, since these taxa require stable

coarse substrates on which to settle.

There may be natural variability in the densities of

larvae available for recruitment between locations.

However, this is considered very unlikely given the

close proximity of the sites.

The loss of adult sessile populations at the treatment

site may have reduced the potential for juvenile

recruitment. For example, B. crenatu.s may require the

presence of adult populations in order to stimulate

settlement, as has been observed for B. balanoides (see

Rodriguez et aL, 1993). In the case of D. grossularia,

larvae are not transported in the plankton but settle

within a few metres of their parents, often on the tunics

of other ascidians (Svane & Young, 1989). Consequently, since many adults were removed by dredging,

the source of juveniles which recruit locally (within 100

m) was likely to have been significantly reduced.

To date, sampling has covered the immediate postdredging period and, while it is evident that substantial

recolonization has taken place, the community structure

at the dredged area has not yet returned to its predredging state. Surveys at these sites are continuing, and

the results will be reported at a later date.

Bray, i. R.. & Curtis, J. T. (1957). An ordination of the upland forest

contmunities of southern Wisconsin. Leo!. Monogr. 27, 325-349.

Dickson, R. & Lee, A. (1972). Study of effects of marine gravel

extraction on the topography of the sea bed. ICES, C.M. 197 21E:25.

18 pp.

Eleftheriou, A. & Basford, D. J. (1989). The macrofauna of the

offshore northern North Sea. I. Mar. Biol. Ass. UK 69, 123-143.

Holme, N. A. & McIntyre A. D. (eds) (1984). Methods ft,r Study of

Marine Benshos 2nd edn Blackwell, Oxford.

ICES (1975). Report of the Working Group on Effects on Fisheries of

Marine Sand and Gravd Extraction. Co-operative Research Report,

46, 57 pp.

ICES (1992). Report of the Working Group on the Effects of

Extraction of Marine Sediments on Fisheries. Co-operative

Research Report, 182,78 pp.

Kenny, A. J., Rees, H. L & Lees, R. 0. (1991). An Inter-Regional

Comparison of Gravel Assemblages off the English East and South

Coasts: Prelirninsty Resuits. ICES C.M.1991/E:27. 15 pp. (mimeo).

Kruskal, J. B. & Wish, M. (1978). Multidimensional Scaling. Sage,

Beverley Hills, California.

Lees, R. G., Rees, H. L., Lambert, M. A., Rowlatt, S. M. & Limpenny,

D. S. (1990). Benthic Studies in Relation to Dredging Activity off the

Isle of Wight, Southern England. ICES CM1990/E:15. 19 pp.

(mimeo).

Millner, R. S. & Dickson, R. (1977). Physical and Biological Studies of

a Dredging Ground off the East Coast of England. ICES C.M.1977/

E:48. 11 pp. (mimeo).

Rodriguez, S. R, Ojeda, F. P. & Inestrosa, N. C. (1993). Settlement of

benthic marine invertebrates. Mar. EcoL &og. Ser. 97, 193-207.

Pearson, T. H. & Rosenberg. R. (1978). Macrobenthic succession in

relation to organic ennchment and pollution of the marine

environment. Oceanogr. Mar. Biol. Ann. Rev. 16,229-311.

Shelton, R. G. J. & Rolfe, M. S. (1972). The Biological Implications of

Aggregate Extraction: Recent Studies in the English Channel. ICES

C.M.1972/E:26. 12 pp. (mimeo).

Svane, I. B. & Young, C. M. (1989). The ecology and behaviour of

ascidian larvae. Oceanogr. Mar. Biol. Ann. Rev. 27,45-90.

Warren, P. J. (1973). The Fishery for the Pink Shrimp Pandalus

monwgui in the Wash. Ministry of Agriculture, Fisheries and Food,

Lowestoft, Laboratory Leaflet (New Series), 28,46 pp.

Warwick, R. M. & Oarke, K. R. (1993). Increased variability as a

symptom of stress in marine communities. I. Exp. Mar. BioL EcoL

172, 215-226.
Summary of: Kenny, A.J. and H.L. Rees. 1993. Preliminary results on the effects of

marine gravel extraction on benthos: post-dredging recolonisation. ICES Report, Annex VIII.

This report is expanded in Kenny and Rees (1994). Both reports use the same field data.

Site characteristics and Changes (note differences from Kenny & Rees, 1994):

Abundance/0.2 m2 No. of species Biomass (gIm2)

Fig. 6 Fig 5. Fig. 7.

Treatment Reference Treatment Reference Treatment Reference

Featured Taxa:

Polychaete worms showed greatest diversity originally and greatest reduction in species (Fig.

5) then recovery.

Balanus crenatus (CB)2 and Dendrodoa grossularia (Tu) two most abundant species showed

marked reduction, then recovery (See Figs 5-7).

Modiolus modiolus (MB) heaviest mollusc marked reduction, little recovery.

2CB = Crustacea, Barnacle

Tu = Tunicate

MB = Mollusc, Bivalve

Before 230 -230 70 62 182 -80

(March)

After 30 209 30 64 .4 80

(May)

December 53 68
APRIL 1993

PRELIMINARY RESULTS ON THE EFFECTS OF MARINE GRAVEL EXTRACTION ON

BENTHOS: POST-DREDGING RECOLONISATION

AJ Kenny and H L Rees

MAFF Directorate of Fisheries Research

Fisheries Laboratory

Bumham-on-Crouch

Essex

CMO 8HA

INTRODUCTION

Background

During the 1980's the demand for aggregates in the UK steadily increased, primarily as a result

of the boom in the construction industries which required the basic raw materials for "ballast and

concrete. In addition, there was a need for high quality aggregates which could be supplied from the

marine environment. The recent advances in marine mining technologies, the short supply of landbased sources and favourable market economics have paved the way for increased production of

aggregates from marine resources. However, during the 1970's, concern was growing over the

environmental impact of marine aggregate extraction, and in particular the potential threat to benthic

communities and their dependent fisheries (Lart, 1991). Initial research was undertaken by the

Ministry of Agriculture, Fisheries and Food (MAFF) but the impacts on the benthos and the rates of

recolonisation were not fully quantified. Accordingly, in October 1990 a three-year research

programme was initiated by the Crown Estate Commission (CEC) and MAFF to determine: i. the

initial impacts of dredging on the benthos and sediments; ii. the processes of recolonisation post

dredging; iii. the natural faunistic differences between gravels on a wide-scale; and iv. coarse

sediment quantitative sampling methods.

Previous Studies

There are few original scientific investigations which describe the effects of marine aggregate

extraction on benthos. Some of the early observations in the UK were made during the 1970's by

Shelton and Rolfe (1972) and Dickson and Lee (1973) who examined the impacts of suction-anchor

dredging on a shingle bank in the English Channel. Millner and Dickson (1977) examined the

impacts of suction-trailer dredging off Southwold in the Southern North Sea. More recently,

investigations have been undertaken off the Isle of Wight (Lees et a!, 1990) and off Dieppe (Desprez

et a!, 1992) in the English Channel. In addition, a comprehensive study of the effects of suctiontrailer dredging on the benthic communities and seabed topography has been made at an experimental

ite on the Kiaverbank in the Dutch sector of the central Southern North Sea (Sips and Waardenburg,

989; Von Moorsel and Waardenburg, 1990, 1991).
METHODS

Selection of an Experimental Dredging Site

A wide-scale survey of gravel communities off the English Eastern and Southern coasts

(Kenny et al, 1991) indicated potential sites for an offshore field experiment. These were located off

North Norfolk, England (Figure 1). The gravel deposits off North Norfolk were found to support a

relatively rich and stable epifaunal community, with the presence of long-lived sessile organisms such

as the bryozoan Flustrafoliacea Chorn wrack') and the hydroid Nemertesia antennina. This site was

therefore considered to be well suited for experimental dredging. However, in order to determine the

exact location of the 'treatment' and 'reference' sites, further sampling using a 3m vibrocore was

undertaken to assess the thickness of the gravel deposits and to ensure that dredging would not

expose an underlying stratum which was different in nature from the superficial substrate. The

treatment site for the offshore dredging experiment was finally selected 17 miles North of Cromer,

North Norfolk in September 1991 (Figure 1).

Experimental Dredging

During 5 days in April 1992, the MV "Sand Harrier", an "H" class commercial suction-trailer

dredger, removed a total of 52,000 tonnes of mixed aggregate representing 11 hopper loads from an

area measuring 500 by 270 metres.

The position and speed of the "Sand Harrier' was monitored using a Sea Information Systems

"Microplot v3.1" installed on Compaq PC linked to a "RoxAnn" seabed sediment discriminator.

Together they displayed a constant real-time image of the dredging operations. High navigational

accuracy was achieved using a Sercel "NR53" differential Global Positioning System (GPS) which had

been previously calibrated against a "range-range" differential GPS operated by BritSurvey. This gave

an almost constant accuracy of ±lOm. Figure 2 shows the track output generated by "Microplot" for

the entire operation, which represents a total of 200 tracks covering approximately 70% of the

experimental area.

Pre- and Post- Dredging Surveys

An array of benthic sampling equipment was used to survey the treatment and reference sites

pre- and post-dredging. Remote sampling of benthos was achieved using a Hamon grab (Figure 3).

The Hamon grab was found to be ideally suited for quantitative sampling of coarse (or compacted)

sediments. It operates by taking a scoop out of the sediment, and the sample bucket is then forced

against a metal plate which prevents the sample from being washed away during retrieval.

In order to obtain an instant view of the seabed and provide detailed information on the

occurrence, distribution and behaviour of benthic organisms, an underwater camera sledge was used.

The sledge was towed for -1 hour along a transect through the treatment and reference sites. In
addition, an acoustic map of the dredged site was generated using a EG+G dual frequency (100kHz,

500kHz) side-scan sonar.

Field Procedures

Hamon grab stations were randomly located within the defined boundaries of the ireatment

and reference sites. Samples were washed over 5mm and 1mm square mesh sieves so as to remove

excess sediment and obtain all the colonial and solitary benthos. The benthos was fixed in a 4-6%

buffered formaldehyde solution (diluted with sea water) with 'Rose Bengal" (a vital stain) and stored

for laboratory identification and enumeration. In addition a 1 litre sub-sample was taken for particle

size analysis.

The underwater camera sledge was fitted with a television camera linked via an umbilical to a

TV monitor and U-matic video recorder present on the RV. A single lens Reflex (SLR) camera

loaded with a 200 exposure colour 35mm film pre-set to take one exposure every 20 seconds was also

attached.

Laboratory Procedures

Hamon grab samples were first washed with fresh water over a 1mm mesh sieve in a fume

cupboard to remove excess formaldehyde solution. Samples were then sorted on plastic trays and

specimens were placed into jars or petri dishes containing a preservative mixture of 70% methanol

(GPR) , 10% glycerol and 20% tap-water. For each species a representative specimen was recorded,

preserved and stored separately in a glass vial to establish a reference collection and provide a means

for the verification of species identifications. Whenever possible specimens were identified to species

level using the standard taxonomic keys.

Partial wet-weights for each species were determined by placing specimens on a plastic tray

covered with white blotting paper for 12 hours before measuring their weights on a Sartorius 2004

MP five figure balance. Biomass estimates were then calculated from partial-wet weights using

conversion factors given in Eleftheriou and Basford (1989).

Sediment sub-samples were analysed for their particle size distributions according to the

Udden-Wentworth Phi Classification where Phi(0) = -log2d and d is the particle diameter in

millimetres. Each sample was first wet sieved on a 63 micron mesh sieve to provide an estimate of the

fines fraction (<63 microns). The remaining sample was then oven dried for approximately 12 hours

at 1000 C and allowed to cool to room temperature before being sieved through a stack of geological

test-sieves ranging from -6 phi (64mm) to +4 phi (0.0063mm). A weight for each size fraction was

measured using a Sartorius top-pan balance to an accuracy of ±0.01g.
RESULTS

Physical Observations

Particle size data for 6 samples taken from the treatment site (Cruise COR 4/92) in March

1992, 4 weeks before dredging, were compared to 6 samples taken 2 weeks after dredging (Cruise

COR 6/92) in May 1992. Results showed that the gravel content (>2mm) of the sediment increased

from 36% to 56% (Figure 4).

Upon examination of the seabed using side-scan sonar and UW TV it was apparent that the

dredge tracks have become infilled with sand, suggesting a redistribution of sediment has occurred.

The action of the draghead on the seabed has agitated and vibrated the sediment to such an extent that

gravel (>2mm) has consolidated to form ridges between furrows of sand. Inspection, by SCUBA

divers, of the sand accumulations within the tracks showed that the deposits are superficial sand-ripple

features, 1-2cm deep. In addition, the apparent increase in the gravel content at the treatment site

may be caused by the preferential removal of sand by the suction action of the draghead.

Biological Observations

The total number of species recorded from 5 Hamon grab samples taken at the treatment and

reference sites pre- and post-dredging are shown by major phyla in Figure 5. The total numbers of

species 4 weeks before dredging (Cruise COR 4/92) at the treatment and reference sites were broadly

similar at 70 and 62 species, respectively. However, 2 weeks after dredging (Cruise COR 6/92) the

number of species at the treatment site had fallen to 30 (the polychaetes showed the most noticeable

reduction from 35 to 16 species). At the reference site, the number of species has remained generally

constant, having only increased slightly from May (64 species) to December (68 species). However,

at the treatment site the number of species has increased from May (30 species) to December (53

species), which suggests that some readjustment or recolonisation has occurred.

The impact of dredging is more apparent when the abundance data are compared from each

site, pre- and post-dredging. (Figure 6). The total abundance of animals recorded at the treatment and

reference sites 4 weeks before dredging (Cruise COR 4/92) are broadly similar at 230/0.2m2.

However, a dramatic reduction in the abundance has occurred at the treatment site post-dredging

(30/0.2m2), compared to the reference site (209/0.2m2). The crustaceans and 'others" phyla were

numerically dominated by the barnacle Ba/anus crenatus and the sea-squirt Dendrodoa grossularia.

Both showed an increase in abundance from May to December as a result of Summer recruitment.

However, the increase at the reference site was greater than that at the treatment site (possible

explanations are given below).

Biomass data for each site pre- and post-dredging (Figure 7) support the observations made

on the abundance data (Figure 6). A large reduction in the biomass has occurred at the treatment site

post-dredging: from 182g(AFDW)/m2 in March (Cruise COR 4/92) to O.4g(AFDW)/m2 in May

(Cruise COR 6/92). However, at the reference site the biomass figures remain high at
sOg(AFDW)/m2. At the treatment site in December, B. crenatus and D. grossularia contribute very

little to the biomass, although they are present in relatively large numbers (Figure 6). suggesting they

are new recruits. However, at the reference site the biomass figures for December are relatively large,

reflecting the mixed populations of adults and juveniles of B. crenatus and D. grossularia present.

DISCUSSION

Dredging at the experimental site in April 1992 preceded the natural Summer recruitment of

benthos. The 'opportunists' 1). grossularia and B. crenatus, which were numerically dominant before

dredging, showed the greatest increase in abundance post-dredging. However, the increase was

greatest at the reference site. This may be explained by a combination of the following: i. The

treatment site is physically stressed compared to the reference site, due to deposits of mobile sand

being present within the dredge tracks, thereby reducing the recruitment success of sessile epibenthos

such as Sabellaria spinulosa, D. grossularia and B. crenatus; ii. there may be spatial differences in

recruitment success between the reference and treatment sites such that a larger settlement has

occurred at the reference site; iii. the loss of adult sessile epibenthic populations at the treatment site

has reduced the recruitment potential of juveniles, since the "cues" to settle are no longer present.

For example, B. crenatus may require the presence of adult populations in order to stimulate

settlement, as has been observed for B. balanoldes (Stubbings, 1975). In addition, as many adults

have been removed by dredging the source of juveniles which recruit locally (within lOOm) is reduced;

for example, I). grossularia larvae are not transported in the plankton but settlewithin a few metres

of their parents (Svane and Young, 1989).

The effects of seasonality should be borne in mind. The data cover a period of seven months

post-dredging and mortalities will have occurred during the winter of 1992, thereby reducing the

observed gains in abundance at the treatment site.

It remains to be seen whether the populations at the treatment site adjust to the newly-created

physical regime by shifting from a relatively stable community to one characteristic of a more mobile

sediment.

The results from future surveys, planned for 1993 and 1994, will help to further clarify the

processes of recolonisatjon, and the resultant community structure, post-dredging.

REFERENCES

DESPREZ, M. etal, (1992). Ten Years of Biosedimentary Monitoring at a Marine Gravel

Extraction Site off Dieppe (Eastern English Channel). Poster presented at Le Symposium

Manche: Flux et Processes a l'échelle d'une mer macrotidale, 2-4 September 1992, Brest, France, ppl.

Dickson, R., and Lee, A., (1973). Gravel extraction: effects on seabed topography. Offshore

Services. Vol. 6, No. 6, August 1973, pp 32-39, Vol. 6, No. 7, September 1973, pp 56- 61.
Eleftheriou, A., and Basford, D. J., (1989). The Macrofauna of the Offshore Northern North Sea.

Journal of the Marine Biological Association. U.K.69, 123-143.

Holme, N. A., and McIntyre, A. D., (1984). Methods for the Study of Marine Benthos. 2nd

edition. Blackwell. Oxford.

Kenny, A. J., Rees, H. R., and Lees, R. G. (1991). An Inter-Regional Comparison of Gravel

Assemblages off the English East and South Coasts: Preliminary Results. ICES. CM.

1991/E:27, l5pp (Mimeo).

Lart, W. J., (1991). Aggregate Dredging; Fishery Perspectives. Sea Fish Industry Authority.

Seafish Report No.404. pp48.

Lees, R. G. et a!, (1990). Benthic Studies in relation to Dredging Activity off the Isle of Wight,

Southern England. ICES. CM. 1990/E:15, ppl9 (Mimeo).

Miliner, R. S., and Dickson, R., (1977). Physical and Bioligical Studies of a Dredging Ground off

the East Coast of England. ICES. C. M. 1977/E:48, ppll. (Mimeo).

Oele, E. (1978). Sand and Gravel from Shallow Seas. Geol. Mijnbouw. 57,45-54.

Shelton, R. G. J., and Rolfe, M. S., (1972). The Biological Implications of Aggregate Extraction:

recent studies in the English Channel. ICES. CM 1972/E:26, ppl2 (Mimeo).

Sips, H. J. J., and Waardenburg, H. W., (1989). The Macrobenthic Community of Gravel

Deposits in the Dutch part of the North Sea (Klaverbank): ecological impact of gravel

extraction. Bureau Waardenburg by, Culemborg, Netherlands. pp34.

Stubbings, H. G., (1975). Balanus balanoides. Liverpool Mar. Biol. Comm. Mem. No 37.

Svane, I. B., and Young, C. M., (1989). The Ecology and Behaviour of Ascidian Larvae.

Oceanography and Marine Biology Annual Review. 27,45-90.

Van Moorsel, G. W. N. M., and Waardenburg, H. W., (1990). Impact of gravel extraction on

geomorphology and the macrobenthic community of the Kiaverbank (North Sea) in

1989. Bureau Waardenburg by, Culemborg, Netherlands. pp53.

Van Moorsel, G. W. N. M., and Waardenburg, H. W., (1991). Short-term recovery of

geomorphology and macrobenthos of the Klaverbank (North Sea) after gravel

extraction. Bureau Waardenburg by, Culemborg, Netherlands. pp54.
54.00

53.90-

53.80-

53.70-

53.60

53. 50

53.40-

53.30

53.20

53.10

53.00-

52.90-

52.80-

52.70-

52.60-

52.50

SRVEY AREA

I I

I 0 0 0 0 0 0 0

(0 1 C' 0 N (0 0 ('4 (0

d a a d 0 0 0

I

I

Figure 1: Benthic survey off North Norfolk (small box) in order to locate the

experimental dredging and reference sites (large box).
Figure 2: Tracks generat by the suction trailer dredger MV "Sand dredoed Harrier" at the

experimental dredgj site Showing 7O% ofthe seabed area has been
Lifting

arm

Release

hook

Scoop

Figure 3: Schematic diagram of the Hamon grab (taken from Holme and McIntyre,

1984, after Oele, 1978).
100

90 -

80

T 70 -

60 T

50

40-

30

20

10

0

PSA Pre & Post-Dredging

C) (0 (0 CJ - - Lr)

r_, -

'I

Particle Size (mm)

Figure 4: Average cumulative particle size distribution curves for samples taken before dredging (Cruise CUR 4/92) and post-dredging (Cruise CUR 6/92).

COR4/92

-°---- COR6/92
35

30

25

20

15

10

35

5

30

25

20

15

10

(1,

p

TREATMENT SITE SPECIES NUMBERS/i .05m

REFERENCE SITE SPECIES NUMBERS/1.05m

5

LJ

c

J- (I, =

COR14/92 DECEMBER

CIR8/92 AUGUST

COR6/92 MAY

COR4/92 MARCH

C0R14192 DECEMBER

CIR8/92 AUGUST

COR6/92 MAY

C0R4192 MARCH

Figure 5: Number of species by major phyla for samples taken before dredging

(Cruise COR 4/92) and post-dredging (Cruises COR 6/92, CIR 8/92 and

COR 14/92) from the treatment and reference sites.
9000

8000

7000

6000

5000

4000

3000

2000

1000

0

9000

8000

7000

6000

5000

4000

3000

2000

1000

0

TREATMENT SITE ABUNDANCE 1.05/rn

Cl, C-)

LJ=

Q

REFERENCE SITE ABUNDANCE/1.05m

COR14/92 DECEMBER

CIR8/92 AUGUST

COR6/92 MAY

COR4/92 MARCH

C0R14192 DECEMBER

CIR8/92 AUGUST

COR6/92 MAY

COR4/92 MARCH

Figure 6: Abundance by major phyla for samples taken before dredging (Cruise COR

4/92) and post-dredging (Cruises CUR 6/92, CIR 8/92 and CUR 14/92)

from the treatment and reference sites.
120

100

80

60

40

20

120

100

80

60

40

20

0

TREATMENT SITE BIOMASS g(AFDW)/m

0

COo

o

0

0

0C,, 0

REFERENCE SITE BIOMASS g(AFDW)/m

CO

U.S 0

CORI 4/92 DECEMBER

CIR8/92 AUGUST

CORo/92 MAY

COR4/92 MARCH

COR14/92 DECEMBER

CR8/92 AUGUST

CORo/92 MAY

COR4/92 MARCH

Figure 7: Biomass by major phyla for samples taken before dredging (Cruise COR

4/92) and post-dredging (Cniises COR 6/92. CIR 8/92 and COR 14/92)

from the treatment and reference sites.
Appendix 4

Excerpts from: Saloman, C.H., S.P. Naughton and J.L. Taylor,

1982. Benthic Community response to dredging borrow pits.

Panama City Beach, Florida. M.R. 82-3. U.S. Army Corps of

Coastal Engineering Research Center, Fort Belvoir, Va.: 139 p.
Summary of:

Saloman, C.H., S.P. Naughton and J.L. Taylor. 1982. Benthic community response to dredging

borrow pits, Panama City Beach, Florida. M.R. 82-3. U.S. Army Corps of Coastal Engineering

Research Center. Fort Belvoir, Va. 138 p.

Location : Gulf Coast of Florida.

Equipment: Diver-operated 0.0156 m-2 core.

Penetration 23 cm

Sieve 0.7 mm

Borrow Pit Depth: 3-5 m?

Collection Date: 1974-1977

Collection Period Varied

Deposit Description Sand

General Site Characteristics and Changes:

Deposit Mean Organisms! No. of Species Biomass

(mm) m2

Before -0.17-0.2 1506-7178 15-120 None

After 0.177-0.3 324-4037 20-114 None

Controls 1408-5576 53-1 12 None

(After)

Biological Characteristics:

Total # of Species = 362 Total # individuals = 58,068

Phyla # Phyla %

Annelids 152 (42%) Annelids 55%

Arthropods 108 (30%) Mollusks 19%

Mollusks 69 (19%) Arthropods 18%

Remaining 33 species (9%) divided Cnidaria 2%

among 11 groups

Cephalochordata 2%

Nematoda 1%

Echinodermata 1%

Remaining 7 phyla comprised 2%

See Table 3 for numerically dominant species.
20 species (first collection after dredging).

114 species (1 year after dredging).

Number of species collected over 3 weeks after dredging (3 weekly samplings):

Total#: 81

Annelids 35 (43%)

Arthropods 24 (30%)

Mollusks 9 (11%)

Echinoderms 3 (4%)

9 other phyla 10 (13%)

Dominant species after dredging:

1 week after 2 weeks after 3 weeks after

(total abundance 81) (total abundance 279) (total abundance 534)

Lumbrineris 38 (47%) Lumbrineris 113 (40%) Lumbrineris 170 (32%)

cruzensis PB1 cruzensis PE cruzensis PE

Tellina 12(15%) Prionospio 43 (15%) Prionospio 114(21%)

versicolor MB cristata PS cristata PS

Scoloplos 5 (6%) Branchiostom 32 (11%) Pseudoplalyi 31(6%)

armiger PS afloridae CC schnopus sp.

CA

Onuphis 4 (5%) Tellina 18 (6%) Ampelisca 27 (5%)

nebulosa PE versicolor MB verrilli CA

Protohaustoriu 4 (5%) Scoloplos 7 (3%) Tellina 23 (4%)

s sp. CA armiger PS versicolor

MB

FE = Polychaete, Errant.

MB = Mollusc, Bivalve

PS = Polychaete, Sedentary

CA = Crustacea, Amphipod

C = Cephalochordata
Number of Species:

1 week

Annelids 10(48%)

Arthropods 4(19%)

Molluscs 2(9.5%)

5 phyla 5(24%)

Totals: 21

1 week

Annelids 54(67%)

Molluscs 13(16%)

Arthropods 7(9%)

5 other phyla have br 2

inds.

Totals: 81

2 weeks

Annelids 18(44%)

Arthropods 14(34%)

Molluscs 3 (75)

6 phyla 6(15%)

41

Total Abundance.

2 weeks

Annelids 185(66%)

Cephalocordata 32(12%)

Arthropods 29(10%)

Molluscs 23(8%)

3 weeks

Annelids 27(45%)

Arthropods 17(28%)

Molluscs 7(12%)

8 phyla 9(15%)

60

3 weeks

Annelids 390 (73%)

Arthropods 76(14%)

Molluscs 40(8%)

279 534
ABSTRACT (re-typed from original manuscript of Saloman et al. 1982)

This report gives biological and physical oceanographic data from baseline work, and

studies of dredged and undredged sediments before and after dredging (9-meter contour) for

beach nourishment at Panama City Beach, Florida. These studies were designed to show major

short-term environmental effects of offshore dredging and included analyses of hydrology,

sediments and benthos.

Hydrological measurements were limited to water temperature and salinity. Analysis of

surface sediments included particle-size distribution, carbon chemistry, and statistical properties

of mean grain size, sorting, skewness, and kurtosis. Average and extreme periods of water

temperature and salinity were recorded. Regional nearshore sediments proved to be fine sand,

containing less than 1 percent silt-clay, that was moderately well to well sorted, symmetrical to

coarsely skewed, and leptokurtic. Total carbon content averaged less than 0.30 percent, and most

of that occurred in the form of carbonate deposits. Over a postdredging study period of 1 year,

sediment samples from borrow pits showed little variation from these general features.

In studies of the benthos, 362 species and 58,068 individuals were recorded among 14

invertebrate phyla and bony fishes. Dominant groups by species and abundance included

annelida, mollusca, and arthropoda (crustacea). Faunal comparisons between dredged and

undredged areas were made on the basis of species richness and abundance, the Shannon-Weaver

index of diversity (H'), Pielou's index of equitability (J'), Morisita's index of faunal similarity

(together with matrices and classification diagrams derived from that index), and two statistical

derivations, based on diversity and abundance data, that were designed to show sample-to-sample

faunal variations and the time period required for faunal recovery in borrow pits. Information

obtained from these procedures showed that recovery began soon after dredging was complete,

or nearly so, within 1 year.

These results were similar in most respects to those from study of offshore dredging

elsewhere in comparable geographic settings. Even so, the need for close association between

ecological research and coastal engineering programs is emphasized.
PHILIPS

INLET

LAKE POWELL

SO

:0

'4444f4

4) . So 64( S.', I

4, A

GULF OF

MEXICO

WEST BAY

STUDY AREA

PANAMA CITY

Figure 1. Study area at Panama City Beach, Florida, showing stations 1 to 6,

July 1977.

SAMPLING STATIONS

CONTROL

O HOLE

f. I

)

NAUTICAL MILES

0 1 2 3 4 5 H- HH ii

HFHFHFHI ST. ANDREWS 0 2 4 6 8 10 KILOMETERS STATE PARK SHLL

ISLAND - 0S

B6 $?SOW. 45 40
lbVIl?ECI I1OTTnLG 5 2CpGmTc LGJJLG2Guç1çTOu o 2Jwb1TLJ b1u5 hJUJJ9 C'rI'' F°'-Tq (Coii1or) 2J?V1IOI12 i 109' bO2ICOII2IKflCIIOII' 40 Eb1ICV1E2 bE ?VWIE III 0 (ExbEIwEI1Lvr) 2lVlIOkl2 i 10 9' bO2lCOIl2lbflClIOM' 40 Eb1ICV1E2 bEb 2VWIE U (coI.11or) ?LVIIOII i' bKE YIID bO2ICOIl?IISflCIIOII' 35' 39'OK J9 KEbfICVIE2 bE ?VWbE It 0 (EXbEIWEI.l1V1) ?IVIIOII i' bE VIID bO21COI421flC1IOI'1' 35' 39' O 19 KEbICV1E2 bEts ?VWbIE I S (co4I0fl 2IV10I42 V VI4D 8' bEC0V21flC1O4' 4-EbrcviE2 bE 2VWbE C0flb 2VWbE 2AWBOI DE2CIb1I0I1 CEOOVbHIC m rr'TI (U 0I-TI, (U40a-a-'TI 0I-Tn 3? KI10WE1E2 - 3 =-II-0 'UD4 Tn4(U I0-' 0 0 Tn-' (U 0 Tn I-Tn 1 (U 1 0I.- Tn 0 40 I-a- IOCVIIOII2 05 0I-ri, 1 1 m00-Ia- 0m-' a- rna-a-'TI ?1V1IOI1 8 9 2 V 4 3 5 o eonb o o S o o o
Table 2.

NOV.

Textural and statistical properties of sediments in control (undredged bottom) and experimental (borrow pit) samples taken 1 year after dredging at stations 1 to 6 along the 9-meter depth contour off Pana City Beach, -Florida, July 1977. Textural Statistical Station Granule

(pct)

Sand

(pct) Silt-clay

(per) Mean grain size

(phi)

Std. dev.

(phi)

Skewness Kurtosis

1

Control 99.70 0.30 2.45 0.45 -0.19 1.18 Experimental 98.64 1.36 2.50 0.53 -0.00 1.39

2 Control 99.65 0.35 2.45 0.44 -0.18 1.15 Experimental 99.80 0.20 2.43 0.48 -0.19 1.21

3 Control 99.88 0.12 2.21 0.62 -0.32 1.11

Experimental 0.92 98.96 0.11 1.75 1.06 -0.46 0.82

4

Control 99.86 0.14 2.24 0.61 -0.31 1.16 Experimental 0.08 99.81 0.11 2.01 0.83 -0.41 0.95

S Control 99.86 0.14 2.31 0.59 -0.33 1.34

Experimental 99.86 0.14 2.26 0.58 -0.28 1.11

6 Control 0.34 99.52 0.14 2.11 0.76 -0.40 1.09

Experimental 0.14 99.76 0.11 2.31 0.61 -0.34 1.39

Table 1. Water temperature and salinity at stations A and B before the 1974-75 dredging, and

at station 1 before and after the 1976

dredging for beach nourishment at Panama City Beach, Florida.

Station Date Water Salinity

Temp.

(°C) (ppt)

1974 A 18 Nov. 21.0 34.5

B 18 Nov. 20.8 34.3

1975

A 20 Feb 17.4 34.4

B 20 Feb. 17.5 33.9

A 20 May 26.2 32.2

B 20 May 26.0 32.2

A 12 Aug. 28.3 26.2

B 12 Aug. 28.5 26.1

1976

(before) Apr. 20.2 33.3

May 20.2 34.9

June 25.7 32.3

July 28.0 33.3

Aug. 27.0 35.3

Sept. 27.8 32.6

Oct. 24.9 33.1

Nov. 18.0 33.2

Dec. 12.5 34.1

1977

(after) Jan. 12.4 33.3

Feb. 9.0 34.3

Mar. 14.3 34.4

Apr. 22.4 33.5

May 21.8 34.3

June -25.7 32.1

July 27.5 33.6

Aug. 29.0 35.3

Sept. 27.7 32.6

Oct. 25.0 33.1
Table 3. Species in dominant phyla (listed alphabetically) that were numerically abundant at one or more base-line or control stations offshore Panama City Beach, Florida, November 1974

to November 1977.

MOLLUSCA

ActeocuLa candet

Cyltchnetla btdentata Diasoma variwn

Ervilia content ri to

Lepton sp. LLfcjna multilineata

NNELIDA

Armon1in agUs Armandtu moculata

Bronia welifleetensis

Ceratonereis irritabilis

Chone sp. Dispw uncinata Eteone lactea

Glycera americana

Goniada lit torea

Haploscoloplos fouosus

I.,urnbrineris cruzensis Lujr.brinerts teiiuis Lwr.brirtert tetraura

Magelona rio;ai Magelona sp. Mesochaetopterus sagittarius

PRTH1OPODA

AconthohausEorius sp. Albunea paret ii Ampeltsca abdita Ampelisca verritli Cyclaspis varians CyclaspLs sp. Ericttthonjus sp. Lepldactyjus sp.

Table 4. Species richness, abundance, diversity (H'), and equitability

(3') and base-line stations offshore Panama City Beach, Florida, November 1974 to July 1976. Station Date Replicates Species 1ndividual H'

per sample per

(No.) (No.) (No.)

Natica pusilla Periploma margarttuceum

Pitar stmpsoni Strigilta mirabilis

reuino texono

Tettina verswo tar

Nephtys buceru Nephtys picta Onuphis eremita ocutata Onuphis nebutosa

Owenio fusiformts

Paroonides lyra Paroorus furgens Paraprionospto pinnata Prionospu., crstata Rullierir,ereis mexcuno

Scolelepis terana Scoloplos armL per Spio pettiboneue Spiop hones bombyx Unidentified Oligochaete

Monocutodes sp. OyurostyLis smtthj ?rocesso hemphilli Protohaustorus so.

Pseudohau.storius ap. Pseudoplatyischnop us sp. SyncPtelidtum sp. Unidentified Ostracod

Nov. 1974

Feb. 1975

May 1975

Aug. 1975

4 15

27

41

43

2,064

3,008

4,784

3,888

1.9

2.2

2.8

3.1

0.7

0.7

0.8

0.8

Avg. 32 3,436 2.5 0.8

Range 15 to 43 2,064 to 4,7R 1.9 to 3.1 0.7 to 0.8

8 Nov. 1974 4 27 3,808 1.9 0.6

Feb. 1975 26 3.984 2.3 0.7

May 1975 28 5.344 2.3 0.7

Aug. 1975 47 5,248 3.0 0.8

Avg. 32 4,596 2.4 0.7

Range 26 to 47 3,808 to 5,344 1.9 to 3.0 0.6 to 0.8

1 Apr. 1976 32 67 1,506 2.5 0.6

June 1976 36 94 1,902 3.5 0.8

July 1976 36 120 7,178 3.1 0.6

Avg. 94 3,529 3.0 0.7

Range 67 to 120 1,506 to 7,178 2.5 to 3.5 0.6 to 0.8

Overall

Avg. 49 3,883 2.6 0.7

Range 15 to 120 1,506 to 7,178 1.9 to 3.5 0.6 to 0.8
Table 5. Species richness, abundance, diversity (H'), and

equitability (Jt) at control stations offshore

Panama City Beach, Florida, August 1976 to November

1977.

10 Aug. 1976 16 72 5,576 2.4 0.6 18 Aug. 1976 80 5,500 2.8 0.6 24 Aug. 1976 84 4,836 2.9 0.6 1 Sept. 1976 74 3,080 2.9 0.7 8 Sept. 1976 83 2,260 3.4 0.8 21 Sept. 1976 89 3,128 3.0 0.7 4 Oct. 1976 87 3,116 3.3 0.7 18 Oct. 1976 77 3,912 2.6 0.6

1 Nov. 1976 67 3,020 2.6 0.6

1 Dec. 1976 74 3,080 3.0 0.7

5 Jan. 1977 56 1,724 3.0 0.8

2 Feb. 1977 53 1,516 3.1 0.8

1 Mar. 1977 64 2,360 3.1 0.7 1 Apr. 1977 57 2,632 3.1 0.8 2 May 1977 55 2,572 2.7 0.7 1 June 1977 55 1,976 3.3 0.8 5 July 1977 64 3,264 3.1 0.7

2 Aug. 1977 80 5,168 3.0 0.7 1 Sept. 1977 70 3,572 2.9 0.7

3 Oct. 1977 64 2,112 2.8 0.7 1 Nov. 1977 72 2,904 3.0 0.7

Avg. 70 3,205 3.0 0.7 Range 53 to 89 1,515 to 5,576 2.4 to 3.3 0.6 to 0.8

1 11 Jul. 1977 40 99 3,365 3.2 0.7

2 15 Jul. 1977 40 112 3,750 3.4 0.7

3 25 Jul. 1977 40 105 4,326 3.2 0.7

4 26 Jul. 1977 40 74 4,050 2.9 0.7

5 27 Jul. 1977 40 57 1,408 3.0 0.7

6 28 Jul. 1977 40 66 2,483 3.0 0.7

Avg. 86 2,817 3.1 0.7 Range 57 to 112 1,408 to 4,326 2.9 to 3.4 0.6 to 0.8

Overall

Avg. 74 3,119 3.0 0.7 Range 53 to 112 1,408 to 5,576 2.4 to 3.4 0.6 to 0.8

Station Date Replicates Species Individuals H' per sample per m2 No. (No.) (No.)
Table 6. Species richness, abundance, diversity (Ht), and

equitability (J') at experimental stations offshore

Panama City Beach, Florida, August 1976 to November

1977.

Station Date Replicates

per sample

(No.)

Species

(No.)

Individuals

per

(No.)

H' J,

1 10 Aug. 1976 16 20 324 2.3 0.7

18 Aug. 1976 38 976 2.2 0.6

24 Aug. 1976 60 2,136 2.6 0.6

1 Sept. 1976 38 1,612 2.1 0.6 8 Sept. 1976 47 1,344 2.7 0.7 21 Sept. 1976 45 924 2.9 0.8

4 Oct. 1976 85 2,440 3.7 0.8 18 Oct. 1976 46 1,124 2.9 0.8 1 Nov. 1976 55 2,044 2.5 0.6

1 Dec. 1976 54 3,540 2.3 0.6

5 Jan. 1977 36 2,192 1.8 0.5

2 Feb. 1977 44 2,212 1.9 O.i

1 Mar. 1977 62 3,732 2.6 0.6

1 Apr. 1977 52 3,144 2.2 0.6

2 May 1977 54 1,656 2.8 0.7

1 June 1977 69 3,256 3.2 0.8

5 July 1977 49 1,964 2.7 0.7

2 Aug. 1977 70 2,920 3.2 0.8 1 Sept. 1977 32 440 2.9 0.8

3 Oct. 1977 61 1,588 3.1 0.8

1 Nov. 1977 54 1,220 2.9 0.7

Avg. 51 1,942 2.6 0.7

Range 20 to 85 324 to 3,732 1.8 to 3.7 0.5 to 0.8

1 11 July 1977 40 81 2,422 2.9 0.7

2 15 July 1977 40 114 3,862 3.5 0.7

3 25 July 1977 40 98 4,037 3.3 0.7

4 26 July 1977 40 94 2,587 3.4 0.8

5 27 July 1977 40 80 2,644 2.9 0.7

6 28 July 1977 40 83 3,034 3.4 0.8

Avg. 92 3,101 3.2 0.7 Range 80 to 114 2,422 to 4,037 2.9 to 3.5 0.7 to 0.8

Overall

Avg. 60 2,200 2.8 0.7

Range 20 to 114 324 to 4,037 1.8 to 3.7 0.5 to 0.8
Appendix 5

Excerpts from. Padan, J.W. 1977. New England Offshore

Mining Environmental Study (Project NOMES) U.S. Dept. of

Commerce, NOAA: 139 p.
Summary of:

Padan, J.W. 1977. New England Offshore Mining Environmental Study (Project NOMES) U.S.

Dept. of Commerce. NOAA: 139 p.

Location: Atlantic Ocean, Massachusetts Bay off Boston.

Mine area. Cobble stations D2, D3 and C6.

Reference cobble area Cli

Other stations - finer sediments

Water depth: 20m.

Equipment: SCUBA diving.

Grab - not specified.

Collection Period: 1973

Site Characteristics: Cobble areas only - No data extractable

Biological Characteristics:

Species Abundance/rn2

Sample

station

Inverts. Motiles Motiles

D2 31 31 1,640

D3 113 101 4,120

C6 65 39.23 3,138

Cli 89.25 83.25 7,535
Top 5 ranks of the most common motile organisms, based on mean number of animals:

PS = Polychaete, Sedentary

CA = Crustacea, Amphipod

PE = Polychaete, Errant

EE = Echinoderm, Echinoid.

Cobble

substrate stations

C6 (D)T2 (D)T3 Cli

Number of

times sampled

4 1 1 4

See Table 6

for more

details

Euclymene

collaris PS'

E. collaris PS E. collaris PS U irrorata CA

Unicola irrorata

CA2

Phyllodoce

groenlandica

E. dispar PE E. collaris PS

PE

Exo gone dispar Syllis armillis U irrorata CA E. dispar PE

PE3 PE

Glycera capitata G. capitata PE Corophium Notomastus

PE crassicorne luridus PS

CA

Strongylocentrot Tharyx acutus Spirorbis Tharyx acutus

us

droebachiensis

PS spirillum PS PS

EE4
Mean ranking of 5 dominant higher order taxa (see Table 7):

(5.0)

Mean ranking of feeding type (see Table 8):

Cobble C.6 C.11

Scraper/Omnivore (1.3)

Deposit/Direct (1.6)

Predator/Epi/Micro (3.0)

Deposit/Indirect (4.0)

SuspensionlFilter (5.0)

Deposit/Indirect (1.0)

Scraper/Omnivore (2.0)

SuspensionlFilter (3.0)

Predator/Epi/Micro (4.3)

Predator/InlGeneral (4.8)

Scraper/Omnivore (1.0)

Deposit/Indirect (2.5)

Deposit/Direct (3.0)

SuspensionlFilter (4.0)

Predator/Epi/Micro (5.0)

Deposit/Direct (1.0)

Scraper/Onmivore (1 .8)

SuspensionlFilter (4.0)

Predator/In/General (4.0)

Deposit/Indirect (4.2)

Cobble Stations Sand Stations

C6 Cli Sb M9

Polychaeta( 1.0) Polychaeta (1.0) Polychaeta (1.2) Polychaeta (1.0)

Amphipoda (2.0) Amphipoda (1.75) Amphipoda (1.8) Amphipoda (2.0)

Ophiuroidea (3.0) Bivalvia (3.25) Bivalvia (3.0) Bivalvia (3.0)

Echinoidea (3.75) Gastropoda (4.0) Cumacea (5.2) Cumacea (4.6)

Bivalvia (4.0) Polyplacophora Anthozoa (5.4) Isopoda (5.0)

Sand M.9 S. 10
SUMMARY

The New England Offshore Mining Environmental Study (Project NOMES) was

begun in 1972 in order to resolve the marine environmental impact uncertainties that had inspired at many levels of government legal and de facto mora- toriums on marine mining. It was a joint study sponsored by the Cc*mnonwealth

of Massachusetts and the National Oceanic and Atmospheric Administration.

Plans had called for a 1-year study of baseline conditions at a sand and

gravel deposit centered in Massachusetts Bay at 40°21'4l" N., 70047t10* W.,

followed by a period of well-monitored commercial-scale mining. Two years of

post-experiment monitoring were planned to document

mining-induced changes in

the seafloor and water column as well as their subsequent alteration by

natural processes.

The project was terminated in July 1973 as a result of the failure of

the Commonwealth to arrange for a suitable site for the disposal of the three-quarters of a million cubic meters of sand and gravel to be mined during the planned spring 1974 test.

Nevertheless, all principal investigators were funded through a project wrap-up phase, and two were funded long

enough to permit them to study baseline conditions in two important aspects

of marine life (i.e., benthos, phytoplankton) for a full year. The purpose of this report is to consolidate and present the findings in

such a manner that the NONES experience can be considered an informational

and procedural point of departure for studies preceding the next continental

sand and gravel venture, whether it be an experiment or a commercial

mining

operation. Studies are reported in four areas of oceanography: biological

(benthos, phytoplankton, turbidity experiments), geological (bathymetry,

stratigraphy, core samples), chemical (nutrients, suspended solids), and

physical (temperature and salinity, currents and dispersion, and light penetration).

Over 650 species of benthic invertebrates were sampled by scuba diving

at a number of stations arranged in a grid pattern around the test site. The

most important result of this part of the project was the documentation of

the natural variability it the system. The existence of month-to-month var- iation at each station was the most consistent finding. From month to month

there was up to 100% variation in the number of species at a station. In

fact, the species numerically dominant one month frequently were absent fr

collections in the next month. The species tended to change with substrate

type. This is a heterogeneous environment and only a minority of the species

appeared to be specialists for substrate type. Indicator species, if it is

possible to use them in the future, will most likely have to be selected for

each station and not from general substrate types. In a future study of

offshore sand and gravel mining, benthic community studies should utilize

permanent stations, as was done in NOMES, but the sampling scheme should be

altered. A limited number (two to four) of stations should be sampled in- tensively on a quarterly schedule. Fifteen to twenty replicate samples

'hould be taken at each station every three months and analyzed separately so

hat aggregated distributions can be identified. This may provide a more alistic picture of community organization and dynamics than monthly samo- ng with fewer

replicates would.
- 1240

-Test Ste

8os,on /rbo, ±

S

10°54'W

to.

Figure 1. Test site in Massachusetts Bay.

?0°46W

Figure 5. Stations selected for the benthic community study in Massachusetts Bay, identified by substrate type and number.

4224N

42020 N

Scale l8O.0OO HI l i NoutiCol Miles

!

H9 -

\

'rst Site

,' _ç ' H14

%Q -

0M7

-, , 'Boston

- Light

HRockLedge

C-Cobble

D-Mne Area (Cobble)

H8 '---' M 8

S-Sand

M-Mud

-Buoy

lll2 -

oM8
Table 3. Characteristics of Permanent BenthIc Sampling Stations

HArd 1 7 miles MW lSm large rock outcropping clean Hard 8 1.4 miles SSW llui cobble and large boulders moderate silt Hard 9 1.5 miles NNW 25m cobble and large boulders moderate silt Hard 12 2.5 miles SE 18m cobble and large boulders moderate silt Hard 14 0.5 miles NE 25m cobble and large boulders clean Cobble 6 within dredge area 32m gravel with large rocks clean Cobble 11 2.2 miles SSW 27m gravel heavy silt Mud 6 0.5 miles SW 3Sm mud heavy silt

Mud 7 2.1 miles SW 29m mud heavy silt Mud 8 1.8 miles SSE 40m gravel with mud matrix heavy silt Mud 9 3.25 miles 55W iBm muddy sand heavy silt Sand 10 2.5 miles SSW 23m sand moderate silt

Table 4. Sediment Composition Analyses For The Soft-Substrate Stations

in the Benthic Community Study

Station

Core

No. January 1973 April 1973 Jwe 1973 Mean for All Months

% SandS SiltS ClayS Sand

S SiltS Clay

S Sand

S Silt

S ClayS S Sand Silt Clay

116 1

2 Mean

29.0 61.0 45.0

44.8

24.4

34.6

26.2

14.6

20.4

44.1

51.8 47.9

35.1

32.5 33.8

20.9

15.7

18.3 46.5 34.2 19.3

117 1

34 Mean

80.1 8.

77.5

78.7 79.6

15.6 140

17.1

16.0

15.7

4.3

7.9 5.4 5.3

4.7

79.5

82.4

81.0

14.8

11.5

13.2

5.6 6.1

5.9

68.5 82.8

75.6

23.6

11.4

17.5

7.9

5.8

6.8 78.7 15.5 5.8

118 12

3 Mean

65.1

67.7 70.0

67.6

24.8

20.8

19.2 21.6

10.1 11.5

10.8

10.8

54.0 62.6

58.3

43.9

26.6

35.2

2.2 10.8

6.5

67.5

78.9

73.2

23.8

15.5

19.7

3.7 5.6

7.2 66.3 25.5 8.2

119 12 Mean95.4 94.6

95.0

2.6

3.2 2.9

2.0

2.2 2.1 95.0 2.9 2.1 510 12 Mean

99.2

99.2

0.2

0.2

0.6

0.6

78.7

98.6

88.7

20.9

0.7

10.8

0.4

0.7

0.5 93.5 5.5 0.6

Site DiStaflce and Direction Depth Substrate Silt Cover from Dredge Site
Table 5. Total Numbers of Species (Parts a. and b.) and of Densities

(Part c.) of Benthic Invertebrates

a. Ootai ni,b.r of invertehrate species collected on each cruise at each station.

Statnon Cruise Ho. Mean

14 15 16 17 1.8 19 20 21 Hi 51 89 6 6 38.0

118 87 87 105 146 106 138 111.5

549 131 99 1.45 125.0 H12 60 92 64 112 82.0

914 121 66 92 134 142 111.0 C6 37 81 76 66 65.0

02 31 31.0

03 113 113.0 Cli 109 78 50 80 89.25

96 62 67 61 52 79 81 67.0 Ml 69 56 29 60 57 98 81 73 56 70 68 69.6

49 78 59 79 98 90.4

810 90 42 58 67 '76 66.6

b. Tctal nusber Of motile species collected on each cruise at escis station.

Station Cruise HO. Mean

14 15 16 17 18 19 20 21. Hi 31 65 6 6 27.0

98 57 73 75 104 73 102 80.7

49 85 77 109 90.3

912 47 70 54 95 66.5

914 75 58 67 97 120 83.4 C6 32 73 73 59 59.25

02 31 31.0

03 101 101.0 Cli 101 75 83 74 83.25

96 52 56 55 42 72 77 59.0

97 57 52 26 57 50 48.4

98 77 68 48 64 65 64.4

549 71 56 72 82 83 72.8

810 86 42 49 62 74 62.6

c. Total flusters/rn2 of motile species for each cruise for each Station.

St5tio Cruise No. 'lean

14 1.5 16 17 18 19 20 21 HI. 4517.0 7182.1 528.0 1148.0 3343.8

NB 3616.2 10728.9 6295.9 13938.9 11146.2 8427.4 9025.6

89 3080.8 7014.4 7611.4 6168.9

812 3096.0 12236.8 4394.1 11513.0 7810.0

914 2446.2 2816.0 3896.2 7374.6 5043.6 4315.3 C6 479.6 4465.0 3497.5 4112.9 3138.8

02 1640.0 1640.0 D3 4120.0 4120.0 cli 5715.0 2427.5 11140.0 10859.5 7S35.5

96 2231.6 735.7 3262.3 1039.1 2030.9 5792.8 2515.4

547 643.9 920.0 305.5 1254.6 762.0 777.2

98 2977.0 2046.1 336.4 639.7 2139.6 1627.8

119 2666.8 2426.4 3961.5 5410.4 7397.2 4372.5

Sb 2261.1 1415.9 1123.8 1914.7 1690.4 1601.2
Table 6. Rankings of the Most Coamon Motile Benthic Species, Based on the Mean Number of Animals and Grouped by SamplinqStaeion Substrate Type

Spirorbis spirillum 5 Spirorbis borealis Moelleria costulata 9

Notomastus luridus 4 Aricidsa jeffrsyeii 9 Nephtys ciliate lo

Hard Substrate Stations: Hi 1(8 1(9 H12 1(14 Number of Times Sampled: 3 6 3 4 S Spirorbis spirillmi 3 1 1 1 1 Spirorbis borealis 2 2 2 2 Caprella Septentztona1s 1 3 4 4 Vdiolus siodiolus 2 4 Xschyrocerus anguipes 9 5 5 8 9

Pontogeneia inerms 7 6 3 Ophiopholis aculeata 7 6 6 Tonicella rubra 8 8 7 10 Achelia spinosa 9

Cucuinaria frondosus 10

Jesse fulcata 4 Caprella linearis 8

L.acuna pallidula 6 Nereis pelagica 5 Lepidonotus squamatus 3.0 Spirorbis violaceus 7 Musculus niger 9

Sympleustes glabex 3 8

Metopeila angusta 4

Strongylocentrotus droebachiensis 10 6 5 Pectinaria granulata 9 Anomie simplex 10 7 Lacuna vincta 5 Salamis balanoides 3

Soft Substrate Stations:

Number of Times Sampled: Mud

1(66

1(7

5

1(8

5 Sand M9

5 510

5 Ninoe nigripes 1 1 2 Maldane sarsi 2 Spio setosa 3 1

Sternaspis scutata 4 Nucula delphinodonta 5 9

Scoloplos fragilis 6 3 5 7 Travisia carnea 7 Edotea siontosa 8 2 4 Pholoe minute 9 10 3 6 Periploina papyratiusi 10

Diastylis sculpte 4 8 10 Nephtys ciliata 5 6 Owenia fusiformis 6 1 Modiolus siodiolus 7 2 3 Photis reinhardi 8 4 Nasserius trivittata 9

Thracia siyopsis 6 Asarte undata 7 Cerastoderma pinnula turn 8 Paraonis gracilis 10

2

Cobble Substrate Stations: C6 T2 T3 Cli

Number of Times Sampled: 4 1 1 4 Euclymene collaris 1 1 1 2 Unicola irrorata 2 3 1 Exogone dispar 3 7 2 3 Glycera capitata 4 4 10*

Strongylocentrotus droebacbiensis 5 8

Phyllodoce mucosa 6 7 8

Ischnochiton alba 7 10

Spio setosa 8 10

Coroph.iuin crassicorne 9 4 Euchone rubrocincta 10 9 7 Phyllodocs groenlandica 2 Syllis a.rmullis 3 Tharyx acutus 5 5 Owenia fusiformts 6 6

Pholoe minute a -
Table 7. Mean Ranking of Numeiic11y Dominant Taxonomic Groups,

Arranged According to Substrate Types

Hard Substrate

Mud Substrate

Taxonomic Group Hi H8* H9 H12 H14

Polychaeta 2.5 1.0 1.0 1.0 1.0

Axnphipoda 1.25 2.5 2.0 2.25 2.8

Bivalvia 2.0 3.7 2.0 3.75 2.8

Gastropoda 3.7 4.7 5.0 4.0 4.6

As teroidea 4.0 5.0 6.0 6.75 9.2

Ophiuroidea 5.5 6.2 3.5 5.75

Po lyplacophora 9.5 6.8 6.2 4.25 6.0

Ho lothuro idea 6.5 7.25 11.3 10.0 11.0

Pantopoda 7.0 7.8 8.3 9.5 9.5

Isopoda 8.0 8.0 10.5 10.25 10.5

Cobble Substrate

C6* Cli

Poiychaeta 10 1.25

Ainphipoda 2.0 1.75

Ophiuroidea 3.0 9.0

Echinoidea 3.75 7.5

Bivalvia 4.0 3.25

Polypiacophora 5.25 5.0

Gastropoda 5.5 4.0

Anthozoa 7.0 8.0

Brachyura 7.0 10.0

Sipunculida 7.0

M6* Mi M8 M9

Polychaeta 1.0 1.0 1.0 1.0

Bivalvia 2.0 2.8 2.4 3.0

Isopoda 3.2 2.8 2.8 5.0

xnphipoda 4.3 4.2 3.8 2.0

Gastropoda 4.8 5.8 5.8 6.2

Cumacea 5.2 4.4 5.4 4.6

.Anthozoa 7.0 7.0 7.2 6.2

Brachyura 7.0 8.0 8.0 8.0

Asteroidea 7.0 8.0 7.3 8.0

Aplacophora 7.25 5.0

Sand Substrate

S10 M9

Poiychaeta 1.2 1.0

Ainphipoda 1.8 2.0

Bivalvia 3.0 3.0

Cuinacea 5.2 4.6

Anthozoa 5.4 6.2

Gastropoda 5.6 6.2

Isopoda 6.8 5.0

Polyplacophora 7.0

Echinoidea 7.4 7.25

Holothuroidea 7.5 9.3
Ta2. 8. Man Raniting of Feeding Types, Grouped According to

Substrate Types

Feeding rypes*

Hard Substrate

Hi H8 H9 Hl2 H14

Kay filters suspended particles

2 bites or rasps attached plants and animals

captures suspended animals

" predator on small motile epifauna

predator on large motile and sessile epifauna

6 sorts out small particles of sediment to ingest

ingests sediment as it burrows

nonspecialized predator on infauna

9 rasping predator on sessiie epifauna

O predator on infaunal mollusc*

Supension/Pilter1 2.0 1.0 1.0 1.0 1.0 Scraper/Omnivore2 2.0 2.0 2.3 2.0 Suspension/Predato3 1.5 3.6 5.0 4.3 3.3 Predator/Epi/Micro" 3.0 4.0 4.0 4.3 4.8 Scraper/Herbivore 5.8 5.5 4.5 5.0

Predato: pi/Macro5 2.5 5.8 6.0 6.5 8.0

Deposit/Indirect6 6.2 5.0 5.0 5.5 Parasite 7.8 7.5 8.8 9.8 Deposit/Direct7 8.8 8.5 10.3 9.2 Predator/In/General8 10.2 10.5 8.8 9.5 PredatoriEpi/Scraper9 3.0 10.8 9.5 10.0 3.0.0 Predator/In/Mollusc10 12.0 12.0 11.8 10.0

Mud Substrate

Feeding 'rypes M6 HI H8

Deposit/Direct 1.6 6.0 5.4 Deposit/Indirect 1.8 3.2 1.8 Predator/In/General 2.6 1.0 3.0 Suspension/Filter 4.2 3.0 2.8 Scraper/nivore 5.0 2.8 3.6 PredatOr/Epi/Micro 5.6 5.4 4.6

Predator/In/Mollusc 6.6 7.6 9.2 Suspension/Predator 7.6 6.8 8.0 Predator/Epi/Macro 7.6 7.2 8.0

Scraper/Herbivore 7.8 7.6 7.2

Parasite 8.2 7.2 8.4

Predator/Epi/Scraper 8.2 1.6 9.0

Sand Substrate

M9 510 Deposit/Direct 6.3 1.2 Scraper/Omnivore 2.0 1.8 Suspension/Filter 3.0 4.0 Predator/In/General 4.8 4.0 Deposit/Indirect 1.0 4.2 Predator/Epi/Micro 4.3 5.8 Predator/In/Moilusc 6.8 7.0

PredatorfEpi/Macro 8.3 8.4

Suspension/Predator 8.8 8.6

Scraper/Herbivore 8.8 8.8 Predator/Epi/Scraper 8.8 9 0

ParasiteS 8.5 9.2

Cobble Substrate

C6 Cli Scraper/Omnivore 1.3 1.0 Deposit/Indirect 4.0 2.5 Deposit/Direct 1.6 3.0 Suspension/Filter 5.0 4.0 Predator/Epi/Micro 3.0 5.0 Predator/In/General 6. 0 5. 3 Scraper/Herbivore 7.0 7.5 Suspension/Predator 8.3 8.0 Predator/Epi/Macro 9.3 8.0 Parasite 11.0 9.3 Predator/In/Mollusc 9.3 10.0 Predator/Epi/Scraper 10.3 10.3
Table 13. Species Characteristic of Specific Substrate Types, Ranked

According to the Total Number of Individuals per Square Meter*

1 Spirorbis vi.ol.ceus

2 Lacuna pallidula

3 Sualus fabricii

4 Tonicella sarvorea

S Maldanopsis elongate

6 Idotea phospisorea

7 Aaphitrite cirrata

A .Janira alt.. 9 Polycera lessonii 10 Probolojdes Solmesi 11 Mitrella rosacea

12 Nicolea venustula

13 Pot.aaj.11a reniforis

14 Colus stiapsoni 15 Aequipecten irradians

16 Mitrella dissi.ailis

1 Asterias rubens

1 Lacuna vincta

2 Anosia aculeatm

3 Velutina laeviqat.a

4 Nyuç4ion grossipes

S Acmaea testudinalis

6 C1yenel1a torquata

1 L.a1ssros quadriplica

* + 622.8

0 0 454.9

+ * * 206.5

o .. * 174.9

.. .- 144.8

+ 0 96.4

* 0 89.7

0 54.0

56.1

* 0 42.3

+ 36.8

29.6

o * o 16.1

14.7

o 13.4

13.3

+ ..+ 1237.4

o + + 394.3

103.1

o 0 0 0 101.0

* * 0 + 83.4

o * 34.6

* Data fro. all cruises end all stations were used.

Note indicates 10 or sore animals per station. o indicates less than 10 anisals per station.

0 2.5

o 0 1.5

o 2.5

o 1.6

o 0.7

o 7 5.0

2.5

1 Maldane sarsi * 7 1844.1

2 Thracia .yopsis 0 326.S

3 Chiridotea tuftsi o 194.1

4 Tellina aqilis 0 147.3

S Cysthura polite * 0 49.1

6 Anachis halia.cti 0 25.9

7 Hipposedon propxnquus o o o 20.8

B Apistobranchus tullbergi 0 0 19.5

9 Eudorella trunculata 16.7

0 2.3 0 2.5 100.2

Substrate Type Hard Cobble Nod Station Nber 81 88 89 812 814 C6 T7 TI dl P16 M7 , P19 110

total Total Total Raha Species Animals Animals Sn tea is

1 Tonicella rubra 2081.3 o 3.3 0 2.5

2 Alvania castanea 657.4 o o 5.0 o 0.7

3 Dodecaceria conch.srum 351.4 0 2.1 0 0.7

4 Musculus discors o * 208.1 o o 5.8 0 0 2.5

S Cirratulus cirratus + 105.2 o 3.3 0 0.7

6 Flabelligera affinis . + o 61.5 o o 5.0 0 0.8

7 Corophi bon.ili . o 49.6 0 1.0 0.8

1 Syrrhoe crenulata 0 2.7 0 55.0

2 I4000culodes tuherculatus 0 5.3 0 40.0

3 Lbrtneris tenuis 0 3.2 0 37. 5

i Scolelepides vcrjdjs 27.5 o 0.3

2 Lora pleurotosania 20.5 0 0 0 0 5.5

1 crichthonius rubricornis o 2.7 * 0 42.5 o 0.8

2 Phoxichildi fesoratue 0 2.7 33.3 1.4

3 Anonyx sa.rsx o 2.7 e * 15.0 o 1.7

4 Lora turricula 0 2.7 10.0 2.7

1 Spiophanes bosbyx o 3.3 314.9

2 Nephtys picta 5.0 0 90.9

3 Ralcampa duodeciacirrat.a o 2.5 40.8

4 Praxillura ornate o 3.3 11.7

1 Lunatia iacu.lata 46.4

2 Polycirrus exia.ius 12.5

j 3 Odontosyllis fulgurans 10.5

1 Sternaspis Scutata 0 2.3 0 Q 0 1238.6

2 Artica islandica 0 2.7 0 0 310.0

3 Asinopsis orbiculatus 5.3 * * 165.8

4 E4otea triloba 0 5.4 * 0 35.7

S Ptilanthura tenuis 0 5.3 31.2

6 Cheetoderea nitidulue 0 4.0 0 27.5
The aquarium turbidity studies nottd above were to have been conducted

in a Massachusetts Bay aquarium complex as well. Other laboratory investigations were to have involved effects of the discharge plume on phytoplankton, benthic algae. zooplankton, and finfish.

3.4.2 Experimental Mining

The mining test had been scheduled for a 6-week period in late spring

1974. This period was selected to coincide with the sensitive "bloom" period

of phytoplankton to learn whether or not mining would have any impact on this

phenomenon.

The dredge selected for the field operation was the 27,000-metric-ton

"Hydrobarge" Ezra Sensibar (fig. 4). The 155-rn-long, 23-rn-wide hopper-type

barge is both self-loading and -unloading and has a capacity of 12,000 m3.

Mining is accomplished through suction provided by two 84-cm centrifugal

dredge pumps amidships. Valving directs the incoming slurry (composed of

about 10 in3 of water for each cubic meter of aggregate) to individual hoppers. Suspended sediments are allowed to overf low the hoppers as each is

filled with aggregate.

The bathymetry of the selected site is such that excavations were possible over the full range of deposit depths of 3 to 6.7 rn--typically about

4.6 in. A total of three-quarters of a million cubic meters of material was

to have been removed over the 6-week experimental period from a deposit distributed over approximately 25 hectares. The dredge was to have conducted

three 2-hr operations every 2 days. out 14 hr may have been required

between operations for transit, of floading, arid return, depending on the

distance to the discharge point.

Mining operations were to have been monitored to determine the change in

bottom topography, the solids content, size distribution and volumetric

overflow of suspended sediment, and the progress of the sediment concentration in the resulting plume of material discharged from the barge. The silt

Figure 4. Hydrobarqe Ezra Sensibar.

15
In none of the experiments did the 10-day LC5O, LC2O, ur LC1O

values bear any predictable mathematical relationship to one another. This

illustrates the necessity for studying the tolerance of the most sensitive

members of a population.

Tolerance to suspended bentonite seemed to be correlated with

normal habitat of the organisms, but no phylogenetic correlations were apparent. No species living primarily in close association with mud bottoms was

found to be sensitive. All sensitive test species were either invertebrates

occurring predominantly on sandy bottoms or in fouling communities, or fish

not intimately associated with the bottom.

The results indicate that the biological impact of high concentrations of suspended solids would be less severe in winter than in summer. The

typically higher dissolved oxygen levels would increase the survival ability

of all species studied. Low temperatures would increase the suspended-solids

tolerance of the invertebrates, but slightly decrease that of the fish.

However, this slight reduction would likely be offset by the increased tolerance at high dissolved oxygen levels.

The primary emphasis of this study was mortality of adult macrofauna. It cannot be overemphasized that low mortality of adults in 10 days

does not imply the absence of ecologically significant effects. Reduced

reproductive success, in terms of spawning adults, eggs, larvae, or juveniles, may be of greater ecological importance than the death of part of the

existing population.

4.2 Geological Oceanography

This section discusses the characteristics of the seafloor in the test

area, stratigraphy of the upper strata as revealed through subbottom profiling, and sediment properties based upon analyses of vibracores. A prime

objective of Project NOMES was a consideration of the potential of test

mining to release trace metals or other pollutants to the water column.

Additional objectives included the delineation of the deposit in adequate

detail to assure that it was commercial in character and that a prediction

could be made as to the nature of the discharge plume at the point of discharge from the hydraulic dredge.

4.2.1 Bathymetry

The bathymetric measurements obtained during subbottom profiling, which

are discussed later, revealed an irregular bottom topography with NNW.-

trending ridges interspersed with circular and eliptical depressions (fig.

19). The area to be excavated during the planned test mining (shaded area in

fig. 19) had a relief of 3.4 m and so was considered a realistic choice for

operation of the hydraulic dredge.

The shape of the seafloor is characteristic of an area that has experienced glacial scouring and sediment deposition, as well as post-glacial

stream channeling and subsequent modification of bottom contours by advancing

post-glacial seas.

55
56

70048 70°47 70°46W

Figure 19. NOMES Project area and area of planned test mining.

Contours in enlarged section show water depths in meters,

referenced to high water.
4.2.2 Stratigraphy

A pre-NOMES survey conducted for the Massachusetts Division of Mineral

Resources in early 1972 located the NOMES sand and gravel deposit through

subbottom profiling, side-scan sonar, arid two 12-in vibracores. In August

1972, 37 additional subbottan profiles were run, and 31 4-rn vibracores were

drilled in the vicinity of the deposit. Figures 20 and 21 show the locations

of the core sites and subbottan profile tracklines.

For the purpose of identifying those areas within the NOMES deposit that

offered characteristics with the best potential for test mining (i.e., thickness of aggregate, coimnercial quality of sediment, water depth at mean high

water, and distance to glacial till outcroppirigs) three sediment distributior

maps were prepared: surficial, subsurface (-1.5 in), and subsurface (-3 in).

Core analysis data, reported below, augmented the geophysical records in the

preparation of the maps.

The surficial sediment map (fig. 22) was produced from subbottom profiling records, core sample analyses, and information from television observations, as well as diver samples and observations. Classification of sediment types was based on the scheme shown in Table 21, which was modified frcr

a system in use in the Gulf of Mexico (Louisiana Wild Life and Fisheries

Commission, 1971). From figure 22 it can be seen that the seafloor in the

test site area consists of a patchwork of mixtures of sand, gravel, mud, and

glacial till.

Table 21. Sediment Classification

57

% Gravel % Sand % Mud

Gravel 75 - 100 0 - 25 0 - 25

Sandy Gravel 33.5 - 75 12.5 - 50 0 - 33.5

Muddy Gravel 33.5 - 75 0 - 33.5 12.5 - 50

Sand 0 - 25 75 - 100 0 - 25

Gravelly sand 12.5 - 50 33.5 - 75 0 - 33.5

Muddy Sand 0 - 33.5 33.5 - 75 12.5 - 50

Mud 0 - 25 0 - 25 75 - 100

Gravelly Mud 12.5 - 50 0 - 33.5 33.5 - 75

Sandy Mud 0 - 33.5 12.5 - 50 33.5 - 75

Till Mixture of boulders, cobbles, gravel, sand, mud
A A A

A ADA A

A AAAA

A AA

A 4-rn Vibrocore

0 2-rn Vibracore

T I 4222N

58

70°49'N 70°46W 70'49* T0°46' !igure 20. Locations of the vibra- Figure 21. Ship tracks of the subcores taken in the NOMES site for bottom profiles.

WOAA (4-rn) and for the Masse chusetts Division of Mineral Resources (12-rn).

The sediment distribution 1.5 in beneath the seafloor (fig. 23) is similar to that of the seafloor in that sandy gravel and gravelly sand are still

distinct; also the muddy area remains in the northeast corner of the deposit.

The overall size of the deposit is smaller, however.

The sediment distribution in beneath the seafloor (fig. 24) shows a

greatly diminshed area of sandy gravel. The extent of glacial till, which

underlies most of the test site area, becomes more evident.

The subbottom profiling records, coupled with the core analyses, permitted an estimate of the thickness of the area to be mined (fig. 25). It appeared that the deposit contained over 5 million cubic meters of sand and

gravel, considerably more than the three-quarter million planned for excava- tion during the mining test.

Although the main interest in the stratigraphy of the NOMES area was the

upper few meters, the subbottom profiling records revealed several distinct

formations: Carboniferous Cambridge argillite, undifferentiated Pleistocene

glacial drift, two Pleistocene marine "clays," and Holocene sand and gravel

(fig. 26). The section lines used to develop figure 26 are shown in figure

27.

4 2° 20 N

A

A

A

A

A

A A
10 °4 8 'N 70°46N

42°21'N

42° 20' N

42°21'N

42° 20' N

59

70°48 'N

EJ

10°46N

Till

Mud

Sandy Mud or Muddy Sand

Sand

Gravelly Sand

Sandy Gravel

Figure 22. Surficial sediment Figure 23. Subsurface (-1.5 m)

distribution in NOMES Project sediment distribution in NOMES area. Project area.

10°48'W 10°46'W

Figure 24. Subsurface (-3 m)

sediment distribution in

NOMES Project area.
60

70°48 70047' 70'46

Figure 25. Aggregate isopach map of the experiment area

(deposit contoured in meters).

The Cambridge argillite outcrops on several Boston Harbor Islands and is

believed to underlie the NOMES area as bedrock. It is overlain by Pleistocene glacial till, a heterogeneous mixture of boulders, gravel, sand, silt,

and clay ranging in thickness from a thin veneer to nearly 30 m. Two marine

clays, separated by an erosional uncomformity, overlie the glacial till.

The "NOMES deposit" appears to be a gradational feature resting on, and

yet geologically part of, the upper marine clay.

4.2.3 Core Analyses

Of the 31 cores acquired by vibratory coring (fig. 20), 10 were selected

for detailed laboratory characterization. Mineralogy, chemistry, and trace

metal properties were examined.
Marine Sand and Gravel

Holocene Marine Sandy Mud

Pleistocene Marine Sandy Mud

Undifferentiated Glacial Tills

Cambridge Argillite

Figure 26. Selected geological cross sections through the NOMES site.

61
Figure 27. Locations of

tracklines used as a

basis for cross sections

in Figure 26.

Mineralogy

The general appearance of the -10, +230 mesh material from several

strata of all 10 cores was similar; therefore, only a single description is

given. The particles were subrounded to subangular. The predominant minerals and their percentages were as follows:

Feldspar, 25-30%

Quartz, 20-30%

Hornblende, 15-20%

420 21 N

42°20N

62

(Primarily plagioclase and

some orthoclase)

(50% clear-to--milky, and

50% stained dull yellow)

Mica, 5-10% (Biotite and muscovite)

In addition, the minor minerals included some opaques, garnet, tourinaline, and possibly olivine. There were a few rock fragments that appeared to

be granitic grains. The organics included about 5% shell fragments plus

Foraminifera tests.

The predominant -230 mesh mineral was quartz.

Sedimentological characteristics are not included in this report but

generally reveal what we would expect from a heterogeneous glacial deposit:

very poorly sorted sediments.

O°46\N
on

Stratum Depth -230 CoD(b) 0rgaiuc C(d) Sulfide Phosphorus Grease

flumber (Inches) Color Description Mesh (g/kg) Carbon (meq/lOOg) (ag/kg) (mg/kg) (ag/kg)

7-1-2-1-I 0-20 2.5YN/4 Sand-fine to medium

coarse. Gravel-to 1.5 inches.

Shell Fragments-very

coarse. 7-1-2-2-i 20-37 2.5YN/4 Pine sand-no gravel.

7-1-2-2-2 37-44 2.SYN/4 Pine sand & lS% 0.75"

gravel.

7-1-2-3-I 44-54 2.5YN/4 Coarse sand & .15% clay.

7-1-2-3-2 54-70 2.SYN/4 Fine gravel C '40% clay.

Becomtng gravelly to 0.5".

7-2-2-4-1 70-80 SYS/2 Clay Layer with inter- mittent, discontinuous

sand lenses.

7-1-2-4-2 80-85 5Y5/2 Medium fine sand.

7-1-2-4-3 85-102 5Y5/2 Clay layer with inter- mttent diacontinuoua

sand lenses.

7-1-2-5-I 102-132 5Y5/2 Clay layer with inter- mittent. discontinuous

sand lenses.

7-K-l-l-1 0-14 5Y4/2 Fine sand.

7-K-l-l-2 14-56 5Y4/2 Coarse sand and gravel to 2. Gravel becoming very coarse to 3". (Ex- cellent fill material.)

7-M-l-l-1 0-9 S'f512 Pine sand with few

shells. Becoming

coarse sand.

7-14-1-1-2 9-13 5Y5/2 Coarse sand and graveL

to 0.75; well graded.

7-14-1-1-3 13-29 5Y5/2 Coarse gravel to 2.5"; very well graded.

2.5YN/4 Small aunt of clay.

8-1-1-1-1 0-12 5y5/1 Fine sand and shell

fragments.

8-1-1-1-2 12-30 5Y5/l Sand and gravel to 2".

Well graded; sand

becoming coarse.

8-1-1-2-i 30-39 5Y5/2 Sand and gravel to

1.5". Well graded. Some

shell fragments.

8-1-1-2-2 39-50 2.5Y5/2 Pine sateS and shell

fragments.

8-1-1-2-3 50-60 2.sys/3 Becoming gravel to 2". Shell fragments to 0.5.

Table 22. Chemical Analses of Core Strata (a)

64

(%) (%) Core 7-I-I

3.54 30.8 1.15 61.4 (0.05 1.2 <50

62.0

32.0

24.9

27.4

15.4

20.5

8.6 0.32 11.0

11.3

Core 7-K-lw

4.40 30.7 1.15 43.1 0.19

26.0

Core 7-M-1

3.81 26.2 0.98 49.9 0.05 1.9 <50

41.4

25.2

Core 8-I-lw

4.50 36.3 1.36 53.6 0.86 3.4 <50

61.2

57.1

.339 1.27 68.5

56.3
Stratum Depth HuIsoer (Inches) Color Description

Sand and gravel to 1.S. well graded.

Sand with 10-15% gravel and large shell frag- ments. Very coarse sand-no gravel-large shell

fragments.

Consi4erable clay and silt

Silty clay.

Excellent sand and

gravel to 2.5"

Well graded.

Fine sand and .25%

gravel to 1.5". Heavy

shell fragments.

Silty clay and fine

sand.

Old beach sand.

Coarse sand and

gravel to 1.5". Well graded.

Clayey sand and '.20%

gravel to 1.5" with

shell fragments. Changing to less gravel.

Very stiff silty clay with occasional fine

sand lenses.

Table 22. (Continued)

-230 coo organtc° (d) Su1fid Pipboru.1

Mesh (g/kq) Carbon (meg/bOg) (mg/kg) (mg/kg)

(5) CS) Core

65

All results based on oven dry weight (103"-105C) Chemical Oxygen Demand of -230 mesh material.

Cc) Calculated from COD results.

Cd) Cation Exchange Capacity of -230 mesh material. These results are high in some cases. Se. text.

Based on "grab" samples taken prior to screening. Total phosphorus after destruction of organic matter--based on -230 mesh material.

(gi Freon extractable from -230 mesh material.

Oil

Grease

(mg/kg)

0.94 55.3 2.07 61.3 0.36 2.0 <50

34.0

65.0

37.2 1.40 17.0 1.8

29.9 1.12 15.8 0.84

Core

2.30 14.7 0.55 35.0 0.48 0.90 <50

(a) Core 5-0-1

5.00 33.6 1.26 57.7 0.34 14. <10

28.6

Core l0-G-1

2.32 17.1 0.64 30.3 1.4 2.5

30.0

Core 1l-E-1

14.7 17.5 0.66 54.0 0.37 1.5 <5

11.8

8-74-2-1-1 0-18 5Y4/2

8-74-2-1-2 18-40 2.5Y4/2

8-74-2-2-1 40-74 2.5Y4/2

8-74-2-3-1 74-84 2.5Y412

8-74-2-4-1 84-110 2.5Y4/2

8-0-1-1-1 0-28 5Y6/2

9-1-1-1-1 2-38 5Y5/1

9-1-1-3-I 38-45 2.5YH/5

10-G-l-l-1 0-10 5Y5/l

10-0-1-1-2 10-18 5Y5/1

ll-E-1-l-1 0-23 2.5Y74/4

ll-E-1-2-1 23-83 2.SYN/5

8-IC-L-l-1 0-11 5Y5/2 Well graded sand and 4.12 44.7 1.68 50.4 1.15 2.4

gravel to 2.

8-K-l-1-2 11-22 5Y4/l Gravel as above but 32.5

sand becoming finer.

8-K-1-2-1 22-41 2.SyN/4 Medium coarse sand and 27.5

gravel to 2" Fine sand

layers of 2-3" with

small shell rqmects and

slight color change from

light to dark gray.

Core 8742(a)
idea]. dredge material is considered to be 40% sand and 60% gravel. - purities normally consist of clay, silt, fine sand, and shells. Their

significance will be discussed below, but generally 5% impurities is

considered a maximum for a deposit to be mineable.

Both question (1) and question (2) are answered by probing the depostt

with a large-diameter (about 77 cm.) drill sampling tool. This provjdeg I

bulk sample which, although not undisturbed, is adequate for deposit evalul

tion. Sometimes bulk sampling is done by the actual mining of small quantt

ties of sediment. In either case, the amount of material removed from th. Joposit is limited to a small amount under terms of the UK prospecting license.

There is no indication of any adverse environmental impact associated

with the steps in the exploration phase: bathymetry, acoustic sub-bottom

profiling, drill sampling, and bulk sampling by mining vessel. However, the

final two steps, because thay are conducted by vessels at anchor, could pose

a navigation hazard. In addition, the final step could provide a miniature

version of the impact associated with the mining phase.

Excavation

Sand and gravel are economically mined from the seafloor in several

ways: clam-shell barge, bucket-ladder dredge, and suction dredge. The clam

shell technique is gradually phasing out as uneconomic in Europe, where

deposits are mined far from shore by large ocean-going hopper dredges, but

it supplies over one-half of Japan's production, where numerous small

dredges mine fairly close to shore. The bucket ladder dredge is best suited

for digging hard bottom formations. The hardware (not to mention the great

capital investment) required is not needed for most sand and gravel deposits.

In addition, the dredge is relatively unstable for ocean operations in most

parts of the world.

The bulk of the 80-vessel UK marine mining fleet consists of suction

hopper dredges. Cargo capacities range from about 500 to about 10,000 tons.

The trend is toward larger and larger dredges to reduce the cost per unit of

material dredged.

Recovery of sand and gravel is done by use of one or more high-head

centrifugal pumps dredging a slurry of solids from the seafloor (up to

about 30 meters beneath the ocean surface) through a suction pipe. The

slurry, about 10% solids, is fed to the hopper(s) where most of the solids

remain. The excess water flows overboard, along with fine particles

trapped in supens1on.

Some dredges recover from a single point while at anchor. This results

in the creation of a pit 10 or more feet deep initially, and finally, a

pockmarked deposit (fig. 3, p. 7). Some dredges recover while drifting with

the changing tidal current, while at anchor. This results in the creation

of a crescent-shaped trench about one meter deep. Eventually, the deposit

becomes laced with overlapping crescent-shaped trenches. Other dredges

112
recover while drifting unanchored.* This results in numerous shallow

trenches, each about 30 cm. in depth (fig. 3).

Most UK mining operations are governed by the tides, operating on a

24-hour cycle whereby the dredges take advantage of high tides for leaving

and returning to normally shallow-water cargo discharge points in estuaries

or rivers. Generally, a dredge leaves port at the start of ebb tide, steams

to its lease area, fills its hopper in a matter of 1 to 3 hours, returns on

the flood tide, discharges in 1 to 2 hours, and leaves for sea--all within

24 hours. If the draft of the dredge is not a critical factor, and the

lease area is not too far away--l30 Jan. is not uncommon--the cycle may then

occupy less than 24 hours.

The majority of deposits worked in the UK are relatively near shore

(within 30 km.), comparatively close to market, generally in 20 to 30 meters

of water, and from 1 to 10 meters thick.

The mining operation can impact the environment in a variety of ways

as described in this report. The main UK fears concern damage to the coastline and to marine life. Interference with navigation and communications

is minimized by not permitting mining within 0.8 km. of shipping lanes,

submarine cables, pipelines, and marker buoys.

Coastal erosion can be caused in four ways: (1) slumping of the beach

profile; (2) changing wave refraction patterns; (3) reduced protection from

big waves by the removal of offshore banks; and (4) the removal of material

normally part of the onshore-offshore sediment transport budget. The UK

has sufficient experience to be able to avoid coastal problems from marine

mining.

The main enigma concerns the effect on marine organisms of the

turbidity plume and resultant blanket of fines. However, the industry has

gone on for so long (since 1926) that there is no way to gain a "before"

characterization of marine communities. Therefore, the main focus has been

on insuring that mining does not occur on known spawning grounds, such as the

clean gravel substrate where herring spawn (International Council for

Exploration of the Sea, 1975).

Shipboard Treatment

Most dredges utilize a coarse-grid steel framework across the opening

of the suction head. This prevents large rocks from entering the suction

pipe. In addition, the coarser sizes are screened off and rejected after

passing through the pump. At the other end of the particle size distribution

* It has been predicted that the cutterhead pipeline dredge may find

application in the United States, in those cases where mining is done

within a few kilometers from shore (National Research Council, 1975).

This would in effect transport the discharge plume to shore.

113
spectrum, fine material is washed overboard, as described in this report.

All sizes in-between are retained in the hopper if the market demand Coincides

with the composition of the deposit. Usually this is not the case and so

newer dredges are equipped with vibrating screens whereby all or part of the

material--usually the sand fraction--is dumped back into the ocean. On the

average, the ratio of sand to gravel mined in the UK is about 70:30. An

average market mix requires about 40:60, so gravel dredges frequently dump

overboard 2 to 3 tons of sand for every ton of gravel recovered.

Specialized large-capacity dredges of advanced design are being developed

in increasing numbers. Such dredges are equipped with highly automated shipboard treatment plants capable of producing a wide range of washed and sized

aggregate products at sea and then transporting the cargo to distant ports

and unloading a desired sized product or special mix with an automated selfdischarging system.

There is no knowledge of the impact of shipboard treatment. But inasmuch

as no chemicals are utilized, there are probably but two effects: (1) the

dumping of large volumes of sand onto the lease area with a gravel/sand

substrate; and, (2) the washing overboard of a greater percentage of fines.

The former would alter the bottom habitat but not cause a turbidity plume.

The latter would add to the initial turbidity plume problem, but eliminate

the washing problem at shoreside.

Transportation to Shore

During transit from dredge site to discharge port, the aggregate settles

in the hopper (s) and water is drawn off and pumped overboard. The amount of

water lost by this means can be about 10% by weight of the total load. Some

very fine particles, in suspension because of the ztion of the dredge, are

lost overboard in this process.

Shoreside Processing

Shore-based support facilities for the dredges include wharves, stockpiling and processing facilities, and treatment plants. The exact arrangement depends upon the nature of the unloading technique, the processing

required to produce marketable aggregate, and the treatment required to clean

up waste water.

Some dredges pump ashore with either shore-based pumps or shipboard

pumps. In either case the hopper full of sand and gravel is reflooded with

river water, and the resultant slurry drawn through a duct in the bottom of

the hopper arid then into a discharge pipe. From there it is pumped into a

large settling tank or pond. The overflow water passes through several

stages of settling tanks before it is clean enough to return to the river

(or estuary).

114
Dry discharging is accomplished by clam-shell grabs, elevators and belt

conveyors, or scraper-buckets. Self-discharging by scraper buckets, coupled

with over-the-side conveyor belts, has been well received in the UK as the

most efficient and economical system available. With this system, scraper

buckets are rapidly hauled up ramps at the forward part of the hopper and

then emptied into an elevated hopper which feeds an over-the-side conveyor

belt that carries the material ashore.

Shores ide sand and gravel treathent plants are located near dockside

cargo-discharge points, where the material is washed and screened into appropriate sizes which then commonly are blended in specified proportions for

local markets.

Excessively large stones are crushed for sale as crushed stone. The

remaining sand and gravel is washed (just as it is in inland operations), as

it is screened into desired sizes, in order to remove excessive fine material

which interferes with the cement-aggregate bond in concrete.

Most UK plants utilize recirculated water for washing, although some use

water pumped from a river or estuary. If the water is to be discharged into

the environment, and not recycled, large settling tanks are used for cleaning

up the water.

Standards have been developed in the UK for salt and shells--two obvious

impurities in sea-won aggregate. The sole concern with shell, based on tests

by the British Standards Institution, the Greater London Council, and others,

involves excessive amounts of hollow shell that reduce concrete strength.

Although salt is thought to accelerate the rate of curing--at the expense

of strength--the salt water content of sea-won aggregate is not considered to

be a problem. The washing required to remove the fines also removes enough.

salt that no extra washing is required.

Present UK specifications for sea-dredged aggregates are based largely on

standards developed by the Greater London Council, as summarized below:

r The sodium chloride content of the fine and coarse

aggregate must not exceed 0.10% and 0.03%, respectively,

by weight of dry aggregate.

The total sodium chloride content derived from the

aggregates can exceed the above amounts as long as

it is not greater than 0.32% by weight of the cement

in the mix.

Shells that are hollow or of unsuitable shape, in

quantities sufficient to adversely affect the

permeability or other qualities of the concrete,

shall not be permitted.

115
4) The shell content of the aggregate shall not exceed

the following allowable dry-weight percentages of

shell: 2% in the 11/2-inch fraction, 5% in the

3/u-inch fraction, 15% in the 3/8-inch fraction,

and 30% in the sand (minus 3/16.-inch) fraction.

The above specifications, which were said to be subject to amendment at

the time of their release (1968), have generally prevailed throughout the

industry. However, thay have become somewhat more definitive with respect

to shell, and less rigid in the case of sodium chloride content. By late

1970, limits set forth by such bodies as the Greater London Council for the

new London Bridge contract have been generally accepted by industry in the

London area as well as in most other dredging areas in the UK. These

standards are as follows:

Maximum % by Weight

Transportation to Market

Truck transport has been used in the UK to connect the shoreside

processing facilities with urban market outlets. In addition, ready-mixedconcrete plants are now being located alongside sea dredging discharge points.

A recent trend has been the establishment of secondary distribution

centers in inland metropolitan areas. These secondary centers are supplied

by rail from the primary dredge discharge points, using special rail

aggregate-container cars.

116

11/2-inch straight 2 0.1

3/it-inch straight 5 0.1

3/ti-inch graded 10 0.1

3/8-inch sand 15 0.1

3/16-inch sand 30 0.2

Fraction Shell Sodium Chloride
Appendix 6

Excerpts from: Jewett, S.C., L.A. Gardner, P.C. Garvin, and

C.E. Sweeney. 1991. Nome Offshore Placer Project (Western

Gold Exploration and Mining Company, Limited Partnership.

Nome, Alaska) ENSR Consulting & Engineering. Doc. Num.

7235-010.
Summary of:

Jewett, S.C., L.A. Gardner, P.C. Garvin, and C.E. Sweeney. 1991. Nome offshore placer project.

(Western Gold Exploration and Mining Company, Limited Partnership. Nome, Alaska). ENSR

Consulting & Engineering. Document Number 7235-0 10.

Location: Bering Sea, Norton Sound off Nome, Alaska.

Collection period: September 1985 - June 1990.

Table 1. General Site Characteristics and Changes:

Gravel percentages (>2 mm) present at Cobble and Sand sites

Table 2. Biological Characteristics:

* High biomass due to presence of sea urchin Strongylocentrotus droebachiensis.

Stations Cobble Sand

Post-Dredging R6 42.07-97.04 0.12-13.04

Dredge Impact R7 50.94-86.11 0.02-24.07

Pre-Dredging D8 34.2

Controls C2 29.22-92.81 0-18.60

C3 51.93-61.15 0.03-27.60

S2 42.39-98.36 0.41-23.75

S3 53.37-61.82 0.42-9.50

Cobble data 1990

R6 (mined R7(mined S3 C3

1986) 1987-1990)

Total # of Taxa 63 38 84 83

Average density 746 408 1497 3997

(= indiv/m2)

Average Biomass

(g/m2)

104.2* 0.6 27.2 32.5
(Table 2 cont.) Sand data 1990 (dredging initiated at R6 in 1986, and at R7 in 1987)

Station R7 cobble

(dredging initiated nearby in 1987)

Gravel % Number of Number of

organisms/rn2 species

Biomass/m2

1987 50.94 60.2 57 17.3

(Dredging

occurring)

1988 79.86 363 34 0.6

1990 86.11 403 38 0.6

Station S3 cobble/C3

1986 55.28 3243 79 106.8

1987 56.45 2703 76 21.1

1988 53.37/ 2462/ 75/ 17.0/

51.93 1007 64 26.5

1989 57.07 1317 70 1.6

1990 61.82/ 1497/ 84/ 27.2/

61.13 3997 83 32.5

Station R6 cobble

1988 75.17 578 32 0.7

1989 42.07 997 49 4.4

1990 97.04 746 63 104.2

R6 R7 S3 C3

Total#ofTaxa 67 30 81 67

Average density 1975 328 7113 1768

(# indiv/m2)

Average 20.4 1.0 13.8 21.9

Biomass (g/m2)
(Table 2 cont.) Station R6 sand

Sand % Number of Number Biomass/m2

organisms! of species

m2

1987 93.94 160 21 0.8

1988 86.37 1253 53 7.2

1989 99.47 1117 48 13.6

1990 98.38 1975 67 20.4

Station C3 sand

1987 97.61 1595 66 35.3

1988 72.09 845 56 10.4

1989 98.60 1050 63 19.6

1990 97.18 17.68 67 21.9
Table 3. Year and depth of dredging

* Station R7 was dredged in 1988 and was located in the area affected by dredging in 1987,

1989 and 1990

STATION DEPTH (m) YEAR DREDGED

R6 10.7 to 12.2 1986

R7 10.7 to 12.2 1987 through 1990*

D8 18.3 1988

C2 7.0 to 7.6 control Station not dredged

C3 10.7 to 12.2 control Station not dredged

S2 7.0 to 7.6

Subsistence Station not

dredged

S3 10.7 to 12.2

Subsistence Station not

dredged
Table 4. Most abundant species (numbers/rn2) at cobble stations

CA = Crustacean, aznphipod, CC = Crustacean. cumacea, EE = Echinoderm, Echinoid, E0 Echinoderm, ophiuroid, MB = Mollusc, bivalve, MG = Mollusc, gastropod, N = Nematean, P = Polychaete.

Station June 1986 June 1987 June 1988 June 1989 June 1990 C-2 - Cobble Cerebratulus spp. (N) 68 lschyrocerus spp, (CA) 632 D. craterodmeta (EO) 87 Mytilus edulis (MB) 425 H. arctica (MB) 200 7.0 - 7.6 m Diamphioda craterodmeta (80) 45 Travisia pupa (P) 45 Pholoe minuta (P) 78 AutolyWs sp. (P) 220 Myriochele oculata (F) 102 Clinocardium californiense (MB) 42 Hiatella arctica (MB) 23 Cerebratulus spp. (N) 67 lschyrocerus spp. (CA) 63 C. califonsiense (MB) 67 Strongylocentrotus Glycinde spp. P) 22 Autolytus sp (P) 63 Lamprops quadriplicata (CC) 50 Glycinde spp. (P) 35 droebachiensis (EE) 37 Bathymedon spp. (CA) 22 Glycinde spp. (P) 60 Cerebratulus spp. (N) 42 Cerebratulus spp. (N) 22 Glycinde spp. (P) 28 S2..Cobble Cerebratulus spp. (N) 102 T. alternate (P) 140 D. craterodmeta (00) 168 Onchidoris muricata (MG) 105 M. occulata (P) 182 7.0 - 7.6 m Spirorbis spp. (P) 50 Spirorbis spp. (P) 138 Cerebratulus spp. (N) 118 Glycinde spp. (P) 92 C californiense (MB) 70 Typosyllis alternata (P) 47 H, arctica (MB) 72 P. minuta (F) 82 Tellina lutea (MB) 80 Glycinde spp. (P) 68 C. californiense (MB) 45 Cerebratutus spp. (N) 53 C. californiense (MB) 47 P. minuta (P) 60 Magelona sacculata (P) 62 Spirorbis abnonnis (P) 30 S. abnormis (P) 52 Autolytus prismaticus (P) 32 D. craterodmeta (ED) 47 1. lutes (MB) 47 C3 - Cobble D. craterodmeta (00) 213 M. oculata (F) 2285 10.7 - 12.2 m M. oculata (P) 115 0. craterodmeta (E0) 297 C. Californiense (MB) 77 Glycinde spp. (P) 163 Tharyx secundus (P) 67 Cerebratulus spp (N) 145 Yoldia spp. (MB) 67 C. californiense (MB) 95 S3 - Cobble D. craterodmeta (80) 317 D craterodmeta (E0) 317 D. craterodmeta (80) 253 Polydora sp. (P) 412 0. craterodoseta (E0) 353 10.7 - 12.2 m T. alternate (P) 277 T. alternata (P) 277 P. minuta (P) 225 M. oculata (P) 145 M. oculata (P) 287 Ampharele acutifrons (I') 213 A. acutifrons (P) 213 Cerebratulus spp. (N) 207 Cerebratulus spp. (N) 68 Glycinde spp. (F) 147 P. minuta (P) 197 P. minuta (P) 197 A. acutifrons (P) 197 Echiurus echiurus alaskanus (E0) 40 Cerebratulus spp. (N) 65 M. oculata (P) 163 M oculata (P) 163 1. secundus (P) 167 P. minute (P) 35 C. califomiense (MB) 48 Diastyllis alaskensis (CC) 35 R6 - Cobble D. craterodmeta (EO) 110 Autolytus sp. (P) 273 D. craterodmeta (E0) 272 Dredged in 86 Yoldia spp. (MB) 103 M. edulis (MB) 117 M. oculata(P) 103 10.7 - 12.2 m C. californiense (MB) 68 Onchidoris muricata (MG) 83 C. californiense (B) 25 P minuta (F) 42 D. craterodmeta (E0) 53 S. droebachiensis (EE) 22 Spio fihicornis (F) 32 H. aJctica (MB) 45 Spirobis spp. (F) 22 Cerebratulus spp. (N) 22 Ri - Cobble Cerebratulus spp. (N) 110 P.minuta(P) 75 C. californiense (MB) 197 Dredged 87-90 D. craterodmeta (E0) 70 C. californiense (MB) 37 0. craterodmeta (00) 87 10.7 - 12.2 m T. alternata (P) 38 0. craterodmeta (E0) 23 Atylus collingi (CA) IS Heteromastus filiformis (F) 35 Yoldia spp. (MB) 23 lschyrocerus spp. (CA) 10 Glycinde spp. (P) 35 Mya up. (MB) IS M. oculata (P) 10
Table 5. Most abundant species (numbers/rn2) at sand stations

CA = Cruatacean. amphipod. CC = Cruatacean, cumacea, FE = Echinodenn, Echinoid, E0 = Echinoderm, Ophiuroid, MB Mollusc, bivalve, MG Molluac, gaslropod, N = Ncmcrtean, P = Polychaete, T Tunicate.

Station Sept 1985 June 1986 June 1987 June 1988 June 1989 June 1990 C2 - Sand Spiophanes bombyx (P) 337 M sacculala (P) 635 M. sacculata P) 395 M. sacculata (1') 887 M. sacculata (P) 780 M. aacculata (F)

Magelona saccutata (P) 327 Spiophanes bombyx (P) 295 Travisia pupa (P) 120 M. ama (MB) 248 T. lutea (MB) 632 Glycinde app. (F)

Macoma ama (MB) 235 1. lutea (MB) 272 M. lama (MB) 92 S. bombyx (P) 202 Glycinde app. (F) 317 Scoloploa armiger (F)

Echinarachniva pai'ma (SD) 168 Macoma lama (MB) 265 T. lutea (MB) 85 Siliqua alta (MB) 148 M. lama (MB) 187 T. lutea (MB)

Tellina lutea (MB) 157 Glycinde app. (P) 152 S. bombyx (MB) 68 T. pupa (P) 143 S. bombyx (F) 187 S. bombyx (F) S2 - Sand M. sacculata (P) 830 M. sacculata (P) 262 M. sacculata (F) 570 Spisula polynyma (MB) 543 M. sacculata (P) 720 M. sacculata (F)

M. ama (MB) 177 5. anniger (P) 60 S. bombyx (P) 178 1. lutea (MB) 90 T. lutra (MB) 627 1, lutea (MB)

S. armigcr (P) 137 Diastylis alaskensis (CC) T. lutes (MB) 120 E. parnsa (SD) 60 S. bombyx (F) 360 S. armigcr (F) 1. lutea (MB) 128 Cerebratulus app. (N) 48 Euhaustorius eous (CA) 87 Cerebratulus app. (N) 60 Glycinde app. (P) 278 Myriochele oculata (F)

cilycinde app. (P) 93 Lamprops quadriplicata (CC) 47 Acanthostepheia behringiensis (CA) S. Alta (MB) 50 Macoma app. (MB) 270 S. bombyx (F) 65

32 C) - Sand M. sacculata (F) 202 M, oculata (F) 203 Glycinde app (P) 207 M. oculata (F)

M. oculata (P) 130 S. armiger (0) 73 M. aacculata (P) 130 M. aacculata (F) S. bombyx (F) 127 Glycinde app (P) 73 S. polynyma (MB) 127 Glycinde app. (F) T. pupa (F) 125 M. sacculata (P) 60 Apistobranchus ornatua (F) 105 D craterodmeta (E0)

E,parma(SD) 117 Yoldia app (MB) 53 T. lutea (MB) 47 S. amsiger (F) S3 - Sand M oculata (P) 308 S. bombyx (F) 465 M. oculata (P) 1027 T. lutea (MB) 332 M. oculata (F)

Cerebratulus app (N) 278 M. oculata (0) 280 E. parole (SD) 512 F. patina (SD) 308 S. bombyx (F)

Typosyillis altcrnata (P) 145 Heteromastus fihiformis (0) 268 Glycinde app. (P) 327 Glycinde app (F) 232 Glycinde app. (F)

Glycinde app. (P) 140 M. sacculata(P) 157 H. filiformis (P) 302 D. caterodmtta (E0) 212 T. lutea (MB)

D. alaskensis (CC) 82 0. parma (SD) 123 Cistcnidea granulata (F) 207 Machaironyx muelleri (CA) 193 Diamphiodea. craterodmela

(00) R6 - Sand Dredged 1986 D. alaskensia (CC) 57 Tharyx aecundus (P) 360 M. sacculata (P) 223 M. oculata (P)

L. quadriplicata (CC) 20 M. oculata (0) 225 T. lutea (MB) 213 M. sacculata (F)

Glycinde spp (F) Is Cerebratulus spp.(N) 105 S. polynyma (MB) 183 Glycinde app. (P)

A. behringiensis (CA) to H. filiforsnis (F) 97 Glycinde app. (F) 72 S. armiger (P)

Nephtys Iongosctosa (P) 7 Glycinde app. (F) 55 Spisula sp. (MB) 55 A. ornatus (F)

Campylaspis umbenais (CC) 7 R7 - Sand Dredged 1987 Pholoe minuta (P) 93 Glycinde app. (F) 30 D. craterodmeta (00)

Nephtys ap (P) 22 HiateIla arctica (MB) 22 Glycinde app. (0)

Mya sp. (MB) 22 Mytilus edulis (MB) 20 D. alaskensia (CC) Clinocardium califomiense M. sacculala (P) 13 M. sacculata (F)

(MB) 18 Molgula up. (T) 13 M. oculata(P)

Monoculodes sp. (CA) 17
Table 6

Shannon Diversity index (H'), Simpson's Index (0), species richness. denlsty (indiv./m2), biomass (g/m2), and number of taxa of benthic invertebrates collected from Sand (s) and cobble (c)

near Nome, /Jaska during 1985 through 1990.

NS Not Sampled

180 = To be determined

Year

Sand Stations Cobble Stations

C2s S2s C3s S3s R6s 1175 C2c S2c C3c S3c R6c R7c 06c

Shannon (H') 1990 TBD TBD 2.54 1.38 2.15 2.36 TBD TBD 1.68 2.83 2.46 1.71 NS

Simpson (0) 0.83 0.42 0.69 0.85 0.56 0.87 0.80 0.67

Species flchness 7.53 8.38 7.53 4.94 8.51 9.27 7.87 4.43

Density (indIv.m2) 1768 7113 1975 328 3997 1497 746 408

Biomass (g/m ) 21.9 13.8 20.4 1.0 32.5 27.2 104.2 0.6

No. Taxa 67 81 67 30 83 84 63 38

Shannon (H') 1989 2.34 2.70 2.88 3.47 2.65 2.90 2.17 3.25 NS 2.44 2.27 NS NS

Simpson (0) 0.84 0.89 0.90 0.96 0.88 0.93 0.78 0.93 0.79 0.81

Species richness 5.80 7.33 7.85 10.13 6.47 6.24 6.06 10.40 8.31 5.59

Density (indiv.m2) 2813 3591 1050 3427 1117 530 1208 1088 1317 997

Biomass (g/m ) 62.2 76.1 19.6 47.4 13.6 0.9 23.5 7.2 1.6 4.4 No. Taxa 57 71 63 92 48 40 55 76 70 49

Shannon (H') 1988 2.34 2.18 2.96 3.24 2.42 2.44 3.22 2.69 2.82 3.12 2.20 2.64 3

Simpson (0) 0.82 0.72 0.91 0.92 0.84 0.84 0.94 0.88 0.90 0.94 0.84 0.89 0

Species rIchness 5.10 6.04 7.58 9.69 6.60 4.96 8.52 7.53 7.78 8.81 4.50 5.08 14 Density (lndiv.m2) 2437 1277 845 5050 1253 357 995 740 1007 2462 578 363 5780

Biomass (g/m ) 93.9 4.9 10.4 30.8 7.2 0.3 35.4 14.2 26.5 17.0 0.7 0.6 101 No.Taxa 44 47 56 93 53 34 62 54 64 75 32 34 134

Shannon (H') 1987 2.60 2.51 3.22 3.11 2.28 NS 1.81 2.74 NS 3.13 NS 3.09 NS

Simpson (0) 0.86 0.82 0.94 0.92 0.83 0.57 0.90 0.93 0.93

Species richness 5.93 6.92 8.48 8.49 4.21 7.54 6.75 8.98 8.58

Density (indiv.m2) 1208 1497 1595 2485 160 1030 752 2703 602

Biomass (g/m ) 59.9 39.4 35.3 58.8 0.8 12.6 51.6 21.1 17.3

No. Taxa 43 52 66 66 21 59 48 76 57

Shannon (H') 1986 2.40 2.56 NS 2.89 NS NS 3.06 2.70 NS 2.50 NS NS NS

Simpson (0) 0.86 0.84 0.91 0.94 0.90 0.83

Species richness 4.46 5.46 7.86 6.35 5.58 9.01

Density (indiv.Lm2) 2193 707 1767 495 448 3243

Biomass (g/m) 91.5 17.1 10.8 77.0 30.6 106.8

No.Taxa 34 36 61 43 39 79

Shannon (H') 1985 2.40 2.21 NS NS NS NS NS NS NS NS NS NS NS

Simpson (0) 0.88 0.78

Species richness 4.19 4.55

Density (indtv.m2) 1289 1438

Biomass (g/m ) 33.4 41.3

No. Taxa 32 37
Table 7

Example of WestGold's Waste Discharge Permit

and Effluent Monitoring Requirement 1990

Process Tailings Launders (DIscharge 003)

1. The combined discharge of the process tailings launders shall be limited and monitored by

the Permittee as specified below:

Flow (MGD)

Settleable Solids

small fraction (m3lhr)

Suspended Solids (mgIL)1

Arsenic (pg/L)2'3

Cadmium (pg/L)2'3

Copper (pgIL)23

Lead (pgIL)23

Mercury (pg/L)23

Nickel (pgIL)2'3

Zinc (pg/L)2'3

35

15,000

234

167.4

52.2

100.8

0.45

149.4

1,548

45

413

30,000

1,242

774

52.2

2,520

37.8

1.350

1,710

Estimate:

3 times per day

Grab:

3 times per day

Grab:

3 times per day

Grab:

2 times per week

Grab:

2 times per week

Grab:

2 times per week

Grab:

2 times per week

Grab:

2 times per week

Grab:

2 times per week

Grab:

2 times per week

Average monthly:

maximum daily

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

Average monthly:

maximum

1/ Suspended solids is defined herein as the combination of clay and silt particles.

Metal concentrations shall be measured and reported as both total recoverable and total dissolved metals.

EPA may reduce the monitoring requirements after the development of a substantial, reliable record of metals

concentrations in the discharge plume.

Total recoverable and total dissolved metals shall be measured in accordance with EPA-approved methods

which meet the requirements of 40 CFR Part 136 to achieve detection limits for (or at least) As (1 pgIL), Cd

(1 pgIL), Pb (1 pgIL), Hg (0.05 pg/L), Ni (1 pgIL) and Zn (8.6 pgIL).

There shall be no discharge of dredge material from the tailings process launder to waters

of less than 8 m depth at MLLW.

The average daily discharge of process tailings shall not exceed 10% solids,

of which no more than 15% shall be silts and clays.

Effluent Characteristics Discharge Limitations Monitoring Requirements

Sampling Method Monthly Reporting

Average Maximum and Frequency Values
Table 8

Example of WestGold's Waste Discharge Permit

and Effluent Monitoring Requirement 1990

Gold Table Sluice Tailings Launder (Discharge 004)

1. The discharge shall be limited and monitored by the Permittee as specified below:

1! Metal concentrations shall be measured and reported as both total recoverable and total dissolved metals.

EPA may reduce the monitoring requirements after the development of a substantial, reliable record of metals

concentrations in the discharge plume.

2/ Total recoverable and total dissolved metals shall be measured in

accordance with EPA-approved methods which meet the requirements of

40 CFR Part 136 to achieve detection limited for (of at least) Cu (1 tgIl), and

Hg (0.05 pg/I).

Effluent Characteristics Discharge Limitations Monitoring Requirements

Average Maximum

Sampling Method Monthly Reporting

and Frequency Values

Flow (MGD) 0.34 Estimate: Average monthly:

daily maximum daily

Suspended Solids (mg/L) 15,000 30,000 Grab: Average monthly:

daily maximum

Copper (ig/L)1'2 52.2 52.2 Grab: Average monthly:

2 times per week maximum

Mercury (tg/L)1'2 0.45 37.8 Grab: Average monthly:

2 times per week maximum
Marine Mining Technologies

and Mitigation Techniques

A detailed analysis with respect to

the mining of specific offshore mineral commodities

July 1996

Executive Summary

Contract No. 14-35-0001-30723

U.S. DeparLment of the Interior

Minerals Management Service

Office of International Activities and Marine Minerals (INTERMAR)

OCS Report

MMS 95-0003
DISCLAIMER

This report has been reviewed by the Minerals Management

Service and approved for publication. Approval does not signify

that the contents necessarily reflect the views and policies of the

Service, nor does mention of trade names or commercial products

constitute endorsement or recommendation for use.
TABLE OF CONTENTS

1.0 Introduction I

1.1 Purpose and Objectives 1

1.2 Project Team 1

1.3 Approach and Methodology 2

1.3.1 Marine Mining Scenarios 2

1.3.2 Mitigation Analysis 4

1.4 Structure of the Report 4

2.0 Summary of Environmental Issues 6

3.0 Summary of Mining Scenarios 9

4.0 Summary of Mitigation of the Environmental Impacts of Marine Mining 14

5.0 Summary of Application of Mitigation Techniques 15

6.0 General Recommendations 21

1
1.0 Introduction

This report was commissioned by the Minerals Management Service (MMS), U.S.

Department of Interior (DOT). The MMS is responsible for the regulation of mineral exploration and

development on submerged Federal Lands of the U.S. Outer Continental Shelf (OCS).

Environmental protection is an integral part of the mandate of the MMS, and regulatory requirements

exist for the enactment of mitigation measures during the conduct of all offshore mineral exploration

and development activities. In keeping with the requirements of the National Environmental Policy

Act (NEPA) and Council of Environmental Quality (CEQ) regulations, the MMS performs

environmental analysis and assessment of proposed offshore mineral developments on a case-by-case

basis. This document provides the MMS with practical guidance in prescribing mitigation measures

for future marine mining developments on the U.S. OCS.

1.1 Purpose and Objectives

The purpose of this study was to undertake a detailed analysis of available and proposed

marine mining technologies and mitigation techniques with respect to the mining of specific mineral

commodities on the U.S. Outer Continental Shelf.

The objectives of the study were:

To develop realistic scenarios for marine mining activities that are likely to occur on the U.S.

OCS within a ten-year time span

To make predictions of the environmental impacts associated with the marine mining

scenarios

To establish guidelines for mitigation of the anticipated environmental impacts

To perform cost-benefit analyses of available and proposed techniques for mitigation of the

environmental impacts of marine mining

To formulate recommendations as to the most appropriate mitigation techniques for the

scenarios and similar marine mining developments.

1.2 Project Team

To meet the above stated objectives, a multi-disciplinary team was formed consisting of

environmental scientists and marine mining and dredging specialists. The Project Team consisted

3f four principal groups, or nodes, which included a Management node, a Mining Engineering node,

a Physical Environmental Effects node and a BiologicallChemical Environmental Effects node.

1
The Management node was based at C-CORE - Centre for Cold Ocean Resources

Engineering, Memorial University of Newfoundland, Canada. The Mining Engineering node was

led by Richard Garnett, Ph.D., C.Eng., an international mining consultant based in Oakville, Ontario,

Canada. The Physical Environmental Effects node was headed by Laurie Davidson, M.Sc., Physical

Oceanographer and President of Seabome Information Technologies Ltd., located in St. John's,

Newfoundland, Canada. The BiologicallChemical Effects node was directed by Derek Ellis, Ph.D.,

R.P.Bio., Professor of Biology at the University of Victoria, Canada, and Principal of Derek V. Ellis

Ltd., marine environmental consultants.

This report was authored primarily by the node leaders. Supporting contributions were made

by Tom Pederson, Ph.D. (environmental chemist; University of British Columbia), Jack Littlepage,

Ph.D. (pelagic biologist; University of Victoria), Robert Jantzen (dredging engineer; Jantzen

Engineering Co., Inc.), Donald Hodgins, Ph.D. (physical oceanographer; Seaconsult Ltd.), and

Laurence Davis, M.Sc. (Project Manager and marine geoscientist; C-CORE).

1.3 Approach and Methodology

1.3.1 Marine Mining Scenarios

A variety of mineral commodities exist on the U.S. OCS and are potential targets for future

exploitation. These include sand resources suitable for beach nourishment, marine aggregates, heavy

minerals, precious metals, phosphorites, and others. Their exploitation will inevitably result in

environmental impacts that will vary in degree depending on factors such as the scale of operation,

the extraction technology used, and the nature of the geological and environmental setting.

The fundamental questions dealt with in this study were: what are the likely environmental

impacts associated with mining of specific mineral commodities on the U.S. OCS, and how might

those impacts be mitigated in a cost-effective manner? These questions are addressed through

analysis of a number of marine mining scenarios detailing the scale of operation, extraction

technology, and environmental setting of marine mining operations targeting a range of mineral

commodities.

The mining scenarios were developed in close consultation with the MMS, and were selected

based on the following criteria:

Potential for near-term development: only mineral commodities likely to be exploited within

a ten-year time span were considered. Other mineral commodities were excluded on the

basis of market and technological constraints.

Commodity resource information: much of the available information pertaining to the

distribution of mineral commodities on the U.S. OCS is regional and lacks sufficient

resolution to derive reserve or even resource estimates. However, there is sufficient detail

2
3

to at least target general areas of interest (scale of kilometres) and sometimes more specific

areas.

Proximity to markets and market demand: the proximity to the commodity market, and the

market demand, are important considerations for all potential marine mining developments.

The MMS provided valuable advice with respect to market requirements, which helped

considerably in determining the most appropriate site-selections for the mining scenarios.

Availability of environmental data: a final but secondary consideration in selecting the

marine mining scenarios was the availability of local environmental data. Where

environmental data were sparse or lacking, it was possible to make appropriate

generalizations about distributions of biological assemblages and about likely ecological

responses based on regional information and knowledge of the habitat.

A total of five marine mining scenarios were selected and developed in detail. These include:

Precious metal mining, Alaska - bucket ladder dredge

Precious metal mining, Alaska - underwater miner

Marine aggregate mining, Massachusetts Bay

Heavy mineral mining, offshore Virginia

Beach nourishment, Ocean City, Maryland

The first scenario is a case history of a large-scale placer gold mining operation offshore

Nome, Alaska, carried out by Western Gold Exploration and Mining Company (WestGold) Limited

Partnership during the late 1980's. Though the scale of operation and use of a bucket-ladder dredge

are not likely to be repeated in any future offshore mining activities in the region, the scenario is

highly instructive in that it documents real experience with the practice of marine mining

environmental impact mitigation. It is also the only large-scale marine mining development to occur

in U.S. waters to date.

The second scenario, which involves precious metals mining using an underwater mining

vehicle, is based on actual production trials carried out towards the end of the WestGold project life.

The two mining methods are at opposite ends of the spectrum in terms of scale of operation and

precision of extraction, and therefore provide a useful contrast of the effects of using different

extraction techniques to mine the same mineral commodity.

Offshore aggregate mining, the subject of the third scenario, is thought to be a likely prospect

in the near-term. The first markets to be served by such an industry will probably be the greatermetropolitan areas of Boston and New York. Recognizing that marine aggregate resources in the

region are potentially widespread, the choice of a location in Massachusetts Bay for a hypothetical

aggregate mining operation was based mainly on the availability of environmental information. A

comprehensive baseline environmental study was carried out in the early 1970's in preparation for

an experimental offshore aggregate mining project. The experiment did not proceed, but the baseline

environmental information gathered serves as a useful resource for this study.
Although the baseline study provides what is still perhaps the most complete site-specific

assessment of aggregate resources in the region, it reveals that the geology of the site under

consideration is not particularly conducive to economic dredging operation. The sediments are

poorly sorted and contain, on average, higher proportions of silt and clay than are normally desirable

for an aggregate target. Therefore, the hypothetical marine aggregate mining operation presented

in this report represents a worst case scenario both environmentally and economically. As a marginal

operation the aggregate mining scenario is a sensitive monitor of the economic costs of mitigation.

The fourth scenario, also hypothetical, involves the dredging of heavy mineral sands in

Federal waters off Virginia Beach, Virginia. In this case the site selection for the scenario was based

mainly on commodity resource information. Concentrations of industrial heavy minerals are known

to exist offshore Virginia as well-sorted placer deposits. The geology and environmental setting are

thought to be representative of most areas on the U.S. OCS where heavy minerals could be targeted.

Any proposal for offshore mining development in the area would have to address potential conflicts

with other uses, including commercial fishing, ocean dumping, and military practice.

The fifth scenario considers the use of sand resources in Federal waters (seaward of the state

boundary) for beach nourishment. The site selected for the scenario, Ocean City, Maryland, has

undergone repeated coastal restoration projects, the most recent was in the fall of 1994. Borrow sites

have moved progressively offshore and the need for sand resources from Federal waters appears

imminent.

1.3.2 Mitigation Analysis

One of the key objectives of this study was to develop recommendations as to the most

appropriate techniques for mitigation of environmental impacts associated with mining of specific

mineral commodities on the U.S. OCS. The recommendations are derived from analysis of a broad

range of available and proposed mitigation measures with consideration given to their relative cost,

effectiveness and practicality. The selection of mitigation techniques for analysis was based upon

the environmental impacts and mitigation guidelines, or objectives, identified for each of the mining

scenarios.

1.4 Structure of the Report

Section 2, Environmental Issues, provides a detailed overview of the environmental impacts

of marine mining with special emphasis on key issues relevant to the marine mining scenarios.

Issues relating to modelling, and to environmental monitoring and quality assurance/quality control

(QA/QC) protocols, are also introduced and discussed in the context of environmental impact

mitigation.

The marine mining scenarios are presented in Section 3. Details are provided of their

environmental settings, the commodity resource characteristics, and the extraction and processing

4
methods. Predictions of the potential environmental impacts of each extraction technology are

provided along with guidelines for mitigation of those impacts.

In Section 4, techniques for mitigation of the environmental impacts of marine mining are

reviewed and in Section 5 these are applied in detail to the marine mining scenarios.

Recommendations are made as to the most appropriate selection of mitigation measures for each

mining scenario. A summary and recommendations are presented in Section 6.

5
2.0 Summary of Environmental Issues

Physical environmental processes affect marine mimng operations in two ways. Firstly, these

processes constrain or control the actual conduct of the miiiing operation, and thus are significant

in determining the potential environmental impacts of the operation. Factors exercising control over

mining operations may vary in importance with the mode of mining, but in most instances include

wind, waves, currents and ice. Temperature, precipitation and water level are usually of secondary

interest, except possibly in locations where unusually high tides or tidal ranges might impose

operating constraints. Seasonal as well as short time scale changes in these conditions combine to

determine both the annual operating season for a mining activity, and dictate the viability of

particular operations or activities on a particular day.

Principal sources of potential environmental impact resulting from mining operations include

processes such as the physical disruption of the seabed, the deposition of disturbed and/or mined

materials over undisturbed portions of the seabed, and the dispersion of particulate material through

the water column. Thus, the second physical environmental issue is the role of physical processes

in determining the distribution and motion of this particulate load through the water column, the

timing and location of the deposition of this load on the seabed, and the possible resuspension and

redeposition of unconsolidated materials after initial deposition on the sea floor. The magnitude and

variability of horizontal currents, as well as the grain-size distribution of the particulates and the

particulate cohesive properties, ultimately dictates the residence time in the water column, and the

range of horizontal transport of the particles before they are eventually redeposited on the seabed.

Marine organic matter in seabed deposits fuels a suite of bacterially-mediated postdepositional (diagenetic) reactions in which the organic material is progressively degraded as

oxidants are consumed. The reactions yield a number of products which may have direct or indirect

environmental implications for mining.

Organic matter represents a highly reduced form of carbon which is rich in electrons, and is

therefore a primary energy source. Over most of the seabed, marine organic materials are degraded

by aerobic bacteria, wherein oxygen is the terminal electron acceptor in the reaction CH2O + 02 =

CO2 + H20. Where oxygen is absent, the bacterial community takes advantage of other electron

acceptors, which (in thermodynamic order) include nitrate, manganese and iron oxides and sulphate.

Thus, aerobes are succeeded by nitrate-reducing bacteria, and later by sulphate-reducing bacteria as

long as degradable organic matter remains. This suite of reactions leads to a biogeochemical

zonation in sediments. Anoxia results where oxygen supply via advection or diffusion is exceeded

by demand of aerobes and by inorganic consumption of oxygen by a reaction with reduced species

such as Fe2t Anoxic conditions at relatively shallow depths (a few centimetres or less) are typical

in most coastal and inner continental shelf sediments.

Benthic (seabed) organisms are the base feedstock for fishery and wildlife resources such as

seabed feeding fish, diving waterfowl and marine mammals, and also contribute to the larval

biodiversity of the water column. Their populations allow calculations of biodiversity, and

6
biodiversity can be used as a yardstick for gauging environmental impact. Biodiversity is measured

by (1) how many different kinds of organisms are present (Species diversity), and (2) the relative

numbers of each (Numerical diversity).

Two sets of risks arise from dredging operations: (1) extraction risks (i.e. removal of habitat

material with some surrounding disturbance), and (2) discharge risks (i.e. deposition of discharged

tailings). Extraction removes sediment habitat and the organisms living within the sediment

(infauna), or on it (epifauna). The burrowing forms are relatively immobile and cannot swim away

from the cutting face of the mining excavation. These benthic organisms include fish and prawn

resource species that maintain burrows, and many species of smaller creatures which can function

as feedstock for resource species.

Epifauna come in two forms. The mobile epifauna generally live on the surface of the

deposits. They may be able to remove themselves from the immediate dredge area. Examples

include flatfish, crabs, sea cucumbers and sea urchins (almost immobile), scallops, and other

resource species. The other epifaunal forms are sessile, and are attached to large cobbles, boulders

or rock outcrops. They are generally attached so firmly that they are only removed if their host

substrate is removed. In general, seabed mining will have an impact on resource species, their

feedstock and habitat.

A discharged tailing slurry will fall through the water column, with some dispersion of fines),

and will eventually reach the seabed as a density current. After initial impact, the tailing will spread

in some pattern as a seabed dispersal field. During this spread, deposition occurs, with coarser

particles settling before the finer. Large amounts of tailings may smother and kill organisms over

a broad area. Smothering kills benthic organisms by preventing them from breathing or feeding. In

general the ability of benthos to avoid smothering, or to burrow up faster than smothering deposits

can settle, is not known.

Natural annual fluctuations in biodiversity (both number of species, and number of

organisms) are enormous. Such natural fluctuations cause great difficulty in describing ambient

faunas and environmental impact on benthos. This applies to measuring biodiversity before dredging

operations start, assessing changes derived from the dredging operations, and assessing the progress

of recolonisation after dredging.

Spatial differences in biodiversity on uniform deposits means that the fauna of Dredging

(Test) and nearby Reference sites will inevitably differ to some extent from natural causes, and from

year to year. There may be no cost-effective way to obtain accurate and precise biodiversity

estimates of benthos with different and varying patchiness. Crude quantitative estimates may have

to be accepted.

After a deposit is mined the seafloor is recolonised by benthic organisms. Times for recovery

of a reasonable biodiversity depends on the sediment type. Fine-grained tailings may need only 1

7
year before achieving a recovery level biodiversity, medium-grained deposits 1 - 3 years, and coarsegrained deposits 5 or more years.

During marine mining a number of general impact mitigation objectives are desirable:

Water column:

minimize elevations of surface water turbidity

Seabed:

minimize dredge course overlaps and gaps

minimize return of tailings outside dredge footprint

leave surface of similar grade

no release of contaminants

no anoxic dredge furrows

recovery to a successional community within time scale of: silts - 2 years, sands - 3

years, gravels - 5 years

avoid sensitive areas - endangered species, isolated breeding grounds, etc.

Fisheries and Wildlife Resources:

avoid sensitive habitats, such as finfish and shellfish spawning grounds, and wildlife

migration staging grounds or intense migration routes

design quiet vessels to Best Practical Technology

8
3.0 Swuinary of Mining Scenarios

Five marine mining scenarios are discussed. Details of the marine conditions affecting the

location of each scenario are provided. The scenarios provide details on all aspects of the mining

operations, including the extraction technologies, dredged material processing methods, and effluent

types and constituents. Information relating to the geological and environmental settings is also

provided. At the end of each scenario predictions of environmental impact are given, potential

mitigation objectives are identified, and criteria for appraising the mitigation method are proposed.

1) Precious metal (gold) mining offshore Alaska using a bucket ladder dredge

This scenario is a discussion of actual experiences during a gold mining operation offshore

Nome. Dredging for gold has been attempted several times this century but only WestGold achieved

a significant production using a bucket ladder dredge from 1986 through 1990. The exploration

efforts and the environmental research of WestGold are fairly well documented in various

publications. Much of the experimental work on reducing the environmental effects of the WestGold

dredging is applicable to other forms of dredging. Therefore the Nome bucket ladder dredging

project constitutes a very useful scenario for study. Dredging was carried out using the world's

largest ocean-going bucket ladder dredge, the BIMA. During the years 1987-1990 the dredge

recovered on average 775 mg gold/rn3. There were extensive pre- and post-dredging surveys to

measure the impact of mining

In general terms the sand bed biodiversity started to recover within 1 year of dredging.

A few new species entered the fauna and later disappeared, but there is no clear successional pattern.

By 1990, four years after dredging at one sample station, the biodiversity was clearly back in the

range of the reference stations. Cobble bed biodiversity appears to have recovered more slowly. The

number of species remained as high as the reference stations, or almost so, even 1 year after

dredging. However, the number of organisms showed a reduction barely reaching the lower end of

the year to year range at the reference stations.

The one fishery species monitored in detail was the red king crab (Paralithodes

camtschatica). It was found that crabs move locally over distances which allowed them to transfer

between dredged and undredged areas. Their movements prior to dredging were apparently random,

although crabs often have seasonal shifts with depth. Crabs were fished from both dredged and

undredged areas, with more competition by boats for fishing sites in the dredged than the undredged

areas.

Water column impact was limited to a turbidity plume extending from the dredge discharge

points. Seabed disturbance was caused by the dredge cuts and deposition of tailings. Dredge

footprints were substantially smoothed by wave and current action within 3-5 years, even on the

coarsest gravellcobble habitat. A level of biodiversity encompassing several tens of species and

several hundred organisms/rn2 had recolonized the impacted areas within 3-5 years, with recovery

slower on coarser fraction gravel and cobble habitat.

9
Precious metals mining offshore Alaska using an underwater mining vehicle

An underwater miner (UIWM) constitutes the opposite extreme of a large bucket ladder

dredge: it has a comparatively very low throughput, is limited in terms of the depth to which it can

dig below the seabed, and has the ability to be very selective in its extraction, both laterally and

vertically. Dilution is minimal but unit volume costs are high. A bucket ladder dredge, by contrast,

relies for its economic success on its digging power and its high throughput, resulting in a low unit

volume cost. It is very non-selective, laterally and vertically, and a surficial deposit worked by it is

heavily diluted in grade.

WestGold deployed a UWM in the Nome area of Alaska in the summer of 1989. This was

the first recorded time that such a piece of equipment had been used for an offshore mineral recovery

program other than remotely operated collectors in the earlier deep ocean manganese nodule projects.

The UWM used was the "Tramrod", model 250, developed and manufactured by Alluvial Mining

of the United Kingdom.

The "Tramrod11 is deployed from an ocean-going barge with a draught of 1.5 m. The UWM

is placed on, and recovered from, the seabed by a truck-mounted 210 tonne-capacity crane. Mined

sediments, to a maximum of 0.25 m in particle size, are suctioned through a 25 cm intake and

pumped to surface through a 0.305 m internal diameter floating pipeline.

Marine aggregate mining in Massachusetts Bay with a trailing-suction hopper dredge

The deposit is located within the 3 mile limit offshore from Boston. It is part of a complex

mixture of glacial products similar to that offshore Nome, Alaska consisting of glacial till, a

heterogeneous mixture of boulders, gravel, sand, silt, and clay ranging in thickness from a thin

veneer to nearly 30 m. Two clay units overlie the glacial till and the deposit appears to be a

gradational feature resting on the upper clay unit. The lithology changes rapidly over short

horizontal distances and with each vertical meter below the seabed. The potential dredge area

reportedly comprises about 5.25 million m3 of sand and gravel in water depths of less than 18 m.

Assuming that 100% extraction is achieved, then the deposit's productive life is 2.9 years. The

minimum required project life is 10 years, preferably 20 years or more. Therefore, in practice,

additional or alternative dredging sites will have to be found before the project is started. Mining

would be by means of a large ocean-going trailing suction hopper dredge equipped with twin trailing

drag-arms. Production would be in the order of 18,000 metric tonnes of aggregate per day.

The fauna of the Massachusetts Bay area were documented in a large study during the

summer of 1973. The bio diversity was rich and variable with differences between epifauna on hard

rock (including on exposed cobble surfaces) and the burrowing infauna. There are habitats of hard,

cobble, sand and mud substrates. Over 650 species were sampled. The number of species per

station ranged dramatically, from 31 to 146 per station, as did the number of organisms/rn2, from 305

to over 11,000. The species complexes found on different substrate types differed but statistical

correlation with deposit type was not possible. The temporal variation was enormous from month

10
to month, making the use of dominant species only possible at specific site locations that cannot be

generalised to a substrate level.

Benthic communities directly in the path of the dredge will be destroyed. The benthos in

undredged areas between the furrow lines will be affected by increased turbidity and sedimentation.

The impacts will depend on the depth of sediment that accumulates and depth of the furrows.

Unlike mining scenarios where most of the sediment extracted is returned to the environment,

in mining aggregates a substantial percentage of the particular size fraction is removed. This will

change the particle size characteristics of the deposits remaining. The larvae which recolonise the

altered environment will then reflect these changes. It is probable that the silt/clay fractions will

increase in the dredged area because the coarser sediments are to be retained by the dredging process.

Soft-bottom communities should recover faster than communities found on coarse sediments,

or the epifauna on cobbles and boulders. Expected times to recovery, based on other studies, are

suggested to be 1+ years on deposits with least cobbles, with slower recovery where substantial

numbers of cobbles are removed.

Environmental impact mitigation objectives should include:

reduction of dredge-derived turbidity through improved outfall design and vessel operations

possible development of alternative enhancement uses for under-sized and over-sized

fractions

return of fines to dredge footprint.

4) Heavy minerals dredging offshore of Virginia Beach, Virginia using a cutter suction dredge

The heavy mineral deposit is contained in sand. Total heavy mineral (THM) concentration

averages about 3% with values up to 3.65% in the uppermost metre. THM content decreases steadily

with increasing depth below the seabed, and the incremental THM content, on average, falls to

slightly less than 3% (the grade most probably required in such an offshore operation) at a depth of

4 m. The deposit would be mined using a cutter suction dredge on either a floating pontoon or jackup platform. Throughput would be in the range of 113,000 to 151,000 litres of slurry per minute,

with approximately 95% of dredged sediments returned to sea as effluent.

It is expected that biodiversity will be high on these sands, as is usual at this latitude and

water depth. At this site prior human disturbance has probably introduced contaminants or otherwise

rendered the ground unsuitable or even dangerous for dredging. There are dump-sites indicated on

hydrographic charts and the area is in the proximity of a Navy firing range. The benthos will

probably show localised impact from such prior uses. This will need determining by site-specific

baseline surveys.

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Extensive pre-dredging sampling will be needed to show if there are areas of recent

disturbance with little benthos, stable areas with a normal variability of the fauna, and areas in which

elevated levels of benthos may occur due to enhancement effects of the nearby disturbance. The

site's close proximity to the City of Virginia Beach may pose further stresses to the environment

from sewage, storm drain run off and industrial run off. Existing status of fishery and fish-nursery

sites would need documenting.

Mining-related impact will be from a combination of the removal of substrate and smothering

from the large amounts of tailings. Benthos directly in the path of the dredges will be destroyed.

Surrounding benthos will be affected by sedimentation and turbidity. Trenches formed by the cutter

head will develop as will elevated areas. Impacts will be extensive as experienced at the WestGold

bucket-ladder dredge operation, with time to recovery determined by the nature of the deposit and

energy level of the site. Smoothing of the seabed should occur from natural wave action. In general

terms recovery on sand beds can be expected within 3 - 5 years but the exact composition of the

fauna will vary from place to place, and may not resemble the original populations. Monitoring the

nature of the recovery due to mining may be difficult because the area already may be affected by

other anthropogenic influences, and these may continue to affect the site.

Environmental impact mitigation objectives should include:

minimization of surface turbidity

reduction of benthic impact by constraining tailings discharge to dredge footprint and nearby

possible development of alternative enhancement projects.

5) Beach nourishment at Ocean City, Maryland, using a cutter suction dredge pumping sand

ashore or, alternately, a trailing-suction hopper dredge transporting sand to a nearshore

pumping station

Ocean City's beach is eroded at a rate of about 1 m per year and requires replenishment, by

agreement with the authorities, every 2 - 3 years. The site is selected so as to provide sand of the size

and type normally found on the adjacent beach that is to be renourished. A previously used site 3.2

km offshore is now exhausted. In 1988 the sailing distance was 4.2 km from the hopper dredge

pump-out shoreline connection to the dredging site where 77% of the available 3.06 million m3 of

sand was in more than 15.3 m of water. The borrow site used in 1994 was 8 km from shore where

the water depths are> 15.3 m. The 1.2 - 1.5 m cut bank of sand is underlain by mud and clay which

must not be disturbed.

Generally, if the borrow area is less than 5 - 6 km from the beach, a cutter suction and

pipeline are used. If the distance is greater than 5 - 6 km a hopper dredge is employed. Pipeline

deployment over greater distances is possible but is dependent upon the prevailing sea conditions

at the site. A cutter suction dredge is more productive than a large hopper dredge because the latter

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cannot approach close to the beach with the prevailing water depths. An additional factor at Ocean

City is the difficult entry and the depth of water at the entrance to, and within, the harbour that

restricts the size of hopper dredge which may enter to take shelter from severe storms.

Sand-bed benthos is characteristically of high biodiversity, with many species scattered

through major taxa, and dynamic in the sense of species changing relative numbers from year to year.

Species diversity ranged up to more than 100 per unit area investigated. Numerical diversity ranged

up to more than 7000/rn2.

The immediate impact from mining will be formation of borrow pits. The depth, width and

length of the borrow pits could be regulated to maintain current flow so that they do not become

settlement basins with consequent anoxia, sulphur-reducing chemistry, and reduced biodiversity.

There will be an immediate loss of virtually all the benthic species present. Mining-related seabed

disturbance may release nutrients and uncover and displace feedstock species temporarily providing

extra food for bottom-feeding resource species. Fishing may improve temporarily downcurrent of

the dredging area and continue for some months afterwards. Benthos may also show a temporary

increase in biodiversity downstream from the dredge site.

The mining footprint probably will start to smooth, and biological recolonisation to start,

within weeks of closure. If closure is during an annual larval settling period, recolonisation will be

most rapid, and may be demonstrable within days of closure. It is to be expected that moderate

biodiversity will have reformed within 1 year.

Environmental impact mitigation objectives should include:

minimizing cut-face turbidity, thereby preventing upwelling to surface

planning the location, depth, width and length of dredge courses to minimize the risk of

anoxic borrow-pits developing.

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4.0 Summary of Mitigation of the Environmental Impacts of Marine Mining

The main environmental effects of marine mining that occur and which must be minimized

or reduced to acceptable limits are:

the creation of water turbidity

the release and spread of contaminants

the distribution of sediments outside the dredge footprint

changes to the seabed lithology and profile

Any one or more of four possible requirements of mitigation may be applicable to a marine

dredging operation. Each of the four approaches requires a procedure, alone or in combination with

others, to be successful:

to avoid completely any environmental impact through location-based, time-based, or scalebased constraints on the dredging, alone or in combination

to remove or lessen a cause or effect, and thereby to minimize the unavoidable environmental

impact, through: (a) restrictions on the dredge discharges and certain operating techniques,

and (b) the requirement that the best and most suitable technology is available

to remediate the environmental impact through post-dredging activities

to compensate for the environmental impact through alternative enhancement of a similar or

related environment.

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5.0 Summary of Application of Mitigation Techniques

Precious metal (gold) mining offshore Alaska using a bucket-ladder dredge

Monitoring of the tailings plume during the mining operation was carried out from the

collection of samples from tailings launder and gold room effluent, and from sea water up-current

and down-current from the dredge. WestGold and its consultants employed a 3-dimensional water

quality and sediment plume computer model referred to as Disposal from a Continuous Discharge

(DIFCD) and developed by the U.S. Army Corps of Engineers Waterways Experiment Station. It

was used in conjunction with a Wave Current Sediment Resuspension Prediction (WCSRP) model.

The DIFCD and WCSRP computer programs proved to be invaluable in predicting the plume

creation behaviour of the tailings discharge.

A large variety of mitigation techniques were considered and many were tried over the 1986

to 1990 operation period of the BIMA dredge. The most important was the control of the tailings

effluent discharge so as to minimize the degree and extent of sea water turbidity. The optimal design

developed from four years of dredge-operating experience comprised fitting the existing 1.8 mdiameter header pipes with cones that terminated in a flanged collar of 0.50 m diameter. A straight

circular steel pipe was attached to the flange and extended vertically down to within about 0.5 m of

the water surface. A flexible hose of the same diameter was affixed to the steel pipe and extended

2.7 m below the water surface. Several other configurations were considered but were not employed.

They were discarded in the design stage after operating experience quickly demonstrated their

impracticality under the prevailing marine and dredging conditions. They included multiport

systems, a form of controlled surface discharge and flexible stinger pipe designed to lie on the

seabed.

precious metals mining offshore Alaska using an underwater mining vehicle

A number of location-related restrictions applied to the operation that were designed mainly

to protect migrating salmon. Dredging could not take place:

within 30 m of the low water mark

within 1.6 km of the mouth of a river used by migrating salmon

in water depths of less than 5 m.

Compared with the large-scale bucket ladder dredge the use of a selective, small scale

operation allowed greater flexibility in mitigating the deleterious effects of the effluent discharges.

The use of an underwater miner also served to mitigate the important impact of changing seabed

profile caused by dredging.

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A novel outfall design, consisting of two large troughs fixed along both sides of the treatment

plant barge, was used by WestGold to collect and discharge the jig tailings effluent. The trough was

perforated along its base to provide discharge ports so that the slurry was released as a series of small

point discharges. The sluny may be discharged in the sea either from above or may be fed down a

series of flexible hoses, with quick-coupling disconnect fittings, capable of discharging at various

depths above the seabed. Diffusion of the effluent around the barge perimeter, coupled with the low

gradient of the flexible subsea hoses, minimized the turbulent kinetic energy of the discharge. This

approach was made especially practical by the comparatively low throughput rate of the underwater

mining vehicle.

3) marine aggregate mining in Massachusetts Bay with a trailing-suction hopper dredge

Seabed Lithology and Mitigation. Repeated excavation of aggregate within a dredge

course eventually may reveal different lithologies. As the workable deposit thins through repeated

removal of sediments, high-spots in the underlying sequences are eventually exposed. These may

be hard bedrock or glacial clay similar to that below the aggregate source in Massachusetts Bay. To

the dredge operator such exposure raises the possibility of unwanted cargo contamination. Overdredging has other effects. Local fishing may be affected if boulders become exposed since the

potential for damaging trawling gear is increased. A layer of the original substrate should be left on

the seabed to increase the ability of the original benthic communities to recolonize.

The mitigative solution is two-fold. Either avoid such concentrations of glacial sediments,

or apply restrictions to the extent that aggregate may be removed only from the top of the vertical

sediment sequence.

Seabed Profile and Mitigation. Even if some care is taken by the avoidance of an

excessively repetitive dredge course and direction, trailer suction hopper dredging does not always

yield a sufficiently flat seabed profile. After final excavation the seabed surface frequently contains

too many peaks and valleys because the drag-heads have a tendency to track the previous excavation.

The objective of dredging should be to plane, rather than to trench, the seabed. Operators should

attempt to dredge courses adapted to the seabed geology, even if the courses are oriented across tidal

flow. Ideally, the courses should be adapted to the contours of equal sediment thickness.

Water Quality Mitigation

Overflow Mitigation: Operational Technique. A practical mitigative technique is to delay

the commencement of dredge hopper overflow. This is achieved by emptying all water out of the

hopper before starting to pump any sediments on board. Water is discharged directly overboard as

the dredge pump is primed. A switching mechanism is then used to direct the slurry to the hopper

once sediments are entrained in the pumping system. As a result overflow does not occur until the

hopper is filled to within 60-70% of its dredged material capacity. The time during which sedimentladen waters are released to the sea is thereby substantially reduced.

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17

Overflow Mitigation: Re-cycling. The drag-heads of some trailing suction hopper dredges

are equipped with water jets that are designed to assist in the liberation of compacted sediments

when dredging hard ground. A small percentage of the hopper overflow water can be pumped to the

drag head jets instead of using clean seawater, reducing slightly the total volume of sediment-laden

overflow.

Overflow Mitigation: Hopper Design. The level of the aggregate surface in a loaded

dredge hopper always must be above the load line in order to prevent surplus water from being

transported to shore in addition to the solids. This can be achieved by modifying the shape of the

hopper to keep much of the contents above the load line. Alternatively, special compal tiiients may

be employed on either side of the hopper.

Overflow Mitigation: Overflow Collection. An anti-turbidity measure proposed for

hopper dredging is to install compartments on both sides of the dredge. These are designed to

intercept and temporarily retain the overflow, allowing partial settling of suspended silts within

them. However, there are potentially serious, practical, limitations:

the retention time needed for substantial settling to occur

the need, at intervals, to dispose of the accumulated silts

the significantly reduced hopper capacity caused from the loss of vessel space by

incorporating settling compartments.

Overflow Mitigation: Effluent Discharge. A relatively simple technique for handling

hopper overflow, called an anti-turbidity overflow system (ATOS), has been developed in Japan.

The overflow collection system is streamlined to minimize the entrapment of air bubbles in the

overflow water. Removal of the air bubbles, which otherwise make the particles buoyant and

prolongs settling, allows the fines to settle at a faster rate.

4) Heavy minerals dredging offshore of Virginia Beach, Virginia using a cutter suction dredge.

Water Quality Mitigation: Cutter-Generated Turbidity

Operational Mitigation. Several operational controls are recommended for the reduction

of sediment resuspension. Most involve the manner of excavation. Very thick single-pass cuts

should be avoided, because the cutter-head tends to become buried. As a result more sediment is

excavated and dislodged than the suction can successfully entrain, leading to high levels of

suspension. A thick deposit is best excavated by means of several horizontal slices, each layer of

excavation being a single thin cut depth.

In other places it may be more economical to excavate and remove most of the sediments

very roughly in a single cut. Final clean-up to the base of the pay zone may be delayed until later
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in the operation. However, the loosened sediments, while awaiting clean-up, may be at risk of

suspension.

Where the mining plan involves dredging to a few meters below the seabed within a defined

block the operation should proceed without excavating vertical walls in each cut. The sides should

be inclined sufficiently so as to inhibit the slumping of loose sediments into the cut. The final walls

of a cut should be terraced by reducing the area of cut with each successively deeper slice. On a

smaller scale the formation of distinct cuts should be minimized. The dredge should be swung and

advanced so as to cover equally all the seabed within the block. The cutter-head should be advanced

by increments that are sufficiently small to ensure that very close concentric arcs are described on

the seafloor with each swing of the cutter.

Design Mitigation. The cutter-head's suction should be sufficiently powerful to collect all

the sediments it disturbs. Pick-up capability could be increased by including water jet booster

systems or ladder-mounted submerged pumps. The cutter-head may be designed in such a way that

the suction is brought closer to the sediments thereby improving the chances of entrainment.

Differences in shape are particularly important if the cutter-head is not completely buried in the

sediments.

The acute angle between the base of the cutter tooth and the upper surface of the sediments

is referred to as the rake angle. Proper design of the cutter with an appropriate rake angle is an

important mitigation factor. If the rake angle is too large, a gouging action throws soft, fine-grained

sediments outward. If the rake angle is too small, however, resuspension occurs through impact.

Water Quality Mitigation: Plant Effluent

The most obvious water turbidity is created by the surface discharge of slurry effluents as

tailings from the treatment plant. Mitigation may be attempted by:

minimizing the possibilities of re-digging of the settled tailings through operational

procedures

improving the nature of the effluent by de-sliming, dewatering or degassing

optimizing the manner in which effluent is discharged by employing sub-surface discharge

and/or discharge diffusion

modifying effluent behaviour in the sea by the use of flocculants.

Operational Mitigation. The mine plan should be designed to avoid significant

encroachment of the dredge on the boundaries of a mining block which previously has been dredged

and the surface of which is now covered with tailings. The deliberate exclusion of areas of low-
grade ground from a first dredge course, on the assumption that they may be dredged as part of a

subsequent program, should be avoided.

De-sliming and Dewatering. Separation of the effluent flow into its component parts of

solids and water allows the materials to be discharged into the sea with less impact. The solids are

not held suspended in the water by the boiling action which otherwise is evident. The water,

although containing some fines, is more rapidly diluted by the surrounding sea water. De-sliming

and dewatering systems include hydrocyclones, hindered settling classifiers, bucket-wheels and

screws and dewatenng screens.

Degassing. It is not known whether methane or hydrogen suiphide are present in seabed

sediments offshore Virginia in the vicinity of the mining site. If either gas does occur degassing

equipment should be employed downstream of the dredge pump to prevent depriming. Both the

pump and degasser must be installed underwater because the water depth is greater than 10 m. To

limit water column turbulence associated with released gases the installation of a conduit from the

degasser to the surface is necessary to allow gases to be vented to the atmosphere.

Effluent Discharge Design. Surface water turbidity and the deposition of tailings on the

seabed can be controlled to a significant extent by using a sub-surface discharge. Such an

arrangement would consist of a reinforced rubber pipe attached to the vessel. It may be deployed

in any attitude ranging from vertical to near horizontal. A reduced effluent discharge velocity could

be achieved by fitting the end of a discharge pipe with a flared opening. This modification may

require some experimentation to achieve the optimum arrangement.

Flocculants. The use of flocculants comprises the only means available of speeding the

settlement of entrained silts. Flocculants have two effects. Firstly, because they are flocculated fine

sand and silt are deposited together, thus preventing the natural separation of the two fractions that

occurs naturally during settling. Secondly, the deposited silt is increased in volume by the

entrapment of water. However, volumetric increase should be minimal if the correct amount of

flocculant is used.

Seabed Profile and Mitigation

Operational Procedures. The creation of elevated ridges of tailings on the seabed along the

dredge course, especially at its start, may be minimized during operations in several ways:

a sufficiently wide cut face must be dredged

the dredge must not be restrained by sidelines which are so short as to cause it to angle

excessively

the dredge must not be angled deliberately and unnecessarily into the corners of the cut face

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(4) the gaining of depth at the start of a deep cut must be undertaken at a slow rate

Discharge Control of the Trommel Oversize. Every effort should be made at sea to re-mix

the oversize and the undersize. This involves discharging the screen oversize as closely as possible

to the solids produced as the cyclone product, without causing unnecessary turbulence.

Alternative Enhancement. Considerable potential exists for alternative enhancement

initiatives when dredging for heavy minerals. In particular, the undersize tailings can be pumped

ashore, if needed, for beach renourishment, the remediation of contaminated areas, and the creation

of wetlands.

5) Beach nourishment at Ocean City, Maryland, using a cutter suction dredge pumping sand

ashore or, alternately, a trailing-suction hopper dredge transporting sand to a nearshore

pumping station

Prevention of Coastal Erosion. To avoid the possibility of erosion both the local

hydrodynamic conditions and the sediment characteristics of the deposit being dredged must be

considered. Wave refraction analyses and wave transformation modelling are a necessary input to

a study. Prevention of erosion may be achieved by adopting a correct licensing procedure and by

restricting the operation of a dredge to specified water depths and/or areas.

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21

6.0 General Recommendations

General recommendations derived from the analysis presented in this report include:

the use of Best Available Technology should be practiced with any marine mining operation.

The use of Best Available Technology is not only beneficial environmentally but in most

cases improves operational efficiency and is therefore of value to the operator.

numerical modelling is a useful and cost-effective tool in the development of effluent

discharge systems, the design of environmental monitoring programs, and the assessment of

the potential risk of coastal erosion. Modelling should be implemented as part of any mining

project that involves substantial discharge of effluents at sea, or of projects that may

potentially affect coastal stability.

well-designed environmental monitoring programs with appropriate QAIQC are a necessary

component of any marine mining project. Monitoring ensures that regulatory constraints are

observed, and also provides the operator with ongoing guidance as to the level of effort

needed to meet those constraints.

operational constraints of location, time and scale need to be determined before the initiation

of any mining project, so that the economic viability of the project can be realistically

assessed. From an operational and economic perspective, constraints on operations that are

based on random occurrences such as sightings of particular species in the mining area are

detrimental and potentially very costly. The possible implications of such constraints should

be carefully considered.

mitigation can in some cases be achieved cost-effectively through changes in operational

procedures, such as the planned order and distribution of dredge courses or the depth and

width of cut. The planning of any new marine mining venture should take into consideration

the range of operational mitigation techniques presented in this report.

many technology-based mitigation techniques, such as sub-surface tailings discharge

systems, dewatering and degassing systems, and onboard feed monitoring systems,

measurably reduce the impacts associated with a mining operation and can be implemented

at reasonable cost. The implementation of these techniques should be considered for any

future offshore mining development.

the potential for alternative enhancement projects should be explored during the planning

stages of future marine mining developments. For example, consideration should be given

to the possibility of using tailings produced by a heavy mineral mining operation for

beneficial purposes, such as beach nourishment or the creation or restoration of coastal

wetlands.