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:MJ~i~~lMining Technologies

.an~I' Mitigation Te.chniques II II I·. - .

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tlhetilnfrlgof specific offshore mineral .~~~1JI0clifi~S

July 1996 I.,.,.A!

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,Executlive Summary

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Contract No. 14-35-0001-30723

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•• U.S. Department of the Interior

.",rgg Minerals Management service

I I : - - Office of International Activities and Marine Minerals (INTERMAR) ,

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DISCLAIMER

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This IrJport has been reviewed by the Minerals Management

Se:rvice and approved for publication. Approval does not signify

that the contents necessarily reflect the views and policies of the

S~rvice, nor does mention of trade names or commerciafproducts

90nstitute endorsement or recommendation for use. I

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TABLE OF CONTENTS

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1.0 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

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! 1:.1 Purpose and Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

1'.2 Project Team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

1.3 Approach and Methodology 2

i 1.3.1 Marine Mining Scenarios '. . . . . . . . . . . .. 2

I i 1.3.2 Mitigation Analysis ,. 4

: VA Structure of the Report 4

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2.0 Srtmm;rry of Environmental Issues '. . . . . . . . . . . .. 6

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3.0 SIIII\m;rry of Mining Scenarios .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 9

"I 4.0 sJnJm;rry of Mitigation of the Environmental Impacts of Marine Mining .. . . . . . . . . . .. 14

I ! 1 " ' 5.0 Sunlmary of Application of Mitigation Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15

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6.0 Genbr~Recommendations 21

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1.0 Intr6duction

. I~hil r~port was commissioned by the Minerals Management Service (MMS), U.S.

Departlnbnl of Interior (DOl). The MMS is responsible for the regulation ofmineral exploration and

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

EnvirolrrheAtai protection is an integral part ofthe mandate ofthe MMS, and regulatory requirements

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

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

Act (¥Pt) 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. jrfus!document provides the MMS with practical guidance in prescribing mitigation measures

for futur6 n\arine mining developments on the U.S. OCS.

i '' I , 1.1 pili-pose and Objectives

I~j purp'ose of this study was to undertake a detailed analysis of avaiiable and proposed I I I .

marine rr1ining'teChnologies and mitigation techniques with respect to the mining ofspecific mineral I I I ;: .

commodities on the U.S. Outer Continental Shelf. I

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1 I !; The objectives ofthe study were:

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(1) I-Fo dev~16p realistic scenarios for marine mining activities that are likely to occur on the U.S. I I I ' I

I OCS within a ten-year time span' .

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(2) II ~o imake predictions of the environmental impacts associated with the marine mining

scenanos

(3) iJo Ltablish guidelines for mitigation of the anticipated environmental 'impacts

(4) i)0 be~oim cost-benefit analyses of available and proposed techniques f~r mitigation of the

I ertvlronmental impacts of marine mining .

I i I . (5) [1;"0 jforinulate recommendations as to the most appropriate mitigation techniques for the

I scenarios and similar marine mining developments. , 1 I . I ,

1.2 Project Team

iT: I th b d b" I . d' . I' ) d ,. f ! ;0 Imeet e a ove state 0 JectlVes, a mu tI- ISCIp mary team was ,orme conSIStIng 0

enviro\unental scientists and marine mining and dredging specialists. The Project Team consisted

offout plirlcipal groups, or nodes, which included a Management node, a Mining Engineering node,

a PhyJic~IEnvir'onmentai Effects node and a Biological/Chemical Environmental Effects node. I • I .

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,; The Management node was based at C-CORE - Centr~ for'Cold Ocean Resources

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

led by Richard Garnett, PhD., 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 Seaborne Information Technologies Ltd., located in St. John's,

Newfoundland, Canada. The Biological/Chemical Effects node was lIirected by Derek Ellis, Ph.D.,

R.P.Bio., Professor of Biology at the University of Victoria, Canada, jrnd Principal of Derek V. Ellis

Ltd., marine environmental consultants. 'I

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:: This report was authored primarily by the node leaders. Supp6rting contributions were made " ' by Tom Pederson, Ph.D. (environmental chemist; University of British Columbia), Jack Littlepage,

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

EJigineering Co., Inc.), Donald Hodgins, Ph.D. (physical oceanogtapher; , Seaconsult Ltd.), and

Laurence Davis, M.Sc. (Project Manager and marine geoscientist; G-CORE).

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1.3 Approach and Methodology ,!

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1.3.1 Marine Mining Scenarios

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

e,:ploitation. These include sand resources suitable for beach nourishIhent, marine aggregates, heavy

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

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

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

II . I · :i, The fundamental questions dealt with in this study were: what are'the likely environmental

inlpacts associated with mining ofspecific mineral commodities on ~the U.S. OCS, and how might

those impacts be mitigated in a cost-effective manner? These qu~stions are addressed through

analysis of a number of marine mining scenarios detailing the ~cale of operation, extraction

technology, and environmental setting of marine mining operation~ targeting a range of mineral

commodities. I '

Ii The mining scenarios were developed in close consultation with the. MMS, and were selected

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Potential for near-term development: only mineral commodities likely to be exploited within , a ten-year time span were considered. Other mineral comJttodities were excluded on the

basis of market and technological constraints. i

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Commodity resource information: much of the available information pertaining to the

distribution of mineral commodities on the U.S. OCS is iregional and lacks sufficient , resolution to derive reserve or even resource estimates. However, there is sufficient detail

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: to at least target general areas ofinterest (scale of kilometres) and sometimes more specific , ' I

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(3) IProhmity to markets and market demand: the proximity to the commodity market, and the

~arket demand, are important considerations for all potential marine milling developments.

1ihd MMS provided valuable advice with respect to market requirements, which helped

c~nkidera.bly in determining the most appropriate site-selections for the mining scenarios.

Av1labilitY of environmental data: a final but secondary consideration in selecting the

marine mining scenarios was the availability of local environmental data. Where

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e~vironmental data were sparse or lacking, it was possible to make appropriate

genbralizations about distributions of biological assemblages and about likely ecological

rfsponses based on regional information and knowledge of the habitat.',

Ii t1tal offive marine mining scenarios were selected and developed in de~l. These include:

I pretous metal mining, Alaska - bucket ladder dredge

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,~ref~ous metal mini~g, Alaska - underwater miner

¥anne, aggregate mmmg, Massachusetts Bay IHeJvy'mineral mining, offshore Virginia ,

I,eJch nourishment, Ocean City, Maryland

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

Nome, Ala1ka, carried out by Western Gold Exploration and Mining Company (WestGold) Limited

Paitnetshipl during the late 1980's, Though the scale of operation and use of a bucket-ladder dredge

are ndt l/k~ly to be repeated in any future offshore mining activities in the region, the scenario is

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

envirobrhe~tal impact mitigation. It is also the only large-scale marine mining development to occur , , I

in U.S. waters to date.

j I' T'h I d . h' h' I . al'.. d .. I ,~secon scenano, w IC mvo ves preCIOUS met s mmmg usmg an un erwater mmmg

vehiclb, is based on actual production trials carried out towards the end of the WestGold project life.

The n\ro' rrlining methods are at opposite ends of the spectrum in terms of scale of operation and I . ;

precision of extraction, and therefore provide a useful contrast of the effects of using different

extrach~n techniques to mine the same mineral commodity.

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l0tfShOre aggregate mining, the subject of the third scenario, is thought to be a likely prospect

in theln6~-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 ate potentially widespread, the choice of a location in Massachusetts Bay for a hypothetical

aggre~ate ruining operation was based mainly on the availability of environmental information. A

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

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

environindntal information gathered serves as a useful resource for this study. ' i I , I

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(1)

(2)

(3)

(4)

(5)

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

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

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

(QAlQc) protocols, are also introduced and discussed in the context of environmental impact

mitigation. I

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

er;vironmental settings, the commodity resource characteristics, and the extraction and processing

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1.4 Structure of the Report

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I ii Although the baseline study provides what is still perhaps the most complete site-specific

a1:sessment of aggregate resources in the region,it reveals that the geology of the site under

consideration is not particularly conducive to economic dredging bperation. The sediments are , poorly sorted and contain, on average, higher proportions ofsilt and clay than are normally desirable

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

in this report represents a worst case scenario both environmentally an~ economically. As a marginal

operation the aggregate mining scenario is a sensitive monito~ of tIie!economic costs of mitigation.

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,: The fourth scenario, also hypothetical, involves the dredgibg 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 ofinduslrial 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 havb to address potential conflicts

with other uses, including commercial fishing, ocean dumping, and bilituy practice.

Ii The fifth scenario considers the use ofsand resources in FedJai waters (seaward of the state

boundary) for beach nourishment. The site selected for the scenarlo, Ocean City, Maryland, has

undergone repeated coastal restoration projects, the most recent was iIi the fall of 1994. Borrow sites . I

have moved progressively offshore and the need for sand resources from Federal waters appears

inlminent.

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i One of the key objectives of this study was to develop recommendations as to the most

aj::propriate techniques for mitigation of environmental impacts assobiated with mining of specific

mineral commodities on the U.S. OCS. The recommendations are dehved'from analysis of a broad

range of available and proposed mitigation measures with consideratibn given to their relative cost,

effectiveness and practicality. The selection of mitigation techniquds for analysis was based upon I .

the environmental impacts and mitigation guidelines, or objectives, identified for each of the mining

scenarios.

1.3.2 Mitigation Analysis
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metholls'. Predictions of the potential environmental impacts of each extraction technology are I I

provid,ed along with guidelines for mitigation of those impacts.

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I IjJ. Section 4, techniques for mitigation of the environmental impacts of marine mining are

revie\1(e~ lmd in Section 5 these are applied in detail to the marine mining scenarios.

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

millini s;cehario. A summary and recommendations are presented in Section 6.

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2.'!) Summary ofEnvironmental Issues

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:i Physical environmental processes affect marine mining operations in two ways. Firstly, these

processes constrain or control the actual conduct of the mining opetation, 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 ofsecondary

interest, except possibly in locations where unusually high tides 6r tidal ranges might impose , , operating constraints. Seasonal as well as short time scale changes ill these conditions combine to

determine both the annual operating season for a mining activid, and dictate the viability of

particular operations or activities on a particular day.

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Ii Principal sources ofpotential environmental impact resulting ffom 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 bf particulate material through

th,~ water column. Thus, the second physical environmental issue is lthe 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 timJ in the water column, and the , range of horizontal transport of the particles before they are eventually redeposited on the seabed.

" I ii'; M' anne orgamc. . b d d . matter ill sea e eposlts fl' ue s a sUIte 0 f b . actenally-med'lated postdepositional (diagenetic) reactions in which the organic material lis progressively degraded as

oxidants are consumed. The reactions yield a number of products which may have direct or indirect

environmental implications for mining. i"

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Organic matter represents a highly reduced fonn of carbon wl\ich is rich in electrons, and is

therefore a primary energy source. Over most of the seabed, marine o'rganic materials are degraded , by aerobic bacteria, wherein oxygen is the terminal electron acceptor in the reaction CH20 + O2 =

CO2 + H2 0. Where oxygen is absent, the bacterial community takJs advantage of other electron

acceptors, which (in thermodynamic order) include nitrate, manganes6 and iron oxides and sulphate.

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

long as degradable organic matter remains. This suite of reactio~s leads to a biogeochemical

zonation in sediments. Anoxia results where oxygen supply via adve:ction or diffusion is exceeded

by demand of aerobes and by inorganic consumption of oxygen by aireaction 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 shelfsediments.

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

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

blodi=i<y ,f "'" w,e, "I,m,. Th,l, "':"tiOO' ,now ""ttiOlli of biodi"mi'y, md

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biodivhr~itY cali be used as a yardstic~-for gauging environmental impact. BiodiversitY is measured

by (I) Ih6wj many different kinds of organisms are present (Species diversity), and (2) the relative

numb~rs!of each (Numerical diversity). :

: I~wi sets of risks arise from dredging operations: (I) extraction risks (i.e.' removal of habitat

materiaJ. ~ifh some surrounding disturbance), and (2) discharge risks (i.e. deposition of discharged

tailing's)] Extraction removes sediment habitat and the organisms living within the sediment

(infamia), dr on it (epifauna). The burrowing forms are relatively immobile and cannot swim away ! . I

from the ciltting face of the mining excavation. These benthic organisms include fish and prawn

resour?e isp~eciesthat maintain burrows, and many species ofsmaller creatures which can function

as feedstock for resource species. ,

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~pifauh<l; It'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

includ~ fI~tfish,' crabs, sea cucumbers and sea urchins (almost immobile), scallops, and other

resourCe [spbcies. 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

sUbstr~~ i~ removed. In general, seabed mining will have an impact on resource species, their

feedstbck. :fud habitat. .

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, IA discharged tailing slurry will fall through the water column, with some dispersion offmes),

and will bvbntiJallY reach the seabed as a densitY current. Mter initial impact, the tailing will spread

in socie~pJtteinas a seabed dispersal field. During this spread, deposition occurs, with coarser

particl~s' sc!ttliilg' before the fmer. Large amounts of tailings may smother and kill organisms over

a broad are~. Smothering kills benthic organisms by preventing them from breathing or feeding. In

ge!1er4 ~elability of benthos to avoid smothering, or to burrow up faster than smothering deposits

can settle, is not known. I' :

Natural ~nual fluctuations in biodiversitY (both number of species, and number of , I I

organis~s) are enormous. Such natural fluctuations cause great difficultY in describing ambient

faiJnas and 'environmental impact on benthos. This applies to measuring biodiversity before dredging

operarlohs ktait, assessing changes derived from the dredging operations, and assessing the progress

ofrecbl6nibtion after dredging.

I :: I . " ! ~patial differences in biodiversitY on uniform deposits means that the fauna of Dredging

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

year to Yek. , There may be no cost-effective way to obtain accurate and precise biodiversity

estimJtes df benthos with different and varying patchiness. Crude quantitative estimates may have - ,. I

to be accepted. -

liM!I f d' .' , d th afl ' I' db b th' , T' f I ter a eposlt IS mme e se oor IS reco omse y en IC organisms. trnes or recovery

of a realorlable biodiversitY depends on the sediment tYPe. Fine-grained tailings may need only I

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year before achieving a recovery level biodiversity, medium-grained deposits 1 - 3 years, and coarsegrained deposits 5 or more years. I

During marine mining a number of general impact mitigatiJ objectives are desirable:

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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 I

minimize elevations ofsurface water turbidity

minimize dredge course overlaps and gaps

minimize return of tailings outside dredge footprint

leave surface ofsimilar 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 '. I . • avoid sensitive areas - endangered species, isolated breeding grounds, etc.

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Fisheries and Wildlife Resources:

Seabed:

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3.0 Srinimary of Mining Scenarios

~iJ marine mining scenarios are discussed, Details of the marine conditions affecting the

lo2atio,n pf~each scenario are provided. The scenarios provide details on all aspects of the mining

operatiorls, Including the extraction technologies, dredged material processing methods, and effluent

types :bti donstituents. Information relating to the geological and environmental settings is also

providhci. ~t the end of each scenario predictions of environmental impact are given, potential

mitigatio~ 6bjectives are identified, and criteria for appraising the mitigation method are proposed.

1)~e~bJ metal (gold) mining offshore Alaska using a bucket ladder dre~ge tILl ".. d' . f al . d' Id'" . ff h , 'I ~lllS scenano IS a ISCUSSlOn 0 actu expenences unng a go mmmg operatton 0 sore

Nome., Dr~dgini for gold has been attempted several times this century but only WestGold achieved

a signifibabt production using a bucket ladder dredge from 1986 through 1990. The exploration

efforts kd the environmental research of WestGold are fairly well documented in various

pUblic~ti6n1. Much of the experimental work on reducing the environmental effects of the WestGold

dredgi~g iJ applicable to other forms of dredging. Therefore the Nome bucket ladder dredging

project dorlstitutes a very useful scenario for study. Dredging was carried orit using the world's,

largest dcdan-going bucket ladder dredge, the BIMA. During the years 1987-1990 the dredge

recovdrdd bn average 775 mg gold/m3

• There were extensive pre- and post-dredging surveys to

measu\-e Ith~ impact ofmining. • . 'I j , : : I jIngeneral terms the sand bed biodiversity started to recoverwi~ 1year of dredging.

A few !new'species entered the fauna and later disappeared, but there is no clear successional pattern. . I I 1

By 199q, four years after dredging at one sample station, the biodiversity was clearly back in the

range bfth6 reference stations. Cobble bed biodiversity appears to have recovered more slowly. The

nuinb6r lot! species remained as high as the reference stations, or almost so: even 1 year after

dredgihg. However, the number of organisms showed a reduction barely reaching the lower end of .. I I j . .

the yearito 'year range at the reference statIOns.

; Ifhi on~ fishery species monitored in detail was the red king ~rab (Paralithodes

camtschatita). It was found that crabs move locally over distances which allowed them to transfer

beiwe~n!drbdged and undredged areas. Their movements prior to dredging were apparently random,

althoJgh d-abs Often have seasonal shifts with depth. Crabs were fished from both dredged and

undredg~d !areas, with more competition by boats for fishing sites in the dredged than the undredged

areas. wi j ':1 ' I' . d b'd' I d' f th' d d d' h ater co umn Impact was tmlte to a tur I Ity P ume exten mg rom e re ge ISC arge

points. !Sbab~d disturbance was caused by the dredge cuts and deposition of tailings. Dredge

footprints Jwere substantially smoothed by wave and current action within 3-5 years, even on the

co'ars6s~ gravel/cobble habitat. A level of biodiversity encompassing several tens of species and

sevenh lIUbdredorganismslm2 had recolonized tlte impacted areas within 3-5 years, with recovery

slowet dn bOaIser fraction gravel and cobble habitat. • , I I" ,

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2) Precious metals mining offshore Alaska using an underwater mining vehicle

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'I An underwater miner (UWM) constitutes the opposite eXtreme of a large bucket ladder

dredge: it has a comparatively very low throughput, is limited in tercis of the depth to which it can

dig below the seabed, and has the ability to be very selective in itsl 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 thro~ghput, resulting in a low unit

volume cost. It is very non-selective, laterally and vertically, and a sJrficial deposit worked by it is

h~iaVilY diluted in grade. i l'

'!' WestGold deployed a UWM in the Nome area of Alaska in the summer of 1989. This was

tli,~ first recorded time that such a piece of equipment had been used fdr an offshore mineral recovery

program other than remotely operated collectors in the earlier deep ocelm mimganese nodule projects.

The UWM used was the "Tramrod", model 250, developed and mariufactured by Alluvial Mining

of the United Kingdom. I '.

I I ' " The "Tramrod" is deployed from an ocean-going barge with a1draught 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 pipdline..

3)Marine aggregate mining in Massachusetts Bay with a trailin~.sUCtion hopper dredge

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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 Norne, Alask~ cOIlSisting 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 an~ 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 seabetl. The potential dredge area

reportedly comprises about 5.25 million m' of sand and gravel in wiater depths of less than 18 m.

A,;suming 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 kore. Therefore, in practice,

additional or alternative dredging sites will have to be found before Ithe 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 tonnds oC:;lggregate per day.

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The fauna of the Massachusetts Bay area were documented in a large study during the

summer of 1973. The biodiversity was rich and variable with differerlces between epifauna on hard

rock (including on exposed cobble surfaces) and the burrowing infau~a. 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/m2

, from 305

to·over 11,000. The species complexes found on different substratb types differed but statistical

correlation with deposit type was not possible. The temporal variatibn WilS enormous from month

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, . to month~ making the use of dominant species only possible at specific site locations that cannot be

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generalised to a substrate level.

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; ~enthic communities directly in the path of the dredge will be destroyed. The benthos in

undredged keas between the furrow lines will be affected by increased turbidity and sedimentation.

The iclpacts will depend on the depth ofsediment that accumulates and depth of the furrows.

, I I ' . Unlike mining scenarios where most ofthe sediment extracted is returned to the environment,

in Ininidg Jggregates a substantial percentage of the particular size fraction is removed. This will

chang6 the !particle size characteristics of the deposits remaining. The larvae which recolonise the

altered dnv'ironment will then reflect these changes. It is probable that the silt/clay fractions will

increJe in the dredged area because the coarsersediments are to be retained by the dredging process.

, 'i 1 ' , ,II Soft-bottom communitiesshould recover faster than communitiesfound on coarse sediments,

or the epif~una on cobbles and boulders. Expected times to recovery, based on other studies, are

suggeSted to be I + years on deposits with least cobbles, with slower recovery where substantial , I ' numbers of cobbles are removed. '

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I. al' ... b" h Id' I d ,nvlronment Impact mlugauon 0 ~ecuves s ou mc u e: "

rldLti;~n of dredge-derived rurbidity through improved outfall design and vessel operations

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(2) II Jos'sible development of alternative enhancement uses for under-sized and over-sized I I fractions

, I i I " . (3) return of fines to dredge footprint.

4) HeaJlnerals dredging offshore of Virginia Beach, Virginia using a cutter suction dredge

, I101 heavy mineral deposit is contained in sand. Total heavy mineral (THM) concentration

average~ atiout 3% with values up to 3.65% in the uppermost metre. THM content decreases steadily

with ipdreLing depth below the seabed, and the incremental THM content, on average, falls to

slightly \es~ th;m3% (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 pl~tfbrrh. Throughput would be in the range of 113,000 to 151,000 litres of slurry per minute,

with ~pprolximately 95% of dredged sediments returned to sea as effluent.

, II it il expected that biodiversity will be high on these sands, as is usual at this latitude and

water ddpth. At this site prior human disturbance has probably introduced contaminants or otherwise

rendeted tlie g'round unsuitable or even dangerous for dredging. There are dump-sites indicated on

hydro~r~phic Icharts and the area is in the proximity of a Navy firing range. The benthos will

probably ~how localised impact from such prior uses. This will need determining by site-specific

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surveys.

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:1 Extensive pre-dredging sampling will be needed to show if there are areas of recent

disturbance with little benthos, stable areas with a nonnal variability df the fauna, and areas in which

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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 furthh stresses to the environment

from sewage, stonn drain run off and industrial run off. Existing status offishery and fish-nursery

sites would need documenting.

II

ii' Mining-related impact will be from a combination ofthe 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. ITrenches fonned by the cutter

head will develop as will elevated areas. hnpacts will be extensive as experienced at the WestGold

bucket-ladder dredge operation, with time to recovery detennined by the nature of the deposit and

energy level of the site. Smoothing ofthe 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 atea already may be affected by

other anthropogenic influences, and these may continue to affect thd site..

ii Environmental impact mitigation objectives should include:

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(l) minimization of surface turbidity

(2) reduction of benthic impact by constraining tailings discharge to dredge footprint and nearby

(3) 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 I m per year and requires replenishment, by

a~:reementwith the authorities, every 2 - 3 years. The site is selected ~o as to provide sand ofthe size

arId type normally found on the adjacent beach that is to be renourish6d. A previously used site 3.2

km offshore is now exhausted. In 1988 the sailing distance was 4.12 kID from the hopper dredge

pl;mp-out shoreline connection to the dredging site where 77% of tJie available 3.06 million m' 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 ofsand is un~erlain by mud and clay which

must not be disturbed. II

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Ii Generally, if the borrow area is less than 5 - 6 km from the beach, a cutter suction and

pipeline are used. Ifthe distance is greater than 5 - 6 km a hopper idredge 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 hdpper dredge because the latter

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cmkOtla~proach close to the beach with the prevailing water depths. An additional factor at Ocean

City is' the tlifficult entry and the depth of water at the entrance to, and within, the harbour that

restricis the! size of hopper dredge which may enter to take shelter from severe storms.

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'i sb6-bed benthos is characteristically of high biodiversity, with many species scattered

through rlJajor t~a, and dynamic in the sense ofspecies changing relative numbers from year to year.

Specie~ diversity ranged up to more than lOOper unit area investigated. Numerical diversity ranged I I I .

up to more than 7000/m2•

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IThe immediate impact from mining will be formation of borrow pits. The depth, width and

length 10~ qe 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.

Th~re ./viiI tie an immediate loss of virtually all the benthic species present. Mining-related seabed

disturb1an'celmay release nutrients and uncover and displace feedstock species temporarily providing

extra fbo~ fpr bottom-feeding resource species. Fishing may improve temporarily downcurrent of

the drddging area and continue for some months afterwards. Benthos may also show a temporary

increaSe In biodiversity downstream from the dredge site. '

: I~e! mining footprint probably will start to smooth, and biological rec~lonisation to start,

within Iweeks of closure. If closure is during an annual larval settling period, recolonisation will be

most r~pid! and :may be demonstrable within days of closure. It is to be expected that moderate

biodiversity will have reformed within I year.

. '+t"h~,"," imp,"' miti"tioo obi"ti", ,tioo1d iociod"

(I) I~inimizing cut-face turbidity, thereby preventing upwelling to surface

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(2) Ip~aD.ning, the location, depth, width and length of dredge courses to minimize the risk of

$okic borrow-pits developing. ' I

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(3) the distribution of sediments outside the dredge footprint

(I) the creation of water turbidity

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(2) the release and spread of contaminants

(4) changes to the seabed lithology and profile ,

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Ii Anyone or more offour possible requirements of mitigation 1IJay be applicable to a marine

dredging operation. Each of the four approaches requires a procedur~, alone or in combination with

others, to be successful:

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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 lmd which must be minimized

01' reduced to acceptable limits are:

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to avoid completely any environmental impact through location-ba'ied, time-based, or scalebased constraints on the dredging, alone or in combination I '.

to remove or lessen a cause or effect, and thereby to minimize the unavoidable environmental

impact, through: (a) restrictions on the dredge discharges anq certrin operating techniques,

and (b) the requirement that the best and most suitable technology is available

to remediate the environmental impact through post-dredgin~ activities

to compensate for the environmental impact through alternatit enl~ancementof a similar or

related environment.

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5.0 SmJmary' of Application of Mitigation Techniques

1) PrebLl metal (gold) mining offshore Alaska using a bucket-ladder dredge

: ;~oLtOrihg of the tailings plume during the mining operation was earned out from the

collectio& df samples from tailings launder and gold room effluent, and from sea water up-current

and down-durrent from the dredge. WestGold and its consultants employed a 3~dimensional water

qUalit~ afd lsediment plume computer model referred to as Disposal from a Continuous Discharge

(OIFCD) and developed by the U.S. Army Corps of Engineers Waterways Experiment Station. It

was usbct in' conjunction with a Wave Current Sediment Resuspension Prediction (WCSRP) model. , , , The DIF\CD and WCSRP computer programs proved to be invaluable in predicting the plume

creatidn behaviour of the tailings discharge. • I '

A11ge variety of mitigation techniques were considered and many were tried over the 1986

to 1990 bpbration period of the BIMA dredge. The most important was the control of the tailings

efflue~t ~is~harge so as to minimize the degree and extent ofsea water turbidity. The optimal design

de~eldp¢d :Crom four years of dredge-operating experience comprised fitting the existing 1.8 mdiameferihdader pipes with cones that terminated in a flanged collar of 0.50 m diameter. A straight

circula'r stebl pipe was attached to the flange and extended vertically down to within about 0.5 m of

the watel: s~rface. A flexible hose of the same diameter was affixed to the steel pipe and extended I I •

2.7 m below the water surface. Several other configurations were considered but were not employed.

They w~re! discarded in the design stage after operating experience quickly demonstrated their

impra¢tipality 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. ,

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'precious

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metals mining offshore Alaska using an underwater mining vehicle

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1number oflocation-related restrictions applied to the operation that wJre designed mainly

to prote~t rhigrating salmon. Dredging could not take place: .

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(1) within 30 m of the low water mark

(2; Ili1n J.6 kID of the mouth of a river used by migrating salmon

I 1 (3) in water depths of less than 5 m. ;

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I <I:o!npared with the large-scale bucket ladder dredge the use of a selective, small scale

operatioh <illowed greater flexibility in mitigating the deleterious effects of the effluent discharges.

The u~e: of an underwater miner also served to mitigate the important impact 'of changing seabed

profil+ qaujised by dredging. I

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A novel outfall design, consisting oftwo large troughs fixed al~ng both sides of the treatment , 1

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,Jas released as a series ofsmall

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, cap~ble of discharging at various

depths above the seabed. Diffusion of the effluent around the barge'pbrimeter, coupled with the low

gradient of the flexible subsea hoses, minimized the turbulent kinetic bnergy of the discharge. This

approach was made especially practical by the comparatively low thrdughput rate of the underwater

mining vehicle.

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3) marine aggregate mining in Massachusetts Bay with a trailing-suction hopper dredge

II Seabed Lithology and Mitigation. Repeated excavation bf aggregate within a dredge I

course eventually may reveal different lithologies. As the workable deposit thins through repeated

removal of sediments, high-spots in the underlying sequences are evbntually exposed. These may

be hard bedrock or glacial clay similar to that below the aggregate soJrce in Massachusetts Bay. To 1

the dredge operator such exposure raises the possibility of unwanted cargo contamination. Overdredging has other effects. Local fishing may be affected if bould6rs become exposed since the

potential for damaging trawling gear is increased. A layer of the origfual substrate should be left on

th,e seabed to increase the ability of the original benthic communitiek to recolonize.

,ii' The mitigative solution is two-fold. Either avoid such conceLra~ons of glacial sediments,

or apply restrictions to the extent that aggregate may be removed onJy from the top of the vertical

sediment sequence.

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il Seabed Profile and Mitigation. Even if some care is tilkenby the avoidance of an

excessively repetitive dredge course and direction, trailer suction hORper dredging does not always

yield a sufficiently flat seabed profile. After final excavation the seabbd surface frequently contains

too many peaks and valleys because the drag-heads have a tendency tol

track the previous excavation.

The objective of dredging should be to plane, rather than to trench, Ithe seabed. Operators should

attempt to dredge courses adapted to the seabed geology, even ifthe cburses are oriented across tidal 1

flow. Ideally, the courses should be adapted to the contours of equal sediment thickness.

Water Quality Mitigation

Overflow Mitigation: Operational Technique. A practical tnitigative 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 di~charged directly overboard as

the dredge pump is primed. l>: switching mechanism is then used to Idirect the slurry to the h~pper once sednnents are entraIned m the purnpmg system. As a result overflow does not occur until the

hopper.is filled to within 60-70% ofits dredged material capacity. The time during which sedimentladen waters are released to the sea is thereby substantially reduced·1

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,I OVt;rflow 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 -. I I J

when dredging hard ground. A small percentage of the hopper overflow water can be pumped to the

drag h~ad j~ts instead of using clean seawater, reducing slightly the total volume of sediment-laden

'rfll I , ' ove Gl~. ,

· idverflow Mitigation: Hopper Design. The level of the aggregate ~urface in a loaded

dredge iJo~per always must be above the load line in order to prevent surplus water from being

trallspbried! to shore in addition to the solids. This can be achieved by modifyillg the shape of the

hoppet t~ kbep much of the contents above the load line. Alternatively, special compartments may

beempl6y~d on either side of the hopper.

! JJrflow Mitigation: Overflow Collection. An anti-turbidity me~ure proposed for

hopper dre~ging is to install compartments on both sides of the dredge. These are designed to

intercdp~ and temporarily retain the overflow, allowing partial settling of suspended silts within

them. IHo~ever, there are potentially serious, practical, limitations: i

(I) JeLtention time needed for substantial settling to occur

(2) JeIneed, at intervals, to dispose of the accumulated silts

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(3) I the significantly reduced hopper capacity caused from the loss of vessel space by

ihcorporating settling compartments. I I

@vlrflow Mitigation: Effluent Discharge. A relatively simple technique for handling

hopper dv~rflow, called an anti-turbidity overflow system (ATOS), has been developed in Japan.

The orernpw collection system is s.treamlined to minimize ~e entrapment of air bubbles in the

overflow 'rater. Removal of the au bubbles. WhICh otherwIse make the partIcles buoyant and

prolorigJ settling, allows the fines to settle at a faster rate. '

4) HJJln~rals dredging offshore ofVirginia Beach, Virginia using a cutter suction dredge. i : I ,

Water. Quality Mitigation: Cutter-Generated Turbidity

bplr:tional Mitigation. Several operational controls are recommended for the reduction

of sedinieAt resuspension. Most involve the manner of excavation. Very thick single-pass cuts

should be ~voided, because the cutter-head tends to become buried. As a result more sediment is

excav~tbd Iand dislodged than the suction can successfully entrain, leading to high levels of

suspebs10n. A thick deposit is best excavated by means of several horizontal slices, each layer of

excav~tioJ being a single thin cut depth.

, Llth~r ~laces it may be more economical to excavate and remove most of the sediments

very roJghIy in a single cut. Final clean-up to the base of the pay zone may be delayed until later

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Water Quality Mitigation: Plant Effluent

in the operation. However, the loosened sediments, while awaiting clean-up, may be at risk of

suspension.

(1) minimizing the possibilities of re-digging of the settled tailings through operational

procedures

The most obvious water turbidity is created by the surface discharge of slurry effluents as

tailings from the treattnent plant. Mitigation may be attempted by:

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" (4)

improving the nature of the effluent by de-sliming, dewatering or degassing ,

optimizing the manner in which effluent is discharged by emblOYil~g sub-surface discharge

and/or discharge diffusion i

modifying effluent behaviour in the sea by the use of flocculants. ' I

Ii Operational Mitigation. The mine plan should be d~Sign~d 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 deliber:lte exclusion of areas of lowI

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Where the mining plan involves dredging to a few meters bel~w 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 sutcessively deeper slice. On a

smaller scale the formation of distinct cuts should be minimized. Th¢ 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 cohcentric arcs are described on

the seafloor with each swing of the cutter.

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Design Mitigation. The cutter-head's suction should be sufficiently powerful to collect all

the sediments it disturbs. Pick-up capability could be increasedibY 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 improvingi the chances of entrainment.

Differences in shape are particularly important if the cutter-head islnot completely buried in the

sediments. ,II

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:1 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 ari appropriate rake angle is an

important mitigation factor. Ifthe rake angle is too large, a gouging attion , throws soft, fine-grained

sediments outward. Ifthe rake angle is too small, however, resuspeilsion'occurs through impact.

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gnide gr?up.d from a first dredge course, on the assumption that they may be dredged as part of a

subsequt,nt:program, should be avoided.

fl' i I ~e-slirning and Dewatering. Separation of the effluent flow into its component parts of

solids ana J..ater allows the materials to be discharged into the sea with less impact. The solids are

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sJspended in the water by the boiling action which otherwise is evident. The water,

althou~h cclntaining some fines, is more rapidly diluted by the surrounding sea 'water. De-sliming

and dew~tt!ring systems include hydrocyclones, hindered settling classifiers, bucket-wheels and , I' I I

screws ahdjdewatering screens.

. ii, IDegassing. It is not known whether methane or hydrogen sulphide are present in seabed

sediments bffshore Virginia in the vicinity of the mining site. If either gas does occur degassing

eqtipJte~t :should be employed downstream of the dredge pump to prevent depriming. Both the

pump bd degasser must be installed underwater because the water depth is greater than 10 m. To

limit Jaier ~ohmm turbulence associated with released gases the installation of a conduit from the

degas~er! toi 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 4ari be controlled to a significant extent by using a sub-surface discharge. Such an

arrangbme~t would consist of a reinforced rubber pipe attached to the vessel. It may be deployed

in any lattitJde ranging from vertical to near horizontal. A reduced effluent discharge velocity could

be acHie~elI by fitting the end of a discharge pipe with a flared opening. This modification may

requirb s'oriIe experimentation to achieve the optimum arrangement. '

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F:locculants. The use of flocculants comprises the only means available of speeding the

settlement bf e~trained silts. Flocculants have two effects. Firstly, because they 'are flocculated fine

sand ahd: silt are deposited together, thus preventing the natural separation of the two fractions that

occur~ riathrally during settling. Secondly, the deposited silt is increased'in volume by the

entrapbent ofwater. However, volumetric increase should be minimal if the correct amount of I I I J

flocculant is used. .

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I I -, . I • , Seabed Profile and Mitigation j

, ~Jrational Procedures. The creation of elevated ridges of tailings on the seabed along the

dredge cbuhe, especially at its start, may be minimized during operations in several ways:

(1) , jsJfficiently wide cut face must be dredged .

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(2) the dredge must not be restrained by sidelines which are so short as to cause it to angle

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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

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:: Discharge Control of the Trommel Oversize. Every effort should be made at sea to re-mix

th;: oversize and the undersize. Ibis involves discharging the screen dversize as closely as possible

to the solids produced as the cyclone product, without causing unneclessaIY turbulence.

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;i Alternative Enhancement. Considerable potential exists, for alternative enhancement

inltiatives when dredging for heavy minerals. In particular, the und6rsize tailings can be pumped . , ashore, if needed, for beach renourishment, the remediation of contaIriinated areas, and the creation

of wetlands.

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5) Beach nourishment at Ocean City, Maryland, using a cutter SUCtiOIl dredge pumping sand

ashore or, alternately, a trailing-suction hopper dredge transp6rting sand to a nearshore

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Ii Prevention of Coastal Erosion. To avoid the possibility of erosion both the local

hy'drodynatnic conditions and the sediment characteristics of the dJposit being dredged must be

considered. Wave refraction analyses and wave transformation mod611ing are a necessaty input to , a ~;tudy. Prevention of erosion may be achieved by adopting a correct licensing procedure and by , re:;tricting the operation of a dredge to specified water depths and/ori areas.

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6.0 General Recommendations ..

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General recommendations derived from the analysis presented in this report include:

Je te ofBest Available Technology should be practiced with anymarin~ mining operatiOn.

~he: 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.

nlJerical modelling is a useful and cost-effective tool in the development of effluent

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discharge systems, the design of environmental monitoring programs, and the assessment of

tlie ~otential risk of coastal erosion. Modelling should be implemented as part of any mining J , .

project that involves substantial discharge of effluents at sea, or of projects that may

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potentially affect coastal stability. ; I . . ~ell-designed environmental monitoring programs with appropriate QNQC are a necessary

cbrrlponeilt of any marine mining project. Monitoring ensures that regulatory constraints are

obs~rved, and also provides the operator with ongoing guidance as to the level of effort

nbeded to meet those constraints. .

operatton II .' al . fl . . d al d b d . db c th···· constramts 0 ocatton, ttme an sc e nee to e etermme e,ore e lmttatlon

of lny mining project, so that the economic viability of the project can be realistically

aksJssed. From an operational and economic perspective, constraints on operations that are

bbbd on random occurrences such as sightings of particular species in the mining area are I : •

d~t1mental and potentially very costly. The possible implications ofsuch constraints should

be carefully considered.

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mitigation can in some cases be achieved cost-effectively through changes in operational

p~obedures, such as the planned order and distribution of dredge courses or the depth and

~dlli cif cut. The planning of any new marine mining venture should take into consideration

, ~e ~ange of operational mitigation techniques presented in this report. '

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(6) IriIany technology-based mitigation techniques, such as sub-surface' tailings discharge Isys1ems, dewatering and degassing systems, and onboard feed monitoring systems,

I me:lsurably reduce the impacts associated with a mining operation and can be implemented

; a~ rbasonable cost. The implementation of these techniques should be'considered for any

futtlre offshore mining development.

leIpote~tial for alternative enhancement projects should be explored during the planning

slages offuture marine mining developments. For example, consideration should be given , I

to the Ipossibility of using tailings produced by a heavy mineral mining operation for

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tle~eficial purposes, such as beach nourishment or the creation or restoration of coastal

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