EPA 540/R-09/002 

September 2008

Mine Waste Technology Program 

Activity III, Project 34 

Bioremediation of Pit Lakes – 

Gilt Edge Mine 

By: 

Brian Park 

MSE Technology Applications, Inc. 

Mike Mansfield Advanced Technology Center 

Butte, Montana 59702 

Under Contract No. DE-AC09-96EW96405 

Through EPA IAG No. DW89938870-01-0 

Diana Bless, EPA Project Officer 

Sustainable Technology Division 

National Risk Management Research Laboratory
 

Cincinnati, Ohio 45268 

This study was conducted in cooperation with 

U.S. Department of Energy
 

Environmental Management Consolidated Business Center 

Cincinnati, Ohio 45202 

National Risk Management Research Laboratory
 

Office of Research and Development 

U.S. Environmental Protection Agency
 

Cincinnati, Ohio 45268 
Geochemical And 

Redox-Mediated Bio-TransformationTM 

for 

In Situ Pit Lake Remediation Of 

Acid- And Metal-Toxic Mine Drainage 

2001-2006 Treatability Study 

Report of Results 

ANCHOR HILL PIT LAKE 


GILT EDGE MINE NPL SITE, SOUTH DAKOTA 

Disclaimer 

This publication is a report of work conducted under the Mine Waste Technology Program that was 

funded by the Environmental Protection Agency and managed by the Department of Energy under the 

authority of an Interagency Agreement. 

Because the Mine Waste Technology Program participated in EPA’s Quality Assurance Program, the 

project plans, laboratory sampling and analyses, and final report of all projects were reviewed to ensure 

adherence to the data quality objectives. The views expressed in this document are solely those of the 

performing organization. The views and opinions of authors expressed herein do not necessarily state or 

reflect those of the United States Government or any agency thereof. 

Reference herein to any specific commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or 

favoring by the United States Government or any agency thereof or its contractors or subcontractors. 

Neither the United States Government nor any agency thereof, nor any of their employees, nor any of 

their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes 

any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results 

of such use of any information, apparatus, product, or process disclosed, or represents that its use would 

not infringe privately owned rights. 

ii 
Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the Nation's 

land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to 

formulate and implement actions leading to a compatible balance between human activities and the ability 

of natural systems to support and nurture life. To meet this mandate, EPA's research program is 

providing data and technical support for solving environmental problems today and building a science 

knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect 

our health, and prevent or reduce environmental risks in the future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency's center for investigation 

of technological and management approaches for preventing and reducing risks from pollution that 

threaten human health and the environment. The focus of the Laboratory's research program is on 

methods and their cost-effectiveness for prevention and control of pollution to air, land, water, and 

subsurface resources; protection of water quality in public water systems; remediation of contaminated 

sites, sediments, and groundwater; prevention and control of indoor air pollution; and restoration of 

ecosystems. NRMRL collaborates with both public and private sector partners to foster technologies that 

reduce the cost of compliance and to anticipate emerging problems. NRMRL's research provides 

solutions to environmental problems by developing and promoting technologies that protect and improve 

the environment; advancing scientific and engineering information to support regulatory and policy 

decisions; and providing the technical support and information transfer to ensure implementation of 

environmental regulations and strategies at the national, state, and community levels. 

This project was conducted under the Mine Waste Technology Program. It was funded by the EPA and 

administered by the U.S. Department of Energy (DOE) in cooperation with various offices and 

laboratories of the DOE and its contractors. It is made available at www.epa.gov/minewastetechnology 

by EPA's Office of Research and Development to assist the user community and to link potential users 

with the researchers. 

Sally Gutierrez, Director 

National Risk Management Research Laboratory 

iii 
Abstract 

The U.S. Environmental Protection Agency (EPA) Region 8 Superfund Office and the EPA National Risk 

Management Research Laboratory (NRMRL) Mine Waste Technology Program (MWTP) conducted a 

field-scale treatability study demonstrating an in situ bio/geochemical treatment technology for 

decontaminating acid/metal-toxic water within the Anchor Hill Pit lake at the Gilt Edge Mine Superfund 

site near Deadwood, South Dakota. The purpose of the project, carried out between March 2001 and May 

2006, was to develop performance data of the treatment approach for potential use in long-term water 

treatment/management activities at the Gilt Edge site, as well as other similar sites. The treatment process 

was applied to approximately 72 million gallons of acidic water, with high concentrations of metals 

(including iron, aluminum, arsenic, selenium, copper, cadmium, and zinc), sulfate, and nitrate, and the pH 

was approximately 3. 

The treatment process involved pit neutralization, then application of nutrients to stimulate biological 

activity. The treatment process was successful and approximately 40 million gallons of treated water that 

met the State of South Dakota’s strict surface water discharge standards were discharged from the Anchor 

Hill Pit during the demonstration. All project objectives were met, and considerable experience and 

insight was gained into how operational aspects of such a remediation technique would have to be 

designed for future efforts. 

iv 
Contents 

Page 

Disclaimer............................................................................................................................................... ii
 

Foreword................................................................................................................................................. iii
 

Abstract................................................................................................................................................... iv
 

Contents .................................................................................................................................................. v
 

Figures .................................................................................................................................................... vi
 

Tables...................................................................................................................................................... vi
 

Acronyms and Abbreviations ................................................................................................................. vii
 

Acknowledgments................................................................................................................................... viii
 

Executive Summary ................................................................................................................................ E-1 
 

1. Introduction.................................................................................................................................... 1
 

1.1 Background ........................................................................................................................ 1
 

1.2 Treatment Description........................................................................................................ 1
 

2. Project Chronology........................................................................................................................ 3
 

3. Implementation .............................................................................................................................. 6
 

3.1 Sampling ............................................................................................................................ 6
 

3.2 Treatment Methods/Materials ............................................................................................ 7
 

3.2.1 Lime Addition....................................................................................................... 7
 

3.2.2 Nutrient Addition – May 2001.............................................................................. 7
 

3.2.3 Nutrient Addition – September 2001 .................................................................... 8
 

3.2.4 Nutrient and Caustic Addition – April 2002......................................................... 8
 

3.2.5 Nutrient, Caustic, and Wood Chip Addition – September 2002........................... 8
 

3.2.6 Fertilizer Addition................................................................................................. 9
 

3.2.7 Discharge of Deep Zone Water, 2004................................................................... 9
 

3.2.8 Evaluation of Use of Anchor Hill Pit for Ongoing Site Water Treatment, 2004.. 9
 

3.2.9 Evaluation of Likelihood of Lake Turnover, 2004 ............................................... 10
 

3.2.10 Discharge of Surface Water, 2005 ........................................................................ 10
 

3.2.11 Discharge of Surface Water, 2006 ........................................................................ 11
 

4. Results/Discussion......................................................................................................................... 15
 

5. Lessons Learned/Quality Assurance Activities ............................................................................. 21
 

5.1 Neutralization..................................................................................................................... 21
 

5.2 Nutrient Addition ............................................................................................................... 22
 

5.3 Monitoring ......................................................................................................................... 22
 

5.4 Discharge ........................................................................................................................... 23
 

5.5 Quality Assurance Activities.............................................................................................. 23
 

5.5.1 Field Data Review................................................................................................. 23
 

5.5.2 Laboratory Data .................................................................................................... 23
 

6. Conclusions/Recommendations..................................................................................................... 25
 

7. References...................................................................................................................................... 26
 

v 
Contents (Cont’d) 

Page 

Appendix A: Technical Systems Review Response from MSE to EPA................................................ A-1 

Appendix B: Evaluation of the Use of Anchor Hill Pit for Ongoing Site Water Treatment ................. B-1 

Appendix C: Evaluation of Options for Eliminating Dissolved Sulfide, and Subsequent Addition of 

Concentrated Hydrogen Peroxide .................................................................................... C-1 

Appendix D: Evaluation of Likelihood of Water Column Turnover..................................................... D-1 

Figures 

1-1. 300-acre Gilt Edge Mine site (left); Anchor Hill Pit lake (center & right). ................................ 2
 

3-1. Photograph indicating the location of sample buoys in the Anchor Hill Pit. ............................... 12
 

3-2. Camera used for video camera observations (left), gas bubbles within suspended bags of chips 

(center), and sludge at bottom of pit (right).................................................................................. 12
 

3-3. Photograph showing Neutra-Mill docking location. .................................................................... 12
 

3-4. Adding molasses (left) and methanol (right) to the pit................................................................. 13
 

3-5. Photograph showing white froth of gas bubbles within the floating wood chips......................... 13
 

3-6. Photograph showing in situ hydrogen peroxide addition. ............................................................ 13
 

3-7. Photograph showing evidence of excess sulfur in the shallow pit water after hydrogen peroxide 

addition......................................................................................................................................... 14
 

4-1. pH versus depth prior to pit lake neutralization............................................................................ 16
 

4-2. pH profiles with depth after initial lime neutralization. ............................................................... 17
 

4-3. pH profile following additional efforts to neutralize the Anchor Hill Pit. ................................... 18
 

4-4. Nitrate data at each sample location throughout the demonstration............................................. 18
 

4-5. Anchor Hill Pit nitrate/nitrite and dissolved copper, cadmium, and zinc concentrations over the 

project duration............................................................................................................................. 19
 

Tables 

1-1. Initial Pit Water Composition and ARARs for the Gilt Edge Mine Site...................................... 2
 

2-1. Chronology of Project Execution ................................................................................................. 3
 

4-1. Water Quality Improvement Over the Course of the Project ....................................................... 20
 

vi 
Acronyms and Abbreviations 

ARARs applicable or relevant and appropriate requirements 

ARD acid rock drainage 

BOD biochemical oxygen demand 

BOR Bureau of Reclamation 

CLP Contract Laboratory Program 

DOE U.S. Department of Energy 

EPA U.S. Environmental Protection Agency 

GWS Green World Science, Inc. 

IAG Interagency Agreement Number 

MSE MSE Technology Applications, Inc. 

MWTP Mine Waste Technology Program 

NRMRL National Risk Management Research Laboratory 

ORP oxidation-reduction potential 

pH negative log of hydrogen ion concentration 

QA quality assurance 

QAPP 

RMBTM

quality assurance project plan 

 Redox-Mediated Biotransformation 

SC specific conductance 

SRB sulfate-reducing bacteria 

TSS total suspended solids 

vii 
Acknowledgments 

This document was prepared by MSE Technology Applications, Inc. (MSE) for the U.S. Environmental 

Protection Agency’s (EPA) Mine Waste Technology Program (MWTP) and the U.S. Department of 

Energy’s (DOE) Environmental Management Consolidated Business Center. For this project, Ms. Diana 

Bless was EPA’s MWTP Project Officer, while Ms. Norma Lewis was EPA’s MWTP Project Manager. 

Mr. Gene Ashby was DOE’s Technical Program Officer. Roger Wilmoth was instrumental in accepting 

this project from EPA Region 8. Ken Wangerud was the site’s Remedial Project Manager for EPA 

Region 8. Ms. Helen Joyce was MSE’s MWTP Program Manager and Brian Park was MSE’s Project 

Manager. Additional project-specific contributors that deserve acknowledgement include: Steve 

Fundingsland, Jim Jonas, Paul Hight, and Marko Adzic (CDM), Jim Harrington (Shepherd-Miller), and 

Joe Harrington (Green World Science). 

viii 
Executive Summary 

The U.S. Environmental Protection Agency (EPA) Region 8 Superfund Office and the EPA National Risk 

Management Research Laboratory (NRMRL) Mine Waste Technology Program (MWTP) conducted a 

field-scale treatability study demonstrating an in situ bio/geo-chemical treatment technology for 

decontaminating acid/metal-toxic water within the Anchor Hill Pit lake at the Gilt Edge Mine Superfund 

site near Deadwood, South Dakota. The purpose of the project, carried out between March 2001 and May 

2006, was to develop performance data on the treatment approach for potential use in long-term water 

treatment/management activities at the Gilt Edge site, as well as other similar sites. The treatment process 

was applied to approximately 72 million gallons of acidic water, with high concentrations of metals 

(including iron, aluminum, arsenic, selenium, copper, cadmium, and zinc), sulfate, and nitrate, and a pH 

of approximately 3. 

The original objectives for the treatability study were to: 

–	 successfully establish anaerobic and chemical-reducing conditions to reduce nitrate- and metalsulfate contaminants; and 

–	 reduce toxic-metal concentrations. 

As these objectives were successfully achieved, and the effectiveness of the treatment was evident, an 

additional objective sought was to: 

–	 achieve discharge standards and release “clean” and “non-toxic” water from Anchor Hill Pit lake. 

All of these objectives were met, and considerable experience and insight were gained into how 

operational aspects of such a remediation technique would have to be designed for future efforts. 

The first step of the planned two-stage treatment consisted of using a Neutra-Mill (essentially a floating 

lime slaker developed by Earth Systems, Pty. of Australia), to neutralize the pit water pH to 

approximately 7 using lime (March 2001-May 2001). Following a short stabilization period, during 

which the pH “settled” to a value of approximately 5.0, a patented process for in situ treatment, using an 

organic formulation of molasses, methanol, and proprietary ingredients was implemented as the second 

treatment step in May 2001 by Green World Science, Inc. The purpose of this step was to induce 

reducing conditions, and stimulate bacterial activity for nitrate, selenium, and sulfate reduction to create a 

stable system and improve water quality. 

In 2002, the project team concluded that the pH 5.0 condition was inhibiting biological activity so the pH 

was raised to 6.0 and wood-chips were added as substrate for bacterial growth. Robust bacterial activity 

rapidly proceeded. By the summer of 2003, denitrification was complete and sulfate reduction was well 

under way. The dissolved form of metals which form sulfide precipitates (e.g., copper, cadmium, zinc) 

had decreased dramatically upon the onset of sulfate reduction. Due to the slow-settling nature of these 

metal sulfide particles, the total metals values of those metals in collected samples were significantly 

higher. In general, water in the pit met applicable South Dakota Ambient Water Quality Standards 

(SDAWQS) with the exception of undisassociated hydrogen sulfide (H2S) (i.e., dissolved H2S as opposed 

to HS-

) and biochemical oxygen demand (BOD); the elevated BOD was due to the presence of remaining 

organic carbon as well as the elevated dissolved sulfide. In addition, the Anchor Hill Pit had become 

meromictic, with a chemocline at a depth of 20 to 30 feet. During the year leading to the summer of 

2004, the pit water column became more strongly stratified, with the surface zone being aerated and 

ES-1 
meeting all applicable water quality standards, and the deep zone having increased dissolved H2S (∼50 

mg/L) due to continuing sulfate reduction. 

While discharge of surface-waters could have begun, it was recognized that the strongly-reducing 

conditions in the deeper zone might have attributes worth maintaining for additional acid rock drainage 

(ARD) loading and treatment. It seemed important to better understand the deep-water chemistry and 

treatment conditions. Because the strongly-reducing deep water condition was believed to be the result of 

overdosing the carbon-nutrient compounds, it was surmised that the most effective metal reduction and 

removal could be accomplished by maintaining this reaction-zone with strong reducing conditions. The 

surface water was thought to be more vulnerable to perturbations, such as contaminated surface runoff 

into the pit, which might easily result in metals such as cadmium or zinc increasing above discharge limit, 

with no possibility of decontamination by sulfide precipitation – as would occur in the deeper zone. The 

deeper zone was, in general, considered to be more stable and controllable; consequently, additional 

ARD-loading for treatment might be more efficient because reducing conditions already existed, making 

it unnecessary to add additional carbon to consume dissolved oxygen and establish anoxic conditions. It 

was envisioned that in the future, contaminated water from the Gilt Edge Mine site might be injected, 

along with nutrients, into the pit below the stable chemocline, with the surface layer simply serving as the 

“protective” layer over what would be the “treatment zone”. Certainly if this approach were pursued, the 

relative densities of the deep zone water and the contaminated water added for treatment would have to be 

considered. The decision was made to focus on discharging water from the deep zone. The elevated H2S 

present in the deep water posed health and safety concerns, which were addressed and managed. 

Mitigation of the deep water chemistry was attempted by pumping the deep zone waters through an airsparging unit, followed by a shallow holding-lagoon to complete BOD reduction and solids settling. 

Since the residual H2S oxidized to elemental sulfur and formed colloidal particles that were very slow to 

settle, attainment of regulated values of total suspended solids (TSS) was difficult. Interestingly, metals 

were not remobilized by this sulfide oxidation, and approximately 150,000 gallons of water was 

successfully discharged in two separate batches in October 2004. 

Even though discharge-quality water was achieved, it was apparent that the time and process phases, 

along with the area requirements for “polishing-lagoons,” made this approach impractical. The notion of 

operationalizing a treatment system that could be in-loading ARD to a deep reaction zone was deemed to 

be fraught with too many uncertainties to warrant further consideration. In addition, the team concluded 

that 1) any operationalized treatment process should assume that high-H2S conditions could likely occur 

during each treatment “batch”, 2) that a method for rapidly mitigating such a condition be sought, and 3) 

attention should be turned to eliminating the high-H2S deep-water concentrations in a “single-step” 

approach of in situ oxidation. The addition of various oxidants was considered, and hydrogen peroxide 

was selected as the preferred method to eliminate the dissolved H2S. Thirty-five thousand gallons of 

hydrogen peroxide was released into the pit below the chemocline in August 2005. The reaction was 

successful, and as had been observed in the oxidation-lagoons, the lake experienced a milky-gray 

coloration as elemental sulfur formed and began to settle. 

In late-fall 2005 and spring 2006, large-volume skimming and decanting of surface-waters meeting waterquality standards was carried out, resulting in the release of approximately 40-million gallons of treated 

water into Strawberry Creek, which runs adjacent to the Gild Edge Mine site. Sampling had shown that 

approximately 25-million additional gallons of dischargeable water was present in the treated pit lake, 

when very large spring runoff and storm events required an emergency transfer of ARD-inventory from 

the Gilt Edge Mine site into the Anchor Hill pit, functionally ending the treatability test in late 

spring 2006. 

ES-2 
1. Introduction 

1.1 Background 

This document is the final report for Mine Waste 

Technology Program (MWTP), Activity III, 

Project 34, Bioremediation of Pit Lakes (Gilt Edge 

Mine). The MWTP is a program funded by the 

U.S. Environmental Protection Agency (EPA) and 

jointly administered by EPA and the U.S. 

Department of Energy through an Interagency 

Agreement (IAG). MSE Technology 

Applications, Inc. (MSE) is the principal 

contractor for the MWTP. 

The EPA Region 8 Superfund office and the EPA 

National Risk Management Research Laboratory 

(NRMRL) MWTP, through MSE, conducted a 

field-scale technology demonstration of an in situ 

treatment of the Anchor Hill Pit lake at the Gilt 

Edge Mine Superfund site near Deadwood, South 

Dakota from March of 2001 through May 2006. 

The project goal was to develop performance data 

of the treatment approach for potential application 

in long-term water treatment/management 

activities at the Gilt Edge site, as well as potential 

application at other similar sites. EPA’s interest in 

the in situ treatment process was to determine if 

“semi-passive” treatment of acid rock drainage 

(ARD) was possible in a pit lake, thereby avoiding 

some portion of the costs of operating a 

conventional water treatment plant. 

In addition to summarizing the execution of the 

project over an approximately five-year period, 

this report also summarizes what was learned over 

the course of the project, and provides a 

description of how a similar effort might be 

conducted in light of the lessons learned during the 

execution of this project. Figure 1-1 (left) shows 

an aerial photograph of the Gilt Edge Mine site, 

Figure 1-1 (center) shows an aerial photograph of 

the Anchor Hill Pit lake, and Figure 1-1 (right) 

shows a contoured drawing of the Anchor Hill 

Pit lake. 

Prior to treatment, the Anchor Hill Pit contained 

approximately 72 million gallons of acidic water, 

with elevated metals, sulfate, selenium, and nitrate 

content, and a pH of approximately 3. Table 1-1 

presents the initial water composition for samples 

collected in March 2001 prior to pit neutralization. 

The table also presents the applicable or relevant 

and appropriate requirements (ARARs) for the 

Gilt Edge Mine site that would have to be 

achieved to discharge water to Strawberry Creek. 

1.2 Treatment Description 

The treatment approach was originally envisioned 

to consist of two steps that would occur within the 

pit lake: (1) neutralize the pit water to nearneutral pH using lime (CaO), applied using a 

Neutra-Mill (essentially a floating lime slaker, 

developed by Earth Systems, Pty. of Australia); 

and (2) utilize Redox-Mediated Biotransformation 

(RMBTM) technology, developed and patented by 

Green World Science, Inc. (GWS), to create 

reducing conditions; stimulate bacterial activity 

for nitrate, selenium, and sulfate reduction; 

improve water quality; and create a stable system. 

The RMBTM process involved addition of nutrients 

to the pit, including methanol, animal feed-grade 

molasses, and phosphoric acid, with the goal of 

stimulating indigenous bacterial activity to first 

reduce nitrate to nitrogen gas, and subsequently 

reduce selenium and sulfate. Bacterial reduction 

of selenium to its elemental state removes it from 

solution, and reduction of sulfate produces sulfide, 

which forms metal sulfide particles with copper, 

cadmium, nickel, lead, and zinc. These metals are 

reduced to dischargeable concentrations at neutral 

pH via precipitation as metal sulfides. The metal 

sulfides were intended to settle to the bottom of 

the pit, where a permanent anoxic zone would be 

maintained, ensuring their long-term stability. 

1
 
Figure 1-1. 300-acre Gilt Edge Mine site (left); Anchor Hill Pit lake (center & right). 

Table 1-1. Initial Pit Water Composition and ARARs for the Gilt Edge Mine Site 

Parameter 

Average Concentration ±

Standard Deviation (n=4 unless 

otherwise noted) (μg/L unless 

otherwise noted) 

South Dakota ARARs Criteria Special Conditions 

Unionized Ammonia as N 

Dissolved Oxygen 

pH 

Total Suspended Solids 

(TSS) 

Acidity as CaCO3 

Chloride 

Nitrate/Nitrite as N 

Sulfate 

No data available 

9.0±0.90 (n=3 shallow samples) 

3.1 SU±0.07 (n=29) 

6±1.2 mg/L 

1115±110 mg/L 

37.3±3.3 mg/L 

85.5±15 mg/L 

3260±30 mg/L 

≤0.02 mg/L 

≥5.0 mg/L 

≥6.5-≤8.8 S.U. 

≤10 mg/L 

N/A 

N/A 

<50 mg/L 

N/A 

30-day average 

30-day average 

24-hr composite 

30-day average 

Contaminant Acute Aquatic Life Value (μg/L) 

Dissolved Aluminum 

Dissolved Antimony 

Dissolved Arsenic 

Dissolved Cadmium 

Dissolved Calcium 

Dissolved Chromium III 

Dissolved Chromium VI 

Dissolved Copper 

Dissolved Iron 

Dissolved Lead 

Dissolved Magnesium 

Dissolved Manganese 

Dissolved Mercury 

Dissolved Selenium 

Dissolved Silver 

Dissolved Zinc 

Cyanide (weak-acid 

dissociable) 

224,000±19,000 

17±0.5 

73±3 

576±25 

506,000±27,000 

No data available 

<500 

43,300±2500 

15,600±

31±2.3 

196,000±6800 

27,100±680 

<0.1 

26±1.6 

<1.3 

14,100±400 

No data available 

N/A 

N/A 

360 

3 

N/A 

1708 

15 

63 

N/A 

281 

N/A 

N/A 

2.1 

20 

37.4 

370 

22 

Hardnessa 

Hardnessa
 

Hardnessa 

Hardnessa 

Hardnessa 

Hardnessa 

a The discharge limits are calculated by formulas incorporating hardness. 

hardness value of 400 mg/L as calcium carbonate. 

The values shown are for the maximum allowable 

2
 
2. Project Chronology 

A project chronology, shown in Table 2-1, elements of the project are presented in other 

presents an overall view of how the project was sections of this report. 

executed. More detailed explanations of selected 

Table 2-1. Chronology of Project Execution 

Date Action Taken 

July 2000 The project was selected for execution by EPA-NRMRL’s MWTP in collaboration with EPA 

Region 8. A project kickoff meeting was held at the Gilt Edge Mine including personnel from EPA 

Region 8, CDM Federal Programs, Inc., the U.S. Bureau of Reclamation, EPA NRMRL, Green 

World Science, Inc., Shepherd-Miller, Inc., South Dakota Department of the Environment and 

Natural Resources (SDDENR), MSE, and the U.S. Department of Energy. Responsibilities were 

assigned as follows: MSE would have a sole-source subcontract with Shepherd-Miller (North 

American licensee for the Neutra-Mill technology) to neutralize the pit with the Neutra-Mill and a 

sole-source subcontract with GWS to implement its RMB technology by dosing nutrients after lime 

neutralization. MSE would also prepare a quality assurance project plan (QAPP) for the project and 

participate in data collection and evaluation activities. EPA Region 8 would cover analytical costs, 

including data validation for the project and provide the lime for the pit lake neutralization. CDM 

would provide quality assurance (QA) oversight and facilitate on-site support along with the U.S. 

Bureau of Reclamation (BOR). Note that at that point in time, the site was operated by the BOR. 

September 2000 Shepherd-Miller personnel visited the site to obtain vertical profiles of physical measurements and 

collect water for lime titrations, with the goal of establishing water depths for future monitoring, and 

estimating lime requirements. Initially, the pit only contained 35 to 40 million gallons. Additional 

water was transferred to the pit in the fall of 2000, including high-nitrate water from the heap leach 

pad, until the final water volume was about 72 million gallons. The purpose of this was to provide 

higher nitrate concentrations to challenge the technology, as well as to simply provide more water 

for treatment. 

February 2001 The project QAPP was finalized by MSE after receiving approval from EPA. 

March 2001 Initial samples and profiles vs. depth were obtained, continuous monitoring equipment was installed, 

and lime neutralization was initiated. Various problems were encountered in delivering lime to the 

Neutra-Mill, as well as Neutra-Mill operation. These are discussed in more detail later in this report. 

May 2001 Lime neutralization was completed around May 10, based on pH profiles indicating near neutral pH 

through the bulk of the water column, with higher pH values near the bottom. Nutrient dosage was 

planned for two weeks later. Immediately prior to adding nutrients, pH profiles were obtained. 

These indicated that the pH of the water column had dropped to approximately 5 over the previous 

two weeks. This is discussed in more detail below. Since trucks containing the nutrients were due 

to arrive imminently, it was decided to proceed with nutrient dosing, which was completed in late 

May. 

June 2001 Continuous monitoring equipment indicated that dissolved oxygen was consumed within several 

weeks of nutrient addition. 

August 2001 EPA-NRMRL performed a technical systems audit on August 29, 2001. There were four findings 

related to QAPP deviations, timely receipt of data so that previous sampling event data was available 

for review, anomalies noted for field pH measurements, and the questionable nature of data gathered 

from the continuous monitoring probes. All findings, observations, and technical comments were 

addressed by MSE in a response to EPA. This response memo is contained in Appendix A. 

September 2001 Additional nutrients were added to the pit. This was justified by GWS by the presence of much 

higher nitrate than originally expected due to the addition of high-nitrate heap leach pad water. 

More nitrate to be reduced requires more carbon to provide the necessary electrons. 

3
 
Date Action Taken 

October 2001- Monitoring of the pit conditions continued. Nitrate reduction was proceeding at a rate much slower 

September 2002 than expected. This was attributed to a combination of lower than optimum pH along with the 

presence of elevated aluminum in solution. 

April 2002 Additional molasses was added to the pit, along with sodium hydroxide to raise the pH of the water 

to near neutral. 

September 2002 A concerted effort was made to place the pit into optimum conditions for biological activity going 

into the winter of 2002-2003. These efforts included raising the pH to neutral with sodium 

hydroxide, as well as adding more nutrients. In addition, wood chips were added with the intention 

that they would become waterlogged and sink, and thereby provide a suitable substrate for bacteria 

to attach to and grow on. 

October 2002 Foam was observed on the water surface, likely due to nitrogen and carbon dioxide gases produced 

from bacterial activity. Samples taken indicated a significant drop in nitrate concentration. 

April 2003 Samples and profiles taken through the winter of 2002-2003 showed rapid progress in nitrate 

reduction. Physical evidence of sulfate reduction was observed after ice-off in April 2003, by the 

presence of black solids and hydrogen sulfide gas aromas whenever the surface water was disturbed. 

April 2003 – Continued monitoring confirmed the progression of sulfate reduction, as evidenced by a dramatic 

April 2004 drop in dissolved concentrations of metals that form sulfide precipitates (e.g., copper, cadmium, 

zinc); the presence of excess sulfide; very low oxidation-reduction potential (ORP) levels; and the 

smell of hydrogen sulfide gas associated with samples. The pit water column had become strongly 

stratified, with aerated water floating on top of strongly reducing, high-sulfide water below. 

April 2004 Discussions occurred relating to potential discharge of water from the pit. These discussions 

focused on how the pit might be used as part of long-term water treatment scenarios at the site. 

Options considered were directly discharging surface water, or filtering and aerating deeper water. 

It was decided to focus on discharging deeper water, since it was thought that the deep zone could be 

better used for long-term treatment. The rationale for this is discussed later in this report. 

May-July 2004 Preliminary filtration tests were performed to evaluate the required filtration of deep water, and a 

filtration apparatus using bag filters was assembled. A shallow aeration/settling pond was created by 

lining an existing pond. Initial transfer of small amounts of water produced issues with hydrogen 

sulfide gas management, along with issues related to total suspended solids. The excess sulfide in 

the water oxidized rapidly to elemental sulfur, which was very difficult to settle or filter. This is 

discussed further below. 

July 2004 Whole effluent toxicity tests of Anchor Hill Pit surface water, as well as water from the shallow 

aeration/settling pond, showed the water was not toxic. 

July-September In order to evaluate potential use of the high-sulfide, anoxic zone in the pit for future water 

2004 treatment, two buckets were suspended in the pit. Both buckets contained four gallons of deep 

Anchor Hill Pit water and one gallon of Surge Pond water, which was neutral pH, containing 

elevated nitrate and a small amount of metals. One bucket had nutrients (methanol, molasses, 

phosphoric acid) added, while the other did not. When these buckets were retrieved after six weeks, 

it was found that the nitrate initially in the bucket with nutrients added had reduced to ammonia. 

This was surprising and had very significant ramifications for use of the deep water zone for water 

treatment. 

August-October The initial 100,000-gallon batch of deep water was aerated and successfully discharged after all 

2004 discharge requirements were met. 

4
 
Date Action Taken 

November 2004 An evaluation was performed of the stability of the stratification in the Anchor Hill pit, indicating 

that turnover was very unlikely. A turnover event would be potentially dangerous due to the 

presence of excess dissolved hydrogen sulfide gas in the deep zone. At approximately the same 

time, the surface of the pit turned a different color, and a sewage-type smell was produced. This 

indicated partial vertical mixing to a slightly deeper depth than mixing had previously occurred. The 

likelihood of a turnover event was evaluated by determination of the Lake and Wedderburn 

numbers, utilizing historical wind speed information from the site along with the estimated vertical 

density gradient in the water column. Results of this analysis indicated that no vertical mixing 

would be expected under average recorded wind speeds of 10 miles per hour (mph). Under 

sustained wind speeds at the maximum recorded value of 35 mph, no hypolimnion mixing would be 

expected, but there would be potential for mixing of the metalimnion with the surface zone. A 

sustained wind speed of 63 mph would be required to produce conditions potentially leading to a 

turnover of the overall water column. This was considered very unlikely, and no further steps were 

deemed necessary to address the issue of elevated H2S in the deep zone; however, reducing the H2S 

levels in the deep zone became an important goal late in the project. 

February 2005 About 4 million gallons of surface water from under the ice on the pit was discharged. This took 

several attempts before a configuration was established that did not draw water from below the 

chemocline. 

May 2005 Since it had been established that utilizing the reducing, anoxic conditions below the chemocline for 

long-term water treatment was not going to be viable, discussions were undertaken regarding how to 

best address the high sulfide levels in the deep water in preparation for project completion. 

Evaluations were performed of using ferrous iron salts, ferric iron salts, and oxidants such as 

hydrogen peroxide or bleach to eliminate the high sulfide levels. It was ultimately decided to utilize 

concentrated hydrogen peroxide due to lower cost and ease of handling. 

July-August About 15 million gallons of surface water meeting applicable South Dakota water quality standards 

2005 was discharged. 

August 2005 Concentrated hydrogen peroxide was added to the pit to eliminate excess sulfide by oxidizing it to 

elemental sulfur. Much of the peroxide initially went directly to the bottom of the pit, leaving a zone 

of untreated water between a depth of about 10 feet down to about 45 feet. Over the ensuing 

months, mixing and reactions slowly occurred. 

October 2005 Similar to the previous year, the color of the water surface changed and a sewage-type smell was 

produced. Once again this probably indicated vertical mixing to a deeper depth than previously
 

encountered. 

March 2006 Analytical data indicated that the pit water no longer contained any sulfide, indicating that hydrogen 

peroxide treatment had been successful. 

April 2006 An additional 15 million gallons of surface water was discharged from the pit. By this time in the 

project, over 40 million gallons of water had been successfully discharged from the pit. 

May 2006 Due to high amounts of runoff late in the winter at the Gilt Edge site, ARD was transferred from the 

Sunday Pit to the Anchor Hill Pit. This effectively ended the monitoring phase of the project. 

5
 
3. Implementation 

The following describes how elements of the 

project were physically conducted and provides 

more details about implementation of each step of 

the project. 

3.1 Sampling 

Sample points were established at two locations 

along the long dimension of the pit. These were 

designated NE for northeast and SW for 

southwest, since the longest dimension of the pit 

runs from southwest to northeast. Orange marine 

buoys marking the locations were attached with 

plastic-coated cable to buckets filled with 

concrete. Based on original profiles taken in 

September 2000 by Shepherd-Miller personnel, a 

thermocline could be expected to extend down to 

the 20- to 25-foot depth in summer. In an attempt 

to provide depth and lateral coverage without 

driving up analytical costs excessively, sample 

points were established at depths of 5 feet and 35 

feet at the northeast location (thereafter designated 

NE05 and NE35) and at depths of 20 feet and 60 

feet at the southwest location (thereafter 

designated SW20 and SW60). Generally 

throughout the project, samples were taken at 

these locations. Occasionally, samples were 

obtained at additional depths to help answer 

questions existing at those times. In addition, 

vertical profiles of temperature, pH, ORP, 

dissolved oxygen, and specific conductance (SC) 

were obtained every five vertical feet at the NE 

and SW locations using either a Hydrolab or YSI 

sonde. After a history was established showing 

the NE and SW profiles were generally identical, 

profiles were typically taken at only the SW 

location since the pit was deeper at that end. 

Sediment samples were also obtained on several 

occasions at the NE and SW locations. 

Sampling was usually performed by attaching a 

tube to the sonde, lowering the sonde to the proper 

depth and location, and using a peristaltic pump to 

collect water from that depth. The pump was 

operated for an extended period prior to collecting 

samples to allow purging of the tube. Several 

sampling events were performed using a 

Kemmerer sampler, in which an open tube is 

dropped to the proper depth, and a “messenger” 

sent down the rope to trip the sampler, capturing 

the tube volume at that depth. Sediment samples 

from the pit bottom were generally obtained using 

an Ekman grab sampler. 

Early in the project, “Troll” data-collecting 

instruments, manufactured by In Situ, Inc., were 

placed in the pit at the NE and SW locations. 

These turned out to require extensive maintenance 

to ensure trustworthy values were obtained, 

particularly after addition of nutrients to the pit 

and the subsequent initiation of biological activity. 

The instruments became coated with organic 

material that looked somewhat like algae. Since 

on-site labor was not available to undertake this 

maintenance, those instruments were only used 

sporadically the first year, and eventually removed 

and used elsewhere at the Gilt Edge site by CDM 

Federal Programs. 

Safety issues associated with sampling while the 

pit was frozen over had to be addressed. For work 

on the ice during neutralization (March-April 

2001), a “walkway” was constructed using a series 

of floats tied together, extending from shore to the 

NE sample point, and from there to the SW sample 

point. Each float was essentially a 4-inch thick, 4­

foot by 8-foot piece of closed cell foam 

sandwiched by two 4-foot by 8-foot sheets of 

plywood. This walkway had a cable running 

down the middle, and personnel were required to 

clip in a harness to this cable while using the 

walkway. This was quite cumbersome since, due 

to the construction of the system, personnel had to 

unclip and re-clip to the cable when stepping from 

one float to the next. This system was used for the 

first two winters. Subsequently, personnel had to 

drag one float with them out to the holes cut in the 

ice for sampling purposes. Ultimately, CDM 

imposed safety procedures that required a certain 

minimum ice thickness for work on the ice. 

The project highlighted the need for innovative 

monitoring methods to better understand the 

6
 
progress of each reaction phase. Analysis and 

interpretation of traditional laboratory data proved 

to be an inadequate means of evaluating the timesensitive reactions that were proceeding in real 

time. In March 2004, a video camera was used to 

explore the Anchor Hill Pit to provide visual 

evidence of what was occurring in the pit. The 

video images captured by the camera (shown in 

Figure 3-2, left) helped the project team determine 

that biological processes were underway as 

evidenced by the gas bubbles seen in the 

suspended bags of wood chips (Figure 3-2, center) 

and also allowed the team to view the sediments at 

the bottom of the pit and also estimate the depth 

the these sediments (Figure 3-2, right). 

3.2 Treatment Methods/Materials 

During the project, several different materials 

were added to the pit lake during the treatment 

process. The following sections describe the 

amount of each material added and the methods 

used to add the materials to the pit. 

3.2.1 Lime Addition 

Lime addition was accomplished with a NeutraMill, developed by Earth Systems, Pty. of 

Australia. A photograph of the Neutra-Mill is 

depicted in Figure 3-3. Shepherd-Miller was the 

North American licensee of that technology at that 

time. Since lime addition ended up being 

performed essentially under winter conditions, this 

turned out to be quite difficult. Initial plans by 

Shepherd-Miller were to have dump trucks deliver 

lime to the work site, but the truck drivers were 

unwilling to back their trucks down the haul road 

into the pit under icy conditions. For this reason, a 

lime silo was rented and placed in an accessible 

location on the rim of the pit above the NeutraMill docking location. The silo had an auger 

system integral to it, and a configuration was 

assembled in which the auger would transfer lime 

to a 6-inch pipe, and the lime would flow through 

the pipe down the pit wall to the working 

elevation, and then across the haul road to the 

Neutra-Mill. Note that this was pebble quicklime, 

and this system did not work. The coarse lime did 

travel down the pit wall, but could not flow across 

the relatively level haul road. Ultimately, a 

conveyor was rented to transfer the pebble 

quicklime across the haul road to the Neutra-Mill. 

Once the problem of delivering lime to the NeutraMill was solved, the Neutra-Mill itself presented 

numerous operational problems. A common 

problem was that the rotating drum would become 

out of balance, which would require a shutdown 

until personnel could redistribute the load so the 

drum could function, until the next shutdown. 

It was anticipated that the Neutra-Mill would be 

capable of delivering, grinding, and disseminating 

three to five tons of lime per hour to the pit lake. 

The throughput was well below this anticipated 

level. In an attempt to achieve a higher 

throughput, the project switched to fine hydrated 

lime delivered in bulk bags. These bulk bags were 

transported by forklift down the haul road to the 

Neutra-Mill, which was then fitted with a small 

hopper and auger. The bulk bag was lifted above 

the hopper, and cut open, spilling the lime into the 

hopper. Water was needed to “sluice” the lime 

from shore to the Neutra-Mill. Throughput never 

exceeded 1.5 tons per hour. 

The Neutra-Mill mixed the lime into the water 

immediately below the platform on which the 

Neutra-Mill sits. In an effort to enhance mixing 

into the pit lake, a portable pump capable of 

transferring approximately 400 gallons per minute 

(gpm) was utilized. The pump inlet drew 

water/lime slurry from beneath the platform, and 

the discharge was through a 4-inch hose with the 

exit placed in the southwest section of the pit. A 

total of 292 tons of lime had been added to the pit 

when neutralization was considered complete. 

3.2.2 Nutrient Addition – May 2001 

Nutrient addition was performed by Green World 

Science in May 2001. This consisted of 

offloading six truckloads of animal feed-grade 

molasses and one truckload of methanol into the 

pit water. These were not blended into the water 

in a complicated manner, they were simply 

offloaded through large hoses manned by the truck 

drivers. Refer to Figure 3-4 (left) for a photograph 

of molasses addition and Figure 3-4 (right) for a 

7
 
photograph of methanol addition. Due to the high 

specific gravity of the molasses (1.5 to 1.6), the 

molasses likely sank immediately to the bottom of 

the pit. The methanol could be seen spreading out 

on top of the water surface, and likely mixed in 

well with the water. 

3.2.3 Nutrient Addition – September 2001 

Two truckloads of molasses were added to the pit 

in the same manner as before – offloaded to the pit 

surface by a hose. 

3.2.4 Nutrient and Caustic Addition – April 

2002 

One truckload of molasses was added to the pit by 

offloading through a hose onto the ice surface (the 

pit was still frozen) at the SW end of the pit. 

Upon the ice melting, the molasses then mixed 

into the pit water column. 

One truckload of 50% sodium hydroxide (caustic) 

was also added because pH values were still not 

optimum for bacterial activity despite the first pit 

lake neutralization with lime that was discussed 

previously. The caustic solution was added by 

gravity flow through a pipe out to the middle of 

the pit, on top of the ice. The solution was quite 

warm; typically 50% sodium hydroxide is 

delivered at 120 °F to ensure adequate flowability. 

This solution melted a hole in the ice, and upon 

contacting the cold water under the ice, the caustic 

then probably sank to the bottom of the pit. 

Anecdotal discussions with others indicated that 

warm 50% caustic forms “globs” when contacting 

cold water and does not mix well. It is possible 

that the caustic sank to the bottom of the pit and 

never reacted with the water, and it is also possible 

that the caustic reacted very slowly with the water. 

No direct evidence of a beneficial pH increase was 

ever noticed in the vertical profiles after this 

addition of caustic. 

3.2.5 Nutrient, Caustic, and Wood Chip 

Addition – September 2002 

Through the spring-summer of 2002 (due to the 

continuing absence of indicators of robust bioreactions) the test-team was concerned that 

“something was missing” for onset of strong 

reducing conditions. Debate focused on three 

contributing factors: 

•	 Dissolved-aluminum concentrations high 

enough to possibly cause adverse toxicity to 

bacteria at the cell membrane, inhibiting 

metabolism, growth and reproduction; 

•	 Insufficient “substrate” for bacterial 

colonization; or, 

•	 continuation of suppressed pH conditions, not 

conducive to robust biology for the sulfatereducing bacteria (SRB). 

The project team decided that the test should 

incorporate changes to optimize the pit conditions 

for all of the above factors, including: 

•	 Adding caustic to raise the pH to ~6.5, thereby 

assuring the precipitation of dissolved 

aluminum, thus minimizing the potential for 

continuing cellular toxicity from aluminum; 

•	 Adding wood chips (by floating woodchips, 

by dropping woodchip-filled netting-bags to 

the sediment interface at the lake-bottom, and 

by installing suspended vertical columns of 

woodchip-filled netting-tubes through the 

water column—spanning the various lakelayers). 

These actions were carried out over the fall of 

2002 and winter-spring of 2002-03. 

Two truckloads of molasses, one truckload of 

methanol, and two truckloads of 50% caustic were 

added to the pit. These were approached much 

differently than previous efforts. A pumping 

system was set up to draw near-surface water from 

the NE end of the pit, and discharge it at the 

surface near the middle of the pit, with the goal of 

inducing some better lateral mixing than would 

normally be expected. The pumping rate was 

approximately 800 gpm. The nutrients and caustic 

were slowly metered into the pumping loop to 

ensure they were well mixed into the water when 

it was discharged to the pit. This avoided the poor 

8
 
mixing observed previously with the caustic 

addition in April 2002, and provided better mixing 

of the nutrients into the water as well. 

Lastly, 96 tons of wood chips were added to the 

pit by offloading dump trucks of wood chips into 

the SW portion of the pit. The purpose of the 

wood chips was to provide bacteria with a 

substrate to attach to and also provide a long-term 

carbon source for the bacteria. It is unclear if 

these wood chips were required or whether the 

reactions would have proceeded without their 

addition, but these wood chips floated for a few 

weeks before becoming waterlogged and sinking. 

Physical indications of nitrate reduction were 

evident nearly immediately; biological “slimes” 

formed within the floating chips, and within 10-14 

days a white froth of gas bubbles appeared and the 

pit-surface was “fizzing.” Figure 3-5 is a 

photograph showing a white froth of gas bubbles 

within floating woodchips. 

3.2.6 Fertilizer Addition 

Monitoring indicated that nitrate reduction was 

complete by early 2003 and that sulfate reduction 

was underway by summer 2003. Fertilizer was 

added to the pit in August 2003 in an attempt to 

stimulate an algae bloom. This was part of the 

overall approach put forth by Shepherd-Miller and 

Green World Science at project inception. The 

idea was to stimulate algal blooms, so that when 

the algae dies and sinks, it can serve as an 

additional nutrient source for the SRB in the 

deeper water. The fertilizer added was granular 

triple super phosphate with the formula 

Ca(H2PO4)2 (17% to 23% P; 44% to 52% P2O5). It 

was added by slowly driving a boat around the pit 

water surface and adding scoopfuls of fertilizer 

into the surface water. The dosage was sufficient 

to provide 1 mg/L of phosphorus to the top 3 feet 

of the water column. An algal bloom was never 

produced, and it is unclear if this fertilizer addition 

had any impact on the overall treatment scheme. 

Once near optimum conditions for treatment were 

established, the reducing conditions did occur and 

the bulk of water in the pit did improve from a 

water quality standpoint. At this point, the project 

team began to focus on treatment options to 

further upgrade the water in the Anchor Hill Pit to 

meet the very strict discharge requirements for the 

site. These were very involved efforts, and only a 

summary will be presented in the body of this 

report, with a more complete description presented 

in Appendix B. 

3.2.7 Discharge of Deep Zone Water, 2004 

Discharge of deep zone water was attempted 

during the summer and fall of 2004. Bench-scale 

tests focused on developing a filtration and 

aeration process to meet discharge requirements. 

Filtration would be required to remove residual 

suspended metal sulfides, and aeration would be 

needed to increase dissolved oxygen and return 

biochemical oxygen demand (BOD) to 

dischargeable levels. Safety issues associated with 

hydrogen sulfide gas release arose and were 

solved. Problems with TSS arose due to oxidation 

of excess sulfide to elemental sulfur during the 

aeration step. Approximately 100,000 gallons 

were ultimately discharged by the further 

processing treatment of the deep zone pit water. 

The difficulties with filtration, hydrogen sulfide, 

and the very fine sulfur solids were significant, 

and pursuing discharge in this manner was not 

practical. Further information about this phase of 

the project can be found in Appendix C. 

3.2.8 Evaluation of Use of Anchor Hill Pit 

for Ongoing Site Water Treatment, 2004 

In parallel with the efforts focused on discharging 

deep Anchor Hill Pit water described previously, 

attention was also focused on ongoing water 

treatment options. It was envisioned that water 

from the site could be injected with required 

nutrients under the existing chemocline, taking 

advantage of existing anoxic conditions and 

elevated sulfide, resulting in rapid denitrification 

followed by sulfate reduction as seen previously in 

the pit. As part of the evaluation of injecting 

water for treatment below the chemocline, bucket 

tests were performed to gain further information 

about what results could be expected. Water from 

the Surge Pond at the site was selected for 

treatment, since it contained elevated nitrate (120 

mg/L as N), was relatively neutral in pH, and had 

9
 
small amounts of dissolved metals present. 

Approximately 1 gallon of Surge Pond water 

along with 4 gallons of deep Anchor Hill Pit water 

was placed in each bucket. Nutrients sufficient to 

completely reduce the nitrate were added to one of 

the buckets, and the other bucket received no 

nutrient addition. The buckets were placed at the 

60-foot depth in the pit in July 2004, to both 

ensure no oxygen passed through the bucket wall, 

and that representative temperatures were 

maintained. The buckets were retrieved six weeks 

later. It was anticipated that some or all nitrate 

would be reduced, and that some or all of the 

existing sulfide would be oxidized. The results 

indicated that sulfide was oxidized, but more 

significantly, nitrate was apparently reduced to 

ammonia. This was surprising, but upon 

investigation, it was found that nitrate 

ammonification is a known process occurring 

under extreme reducing conditions. Hydrogen 

sulfide can serve as an electron donor, along with 

other compounds. It is surmised that a different 

type of nitrate-reducing bacteria were predominant 

in the deep water, relative to the oxic conditions 

existing at the start of the project; however, this 

was never verified. Judging by the lengthy time 

(almost two years) since the completion of 

denitrification, this was in retrospect not 

surprising. However, the reduction of nitrate to 

ammonia has very significant implications for the 

Gilt Edge Mine site, since nitrate is present in 

varying amounts in essentially all waters existing 

on-site. Discharge limits for the site are 50 mg/L 

as N for nitrate, and approximately 3-5 mg/L as N 

for ammonia, depending on temperature and pH. 

These results caused the project team to abandon 

the concept of feeding site waters (with nutrients) 

below the chemocline for treatment, since nitrate 

is present in all site waters, and the risk of 

excessive ammonia production was considered too 

great. Abandoning the addition of site waters 

below the chemocline meant that use of the 

treatment process would have to be accomplished 

on a batch basis, and the deep zone could not be 

used as the primary reactor for ongoing water 

treatment at the Gilt Edge site. 

Based on the risk of nitrate ammonification 

described above, it was surmised that any future 

water treatment within the Anchor Hill Pit would 

have to be performed on a batch basis (i.e., the pit 

would be loaded with so much water, neutralized, 

and dosed with sufficient nutrients to denitrify the 

nitrate present and achieve a suitable amount of 

sulfate reduction to produce dischargeable 

dissolved metals concentrations). After allowing 

some settling time for the metal sulfide 

precipitates, the water would need aeration to 

eliminate any remaining sulfide and BOD, and to 

raise the dissolved oxygen, possibly along with 

some clarification. Aerating the water, and 

settling the sulfur to produce dischargeable water, 

it is not practical to perform for very large 

volumes of water so other ways to oxidize the 

excess sulfide would be employed. 

3.2.9 Evaluation of Likelihood of Lake 

Turnover, 2004 

Concerns about the potential for a turnover-mixing 

event by the meromictic (lake layers that do not 

mix) lake was evaluated in October/November 

2004. This was accomplished by evaluation of the 

Lake and Wedderburn numbers, utilizing historical 

wind speed information from the site along with 

the estimated vertical density gradient in the water 

column. Results were that no vertical mixing 

would be expected under average recorded wind 

speeds of 10 mph. Under sustained wind speeds at 

the maximum recorded value of 35 mph, no 

mixing would be expected, but there would be 

potential for mixing of the metalimnion (layer of 

rapid temperature change) with the surface zone. 

A sustained wind speed of 63 mph would be 

required to produce conditions potentially leading 

to a turnover of the overall water column. This 

was considered very unlikely, and no further steps 

were deemed necessary to address the issue of 

elevated H2S in the deep zone; however, reducing 

the H2S levels in the deep zone did become a 

project objective. A more complete description of 

this effort is provided in Appendix B. 

3.2.10 Discharge of Surface Water, 2005 

Having determined that (a) processing of the 

deeper sulfide-laden water for discharge is 

10
 
difficult and likely not economically viable; (b) 

use of the deep zone for ongoing water treatment 

is not feasible due to the risk of excessive 

ammonia production; and (c) use of the treatment 

technology in the Anchor Hill Pit would have to 

be done on a batch basis, it was decided to begin 

discharge of surface water. This reverted back to 

the original concept of placing a batch volume of 

water in the pit, neutralize as necessary, add 

nutrients, and let the biological treatment 

processes proceed. Surface water would be 

removed at some rate consistent with sulfur 

settling through the water column, and enough 

water would be left in the pit to provide a 

reasonable cover for the less-stable, higher sulfide 

at the bottom of the pit. Excess sulfide would 

probably be present in all batches, due to the need 

for excess sulfide to drive dissolved copper, 

cadmium, and zinc down to very low 

dischargeable levels. However, as the chemocline 

would come closer to the surface as water is 

discharged, remaining sulfide would be oxidized 

to sulfur by interaction with the aerated surface 

zone. The issue would then be whether the sulfur 

residues would settle fast enough to accommodate 

desired discharge rates as water levels drop, and 

the thermo/chemocline readjusts. Developing this 

approach began over the winter of 2004-2005. 

In March 2005, while the pit was still frozen over, 

approximately 4.3 million gallons (equating to 

approximately 1 vertical meter of water column) 

were removed from just under the ice surface, and 

successfully discharged. This was encouraging, 

and plans were made to discharge additional 

surface water and eliminate excess sulfide in the 

summer of 2005. The addition of various oxidants 

was considered to oxidize the excess sulfide to 

sulfur. Hydrogen peroxide was selected as the 

preferred method to eliminate the dissolved H2S. 

Thirty-five thousand gallons of concentrated 

hydrogen peroxide were released into the pit 

(Figure 3-6) in August 2005. The reaction was 

successful, and as had been observed in the 

oxidation-lagoons, the lake experienced a milkygray coloration as elemental sulfur formed and 

began to settle (Figure 3-7). 

Approximately 10 million gallons of water 

meeting discharge standards was successfully 

released from the surface of the pit between midJuly and mid-August 2005. 

3.2.11 Discharge of Surface Water, 2006 

Analytical data indicated that the pit no longer 

contained any sulfide, indicating that the oxidation 

with hydrogen peroxide had been successful. In 

April 2006, an additional 15 million gallons of 

surface water was discharged from the pit. Since 

the project began, over 40 million gallons of water 

had been successfully treated and discharged from 

the Anchor Hill Pit. 

11
 
NE Sample Buoy 

SW Sample Buoy 

Figure 3-1. Photograph indicating the location of sample buoys in the Anchor Hill Pit. 

Figure 3-2. Camera used for video camera observations (left), gas bubbles within suspended bags of chips (center), and 

sludge at bottom of pit (right). 

Figure 3-3. Photograph showing Neutra-Mill docking location. 

12
 
Figure 3-4. Adding molasses (left) and methanol (right) to the pit. 

Figure 3-5. Photograph showing white froth of 

gas bubbles within the floating wood chips. 

Figure 3-6. Photograph showing in situ hydrogen 

peroxide addition. 

13 
Figure 3-7. Photograph showing evidence 

of excess sulfur in the shallow pit water 

after hydrogen peroxide addition. 

14
 
4. Results/Discussion 

At the beginning of this project, the Anchor Hill 

Pit contained about 72 million gallons of acidic 

water, containing high levels of dissolved metals,

selenium, nitrate, and sulfate. Table 4-1, which 

illustrates how water quality improved over the 

course of the project, presents selected data. 

Figure 4-1 presents the pH data prior to pit 

neutralization. 

Neutralization of the pit occurred between March 

and May 2001 using the Neutra-Mill and lime. 

After a several-week stabilization period following 

neutralization, the RMBTM process was initiated in 

May 2001 with the addition of carbon nutrients. 

Additional sodium hydroxide additions were made 

to the pit in response to a significant drop in pH 

(from approximately 7 to 5) observed during the 

post-neutralization stabilization period. Figure 4-2 

shows the pH profiles with depth after lime 

neutralization. Even after the attempt to neutralize 

the pit lake, the bulk of the water was still at a pH 

of approximately 5. 

The RMBTM treatment proceeded slower than 

expected, presumably attributed to starting the 

treatment at a lower-than-optimum pH. Along 

with pH conditions deemed lower than ideal for 

bacterial growth, aluminum dissolved back into 

solution at concentrations (~40 mg/L) that may 

have adversely affected bacteria metabolism. 

Denitrification proceeded slowly through the 

remainder of 2001 and the bulk of 2002. In 

September 2002 a concerted effort was made to 

raise the pit pH to near 7 in order to provide a 

better pH environment for the bacteria and to 

precipitate aluminum from solution. Figure 4-3 

shows the pH profile following additional efforts 

to neutralize the pit. This effort was successful – 

the pH was increased to near neutral, and 

aluminum concentrations decreased to less 

troublesome levels. 

Wood chips were also added to provide a substrate 

for bacterial growth, but the effect of the wood 

chips is unknown. By October 2002, nitrate 

showed a significant drop, and microbial counts 

increased. 

Nitrate concentrations continued to decrease 

through the winter of 2002-2003, with nondetectable levels achieved by March 2003. A plot 

of nitrate/nitrite concentrations at each sample 

location versus time is presented in Figure 4-4. 

Note that nitrate reduction occurred more quickly

in the deep portions of the pit compared to the 

shallower portions of the pit until the pH was 

adjusted to near neutral in September 2002 and 

nitrate reduction proceeded rapidly when more 

optimum conditions for biological activity were 

attained in the pit. 

Thermodynamically, significant sulfate reduction 

was not anticipated to occur until denitrification 

was complete; and sulfate reduction was expected 

to begin as soon as a population of SRB could be 

developed. In mid-April 2003, field personnel 

observed indications that sulfate reduction was 

occurring. Whenever the surface was disturbed, 

they noted black precipitates welling to the surface 

and hydrogen sulfide gas odors. 

Sampling through the summer of 2003 confirmed 

the initiation and progress of sulfate reduction. 

Average dissolved copper, cadmium, and zinc 

concentrations showed a striking decrease. Figure 

4-5 plots these parameters along with nitrate, 

versus time, showing the dramatic decrease in 

these metals (due to the onset of sulfate reduction) 

immediately following denitrification. The pit 

water also showed an increase in alkalinity in the 

deeper water to approximately 400 mg/L (as 

CaCO3), providing further evidence that biological 

nitrate and sulfate reduction was occurring, since 

bicarbonate is a byproduct of both the nitrate and 

sulfate reduction reactions. Data through April 

2006 is plotted. During May 2006, the test 

effectively terminated with addition of ARD to the 

Anchor Hill Pit when precipitation events caused 

water management issues at the Gilt Edge site, 

which prompted the addition of site waters to the 

pit lake. 

15 
Water quality in the Anchor Hill Pit continued to 

improve until it approached discharge standards. 

Both deep pit water and surface water were 

successfully discharged from the pit. Discharge of 

the water was only possible after overcoming 

many technical challenges; however, the most 

significant result of the project was that over 40 

million gallons of water was successfully 

discharged from the Anchor Hill Pit lake. In this 

sense, the project was successful, but there were 

several lessons learned during the project that 

would be taken into account if a second batch of 

water were processed in the Anchor Hill Pit or if a 

similar project were undertaken at another pit lake. 

Depth (ft) 

10 

pH 

NE Buoy 

3/26/01 

SW Buoy 

3/26/01 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

2468 12 

Figure 4-1. pH versus depth prior to pit lake neutralization. 

16 
Depth (ft) 

(c) 3rd Quarter
 

0
 

10
 

20
 

30
 

40 

50 

NE Buoy 

8/29/01 

SW Buoy 

8/29/01 

60
 

70
 

80
 

90
 

2 4 6 8 10 12
 pH
 

Figure 4-2. pH profiles with depth after initial lime neutralization. 

17
 
October-00 

April-01 

November-01 

May-02 

December-02 

June-03 

January-04 

August-04 

February-05 

September-05 

March-06 

October-06 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

Nitrate + Nitrite (mg/l as N) 

NE 1.5m 

NE 10.7m 

SW 6.1m 

SW 18.3m 

Date 

(d) 4th Quarter 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

Depth (ft) 

NE Buoy 

10/24/02 

SW Buoy 

10/24/02 

SW Buoy 

11/8/02 

SW Buoy 

11/13/02 

2 4 6 8 10 

pH 

12 

Figure 4-3. pH profile following additional efforts to neutralize the Anchor Hill Pit. 

Figure 4-4. Nitrate data at each sample location throughout the demonstration. 

18 
October-00 

April-01 

November-01 

May-02 

December-02 

June-03 

January-04 

August-04 

February-05 

September-05 

March-06 

October-06 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

Nitrate + Nitrite (mg/l) 

0.001 

0.01 

0.1 

1 

10 

100 

Dissolved Cu/Cd/Zn (mg/l) 

NO3-NO2 

Cu 

Cd 

Zn 

Figure 4-5. Anchor Hill Pit nitrate/nitrite and dissolved copper, cadmium, and zinc concentrations over the project 

duration. (Note: values averaged over all sampling locations.) 

19
 
Table 4-1. Water Quality Improvement Over the Course of the Project 

Parameter May 2001 January 2003 February 2004 

NE05 NE35 SW20 SW60 NE05 NE35 SW20 SW60 NE05 NE35 SW20 SW60 NE05 

Dissolved 

Aluminum 

49.7 45.2 41.8 40.3 3.38 3.43 4.09 3.51 0.127 0.017 0.012 0.019 0.02 

Dissolved 

Cadmium 

0.315 0.349 0.35 0.345 0.203 0.206 0.198 0.184 0.0038 0.005 0.005 0.005 0.001 

Dissolved 

Copper 

16.3 16.6 16.4 15.9 0.871 1.47 3.3 2.64 0.014 0.008 0.005 0.004 0.01 

Dissolved 

Iron 

0.337 0.0867 0.035 0.0479 1.03 1.27 0.976 0.858 0.03 0.048 0.046 0.034 0.04 

Dissolved 

Zinc 

7.19 7.65 7.61 7.49 3.38 3.43 4.09 3.51 0.127 0.017 0.012 0.019 0.02 

Alkalinity <5 <5 <5 <5 134 134 176 186 106 366 378 404 98 

Nitrate/ 

Nitrite 

68.7 73.2 79.3 71.6 0.4 0.2 3.8 7.21 0.14 0.88 0.14 0.5 0.19 

Sulfate 2540 2700 2230 2510 ND ND ND ND 1900 2400 2100 2300 ND 

Sulfide <5 <1 <1 <1 80 100 81 94 4.5 9 8.7 11 1.9 

20
 
5. Lessons Learned/Quality Assurance Activities 

The following sections are lessons learned over 

the course of the project that may have value to 

others attempting similar work in the future. Also 

included in this section is a discussion of QA 

activities undertaken throughout the project. 

5.1 Neutralization 

•	 If possible, physical profiles should be 

gathered frequently during times of the year 

with rapid temperature change (spring and 

fall) or with potential acidic run-off to more 

confidently understand the trends observed. 

Furthermore, regular physical profiles should 

be gathered for at least one year for an existing 

pit lake prior to initiating treatment such as 

that presented here, for comparison purposes. 

In addition, for pit lakes with uncertain 

groundwater inflow or flow through, the use 

of a bromide background indicator would be 

useful. However, to be useful it must be well 

mixed into the lake, which is easier said than 

done. Ideally, it would be added a year or 

more in advance to allow time for dispersion. 

•	 The Neutra-Mill was not an efficient way to 

neutralize this pit lake due to the relatively 

low throughput and the relatively high 

operator attention required. In fairness, 

personnel at Earth Systems take the position 

that the system was not properly configured 

and operated by Shepherd-Miller. The fact 

remains, however, that it was difficult to 

transfer the lime to the Neutra-Mill without 

“slurrying” it to the Neutra-Mill, and it did 

require constant vigilance in operation. It is 

probably quite suitable for smaller bodies of 

water where the desire is simply to “get in, do 

it, get out”. For a larger body of water, where 

a larger effort is inherently required, a reliable, 

less labor-intensive approach would be 

desirable. One possibility would be to set up a 

mixed tank at an accessible location near the 

lakeshore. Bulk bags of lime could be added 

directly to the mixed tank, which would be 

constantly agitating, with makeup water being 

delivered via a pump immediately adjacent in 

the water body. A second pump could transfer 

lime slurry from the tank to the water body 

through a flexible hose, which could be moved 

occasionally around the lake to facilitate 

mixing. 

•	 In April of 2002, one truckload of sodium 

hydroxide was offloaded on top of the icedover pit lake, with the idea that as the ice 

melted, the caustic would mix in with the 

water. In reality, concentrated caustic is 

delivered at elevated temperature, on the order 

of 120 °F. This caustic was delivered to the 

ice surface, where it promptly melted a hole in 

the ice and probably sank to the pit floor. 

Concentrated caustic does not mix well in cold 

water; it simply forms “globs”. This type of 

neutralization approach should not be 

attempted in a similar, future effort. 

•	 A more effective methodology was used in 

September of 2002, when the concentrated 

caustic was metered into a pumping loop. 

Water was drawn from near the shore at 

approximately 800 gpm, concentrated caustic 

was metered into it at a low rate of around 1 

gpm, and the mixture was delivered to the 

water surface in the middle of the pit. This 

was the approach taken in effectively 

neutralizing the pit prior to the more rapid 

commencement of biological activity. 

•	 The choice of lime versus caustic for in situ 

neutralization depends, in part, on the initial 

sulfate concentration and the desired final 

sulfate concentration. Certainly, liquid caustic 

has some advantages in ease of handling, but 

will not effect the sulfate concentration as lime 

can through the production of a gypsum 

precipitate (CaSO4•2H2O). With the high 

initial sulfate concentration present in the 

Anchor Hill Pit, the use of lime was 

advantageous in producing lower total 

dissolved solids and sulfate values. In the case 

of a hypothetical pit lake with sulfate 

21 
concentrations well below the gypsum 

saturation point, there may be no advantage in 

using lime. 

•	 Neutralization should not be conducted in the 

winter. Due to project-specific circumstances, 

neutralization was performed in the winter, but 

it was extremely difficult. If a pit lake is 

located in a climate where it freezes, then 

neutralization should be conducted in the fall, 

so nutrients can then be added just prior to 

freezing over of the lake. 

•	 If neutralizing to a target pH is attempted, 

adequate time must be allowed for reactions to 

complete, and the need for additional reagent 

dosages are likely. Overdosing of the reagent 

should be considered, as well as the partial use 

of a reagent providing buffering capacity, such 

as soda ash. 

5.2 Nutrient Addition 

•	 Nutrient dosages should be made based on a 

recent analysis of the water to be treated, and 

should be conducted either after the lake 

freezes or immediately before ice forms (if the 

climate is such that ice will form). 

•	 Reduction reactions would likely have 

occurred much faster had neutral pH been 

maintained from the beginning. 

•	 Nutrient dosage must be estimated carefully so 

as to not overdose, avoiding excessive sulfide 

production. 

•	 A good microbial community analysis on the 

initial water would be helpful to determine the 

kinds of bacteria that are present and whether 

bioaugmentation may be necessary. The 

addition of inoculating bacteria to the pit 

should be considered. It was not performed on 

this project because a significant philosophical 

aspect was that only native bacteria would be 

stimulated and utilized for treatment. In other 

applications it should receive serious 

consideration since it might result in more 

rapid treatment. 

•	 The wood chips were a relatively cheap 

material to add to the pit lake to provide 

substrate for bacterial growth, but it is not 

clear if wood chips were required for the 

reactions to proceed. 

•	 If fertilizer is added in an attempt to stimulate 

algae growth, a liquid rather than solid form 

should be utilized. In addition, careful thought 

should be given to the makeup of the fertilizer 

and its interaction with the surface water. In 

this case, only granular triple super phosphate 

[Ca(H2PO4)2] was added. It is likely that a 

small amount of nitrogen should also have 

been added, and further, it is likely that with 

the high amounts of dissolved calcium already 

present in the water, the calcium phosphate 

could not readily dissolve. 

5.3 Monitoring 

•	 In situ physical measurements using a 

datasonde-type apparatus are favored over the 

use of a flow cell and peristaltic pump. This is 

because changes can occur between the deeper 

zones and the water surface. On one sampling 

occasion (October 2001) a pump and flow cell 

were utilized. The temperature of the water in 

situ warmed up as it was pumped through 

warmer water to the surface, and was 

obviously inaccurate. In addition, noticeable 

degassing of (probably) carbon dioxide was 

observed in the pump tubing at the water 

surface. This would tend to result in a pH 

increase. 

•	 If unexpected dissolved oxygen measurements 

are encountered, the presence of other 

dissolved gases (i.e., sulfur dioxide, nitrous 

oxide, nitric oxide, chlorine, etc.) should be 

considered and, if necessary, evaluated. 

•	 The use of charge balance calculations as a 

check of data quality and validity should be 

utilized as necessary. 

22 
•	 If biological treatment is being undertaken, 

appropriate analyses (e.g., SRB counts, total 

direct counts) should be performed prior to 

treatment and regularly thereafter. 

5.4 Discharge 

Extraction and oxidation of deep water is not the 

best way to discharge water. Taking water off the 

oxygenated surface zone avoids the TSS problem 

caused by oxidation of sulfide to elemental sulfur. 

Over 20% of the water initially present in the pit 

was successfully discharged this way. 

5.5 Quality Assurance Activities 

At the beginning of the project, responsibilities 

were assigned to various members of the project 

team. Quality assurance activities were assigned 

to EPA Region 8 and their contractor CDM. The 

following sections discuss data quality issues that 

were identified during the project by CDM and 

other members of the project team, the impact of 

these issues on data quality, and how these issues 

were addressed. 

5.5.1 Field Data Review 

Field data (pH, ORP, temperature, dissolved 

oxygen) was collected by Shepherd-Miller, MSE, 

and CDM during regular sampling events. 

Continuous monitoring probes were also installed 

in the Anchor Hill Pit to give real-time 

information. As treatment proceeded, field data 

was reviewed primarily to identify trends. Data 

anomalies were identified for the continuous 

measurements when these results were compared 

to field data from sampling events. It was decided 

that it was difficult, if not impossible, to maintain 

valid calibration of these probes, so this data was 

only used by the project for identifying trends and 

was not used to determine if project objectives had 

been met. 

There were also occasional issues identified with 

the field data collected at scheduled sampling 

events. The pH data collected by MSE using the 

YSI from the August 2001 sample event should 

also be considered estimated because after 

measurements were collected, all buffers measured 

0.4 pH units above the accepted value for the 

buffer. After this event, the YSI was calibrated at 

each location prior to collecting vertical profiles, 

thus not relying on one calibration for the entire 

sampling event. 

5.5.2 Laboratory Data 

EPA Region 8 began submitting samples to the 

EPA Contract Laboratory Program (CLP). Over 

the course of the project, CDM reviewed field and 

laboratory data as it was generated and prior to 

release to MSE. CDM did validate the data and 

indicate through data qualification if any data had 

data quality issues. The data review process at 

CDM did cause problems, as the data was not 

available for review in a timely fashion so that 

technical adjustments could be made. 

Upon review of the data from 2001-2002, MSE 

personnel noted that the charge balances were not 

within tolerable limits for the samples collected. 

To correct this, EPA Region 8 gave CDM 

approval to send samples to non-CLP labs (Energy 

in Rapid City, South Dakota and Mid-Continent in 

Rapid City, South Dakota). CDM also reviewed 

and validated this data prior to release to MSE. 

The charge balance issue was corrected and 

reporting limits improved after sending samples to 

the non-CLP laboratories. 

Also, as a result of the data anomalies reported by 

MSE, EPA-NRMRL performed a technical 

systems audit on August 29, 2001. There were 

four findings related to QAPP deviations, timely 

receipt of data so that previous sampling event 

data is available for review, anomalies noted for 

field pH measurements, and the questionable 

nature of data gathered from the continuous 

monitoring probes. All findings, observations, and 

technical comments were addressed by MSE in a 

response to EPA. This response memo is 

contained in Appendix A. As mentioned 

previously, CLP laboratories being used at the 

project outset were not providing very defensible 

data because charge balances were not within 

tolerable limits. 

The only relevant laboratory data that was deemed 

unusable by CDM included total lead data from 

23
 
the SW60 sample location collected on 8/29/01. 

Because lead was already below the discharge 

criteria, this did not impact project objectives. 

The most critical sampling events were those that 

would determine if water met the ARARs and 

could be discharged from the Anchor Hill Pit. As 

summarized in Appendix B, much work was 

performed to determine the best way to obtain 

water that met the discharge standards. Data 

relating to discharge requirements was intensely 

reviewed by the project team and the State of 

South Dakota to ensure that the data used to make 

discharge decisions was defensible. CDM did not 

report any data quality issues for these samples. 

24
 
6. Conclusions/Recommendations 

There is clearly the potential to utilize the pit lake 

for treating site waters in batch mode. The 

approach of neutralizing acidic water in the late 

summer/fall followed by adding nutrients after an 

extended stabilization time, will likely yield 

dischargeable water quality at the surface about 

1.5 years later (two winters and one summer). 

Water can then be discharged off the surface. This 

would probably result in significant cost savings 

(20% to 40%) over the water treatment plant, but 

there are many considerations, as discussed in the 

previous section, to ensure that the technology is 

applied appropriately. 

With the successful discharge of Anchor Hill Pit 

surface water in 2005 and 2006, it is possible that 

significantly more surface water could be 

discharged in a like manner. Water would be 

discharged from the surface zone (top 5 to 10 

feet), and then the new surface water would be 

allowed to equilibrate with the atmosphere for 

some period of time. If necessary, the dissolved 

oxygen and BOD of that water would become 

acceptable for discharge, along with settling of any 

precipitates present, be they ferrous sulfide or 

elemental sulfur. Once the surface water met 

applicable standards, it could be similarly 

discharged. 

With the knowledge gained during the project, if a 

new batch of water were ready for treatment, it 

would be accomplished as follows: 

•	 Good initial characterization over a year to 

determine seasonal effects on pit lake water 

quality; 

•	 Bench-scale tests on actual water at 

appropriate conditions (similar to bucket tests 

performed in the pit) to assist in determining 

proper dosages of the various reagents and 

nutrients; 

•	 Faster and more efficient means of pit lake 

neutralization; 

•	 Careful dosage of nutrients in the late fall so 

that biological reactions could occur over the 

winter; 

•	 Detailed microbial community analysis to 

determine nutrient requirements and need for 

bioaugmentation; 

•	 Consideration of adding bacterial inoculum to 

speed the treatment process, particularly if 

initial biological characterization indicated 

that critical bacteria types were not present in 

the initial water; and 

•	 Discharge of water from the surface water 

zone incrementally as treatment proceeds and 

surface water meets discharge standards. 

All original project objectives were eventually 

met, and the project was a success. Also, 

considerable experience and insight was gained 

into how operational aspects of such a remediation 

technique would have to be designed for a future 

application of this technology. In late-fall 2005 

and spring 2006, large-volume skimming and 

decanting of surface-waters meeting water-quality 

standards was carried out, resulting in the 

successful release of approximately 40-million 

gallons into Strawberry Creek over the course of 

the project. 

25
 
7. References 

Lewis, N.M., K.W. Wangerud, B.T. Park, S.D. 

Fundingsland, J.P. Jonas, “Status of in situ 

treatment of Anchor Hill Pit Lake, Gilt Edge Mine 

Superfund site, South Dakota”, in Proceedings for 

ICARD 2003: Sixth International Conference on 

Acid Rock Drainage, July 14-17, 2003, Cairns, 

Queensland, Australia, The Australian Institute of 

Mining and Metallurgy (AussIM), ISBN: 1­

875776-98-2, 2003. 

Lewis, N.M., K.W. Wangerud, B.T. Park, S.D. 

Fundingsland, J.P. Jonas, “Anchor Hill pit lake 

two-stage in situ treatment, Gilt Edge Mine 

superfund site, South Dakota, USA”, in 

Proceedings for Tailings and Mine Waste’03, pp. 

345-358. Swets and Zeitlinger B.V., Lisse, The 

Netherlands, ISBN: 90 5809 593 02, 2003. 

26
 
Appendix A 

Technical Systems Review Response from MSE to EPA 
A-1 
A-2 
A-3 
A-4 
A-5 
A-6 
Appendix B 

Evaluation of the Use of Anchor Hill Pit for Ongoing Site Water Treatment 
Evaluation of Use of Anchor Hill Pit for Ongoing Site Water Treatment 

It was envisioned that water from the site could be injected with required nutrients under the existing 

chemocline, taking advantage of existing anoxic conditions and elevated sulfide, and resulting in rapid 

denitrification followed by sulfate reduction as seen previously in the pit and become an integral part of 

the water treatment strategy for the Gilt Edge Mine site. As part of the evaluation of injecting water for 

treatment below the chemocline, bucket tests were performed to gain further information about what 

results could be expected. Water from the Surge Pond at the site was selected for treatment, since it 

contained elevated nitrate (120 mg/L as N), was relatively neutral in pH, and had small amounts of 

dissolved metals present. Approximately 1 gallon of Surge Pond water along with 4 gallons of deep 

Anchor Hill Pit water was placed in each bucket. Nutrients sufficient to completely reduce the nitrate 

were added to one of the buckets, the other bucket received no nutrient addition. The buckets were placed 

at the 60-foot depth in the pit in July 2004, to ensure that no oxygen passed through the bucket wall, and 

that representative temperatures were maintained. The buckets were retrieved six weeks later. It was 

anticipated that some or all nitrate would be reduced, and that some or all of the existing sulfide would be 

oxidized. The results indicated that sulfide was oxidized, but more significantly, nitrate was apparently 

reduced to ammonia. This was surprising, but upon investigation, it was found that nitrate 

ammonification is a known process occurring under extreme reducing conditions. Hydrogen sulfide can 

serve as an electron donor, along with other compounds. It appeared that a different type of nitratereducing bacteria were predominant in the deep water, relative to the oxic conditions existing at the start 

of the project. Judging by the lengthy time (almost two years) since the completion of denitrification, this 

is in retrospect not surprising. However, the reduction of nitrate to ammonia had very significant 

implications for the Gilt Edge mine site, since nitrate is present in varying amounts in essentially all 

waters existing on-site. Discharge limits for the site are 50 mg/L as N for nitrate, and approximately 3-5 

mg/L as N for ammonia, depending on temperature and pH. These results caused the project team to 

abandon the concept of feeding site waters (with nutrients) below the chemocline for treatment, since 

nitrate is present in all site waters, and the risk of excessive ammonia production was considered too 

great. Abandoning the addition of site waters below the chemocline meant that use of the treatment 

process would have to be accomplished on a batch basis, and the deep zone could not be used as the 

primary reactor for ongoing water treatment at the Gilt Edge site. 

Treatment & Discharge Options: In Situ and Ex-Situ Process Considerations 

Treatment of the entire pit-lake water body obviously did not occur as envisioned at the outset. Except 

for the surface layer, the fine suspended metal-sulfide precipitates in the water did not readily coagulate 

and completely settle. The team’s attention and efforts in 2004 therefore focused on dealing with the fine 

residual suspended solids in the water column, as well as chemistry issues associated with deep zone 

waters. 

Over the course of treatment, the pit lake had become meromictic (i.e., permanently stratified) with the 

upper layer separated from the deeper zone by a strong density gradient (or chemocline). The team 

recognized that the strong redox condition of the deeper zone is where the most robust kinetics for 

“decontamination” occurred, and that the overlying surface layer served to protect and maintain the 

thermodynamic conditions of the deep zone. Even though the upper surface-waters were “clean”, the 

team initially felt that discharge of the surface layer water might compromise the stability and integrity of 

the multi-layered pit-lake reactor. Accordingly, the team felt that the deep zone (containing the largest 

volume of pit-waters) should be targeted as the primary extraction zone—recognizing that the issues 

related to fine residual suspended solids and the inherent chemistry of the deep-waters (residual carbon, 

B-1 
excess sulfide, and dissolved non-metal constituents) would have to be dealt with. The team realized that 

an additional stage of treatment would be necessary to accomplish deep zone water discharge. 

Ex-Situ Deep Zone Water Processing 

The team developed an approach for filtering the water while pumping from the pit, followed by aeration 

in a shallow lagoon. Filtering would remove the fine suspended solids, while surface-exposure and 

aeration would remove dissolved hydrogen sulfide, bio-reduce organic constituents to decrease 

biochemical oxygen demand (BOD), and increase dissolved oxygen, thereby achieving dischargeable 

water. Planning began for filtering of the water, and also for construction of a 0.1-acre, 5-feet deep, 

150,000-gallon lined lagoon to serve as an aeration and oxidation cell. 

Filter testing. In late-January 2004, filtration tests were performed on water from the deep zone in which 

water was filtered through 0.45-, 1-, 5-, 10-, and 20-micron filter cartridges and the filtrates subjected to 

total metals analysis. The purpose of this was to determine approximate filtration requirements for 

sufficiently removing the residual fine metal sulfide precipitates from solution when pumping from the 

deep zone. Results indicated that filtration at a level of 5-microns or less would be needed. While 

discharge metals standards are based on dissolved analytical values, a conservative approach focusing on 

total metals values was followed, since it was thought that suspended metal-sulfide particles might 

oxidize and re-dissolve upon prolonged exposure to air. 

Field-filters and initial pump-runs. A filtration configuration consisting of an initial 25-micron bag filter 

followed by two 1-micron bag filters in parallel was selected as an inexpensive, simple way to filter initial 

batches of water. These bag filters were #2-size (7-inch diameter by 32-inches long), contained in 

standard, off-the-shelf filter housings. The water fed to the filtration system was to be pumped from the 

50-foot depth by a Godwin HL80M pump. The pump-filtration setup and the lagoon are shown in Figure 

B-1. Due to anticipated off-gassing of carbon dioxide and hydrogen sulfide gases released by pumping 

water from the deep zone to the surface, tests were performed at increasing scale to assure worker health 

and safety. An initial pumping test of a small batch of 200 gallons confirmed the formation and release of 

small amounts of hydrogen sulfide gas. As a result, the Site Health & Safety Plan was modified for 

increased instrumentation and monitoring capability, self-contained breathing apparatuses (SCBAs) were 

obtained for emergency contingency in the event of extreme off-gassing, and a weather station was set up 

in the vicinity of the aeration lagoon to assure that pumping into the lagoon was only done under 

sufficient wind conditions to assure rapid dispersion of H2S. 

Figure B-1. (left) Godwin pump and filter-set configuration in the pit; (right) aeration/settling lagoon 

(ASL) with initial 2,000 gallon run 

B-2 
Scale-Up Operations and Results. After addressing the anticipated hydrogen sulfide gas concerns, a 

2000-gallon pumping test was conducted in late June 2004. Initial pumping using the Godwin pump 

produced erratic, surging flow rates, likely due to the anticipated off-gassing of carbon dioxide and 

hydrogen sulfide gases resulting from pressure-differentials when pumping water from the deep zone to 

the surface. A subsequent 20,000-gallon pumping test, also in late-June, at a lower pump speed showed 

much better pump behavior, with a flow of approximately 350 gpm. Filtration performance was not as 

effective as desired, with some suspended solids remaining visible in the filtrate in the lagoon. Following 

both pumping tests, field analyses of sulfide and turbidity were performed in the ensuing days. In both 

cases, the sulfide level decreased and the turbidity increased. This is believed to be caused by the 

formation of elemental sulfur precipitates resulting from the oxidation of residual dissolved sulfide. The 

implication of this was that the aeration lagoon would have to doubly serve as a settling basin upon 

completion of aeration/oxidation. 

Initial Full-Scale Process Discharge. In mid-July an additional 100,000 gallons were pumped from the 

deep zone through the filters to the ASL. Once again the water initially showed a decrease in sulfide and 

increase in turbidity from the oxidation of sulfide to elemental sulfur, as shown in Figure B-2. The time 

required for completion of this process was longer than the previous batches due to (a) the increased 

volume and depth of the water in the lagoon, (b) conservative operations in delaying active aeration to 

allow slower release of small amounts of residual H2S gas, and (c) to gain de-gassing experience. Active 

aeration initially consisted of recirculation of lagoon water with small submersible pumps (too time 

consuming), followed by use of the Godwin pump at approximately 500 gpm. This recirculation occurred 

for about five days, at which time the pumps were shut off to observe settling behavior of the suspended 

sulfur and the anticipated decay in BOD. Measurements were made regularly during the aeration process 

with a multi-parameter probe for pH, temperature, dissolved oxygen, and oxidation-reduction potential at 

the water surface and at the bottom of the lagoon. In addition, the lagoon was sampled regularly for 

BOD, total organic carbon, as well as daily measurement of dissolved sulfide and turbidity. Laboratory 

results for BOD and field results for dissolved oxygen at the top and bottom of the ASL are presented in 

Figure B-3. 

0.001 

0.01 

0.1 

1 

10 

7/12/04 7/17/04 7/22/04 7/27/04 8/1/04 8/6/04 8/11/04 8/16/04 8/21/04 

Time 

Dissolved Sulfide (mg/l) 

0 

25 

50 

75 

100 

125 

150 

175 

200 

Turbidity (NTU) 

Dissolved Sulfide (mg/L) 

Turbidity (NTU) 

Figure B-2. Dissolved sulfide and turbidity in ASL following 100,000-gallon addition 

B-3 
0 

5 

10 

15 

20 

25 

7/17/2004 7/22/2004 7/27/2004 8/1/2004 8/6/2004 8/11/2004 8/16/2004 8/21/2004 

Time 

BOD (mg/l) 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Dissolved Oxygen (mg/l) 

BOD 

DO (Top) 

DO (Bottom) 

Apparent 

probe 

malfunction 

Figure B-3. Biochemical oxygen demand (BOD) and dissolved oxygen in ASL following 

100,000-gallon addition 

Upon observing the expected decreases in dissolved sulfide, turbidity, and BOD in the several weeks 

following the final water transfer, it became apparent that the water in the ASL was approaching 

dischargeable quality. To investigate this, several sampling events occurred to provide further data. The 

first event was on August 3, 2004 with the analytical results presented in Table B-1. 

This dataset was encouraging since all metal values reported were within discharge standards. A further 

sampling event occurred on August 5 assessing discharge parameters at three different depths within the 

ASL. (Note: The opportunity was also taken to collect a sample from the surface of the Anchor Hill Pit 

for comparison.) The results for both of these samples are shown in Table B-2 below. 

B-4 
Table B-1. August 3, 2004 ASL Metals Results 

Sample was collected from 0-2 feet 

All results are from Energy Laboratories, Rapid City, SD 

All units are mg/L 

ASL 8/3/04 SD AWQC 

Analyte Total Dissolved Acute Chronic 

Aluminum 0.58 <0.05 

Antimony <0.01 <0.01 

Arsenic 0.04 0.05 0.36 0.19 

Barium 0.03 0.09 

Beryllium <0.005 <0.005 

Cadmium <0.001 <0.001 0.017 0.003 

Calcium 570 590 

Chromium <0.01 <0.01 0.015 0.01 

Cobalt 0.02 0.02 

Copper 0.03 0.02 0.063 0.037 

Iron 0.3 <0.02 

Lead <0.01 <0.01 0.281 0.011 

Magnesium 110 110 

Manganese 17 16 

Mercury <0.0002 0.0002 0.0021 0.000012 

Nickel 0.01 0.01 4.569 0.508 

Potassium 28 30 

Selenium <0.005 <0.005 0.02 0.005 

Silver <0.005 <0.005 0.037 

Sodium 360 370 

Titanium <0.005 <0.005 

Vanadium <0.01 <0.01 

Zinc 0.04 0.07 0.37 0.338 

B-5 
Table B-2. August 5, 2004 ASL and Anchor Hill Pit Surface Water Compared to Strawberry Creek 

Receiving Standards 

All results are from Mid-Continent Laboratory, Rapid City, SD 

All units are mg/L 

Anchor Hill 

Pit Lake ASL Std. 

1-2 ft 0-1 ft 1-2 ft 2-3 ft 

pH 7.88 7.97 7.96 8.00 >6.5-<8.8 units 

See section 

74:51:01:07--no change 

in receiving water 

greater than 0.5 units 

Conductivity at 

25 °C 2,460 3,550 3,560 3,560 

<2,500 

umhos/c 

m 30-day average 

<4,375 

umhos/c 

m daily maximum 

Temperature Not Analyzed 16 - 27 °C <75 °F 

See section 

74:51:01:31--no temp 

change over spawning 

beds, <4 °F change 

Dissolved oxygen Not Analyzed 4.7 mg/L (8/9/04) >5.0 mg/L 

Total alkalinity as 

CaCO3 

54.4 285 285 279 

<750 mg/L 30-day average 

<1313 mg/L daily maximum 

Total Dissolved 

Solids 

2246 3428 3303 3454 

<2500 mg/L 30-day average 

<4,375 mg/L daily maximum 

Suspended Solids 

13 11 15 22 

<10 mg/L 

24-hr composited 

sample 

<17.5 mg/L Grab sample maximum 

Sodium 

adsorption ratio 2.5 3.5 3.5 3.6 <10 

[Na+]/sqrt(([Ca+2] + 

[Mg+2])/2) all in meq/L 

un-ionized 

ammonia nitrogen 

as N 

(all assuming 

22 °C and pH 8.0) 

<0.0022 

0.055 

(total 

NH3-N 

1.25) 

0.057 

(total 

NH3-N 

1.29) 

0.055 

(total 

NH3-N 

1.26) <0.02 mg/L 30-day average 

<0.0022 0.055 0.057 0.055 

<0.035 

(1.75 times 

the 

applicable 

criterion) mg/L daily maximum 

Nitrates as N 

0.068 <0.05 <0.05 <0.05 

<50 mg/L 30-day average 

<88 mg/L daily maximum 

Undisassociated 

hydrogen sulfide <0.4 (total) 

<0.4 

(total) 

<0.4 

(total) 

<0.4 

(total) <0.002 mg/L 

B-6 
BOD5 

Fecal Coliform 

(May 1-Sept 30) 

TPH 

Oil and Grease 

Parameter 

<10 mg/L 

24-hr composited 

sample Awaiting 

Results 5 (8/3/04) 

<17.5 mg/L Grab sample maximum 

<1,000 /100 ml 

geometric mean based 

on a minimum of 5 

samples obtained 

during separate 24-hour 

periods for any 30-day 

period, and they may 

not exceed this value in 

more than 20% of the 

samples examined in 

this same 30-day period 

Not Analyzed 

<2,000 /100 ml in any one sample 

Not Analyzed 

<10 mg/L 

See section 

74:51:01:10--Cannot 

impart a visible film or 

sheen on the surface of 

the water or adjacent 

shoreline 

Not Analyzed 

<10 mg/L 

See section 

74:51:01:10--Cannot 

impart a visible film or 

sheen on the surface of 

the water or adjacent 

shoreline 

 Dissolved Concentrations 

Acute Chronic 

Arsenic (μg/L) <5 23 23 24 360 190 

Cadmium (μg/L) 2 <1 <1 <1 17 3 

Chromium (III) 

(μg/L) <1 <1 1708 554 

Chromium (VI) 

(μg/L) 

<1 (total) <1 (total) (total) (total) 

15 10 

Copper (μg/L) 16 15 15 16 63 37 

Cyanide (weak acid 

dissociable) (μg/L) <10 <10 <10 <10 22 5.2 

Lead (μg/L) <1 <1 <1 <1 281 10.9 

Mercury (μg/L) <0.2 <0.2 <0.2 <0.2 2.1 0.012 

Nickel (μg/L) 17 23 23 25 4569 508 

Selenium (μg/L) 12 22 23 22 20 5 

Silver (μg/L) <1 <1 <1 <1 37.4 --­

Zinc (μg/L) 119 56 79 95 370 338 

based on dissolved conc (for 

acute) and total recoverable for 

chronic 

B-7 
Calcium, Dissolved 

(mg/L) 319 510 518 497 

Magnesium, 

Dissolved (mg/L) 69.2 107 107 105 

Sodium, Dissolved 

(mg/L) 188 333 334 335 

Potassium, 

Dissolved (mg/L) 12.2 25.9 26 25.8 

Hardness (in mg/L 

CaCO3) 400 

use 25 mg/L as a 

minimum 

use 400 mg/L as a 

maximum 

Note: For cadmium, chromium, copper, lead, nickel, silver, and zinc, the discharge limits are calculated by 

formulas incorporating hardness. The values shown are for the maximum allowable hardness value of 400 

mg/L as calcium carbonate. 

This dataset showed little variation in metals data vertically, but also identified a potential problem. 

Selenium values reported by Mid-Continent Laboratory were typically 0.020 to 0.025 mg/L, as compared 

to nondetectable values at 0.005 mg/L produced by Energy Laboratories. This is significant because the 

chronic discharge limit is 0.005 mg/L and the acute limit is 0.020 mg/L. 

These samples had ammonia-nitrogen concentrations of 1.25-1.30 mg/L, suggesting that the breakdown 

of nitrogen- and carbon-organics manifested itself as increased ammonia-nitrogen. 

With these further encouraging results, it was decided to obtain a sample that would be fully 

representative of ASL discharge water. It was apparent that dissolved oxygen would need to be boosted 

from the approximate 4 mg/L present to above the discharge requirement of 5 mg/L. A decant skimmer 

was set up on the ASL to take water from the upper six-inches, and pump the water through a “riffle run” 

of corrugated PVC pipe to further aerate the water and increase the dissolved oxygen. The riffle run is 

shown in Figure B-4. Approximately 2,000 gallons of ASL water was pumped through this riffle run into 

a tank, with a sample taken from the tank for a comprehensive set of analyses representing the “final 

discharge” waters. For ease of discussion, the team has coined an acronym for the entire post-Anchor 

Hill Pit deep-water treatment process—the SOX process. This acronym represents the chemical- and 

process-operations involved with the removal of residual concentrations of sulfur (S) and organics (O) 

and the addition of oxygen (X). The comprehensive results for the SOX-process waters are presented in 

Table B-3 below. 

B-8 
Figure B-4. “Riffle run” for aerating ASL water prior to 

discharge; the discharge line connects to the fitting on lower 

tank, connecting with a tributary clean water-diversion pipe 

near the base of slope. 

B-9 
Table B-3. August 11, 2004 SOX Process Results Compared to Strawberry Creek Discharge Standards 

SOX Std. 

pH 8.04 >6.5-<8.8 units 

See section 74:51:01:07--no change 

in receiving water greater than 0.5 

units 

Conductivity at 25°C 

3,680 

<2,500 

umhos/c 

m 30-day average 

<4,375 

umhos/c 

m daily maximum 

Temperature 22°C <75 °F 

See section 74:51:01:31--no temp 

change over spawning beds, <4oF 

change 

Dissolved oxygen 6.7 >5.0 mg/L 

Total alkalinity as 

CaCO3 

271 

<750 mg/L 30-day average 

<1313 mg/L daily maximum 

Total Dissolved Solids 

3551 

<2500 mg/L 30-day average 

<4,375 mg/L daily maximum 

Suspended Solids 

13 

<10 mg/L 24-hr composited sample 

<17.5 mg/L Grab sample maximum 

Sodium adsorption ratio 3.6 <10 

[Na+]/sqrt(([Ca+2] + [Mg+2])/2) all 

in meq/L 

Un-ionized ammonia 

nitrogen as N 

0.055 (pH 8.0, 

22°C) (total NH3­

N 1.26 mg/L) <0.02 mg/L 30-day average 

0.055 

<0.035 (1.75 

times the 

applicable 

criterion) mg/L daily maximum 

Nitrates as N 

<0.05 

<50 mg/L 30-day average 

<88 mg/L daily maximum 

Undisassociated 

hydrogen sulfide <0.05 (total) <0.002 mg/L 

BOD5 < 3 <10 mg/L 24-hr composite sample 

<17.5 mg/L Grab sample maximum 

Fecal Coliform (May 1­

Sept 30) 

Not Analyzed 

<1,000 /100mL 

geometric mean based on a 

minimum of 5 samples obtained 

during separate 24-hour periods for 

any 30-day period, and they may 

not exceed this value in more than 

20% of the samples examined in 

this same 30-day period 

<2,000 /100mL in any one sample 

B-10 
TPH 

Not Analyzed 

<10 mg/L 

See section 74:51:01:10--Cannot 

impart a visible film or sheen on the 

surface of the water or adjacent 

shoreline 

Oil and Grease 

Not Analyzed 

<10 mg/L 

See section 74:51:01:10--Cannot 

impart a visible film or sheen on the 

surface of the water or adjacent 

shoreline 

 Dissolved Concentrations 

Parameter Acute Chronic 

Arsenic (μg/L) 23 360 190 

Cadmium (μg/L) <1 17 3 

Chromium (III) (μg/L) 1708 554 

Chromium (VI) (μg/L) <1 (total) 15 10 

Copper (μg/L) 16 63 37 

Cyanide (weak acid 

dissociable) (μg/L) <10 22 5.2 

Lead (μg/L) <1 281 10.9 

Mercury (μg/L) <0.2 2.1 0.012 

Nickel (μg/L) 18 4569 508 

Selenium (μg/L) 11 20 5 

Silver (μg/L) <1 37.4 --­

Zinc (μg/L) <50 370 338 

based on dissolved conc (for acute) 

and total recoverable for chronic 

Calcium, Dissolved 

(mg/L) 526 

Magnesium, Dissolved 

(mg/L) 108 

Sodium, Dissolved 

(mg/L) 345 

Potassium, Dissolved 

(mg/L) NA 

Hardness (in mg/L [actual 1758 

CaCO3) mg/L CaCO3] 400 

use 25 mg/L as a 

minimum 

use 400 mg/L as a 

maximum 

Note: For cadmium, chromium, copper, lead, nickel, silver, and zinc, the discharge limits are calculated by 

formulas incorporating hardness. The values shown are for the maximum allowable hardness value of 400 

mg/L as calcium carbonate. 

B-11 
This SOX process sample showed that the water was in compliance with standards with minor 

exceptions. Conductivity and total dissolved solids are above the 30-day average limit, similar to the 

current situation with the Gilt Edge water treatment plant, and which is pursuant to an interim-waiver for 

total dissolved solids (EPA Interim-ROD, November 2001). Un-ionized ammonia is above both the limit 

of 0.02 mg/L for 30-day average and 0.035 mg/L for daily maximum. Selenium is reported at 0.011 

mg/L, between the chronic value of 0.005 mg/L and the acute value of 0.020 mg/L. BOD results for the 

August 11 SOX-process sample were pending, although previous ASL analyses showed BOD to be 5 

mg/L, below the 10 mg/L limit. 

The continuing elevated un-ionized ammonia levels were somewhat unexpected, since previous analyses 

in the Anchor Hill Pit had indicated that ammonia levels were within discharge levels. Extended aeration 

would likely take care of this problem; however, in the interest of timely discharge operations, repeat 

processing is not preferred. Alternatively, the pH of the water in the ASL could be lowered to 

approximately 7.5 in order to meet the un-ionized ammonia nitrogen requirement.; this is the preferred 

option in order to facilitate discharge. Hydrochloric or muriatic acid would be used to lower the pH of 

the ASL from 8.0 to 7.5 through the addition of a small amount of concentrated acid, thereby achieving 

the un-ionized ammonia-nitrogen limit. 

Biotoxicity Testing Results. Samples of the SOX process water collected August 11 were received 

August 12 at the ASCI Laboratory in Duluth, MN to test for acute toxicity to both juvenile fathead 

minnows and Ceriodaphnia dubia. Based on the control organism performance and reference toxicity 

results, the effluent exposures met minimum performance requirements specified by EPA and produced 

valid toxicity results. The SOX-process sample collected did not cause sufficient lethality to allow 

calculation of a 48-hour (Ceriodaphnia) LC50=>100% (i.e., there was100% organism survival). The 

sample also did not cause sufficient lethality to calculate a 96-hour median lethal concentration for the 

fathead minnows (i.e., there was 100% organism survival). 

To check the toxicity of the surface layer in the Anchor Hill Pit Lake, a surface water sample (2-foot 

depth) from the pit lake was collected at the same time and likewise sent to ASCI for identical testing. 

48-hour Ceriodaphnia results were 100% survival and 96-hour fathead minnow results were 95% survival 

(reported as no observable adverse effect). The toxicity results indicated that neither the deep zone water 

treated with the SOX process, nor the surface water from the Anchor Hill Pit posed toxicity issues for 

receiving waters. 

Successful Discharge of Water from Below Chemocline, Summer/Fall 2004 

Utilizing the results from the SOX process testing, a larger discharge batch from below the chemocline 

was targeted. While the surface water met discharge requirements, it was recognized that the stronglyreducing conditions in the deeper zone might have attributes worth maintaining for future use. While the 

strongly-reducing deep water condition was believed to be the result of overdosing the nutrients, and 

could be better managed if implemented in the future, most effective metal reduction and removal could 

be accomplished by maintaining a condition with strong reducing potential. The surface water was 

thought to be more vulnerable to disruptions such as contaminated surface runoff into the pit, which 

might easily result in metals such as cadmium or zinc increasing above their discharge limit, with no 

possibility of treatment by sulfide precipitation as in the deeper zone. The deeper zone was in general 

considered to be more stable and controllable. In addition, the overall treatment process might be more 

efficient since reducing conditions already existed, and carbon would not be needed to consume dissolved 

B-12 
oxygen and establish anoxic conditions. It was envisioned that in the future, contaminated water from the 

site might be injected, along with nutrients, into the pit below the chemocline, with the surface layer 

simply serving as the “protective” layer over what could be considered the “treatment zone”. Certainly if 

this approach were pursued, the relative densities of the deep zone water and the contaminated water 

added for treatment would have to be considered. The decision was made to focus on discharging water 

from the deep zone. The elevated H2S present in the deep water posed health and safety issues, which 

were addressed and managed. Mitigation of the deep water chemistry was attempted by air sparging 

followed by the use of a lagoon to complete BOD reduction. In addition, the settling of solids as 

previously discussed, is important since the H2S was oxidized to elemental sulfur, forming colloidal 

particles that were very slow to settle. Interestingly, metals were not remobilized by this sulfide 

oxidation, and approximately 150,000 gallons of water pumped from the deep zone was successfully 

discharged in October 2004. 

B-13 
Appendix C 

Evaluation of Options for Eliminating Dissolved Sulfide, and Subsequent Addition of Concentrated 

Hydrogen Peroxide 
Evaluation of Options for Eliminating Dissolved Sulfide, and Subsequent Addition of 

Concentrated Hydrogen Peroxide 

Following the implementation of the neutralization and RMBTM steps, the Anchor Hill Pit contained about 

67 mg/L of excess sulfide in the deep zone below the chemocline (i.e., below a depth of about 30 feet). 

The presence of this excess sulfide was due to overdosing of carbon nutrients (methanol and animal feedgrade molasses) early in the project. Reducing conditions were established, and subsequently, 

denitrification and sulfate reduction occurred. Sulfate reduction continued to occur beyond what would 

have been desired strictly from a water quality standpoint because the sulfide produced was in excess 

stoichiometrically compared to the metals levels that would form sulfides. Sulfate reduction continued 

due to the presence of required reactants (carbon, other nutrients, and sulfate) along with suitable 

environmental conditions. 

The presence of excess sulfide posed several problems: 1) potential health and safety concerns associated 

with possible release of hydrogen sulfide gas if the pit water column were to “turn over” and mix 

vertically; 2) health and safety concerns associated with release of hydrogen sulfide gas when handling 

the deep water for potential discharge; and 3) problems associated with generation of suspended solids 

from oxidation of the excess sulfide to elemental sulfur when handling the deep water for discharge. 

These risks led the project team to consider possible approaches for reducing or eliminating the excess 

sulfide. Consuming the excess sulfide via addition of metals-laden site waters was ruled out due to fears 

of nitrate ammonification occurring. All site waters contain appreciable nitrate (>30 mg/L as N), and 

bucket tests performed in 2004 in which deep Anchor Hill Pit water was mixed with Surge Pond water 

did show evidence of nitrate being reduced to ammonia rather than to nitrogen gas. Site discharge 

standards for ammonia are much lower than those for nitrate, and the only viable process for removing 

ammonia is to oxidize it back to nitrate via biological nitrification. This would raise other batch treatment 

process complications, and therefore this option was not considered further. 

CDM personnel performed bench-scale titration tests in May 2005. These titration tests evaluated the use 

of ferric chloride (FeCl3), ferrous chloride (FeCl2•4H2O), and hydrogen peroxide (H2O2) to eliminate 

excess sulfide. Ferric chloride would oxidize a portion of the sulfide to elemental sulfur while the ferric 

iron would be reduced to ferrous. Subsequently the ferrous iron would remove the remaining sulfide 

from solution as a ferrous sulfide precipitate. Ferrous chloride would remove the excess sulfide as a 

ferrous sulfide precipitate. Hydrogen peroxide would remove the excess sulfide by oxidizing it to 

elemental sulfur, similar to oxidizing it by aeration in the SOX Process in the summer and fall of 2004. 

Results were much as expected. All three reagents were very effective, essentially quantitative, in 

removing sulfide. There had been some interest in what effect the iron salts might have on the pH of the 

mixture, since the ferric chloride solution (10% strength) was found to have a pH of 0.9 and the ferrous 

chloride solution (10% strength) had a pH of 2.1. At a stoichiometric ratio of over 200% to remove the 

67 mg/L sulfide present, both resulted in a pH drop to about 6.5 from a starting pH of 7.0; the drop in pH 

was limited by the strong buffering capacity in the deep Anchor Hill Pit water (alkalinity ∼450 mg/L as 

CaCO3). The addition of excess hydrogen peroxide did not show dissolution of the suspended metal 

sulfide precipitates; this is consistent with what was seen in the SOX Pond in the summer and fall of 

2004. 

Having gained confidence that any of these three reagents should be capable of successfully reducing or 

eliminating excess sulfide in the deep water, the issue then became the determination of the best path to 

take. The following table summarizes some advantages and disadvantages associated with each material. 

C-1 
Table C-1. Comparison of options to oxidize excess sulfide in the Anchor Hill Pit lake. 

FeCl2•4H2O FeCl3 H2O2 

Strength 25% 33% 50% 

Quantity Needed 

(assuming 

46,000,000 gallons 

[current volume 

minus top 20 feet], 67 

mg/L sulfide, 100% 

stoichiometric 

dosage) 

308 wet tons 

(14 truckloads) 

127 wet tons 

(6 truckloads) 

26 wet tons 

(1.2 truckloads) 

Cost (same 

assumptions as for 

quantity) 

∼$59K ∼$23K ∼$16K 

Effectiveness for 

Sulfide Removal 

High High High 

Handling Issues Low pH (10% strength 

was 2.9, 25% strength 

probably less) 

Very low pH (10% 

strength was 0.9, 

33% strength would 

be less) 

pH 4.5 (50% strength), 

strong oxidizer 

Specific Gravity 1.28 (25% strength) 1.33 (33% strength) 1.2 (50% strength) 

Viscosity (water at 

room temperature is 1 

cp, olive oil is about 

100 cp) 

(could not find) ∼12 cp ∼1.6 cp 

Notes If dosed in excess, 

could provide “sink” 

for future sulfide 

generated 

1. Could be an issue 

with rapid 

decomposition 

generating O2 gas 

bubbles, could strip 

H2S or destabilize 

water column. This 

could be avoided by 

adding H2O2 slowly. 

2. Could be somewhat 

overdosed to provide 

some dissolved 

oxygen (e.g., 3-5 

mg/L) to inhibit 

further sulfate 

reduction. 

Several other items considered were: 

C-2 
•	 There are probably sulfide precipitates on the wall of the pit, as the water level in the pit is 

pumped down, and there was a possibility that these could oxidize and degrade the surface water 

quality. 

•	 Addition of hydrogen peroxide essentially performs the SOX Process in the pit rather than the 

aeration pond. 

•	 If hydrogen peroxide were added, the elemental sulfur sludge generated would be very stable, and 

would cover existing metal sulfide sludge in the pit. 

•	 If a small amount of sulfate reduction continues due to the small amount of carbon present near/in 

sediments, additional dosage may be needed. This may not be needed if another batch of water is 

to be treated. 

•	 Some added dosage may end up being needed simply due to inefficiencies of mixing into a large, 

still body of water. 

It was decided to add sufficient 50%-strength hydrogen peroxide to the pit to stoichiometrically oxidize 

the sulfide present. This selection was made based on the fact that hydrogen peroxide would be the 

cheapest, it would be relatively easy to handle, and it would chemically mimic the reactions observed 

during the aeration and discharge of sulfide-laden deep water in summer 2004 (i.e., it would result in 

production of elemental sulfur and should not remobilize metal sulfides). 

On August 17-18, 2005, approximately 3900 gallons of 50% hydrogen peroxide by weight were added to 

the pit in a total of twelve 325-gallon totes. The twelve totes were added approximately four at a time in 

three locations: the southwest end; the approximate middle of the pit; and approximately midway between 

those two locations. They were added by simply draining each tote down 2-inch diameter pipe to a 10­

foot deep pipe at the point of delivery. Initial totes were drained very rapidly, while later totes were 

throttled to drain down in approximately 20 minutes. There were occasional disruptions seen at the water 

surface, resulting from the hydrogen peroxide pushing air ahead of it in the empty pipe between 

offloading totes, and from apparently vigorous reactions occurring in the deeper zone. 

Initial results indicated that the hydrogen peroxide sank through the bulk of the water column and only 

oxidized sulfide below about the 45-foot depth, with excess hydrogen peroxide remaining below that 

depth. Subsequent mixing of deep- and mid-level waters by pumping from a shallower depth in one 

portion of the lake to a deeper depth in another portion of the lake was attempted and appeared to be 

successful, and as of October 2005 the excess sulfide in the deep water was significantly reduced to 

between 10 and 15 mg/L. Profiles of sulfide concentrations with depth are depicted in Figure C-1. By 

March of 2006, excess sulfide had been successfully oxidized (see Figure C-2). 

C-3 
December-99 

April-01 

September-02 

January-04 

May-05 

October-06 

0 

20 

40 

60 

80 

100 

120 

Sulfide (mg/l) 

NE 1.5m 

NE 10.7m 

SW 6.1m 

SW 18.3m 

Date 

0 

10 

20 

30 

40 

50 

60 

70 

80 

0 10 20 30 40 50 60 7 

Sulfide (mg/L) 

Depth

(ft below surface) 

0 

16-Aug-05 

8-Sep-05 

6-Oct-05 

11-Jan-06 

Figure C-1. Change in sulfide with depth after H2O2 dosage. 

Figure C-2. Sulfide concentrations with time at sampling locations. 

C-4 
Appendix D 

Evaluation of Likelihood of Water Column Turnover 
MEMORANDUM (VIA E-MAIL) 

TO: Ken Wangerud – U.S. Environmental Protection Agency (U.S. EPA) 

Richard T. Wilkins – U.S. EPA 

Mark Lawrensen - South Dakota Department of Environmental and Natural Resources 

(SD DENR) 

FROM: Steven D. Fundingsland - CDM Federal Programs Corporation (CDM) 

Marko E. Adzic - CDM 

CC: Brian Park – MSE Technology Applications, Inc. (MSE) 

DATE: November 9, 2004 

RE: Gilt Edge Mine Superfund Site – Anchor Hill Pit Lake 

Gentlemen, 

As you know in the spring of 2001 and through the EPA’s National Risk Management Research 

Laboratory (NRMRL) Mine Waste Technology Program and the EPA Region VIII Superfund office, a 

field scale demonstration project demonstrating a two-step in-situ treatment of acidic and metal laden 

waters was carried out within the Anchor Hill Pit Lake at the Gilt Edge Mine Superfund site. By 

February 2004, waters within the top 2.5 ft of Anchor Hill (AH) Pit Lake achieved dischargeable 

standards (i.e., South Dakota Ambient Water Quality Criteria) and contaminants of concern (COCs) 

were generally decreased by more than 99%. Given the success of the project the U.S. EPA and SD 

DENR have expressed the possibility of releasing dischargeable waters from AH Pit Lake into 

Strawberry Creek. Prior to conducting such activities however, CDM evaluated the potential for deep 

mixing to occur and the subsequent increase in COCs and off-gassing of hydrogen sulfide. 

Since the spring of 2003 a distinct horizontal stratification within AH Pit Lake has been observed. This 

stratification is supported by field recorded measurements (e.g., temperature, conductivity, oxidationreduction potential [ORP] and pH) collected throughout the water column, see attached. In general, the 

observed horizontal stratification within AH Pit Lake is driven by a combination of thermal and 

chemical gradients. As indicated by the attached data with the exception of the fall and winter months 

(i.e., November to January) where temperature differences within the water column are minimized 

and/or reversed, a distinct thermocline and chemocline are observed. As a result, the horizontal 

stratification within AH Pit Lake is driven by both thermal and chemical gradients. Since horizontal 

stratification within AH Pit Lake has been established there is no evidence that turnover and/or deep 

water mixing has occurred. This is supported by the continuous negative ORP environment (i.e., 

reducing conditions) within deeper waters (i.e., depths >20 ft below surface) of AH Pit Lake. As 

demonstrated by Robertson and Imberger (1994), dissolved oxygen (DO) concentrations within lakes 

are strongly dependent upon the degree of deep mixing. Therefore, should deep mixing of AH Pit Lake 

waters have historically occurred an increase in ORP values (i.e., more positive) would have been 

observed. As confirmed by the historical ORP profiles however, this has not occurred since AH Pit 

Lake has become horizontally stratified. Nevertheless to better understand lake-hydrodynamics within 

D-1 
the AH Pit Lake, first order calculations and quantitative indicators such as the Wedderburn and Lake 

Numbers were calculated (Imberger, 2001; Hamblin et al., 1999; and Robertson and Imberger, 1994). 

The Wedderburn Number (W) evaluates the potential for metalimnetic water to uplift into the 

epilimnion; while the Lake Number (LN) evaluates the potential for deeper hypolimnetic water to uplift. 

One of the required input parameters in calculating the aforementioned quantitative lake indicators is 

water density. The presence of horizontal stratification within AH Pit Lake confirms a definite 

difference in water density throughout the water column (i.e., denser waters are near the bottom). These 

density differences are the result of thermal and chemical differences within the water column. 

Thomann and Mueller (1987) have put forth the following relationship that calculates water density as a 

function of temperature and salinity: 

−3 2 ρ = 1+{(10 [(28.14 − 0.0735T − 0.00469T ) + (0.802 − 0.002T )(S − 35)]} 

 (Equation 1) 

Where: ρ = Water density as a function of temperature and salinity (g/cm3

) 

T = Water temperature (oC) 

 S = Salinity (‰) 

As indicated within Equation 1 a measure of salinity is required. Typically and to date, salinity has not 

been analyzed at the Gilt Edge Mine Superfund site. As a result, salinity levels were estimated using the 

relationship between conductivity and total dissolved solids (TDS). Eighty data points (i.e., sampling 

events) were identified where both conductivity and TDS measurements were recorded within AH Pit 

Lake over a period of time from October 26, 2000 to July 8, 2004. To better understand the relationship 

between conductivity and TDS within AH Pit Lake, a scatter plot of the aforementioned data set was 

generated, a best-fit relationship determined, and a correlation coefficient calculated using Excel. A 

summary of this data is presented within Figure 1.0. 
TDS S ≅ 

1,000mg / L perthousand 

Gilt Edge Mine - Anchor Hill Pit Lake 

Total Dissolved Solids as a Function of Conductivity 

y = 1.0572x - 397.79 

R2

 = 0.8269 

n = 80; Correlation = 0.91 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 

Conductivity (mS/cm) 

TDS (mg/L) 

Figure 1.0. Relationship between Measured Total Dissolved Solids and Conductivity within Anchor Hill Pit 

Lake. 

As indicated within Figure 1.0 there is a positive correlation between conductivity and TDS (i.e., r = 

0.91); and with an r2

 value of 0.83 a linear relationship was identified as being the best-fit. Using the 

linear relationship noted within Figure 1.0, TDS levels were calculated for historic profile sampling 

events within AH Pit Lake. Subsequent salinity levels were determined using the following relationship 

(Thomann and Mueller, 1987): 

 (Equation 2) 

Where: S = Salinity (‰) 

TDS = Total dissolved solids (mg/L) 

Corresponding water densities were then calculated for each sampling event and density profiles as a 

function of depth generated. 

In general, there appears to be three distinct layers present within AH Pit Lake. They include: the 

epilimnion (0-5 ft), the metalimnion (5-20 ft) and the hypolimnion (>20 ft). Average water densities for 

the aforementioned layers were calculated to be approximately 1,001.2 kg/m3

, 1,002.3 kg/m3, and 

1003.3 kg/m3

, respectively. The overall average density was calculated to be approximately 1,003 

kg/m3

. 
gδ '×h2 

W = u*

2 

× L 

Since September 2004, using hydrometer measurements water densities have been recorded at depth 

intervals of 3 ft and 25 ft below surface within AH Pit Lake. As indicated by Figure 2.0, for water 

samples collected at depths of 3 ft below surface, a negative correlation between water temperature and 

density (i.e., as water temperature decreases, water density increases) has been observed. On the other 

hand, water temperature does not appear to any have measurable effect on density at a depth of 25 ft. At 

this depth ‘salinity’ would appear to dominate water density effects. These results support the historical 

observations of a diminishing thermocline but a persistent chemocline during the winter months, see 

attached field water quality profiles. 

Water Density at Varying Depths Within Anchor Hill Pit Lake 

999 

1,000 

1,001 

1,002 

1,003 

1,004 

1,005 

1,006 

1,007 

0 2 4 6 8 10 12 14 

Temperature (degrees C) 

Water Density (kg/m 3 ) 

16 

Depth = 3 ft 

Depth = 25 ft 

Note: Data points collected on the same date have been 

color coordinated. Please note that all diamond shaped 

data points are samples collected at 25 ft; while all 

circular data points were collected at 3 ft below water 

surface. 

Figure 2.0. Hydrometer Density Measurements Recorded within Anchor Hill Pit Lake (September 24, 2004 

to November 5, 2004). 

Using the aforementioned calculated average water densities, Wedderburn and Lake Numbers were 

determined. The Wedderburn Number can be used to examine the extent of upwelling of metalimnetic 

water (Imberger, 2001). It is the ratio of the restoring moment about the center of volume of a lake to 

the disturbance moment for two-layer stratification. Mathematically it is expressed as follows 

(Imberger, 2001): 

 (Equation 3) 

Where: W = Wedderburn Number (unit less) 
z (z g − z0 )Mg(1 − T ) 

LN = z 3 z 2 A (1−	 g )ρ 0u*

2
 

z
 

∑z·A·dz 1m 

z g = ×	 ∑A·dz 3.2808 ft 

∑ρ(z)·z·A(z) 1m 

z0 = ×	 ∑ρ(z)·A(z) 3.2808 ft 

h = Depth to thermocline (m) 

u* = Water shear velocity (m/s) 

L = Fetch length (m), and 

The LN is a quantitative index of the dynamic stability of the water column and is defined as the ratio of 

the moments about the water body’s center of volume and of the stabilizing force of gravity (resulting 

from the density stratification) to the destabilizing forces from wind, cooling, inflow, outflow, and 

artificial destratification devices (Robertson and Imberger, 1994). In general, a LN of one indicates that 

the wind energy is just sufficient to deflect the thermocline, a LN << 1 means that lake stratification is 

weak with respect to wind stress and strong seiching would occur on the surface and turbulent mixing 

within the hypolimnion is expected. A LN >> 1 typically means lake stratification is strong and 

dominates forces introduced by the wind. Little to no seiching and/or turbulent mixing within the 

hypolimnion is expected (Imberger, 2001; and Robertson and Imberger, 1994). 

Assuming that wind is the dominating force for mixing (i.e., inflow, outflow, and any artificial 

destratification devices have minimal destabilizing force) the LN can be calculated as follows (Imberger, 

2001; Hamblin et al., 1999; and Robertson and Imberger, 1994): 

 (Equation 4) 

Where: zg = Center of volume (m) 

z0 = Center of mass (m) 

M = Total mass of water (kg) 

zT = Height to thermocline (m) 

z = Total depth (m) 

A = Lake surface area (m2

) 

ρ0 = Average water density (kg/m3

) 

u* = Water shear velocity (m/s) 

g = Acceleration due to gravity (9.81 m/s2

) 

It should be noted that the center of volume (zg) and center of mass (z0) were calculated as follows: 

 (Equation 5) 

Where: zg = Center of volume (m) 

dz = Incremental depth (i.e., 0.5 ft) 

z = Depth (ft) 

A = Incremental surface area (ft2

) 

 (Equation 6) 
28-Jun-03 

17-Aug-03 

6-Oct-03 

25-Nov-03 

14-Jan-04 

4-Mar-04 

23-Apr-04 

12-Jun-04 

1-Aug-04 

20-Sep-04 

Gilt Edge Mine Site: Anchor Hill Pit Lake 

Plot of Average Hourly Wind Speed as a Function of Time 

40 

35 

30 

25 

20 

15 

10 

5 

0 

Wind Speed (mi/hr)

Time 

Where: z0 = Center of mass (m) 

ρ(z) = Water density as a function of depth (kg/m3

) 

z = Depth (m) 

A(z) = Surface area as a function of depth (m2

) 

Meteorological data (i.e., wind speed) collected from the on-site weather station as maintained and 

operated by the State of South Dakota was reviewed for the purpose of this evaluation. Based on data 

collected from September 2, 2003 to August 31, 2004 the average wind speed recorded at the Gilt Edge 

Mine Superfund site during that time was approximately 10 mph (miles per hour). Maximum wind 

speeds of approximately 35 mph were recorded during that same period of time. A plot of the historical 

wind speeds recorded at the site is provided within Figure 3.0. It should be noted that the average wind 

gust recorded at the Gilt Edge Mine was approximately 15 mph. Using Equations 3 and 4, a sensitivity 

analysis with respect to wind speed was conducted for AH Pit Lake. 

Under average wind conditions of 10 mph and the existing water level (i.e., water depth = 89.5 ft) no 

seiching or turbulent mixing within AH Pit Lake is anticipated. Wedderburn and Lake numbers of 12 

and 39 were respectively determined. As a result, under typical conditions AH Pit Lake would appear 

to be a very stable lake and no seiching and/or mixing would be expected. This is confirmed based on 

field data collected to date. Should a sustained wind speed of approximately 35 mph (i.e., site recorded 

maximum) be observed, there is the potential for seiching to occur (W = 1) while no turbulent mixing 

within the hypolimnion is anticipated (LN = 3). 

Figure 3.0. Average Wind Speeds Recorded at the Gilt Edge Mine Superfund Site from September 2003 to 

September 2004 (data recorded by the State of South Dakota). 
To assess the potential for deep mixing to occur within AH Pit Lake a wind speed sensitivity analysis 

was conducted. Based on our findings, turbulent mixing of the hypolimnion is not expected until a 

sustained wind speed of approximately 63 mph was observed over the lake surface. It is important to 

note that the only variable within the sensitivity analysis was wind speed. All other variables such as 

fetch, water depth and depth to thermocline were held constant. It should be noted that the calculated 63 

mph wind speed is the required minimum to generate turbulent mixing within the hypolimnion, and that 

it does not preclude mixing will occur but rather that it is possible. As demonstrated by Robertson and 

Imberger (1994), even during weakly stratified periods (i.e., low lake stability), deep mixing only occurs 

when the wind force is sufficiently strong to cause LN values to drop below one. 

Similar calculations were conducted assuming that 5 ft of water was discharged (i.e., approximately 7.1 

million gallons) from AH Pit Lake. Should such an activity be conducted it is anticipated that AH Pit 

Lake would become a two-layered stratified lake as opposed to the existing three-layered; and that a 

reduced wind speed would be required to stimulate turbulent mixing within the deeper sections of the 

lake (i.e., approximately 31 mph). Recall that the maximum recorded on-site velocities were 

approximately 35 mph. A summary of our findings is presented within Figure 4.0, with a detailed 

summary of calculations attached. 

Gilt Edge Mine: Anchor Hill Pit Lake - Turnover Potential 

Wedderburn and Lake Numbers as a Function of Wind Speed 

0 

1 

10 

100 

1,000 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 

Wind Speed (mi/hr) 

Wedderburn/Lake Number 

W 

LN 

5 ft Wihdrawal 

63.1 

Note: Average wind speed as calculated using data 

collected from the State of South Dakota on-site 

weather station from September 2003 to August 2004 

is approximately 10.1 mi/hr. 

Mixing 

Figure 4.0. Wedderburn and Lake Numbers as a Function of Wind Speed within Anchor Hill Pit Lake. 
⎛ MKA × (VP − P) ⎞ Q = ⎜ ⎟× X H S	 2

⎝ RT ⎠

1/ 3 ⎛ M ⎞ K1 = K2 ⎜

⎜ 2 ⎟	 ⎟ ⎝ M1 ⎠

Based on first-order lake hydrodynamic numbers lake turnover is highly unlikely to occur within AH Pit 

Lake. However, in the unlikely event that deep water mixing and/or turnover could occur, we also 

evaluated the resulting effects for the potential to release hydrogen sulfide (H2S) due to off-gassing (i.e., 

volatization). 

Volatization rate of hydrogen sulfide was calculated using the following relationship as defined by 

Watts (1998): 

 (Equation 7) 

Where: Q = Volatization rate (g/s) 

M = Molecular weight of H2S (i.e., 34.086 g/mol) 

K = Mass transfer coefficient per area (m/s) 

A = Area of AH Pit Lake (i.e., 17, 855 m2 @ current water elevation) 

VP = Vapor pressure (atm) 

P = Partial pressure of H2S in atmosphere (i.e., 0 at time = 0) 

XH2S = Mole fraction of H2S in solution @ 55 mg/L 

R = Ideal gas constant (i.e., 8.21 × 10-5 m3

·atm/mol·K) 

T = Temperature (i.e., 8oC) 

The mass transfer coefficient and vapor pressure for H2S were estimated using the following 

relationships (Watts, 1998): 

 (Equation 8) 

Where: K1 = Mass transfer coefficient for H2S 

K2 = Mass transfer coefficient of water (i.e., 0.83 cm/s) 

M2 = Molecular weight of water (i.e., 18.02 g/mol) 

M1 = Molecular weight of H2S (i.e., 34.086 g/mol) 

VP = H × S	 (Equation 9) 

Where: H = Henry’s constant (i.e., 8.56 × 10-3 atm·m3

/mol)
 

S = Solubility of H2S (i.e., 398 g/100 g) 

The average ambient air temperature of 8oC was calculated from hourly data collected from the on-site 

meteorological station during the summer months of 2004. Data collected during the winter months 

were not considered for two reasons: 1) they would adversely bias the resulting average, and 2) due to 

the presence of ice cover turnover and/or lake mixing would not result from disturbing forces such as 

wind. 

Applying Equation 7 to the AH Pit Lake, it was determined that the maximum H2S concentration that 

could off-gas should a turnover event occur would be approximately 65 ppm. A summary of detailed 
calculations as generated using Excel are attached. The resulting maximum H2S concentration that 

could evolve from AH Pit Lake in the unlikely event of a complete turnover is significantly less than the 

potentially fatal limit of 250 ppm (MSDS, Canadian Centre for Occupational Health and Safety). 

We trust that the above information is sufficient at this time and look forward to hearing from you on 

this matter in the near future. 

MEA/mea 

Attachments (1) 
REFERENCES 

Hamblin, P.F., C.L. Stevens and G.A. Lawrence. (1999). “Simulation of Vertical Transport in Mining 

Pit Lake”. Journal of Hydraulic Engineering. 125(10), 1029-1038. 

Imberger, Jörg. (2001). “Characterizing the Dynamical Regimes of a Lake”. 1-23. 

Robertson, Dale M. and Jörg Imberger. (1994). “Lake Number, a Quantitative Indicator of Mixing Used 

to Estimate Changes In Dissolved Oxygen”. Int. Revue ges. Hydrobiol. 79(2), 159-176. 

Thomann, Robert V. and John A. Mueller. (1987). “Principles of Surface Water Quality Modeling and 

Control”. Harper Collins Publisher. Chapter 4. 

Watts, Richard J. (1998). “Hazardous Wastes: Sources, Pathways, Receptors”. John Wiley & Sons, Inc. 

Chapter 7.