Management and Treatment of Water from Hard Rock Mines

1.0 PURPOSE Index The U.S. Environmental Protection Agency (EPA) Engineering Issues 1.0 PURPOSE are a new series of technology transfer documents that summarize the 2.0 SUMMARY latest available information on selected treatment and site remediation technologies and related issues. They are designed to help remedial 3.0 INTRODUCTION project managers (RPMs), on-scene coordinators (OSCs), contractors, 3.1 Background: Environmental Problems and other site managers understand the type of data and site charac­ at Hard-Rock Mines teristics needed to evaluate a technology for potential applicability to 3.2 Conceptual Models at Hard-Rock their specific sites. Each Engineering Issue document is developed in Mines conjunction with a small group of scientists inside the EPA and with 3.3 The Process of Selecting Remedial outside consultants, and relies on peer-reviewed literature, EPA re­ Technologies ports, Internet sources, current research, and other pertinent informa­ 3.4 Resources for Additional Information tion. The purpose of this document is to present the “state of the sci­ ence” regarding management and treatment of hard-rock mines. 4.0 TECHNOLOGY DESCRIPTIONS 4.1 Source Control Internet links are provided for readers interested in additional infor­ 4.1.1 Capping and Revegetation for mation; these Internet links, verifi ed as accurate at the time of publi­ Source Control cation, are subject to change. 4.1.2 Plugging Drainage Sources and Interception of Drainage by Diversion Wells 2.0 SUMMARY 4.1.3 Prevention of Acid Drainage via Contaminated water draining from hard rock mine sites continues Protective Neutralization to be a water quality problem in many parts of the U.S. The types of 4.1.4 Passivation of Sulfidic Rock water range from strongly acidic water laden with metals, to variable 4.2 Treatment of Contaminated Water water quality in mining pit lakes, to alkaline water being released from 4.2.1 Treatment of Acidic Waters closed cyanide heap leach operations. 4.2.2 Treatment of Neutral and Alkaline Waters Prevention of water contamination at mine sites is usually the best op­ 4.2.3 Treatment of Mine Water with tion and can sometimes be realized by appropriate management of waste Microbial Processes material, or by hydrologic control in underground systems, or by using 4.3 Mine Pit Lake Management a variety of capping methods for waste rock dumps or closed heaps. 4.3.1 Backfilling and Neutralization However, long-term (decades and beyond) treatment is, and will con­ 4.3.2 Bioremediation and Induced tinue to be, required at many sites. Once a contamination source is Stratification of Mine Pit Lakes established (e.g., reactive waste rock dumps), elimination of these as sources of water is often very expensive and technically challenging. 5.0 CONCLUSION Acid drainage remains the most problematic water quality, in large 6.0 ACKNOWLEDGMENTS part due to the ability of acidic water to dissolve a variety of toxic metals (e.g., cadmium, zinc, nickel) and release of that water to sur­ 7.0 ACRONYMS AND ABBREVIATIONS face or ground water. The most common treatment is neutralization 8.0 REFERENCES using lime, or another suitable alkaline agent, followed by oxidation and precipitation of metals. This will also reduce sulfate to near the gypsum solubility limit (approximately 2,000 mg/L, depending on calcium concentrations). A variety of methods have been tive in many cases, further research is required to reduce utilized to add lime to acidic water, and, particularly for water treatment costs and increase the reliability of these large fl ows (100 gal/min) and/or high acidity/metals technologies. loadings, this option is usually the most cost effective. Other methods for treatment of acidic water include a 3.0 INTRODUCTION variety of wetland systems and bioreactors that are based This Engineering Issue document on treatment of min­ on sulfate reducing bacteria that reduce sulfuric acid to ing waters is a practical guide to understanding and se­ hydrogen sulfide, which consumes acidity and allows lecting technologies for the environmental management precipitation of metals as metal sulfi des. These systems of waste materials and effluents at hard-rock mines. For can either utilize the wetland organic carbon or an exog­ the purposes of this discussion, hard-rock mining primar­ enously supplied carbon source (e.g., ethanol) for sulfate ily refers to open pit and underground mines that produce reduction. These systems show particular promise where base metals (e.g., copper, zinc, lead) and precious metals the flows and acidities are relatively low. The advantage (e.g., gold and silver). While drainage from coal mines has of these systems is that they commonly do not require similar water quality issues, coal drainage has been consid­ the same level of monitoring and operational expense as ered extensively in other publications. It responds to the the lime systems. They also can reduce sulfate levels to need for environmental management at new and aban­ well below the gypsum solubility limits, depending on the doned hard-rock mines by providing guidance for select­ characteristics of the bioreactor/wetland system utilized. ing among available technologies for the stabilization of Drainage from precious metals heaps and tailings facili­ mine waste, treatment of mine water, and management of ties offers a different set of challenges. While most of the mine pit lakes. Target audiences are operators, regulators, precious metals heaps and tailings are not acid generating, stakeholders, and technical consultants involved in select­ several examples of acid generating processing wastes ex­ ing technologies for environmental management of hard- ist in western states. In most cases in mine closures, the rock mines. The general contents of this Engineering Issue residual water used in cyanide extraction of precious met­ document are listed above in the Table of Contents. als remains net alkaline, and was continuously recycled The goal of this document is to increase the effi ciency during operation. The soluble constituents were concen­ of decision makers in defining the scope of mine-related trated as water evaporated, and often contain elevated so­ water quality problems and selecting the least expensive dium from the sodium cyanide used in the process. Thus, effective management technology. It begins with technical land application of these fluids should be limited, due to overviews and conceptual models of contaminant sources salts, arsenic and other constituents. Other than ion re­ (i.e., environmental behavior in the dominant hard-rock moval technologies (e.g., reverse osmosis), few cost effec­ mining facilities—waste rock and heap leach facilities, tive methods for treatment and release of these water are tailings impoundments, and pit lakes). A general overview available. In arid regions, evaporation is often the only of remedial technologies (acid neutralization, biologically option available for such heaps and tailings facilities. induced sulfide treatment, and pit lake management) fol­ Mining pit lakes that are derived from open pit mines that lows. With these technical foundations reviewed, specifi c penetrated ground water are particularly prevalent in the remedial technologies are presented individually and de­ precious metals and copper mines in the western United scribed using the context of the feasibility study process— States. The water quality can vary from a highly acidic sys­ a practical framework for selecting remedial technologies tem in high sulfide host rock to slightly alkaline, better based on implementability, effectiveness, and cost. quality water in carbonate host rock. While treatment of pit lakes is potentially expensive, at least one example of 3.1 Background: Environmental Problems at neutralization of an acidic pit lake (Sleeper Pit Lake in Ne­ Hard-Rock Mines vada) has demonstrated that this is technically possible. Few environmental problems are as widely documented as Because of the long-term nature of many of these drainag­ the legacy of historic hard-rock mines. Small mines, oper­ es, methods for cost-effective treatments are still needed. ating in the era before environmental regulation, removed Many of the presently available technologies have been and milled ore primarily from vein deposits, leaving un­ derived from coal mine drainage research. While effec­ vegetated spoils, unsealed adits, and often-acidic seepage

2 Engineering Issue laden with metals. Modern U.S. hard-rock mining is in Acidic drainage is the dominant environmental problem sharp contrast, with closure designs and associated fi nan­ associated with hard-rock mining. Many valuable metals cial bonds for environmental management commonly re­ in ore deposits are bound to sulfide sulfur, forming spar­ quired even before operations begin and water discharges ingly soluble sulfide minerals such as sphalerite (ZnS), permitted only within the constraints of the Clean Water covellite (CuS), or galena (PbS). Acidic drainage forms and Safe Drinking Water Acts. Environmental issues re­ primarily when iron sulfide, pyrite (FeS2), comes in con­ main, of course, and economies of scale have produced tact with water and oxygen, producing dissolved sulfuric larger operations, but today’s mines are better managed— acid and iron. The three steps summarizing this overall water is treated, and waste is capped and revegetated. Clo­ reaction are: sure requirements for modern mines depend on the reg­ 7 2 2 FeS2(S) 2 O2 H2O Fe 2SO4 2 H (3-1) ulatory requirements and the environmental capabilities 2 1 3 1 Fe 4 O2 H Fe 2 H2O (3-2) and risk of the associated mine. 3 Fe 3H2O Fe(OH)3(S) 3H (3-3) Collectively, the economic liabilities and technical Pyrite oxidation liberates soluble iron (Fe2) and acidity challenges of hard-rock mining are immense (if poorly (H) and sulfate, with the primary limit on the oxidation constrained): rate being the availability of oxygen. The oxidation pro­ Although no global estimation of the impact of acid cess also liberates other sulfide-bound metals (e.g., cadmi­ drainage exists, total liability costs for potentially acid- um, zinc, copper, lead, uranium) and metalloids (e.g., ar­ generating wastes at mining sites is estimated to be senic, antimony, selenium). In addition, most metals are US$530 million in Australia, between US$1.2 and more soluble under acidic conditions (i.e., at low pH), so 20.6 billion in the USA, and US$1.3 and 3.3 billion oxidation and acid production tend to be associated with in Canada. Effectively dealing with acid drainage has increasing metal concentrations. The result is that acidic been—and continues to be—a formidable challenge for conditions (low pH) in mine effluent tend to be highly which no global solutions currently exist. Acid drain­ correlated to elevated heavy metal concentrations. age is one of the most serious and potentially enduring The primary offset to acid production in natural systems environmental problems of the mining industry. Left is the consumption of acidity by calcite (CaCO ): unchecked, it can result in such extensive water quality 3 CaCO 2H Ca2 CO H O (3-4) impacts that it could well be this industry’s most harm­ 3 2(G) 2 ful legacy. (INAP, 2004) and precipitating sulfate as calcium sulfate (CaSO4). There remains an enormous need for development and 2 SO4 Ca CaSO4(S) (3-5) evaluation of effective low-cost technologies for stabiliza­ tion and treatment of mine waste. Acidic mine water that is neutralized by reaction with calcite generally contains 1,500–2,200 mg/L sulfate. Fur­ ther, the consumption of acidity in Equation 3-4 increas­ 3.2 Conceptual Models at Hard-Rock Mines es the pH, which tends to decrease total heavy metal con­ The main processes responsible for water quality deg­ centrations as these constituents precipitate or adsorb to radation at hard-rock mines are reviewed here briefl y surfaces. to provide a foundation for understanding remediation Figure 3-1 is a model simulation illustrating how pore technologies. Mine water contamination comes from two water in waste rock or tailings will change as pyrite and sources: release of constituents contained in rock that calcite are consumed during oxidation. The top graph has been mined and chemical reagents used in mining, shows the amount of acid-generating potential (AGP) (as milling, extraction, and ultimate recovery of the valuable pyrite) and acid neutralizing potential (ANP) from cal­ metal or mineral. The largest source of water contamina­ cite (CaCO3) remaining, with increasing oxidation (also tion is nearly always the material being mined. Ore and related to increasing time) shown along the graph from waste rock has generally been isolated from oxygen and left to right. As long as calcite remains, pore water pH re­ water for geologic time frames, and bringing the material mains near neutral and sulfate concentrations are limited to the surface potentially results in reactions that release to below 3,000 mg/L. Once all calcite is consumed, acid contaminants that degrade water quality. buffering ceases, the pH drops to below 3, and dissolved sulfate increases. Results indicate the dramatic change in

Engineering Issue 3 water quality that can occur when excess AGP remains in nologies discussed in this document are presented in a for­ materials. mat that supports the EPA’s feasibility study process. An overview of each technology (Tables 4-1, 4-5, and 4-6) is provided to facilitate early screening of inappropriate op­ tions. Details of each technology are presented in the text, with a focus on identifying those parameters most criti­ cal in evaluating implementability, effectiveness, and cost. Where possible, specific examples of cost and effectiveness under pilot- or full-scale implementation are provided. The feasibility study process provides a framework for se­ lecting from a range of remedial technologies for specifi c site conditions amidst the interests of regulators, stake­ holders, and technology developers. The process begins with a characterization of the problem (e.g., chemicals of concern, risks, exposure paths, identification of remedial Figure 3-1. Oxidation of mine waste and production of sulfate. This goals, etc.), then identifies potential technologies (screen­ idealized 10% sulfide rock contains both pyrite and calcite and ing process), and finally evaluates the feasibility of a short demonstrates how sulfate concentration and pH are changed as list of technologies to select a remedy. the sulfides in the rock are oxidized and the capacity to neutralize the acid is consumed. The primary technical evaluation criteria for feasibility Contaminated mine waters may also be neutral to alka­ under Superfund (EPA, 1988) are: line, depending on the type of rock being mined and the ● Effectiveness—the potential for the alternative to reagents used to selectively extract the valuable substance. achieve remedial goals established for the site. Thus rock with excess calcite will produce pH-neutral ef­ ● Implementability—the ability to comply with tech­ fluent. However, neutralized mine waste effluent can still nical and administrative issues and constraints in­ contain elevated metalloids, such as selenium, arsenic, and volved in implementing a technology at a specifi c site. antimony. Zinc and other heavy metals have also been ob­ ● Cost—typically an estimate of net-present cost for served in pH-neutral mine waters (e.g., the Burleigh Tun­ each technology. nel and Wellington Oro Mine discharges in Colorado). Metal recovery reagents that may present water quality In practice, implementers identify the technologies that issues include a variety of flotation agents for concentra­ can meet their water quality goals (“effectiveness”), elimi­ tion of metals, as well as lixiviants, particularly cyanide. In nate those that can’t be applied for practical reasons (“im­ the latter case, cyanide can form complexes with a variety plementability”), then implement the least expensive op­ of metals that are very weak (e.g., zinc cyanide) to strong tion (“cost”). This document is intended to support this complexes (e.g., cobalt, iron, and mercury cyanides) and technology selection process, providing descriptions of also transformation products of cyanide, particularly thio­ the common environmental technologies for hard-rock cyanate and nitrate. mining and identifying the critical components affecting the feasibility of each.

3.3 The Process of Selecting Remedial Selecting a technology can be more difficult than imple­ Technologies menting it. The critical components in the evaluation and selection of a technology include: Selection of an optimal technology for a specifi c reme­ ● Source defi nition—water flow rates, material mass, diation problem would, ideally, follow from tightly con­ solute concentrations, expected duration, etc. strained algorithms or flow charts. Regrettably, critical de­ cision variables, such as cost per cubic meter to treat water, ● Identification of environmental goals—discharge net percolation through caps, etc., are generally too de­ standards, compliance points, and human or ecologi­ pendent on site-specific conditions to allow direct trans­ cal risk thresholds. fer between projects. Design methods are transferable across ● Identification of applicable technologies—those sites, but not specifi c designs. In response, remedial tech­ technologies potentially capable of meeting goals.

4 Engineering Issue ● Identification of critical parameters—early deter­ ● EPA’s Web site for the EPA/DOE Mine Waste mination of values for parameters that typically drive Technology Program: http://www.epa.gov/ cost or effectiveness. minewastetechnology. ● Impartial evaluation—a feasibility analysis that is The following publications provide additional informa­ completely independent from technology vendors. tion on innovative acid mine drainage treatment technol­ Other feasibility study evaluation criteria that should be ogies and a comprehensive summary of recent closure and considered during the technology selection process are bonding practices and related costs at hard-rock mines, community and regulatory acceptance. These criteria are respectively: covered to a lesser degree in this document than the tech­ ● “Acid Mine Drainage: Innovative Treatment Tech­ nical criteria, but can have a great impact on the fi nal nologies,” by Christine Costello for the EPA (Costel­ selection. lo, 2003): http://clu-in.org/s.focus/c/pub/i/1054/. ● “Hardrock Reclamation Bonding Practices in the 3.4 Resources for Additional Information Western United States,” by the Center for Science in the Public Interest for the National Wildlife Federa­ Below are prominent organizations dedicated to research tion (Kuipers, 2000): http://www.csp2.org on the causes and remedies for management of drainage /REPORTS/Hardrock%20Bonding%20Report.pdf. at hard-rock and coal mines: ● Mine Environment Neutral Drainage (MEND): 4.0 TECHNOLOGY DESCRIPTIONS http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl­ lmsm/mend/default_e.htm. The strategy for management and protection of water ● Acid Drainage Technology Initiative (ADTI): quality at mine sites varies on a site-specifi c basis. The http://www.unr.edu/mines/adti/. range of contaminants that is released into water from one ore may be very different from another ore, and the ● International Network for Acid Prevention (INAP): http://www.inap.com.au/. Includes clear overview of management methods utilized will depend on the volume topics and reports on INAP-funded research. and environmental characteristics. However, there are common threads that are fundamental to water manage­ ● Australian Center for Mining Environmental Re­ ment, and sufficient similarities exist that a generalized search (ACMER): http://www.acmer.com.au/. discussion is useful. While a large variety of metals are ● The U.S. Geological Survey Toxic Substances Hy­ mined in the U.S., nearly 90% of the value of metals (ex­ drology Program section on Hard-Rock Mining cluding iron ore) consists of a group including gold, cop­ Contamination: http://toxics.usgs.gov/topics/ per, zinc, lead, silver, and molybdenum (USGS, 2006). minelands.html. As such, this discussion will primarily consider waters re­ ● The Restoration of Abandoned Mine Sites Technol­ leased from these important metals mines. ogy Database (RAMS tech): http://www.unr.edu/ Specific remedial technologies are divided into three mines/ramstech/techintro.asp. categories: ● U.S. Army Corps of Engineers Restoration of Aban­ 1. Source Control: typically the chemical stabilization doned Mines Sites (RAMs) Western Region: of reactive rock, or physical isolation or diversion of https://www.nwo.usace.army.mil/html/rams/rams. water away from the mine waste. html. Includes project summaries and documents for nine western states 2. Water Treatment: methods of reducing contaminants in mine waters or otherwise managing ● West Virginia University Extension Service Land contaminated water to reduce impacts to humans Reclamation Program: http://www.wvu.edu/ and the environment. ~agexten/landrec/land.htm#ACID. Focuses on acid drainage from coal sites. 3. Pit Lake Management: treated separately here due to the unique physical characteristics of lakes, ● EPA’s Web site for abandoned mine lands: although many elements of source control and http://www.epa.gov/superfund/programs/aml/. Gives treatment also apply. current technology updates on application of new treatment methods.

Engineering Issue 5 The descriptions of individual technologies under each erating rock from the surface. These caps can dramatically category are intended to provide enough information reduce, although not eliminate, net percolation of water to design a feasibility study at a particular site. This in­ into mine waste. cludes specific information on target analytes, treatment Suitable cap materials include topsoil, run-of-mine waste efficiency, examples of field-scale applications, critical pa­ rock, or waste rock amended to improve its performance rameters, and references for additional information. Key (e.g., with nutrients to enhance plant growth, with fi ne information on the feasibility of each technology is also material or tailings to increase water retention, or with summarized in tables. References are provided for sup­ alkali reagents to offset acid-generating potential). Selec­ porting information, and links to reliable Web pages are tion criteria include moisture retention characteristics included throughout. (measured directly or estimated from particle-size distri­ bution), shear strength (measured in a laboratory), and 4.1 Source Control acid-generating potential (AGP) (chemical analysis). Be­ nign waste rock, with or without amendment, is a par­ Treatment of contaminated mine waters is very often a ticularly attractive cap material because it is typically read­ long-term commitment, and resources will be required for ily available and often strong enough to resist erosion on the duration of the flows. Reducing or eliminating these slopes. Vegetation type is entirely site specific, but seed flows through source control methods clearly has long- mixtures typically focus on perennials that are effi cient at term benefits and should always be considered for deter­ extracting water, have deep roots, are drought resistant, mining the appropriate method for surface and ground and are consistent with post-reclamation land use. water protection. However, the uncertainty of successful­ ly implementing source control and costs of preventing Inhibition of oxygen is often cited also as a goal of mine releases of contaminated water also needs to be evaluated waste caps, as oxygen flux is approximately proportional as a component of the water quality management deci­ to acid rock drainage (ARD) formation. Diffusive oxygen sion. In some cases, source control needs to be considered flux into waste rock facilities typically produces several kg early in the mine development; in other cases, it can be sulfate/m2-yr for at least tens of years. Measurements in­ applied to mine sites that have long been closed. side waste rock indicate that oxygen advection through coarse zones may be a larger oxygen source (Andrina et Source control can be applied to two broad categories of al., 2003), so total oxidation rates in mine waste facilities drainage waters: drainage from surface waste facilities and may be several times higher. However, long-term oxygen drainage from underground workings (e.g., adits). Both exclusion has been demonstrated only with subaqueous are ultimately supplied by meteoric (precipitation runon/ disposal. Void space in waste rock is typically 40% (Wil­ runoff) water, so flow rates are related to precipitation. son et al., 2000b), and the high air diffusivity of oxygen However, surface waste facilities generally have a more lo­ (~10,000 times greater than in water) allows rapid oxygen cal response to rainfall and can be managed by appropriate transport. Oxygen-consuming layers (e.g., wood chips) caps, while drainage from underground workings requires are effective, but have a very short life. Although models consideration of the regional ground water system. indicate that water-saturated zones could be maintained, even in semi-arid climates, other variables often make 4.1.1 Capping and Revegetation for Source Control them impractical. Short of subaqueous disposal, no prac­ tical cap designs currently provide complete long-term Capping and revegetation technologies seek to reduce or barriers to oxygen. Wet covers are facilities that maintain eliminate the flow of water and oxygen into surfi cial mine a permanent water body above reactive mine waste—a waste, producing a corresponding decrease in the produc­ form of subaqueous storage. They have been found to tion and transport of solutes out of these potential sourc­ minimize oxidation and release of contaminants; a nu­ es. “Store-and-release” caps are simply vegetated surface merical analysis study utilizing modeling concluded, “a layers of material with a high moisture-retention capac­ water cover alone leads to a reduction of approximately ity that store water in the cap until it can be transpired 99.1%, in the [sulfide] oxidation rate relative to uncov­ or evaporated. The goals are to minimize net percolation, ered tailings” (Romano et al., 2003). However, such caps support vegetation, reduce erosion, and isolate acid-gen­ generally require perpetual management to ensure con­ tinued water saturation of the surface.

6 Engineering Issue One important caveat for cap effectiveness is that reduc­ sis and SoilCover or Vadose/W for more refi ned analy­ ing net percolation in acid-generating material may delay sis—see the review by O’Kane and Barbour, 2003). the onset of impacts, but not the magnitude, as pore wa­ ters become more concentrated in slower-fl owing waste Key Web Site References (Ritchie, 1994). Decreasing flow in mine waste does in ● Overview of dry covers for mine waste, available on­ fact allow more time for attenuation reactions, including line at INAP (O’Kane Consultants, 2003): silicate mineral buffering and precipitation of sulfate salts, http://www.inap.com.au/completed_research_ so reducing net percolation may in many cases reduce en­ projects.htm vironmental impacts from mine waste. However, reduced infiltration is not a guarantee of reduced impacts. ● “Design, Construction, and Performance Monitor­ ing of Cover System for Waste Rock and Tailings,” a Finally, two theoretical technologies potentially offer comprehensive, five-volume design and monitoring walk-away designs. An umbrella design with sloping lay­ report (MEND Report 2.21.4): http://www.nrcan. ers of fine material—the most conductive in unsaturated gc.ca/ms/canmet-mtb/mmsl-lmsm/mend/ waste (Wilson et al., 2000a)—could shed water around mendpubs-e.htm net acid-generating rock (Barbour, 2000). Potential draw­ backs are that this design still requires select handling of acid-generating rock and that low shear strength of fi ne 4.1.2 Plugging Drainage Sources and Interception of materials may limit its applicability on steep slopes. Sec­ Drainage by Diversion Wells 1 ond is a tailing and waste rock blend design (~ 3 tailings and Because these systems often intercept ground water and 2 3 waste rock). This material has shear strength comparable can even change the hydrologic system, source control to waste rock, but with moisture retention high enough to is limited, complicated, and uncertain. Two general ap­ maintain saturated conditions, providing a long-term bar­ proaches are often considered: interception of water from rier to oxygen introduction (Wilson et al., 2000b; Wilson the underground workings and plugging the drainage et al., 2003). Potential limits may include high blending routes from the underground workings. costs, long-term physical stability of blends, and suitabil­ ity of such blends for revegetation. Neither has been dem­ Plugging of drainage routes: Plugging of adits and onstrated in field-scale tests, and they are not considered grouting of drainage pathways have sometimes been further in this document. demonstrated to be effective in reducing the volume of contaminated water from underground mines. The goal Characterization requirements for mine waste caps in­ is to retain the contaminated water in the underground clude the following: workings and allow the ground water table to rise. This ● Climate (daily temperature, precipitation, humidity, approach also is coupled with the expectation that the lo­ potential evaporation, and insolation) cal ground water level will cover the underground work­ ● Reclaimed vegetation mix (post-reclamation species ings to prevent continued oxidation of the rock. While and their root depth and leaf area index) release of contaminated water through new routes is often observed, further management of these sources can po­ ● Availability of suitable cover (waste rock, soil, tail­ ings, and limestone) tentially cover the historic workings and reduce the con­ taminant load in the water considerably. If successful, and ● Physical characteristics of cap (particle-size distribu­ assuming the adit plugs and grouting are stable, the costs tion, Atterburg limits, specific gravity, compaction of treatment can consequently be substantially reduced. curve) The implementability of this technology is highly site ● Hydraulic characteristics of cap (saturated hydraulic specific and requires an understanding of the hydrologic conductivity and soil water characteristic curve) system, as well as the mine workings. While adit plugs can ● Moisture-retention characteristics of proposed cap work well under favorable conditions of geology, hydrol­ material (can be estimated from particle-size distri­ ogy, and mine development, such favorable combinations bution or determined more reliably with pressure- have been found to be rare. plate laboratory hydraulic tests) Interception of ground water: In some cases, drainage Design and analysis of store-and-release caps can be con­ patterns of surface and ground water can be altered to ducted with models (e.g., HELP for screening-level analy­ keep good-quality water away from reactive underground Engineering Issue 7 workings, pits, or waste rock dumps to reduce the vol­ control of net-acid – generating waste using the addition ume of contaminated water that is produced. Each case of neutralizing materials and identifies those factors most requires an extensive study of the hydrologic system and critical in assessing their feasibility. the associated contaminant source. For surface water, sim­ In-situ acid neutralization technologies are based on the ple diversions via channels over areas of infi ltration (e.g., acid base accounting (ABA) of a material. The ABA is the faults and slopes) can reduce the amount of contaminated balance between total acid-generating potential (AGP), water that is generated. Ground water diversions can po­ which is the total amount of acidity that would be pro­ tentially involve two general techniques. The first is to duced if all sulfide in a material is completely oxidized, establish passive drainage systems that take advantage of el­ and total acid-neutralizing potential (ANP), which is the evation differences and ground water system opportuni­ amount of acid that could be consumed by neutralizing ties. In this case, water is drained away from reactive rock minerals. There are numerous methods for analyzing for by drilling water conduits that change hydrologic gradi­ ABA, allowing the flexibility to tailor testing to site con­ ents to limit the amount of water that rinses reactive rock. ditions and budgets. Unfortunately, there are also several The second is to establish in-perpetuity pumping programs systems of ABA nomenclature in use, with no clear stan­ to keep good-quality water from the reactive rock under­ dard emerging. In this document, the convention in which ground workings. In this case, wells are drilled upgradi­ ANP and AGP are converted to CaCO equivalents and ent of reactive rock surfaces to lower the water table to 3 reported in g CaCO /kg rock (i.e., parts per thousand, reduce the contaminant load in the surface or ground wa­ 3 ‰) is used. ABA is described using net-neutralizing po­ ter. Such proposals have been developed for maintaining tential (NNP), defined as ANP – AGP. Thus, NNP has dry pits, reducing flows of water from springs that exist units of ‰ CaCO and is negative for net acid-generating under reactive waste rock dumps, and reducing fl ows that 3 material and positive for net-neutralizing material. pass through underground workings. These techniques can potentially reduce or eliminate the need for water ABA is typically calculated from analysis of sulfi de S treatment. While almost always expensive, these types of and carbonate C, assuming a 1:1 molar ratio of sulfi de S pumping systems, under certain circumstances, can be (AGP) and carbonate C (ANP). less costly than water treatment. However, in establish­ Converting chemical analysis for sulfide S (S 2 ) and car­ ing programs that require very long-term pumping, it is (FeS ) bonate C (C 3 ): necessary to recognize that if the pumping is discontinued (CaCO ) AGP S (10) (3.12) and ground water flows return to the pre-pumping con­ (FeS) ANP C 3 (10) (8.33) dition, the contaminants in the underground workings (CaCO ) will again be mobilized. Additionally, long-term pumping NNP ANP AGP upgradient of the source area, and resultant dewatering, Where may increase the release of heavy metals from a negative S(FeS) Concentration sulfide sulfur in sample (weight % S) geochemical effect. 3.12 molecular weight of CaCO3 / molecular weight of sulfur 3 C(CaCO ) Concentration carbonate carbon in sample (weight % C) 4.1.3 Prevention of Acid Drainage via Protective 8.33 molecular weight of CaCO3 / molecular weight of carbon Neutralization Basic silicate minerals may also contribute to ANP with The detrimental effects of sulfide oxidation in mine waste “silicate neutralization,” consuming acidity in the process can be offset when the material contains excess acid-neu­ of dissolving. Acid neutralization by silicate minerals is typically much slower than reactions with carbonate, and tralizing minerals, such as calcite (CaCO3). Neutralizing minerals react in situ with acidic leachate to neutralize acid­ reaction rates depend strongly on pH, particle size, and . surface area. ity, precipitate most sulfate (as gypsum, CaSO4 2H2O, or other calcium sulfate compounds) and iron (as oxides or Pore water neutralized in-situ by calcite does not ensure sulfates), and reduce dissolved trace metal by inducing ad­ perfect water quality. Neutralization of sulfuric acid by sorption to surfaces. As a result, sulfidic mine waste that calcite can still leave sulfate concentrations greater than contains excess neutralizing potential can, theoretically, 2,000 mg/L. Under oxidizing conditions, iron will pre­ weather into perpetuity without releasing acidic water. cipitate from neutralized water, forming hydroxide and This section describes the technologies for in-situ source sulfate minerals that are effective adsorption substrates for

8 Engineering Issue trace metals, but trace metals in pore water may remain (2) conducting chemical analysis that accurately indicates above remedial goals. However, solute reductions upon material ABA. neutralization can be dramatic, with 10-fold to 1,000­ The number of samples required to adequately defi ne the fold reductions in concentration common. Lime amend­ distribution of acid/base accounting in mine waste de­ ment of acid-generating waste when mixing, dispersion, pends on the size of the unit targeted for treatment, the and other attenuation processes are considered can, under variability in the ABA of the material, and the desired ac­ favorable conditions, produce mine waste that meets ex­ curacy. A 1989 guidance document (SRK, 1989) is one of posure-point water quality standards. However, if lime is the few references to recommend a fi xed number of ABA consumed prior to exhausting the acid-generating capac­ samples based on the size of each geologic unit. When ity or is inefficiently mixed, contaminated acidic water large waste rock or tailings facilities are targeted for treat­ can begin to drain from these sites long into the future. ment, geostatistical analysis may be warranted to identify Fortunately, studies find that ANP is generally a good in­ spatial correlations in ABA. dicator of long-term acid release. A review of 281 kinet­ The dominant analytical methods for acid/base account­ ic tests (various humidity cell and column tests from 53 ing in mine waste, in order of increasing complexity, are: different mines) found no net-neutralizing samples (i.e., NNP 0) that produced acidic leachate (Morin et al., 1. Net acid-generating test (NAG) (Miller et al., 1995). A separate comparison of 307 samples from nine 1997). This is the simplest ABA analysis, reacting hard-rock mines found similar results—NNP (using car­ a sample with hydrogen peroxide to completely bonate carbon for ANP) was a reliable predictor of ac­ oxidize all sulfide minerals, then noting the pH after tual acid release under simulated weathering conditions reaction as an indicator of whether the material (see Figure 4-1). Acid production rates in sulfi dic mine is net-acid generating (pH 4.5) or net acid- waste vary enormously with intrinsic oxidation rates (i.e., neutralizing (pH 4.5). It is rapid and inexpensive, oxidation rate under atmospheric conditions), with low can be conducted in simple field laboratories, and 8 3 6 can be modified slightly to yield more quantitative rates being ~10 kgO2/m s and high rates being ~10 3 information or excess AGP. kgO2/m s (Bennett, 1998). These results indicate that when there is an excess of naturally occurring carbonate 2. Leco furnace method (ASTM, 2003). This is a minerals in mine waste, the neutralizing reactions gener­ rapid method that requires sophisticated equipment ally keep pace with the acid production. but relatively little labor. Results of this method are generally consistent with comparison tests using long-term kinetic tests. 3. Sobek titration method (Sobek et al., 1978). This is the original ABA analytical method, based on titration of samples to determine acid and base concentrations directly. It is labor intensive and thus generally more expensive to conduct than the two methods previously described, but it is generally regarded as the most reliable indicator of long-term acid release potential. Lime and limestone are the most commonly used amend­ ment materials. Lime—in both the processed (CaO) and

hydrated [Ca(OH)2] forms—is more soluble and reacts

more rapidly than calcite [i.e., limestone (CaCO3)], and Figure 4-1. Humidity cell results from nine hard-rock mines: NNP is thus considered to be more effective in controlling ARD vs. final humidity cell pH. (Source: Exponent, 2000) (Evangelou and Zhang, 1995). However, due to their high solubility, lime amendments can be washed quickly from Estimating the acid/base accounting of mine waste has waste rock, thus limiting their long-term effectiveness in two components: (1) obtaining sufficient sampling to unsaturated conditions, where acid production can con­ generate a representative sample of the target material and tinue after the lime is leached out. Thus, an effective waste

Engineering Issue 9 rock amendment strategy would be to use calcite for the excess ANP (i.e., a safety factor of 3, ANP:AGP 3) and also

cap, where long-term maintenance is required, and CaO suggest an ANP greater than 20 ‰ CaCO3 (BLM, 1996). or Ca(OH) for subaqueous waste, where oxidation will 2 Cost considerations are listed below. These costs are esti­ dramatically slow after emplacement of the waste. While mates based on current quoted costs in 2005. However, oxygen diffusion is slowed, it is usually not completely the costs will vary, based on availability of raw materials, eliminated, and in most cases, the rate of contaminant energy costs, and other specific requirements at a site. release will depend on this rate of oxygen penetration to the reactive surfaces. Limestone (crushed to 2 mm, assuming local source): Mine waste amendment is suitable for any materials that ● Delivered from off-site source: $US30–50/tonne can be accessed and subjected to complete mixing with the amendment. In practice, amendment is generally con­ ● Mined and crushed from on-site source at operating sidered for mine waste that (1) can be treated as it is being mine: $US2–3/tonne excavated; (2) is near the surface ( 1 to 2 meters deep), Lime, variable, depending on source and haulage: which can be amended by surface application followed by ● Hydrated lime: $60–140/tonne ripping to mix at depth (may be amenable when vegeta­ ● tion of acid-generating waste is considered); and (3) is to Lime (CaO): $80–$240/tonne be moved for additional purposes. Safety factor for neutralized waste: ● Excavation of large waste rock and tailings and use of ANP greater than 20 ‰ CaCO3 (BLM, 1996) amendment to ensure perpetual in-situ ARD neutral­ ● ANP/AGP 1.2 (i.e., 20% excess-neutralizing ization is often more expensive than other alternatives, potential) including perpetual collection and treatment of acidic Cost to mix amendments into waste rock (mixing seepage. costs only): Field-scale tests indicate that mixing neutralizing minerals ● Complete mixing (Grizzly to separate waste rock and with acid-generating waste may need to be nearly ideal to pug mill to mix): $US0.75–1.50/tonne prevent ARD formation (Mehling et al., 1997) and that ● Surface mixing by ripping in amendment with neutralizing amendments should be 2 mm in diameter or bulldozer (maximum depth ~6 ft.): $US0.04–0.06/ smaller. Field- and large-scale test plots indicate that there tonne-treated rock may need to be as much as a 100% excess of amendment to ensure perpetual acid neutralization. Amendment rates should exceed those estimated solely on the basis of an Performance Data ABA (Day, 1994; Cravotta et al., 1990). However, the Laboratory and field-scale studies demonstrate that the ef­ amount of excess neutralization capacity required to en­ fectiveness of mine waste amendment is affected primarily sure pH-neutral effluent varies from site to site. Mehling by the mixing efficiency and the particle size of neutral­ et al. (1997) summarized three wide-ranging guidelines izing materials. Specifically, mixing at less than complete taken from successful waste rock blending schemes at his­ homogenization can allow acid production, followed by torical coal mining sites: migration of acidic leachate in preferential fl ow paths; 1. NNP 80 ‰ CaCO (Erickson and Hedlin, 1988) neutralizing amendments, particularly limestone, greater 3 than approximately 2 mm in diameter are signifi cantly 2. NNP 10 ‰ CaCO and ANP 15 ‰ CaCO 3 3 less effective at neutralizing acidity. Following are a few (Brady et al., 1990) studies from the literature that illustrate these conclusions. 3. ANP/AGP 2 (Day, 1994) Further, many states have guidance on ABA requirements Not surprisingly, U.S. regulatory guidelines for classify­ for mine waste, suggesting that blending programs are ing waste as non–acid generating also vary widely. Some generally accepted. However, the failure of several fi eld­ state guidelines consider waste to be non–acid generat­ scale, neutralization-blending tests is likely to be a cause ing without additional kinetic testing if it has 20 percent of concern for the scientific community and possibly for excess neutralizing capacity [i.e., a safety factor of 1.2, experienced representatives in industry and the regulatory ANP:AGP ratio 1.2:1 (NDEP, 1990)]. Bureau of Land community. Two cases of mixed success are presented in Management (BLM) guidelines set this criterion at 300% the following table.

10 Engineering Issue Site Name Samatosum Mine in south-central BC (Morin with a solution of silica. It encapsulates metals in an and Location and Hutt, 1996; Mehling et al., 1997) impervious microscopic silica matrix and prevents Experimental Field-scale horizontal layers of acid-generating additional acid generation or metals migration. This Design waste rock; NNP/ANP ratio of 3. technology has been utilized previously for metals- Results The waste rock pile produced acidic leachate contaminated soils. despite being amended to obtain a three-fold excess ANP. Hydraulic short-circuiting was cit­ These technologies are being tested at several sites, but ed as the probable cause of failure of a waste their efficacy has not been thoroughly established. The rock amendment scheme. EPA/DOE (Department of Energy) Mine Waste Tech­ Site Name Kutcho Creek Project, BC (Mehling et al., 1997) nology Program has examined these technologies (http:// and Location www.epa.gov/minewastetechnology/). The future utiliza­ Experimental Field-scale test constructed with 10-cm thick tion of these methods for waste rock requires consideration Design horizontal layers of acid-generating waste rock with net-neutralizing rock to achieve an ANP/ of the following: AGP ratio of 1.1. Two-year duration. ● Hydrologic characteristics of the waste rock: For Results Partial success, with ARD released from net- pre-existing waste rock dumps, how can complete acid – generating comparison, but not from the (or near-complete) coverage of the reactive surfaces net-neutralizing material. However, projections be ensured? Is it sufficient to treat the top of the indicate that ARD was likely from amended lay­ ers, leading to the conclusion that blending was waste rock dump (1–5 meters), or does the entire not effective for preventing ARD in material with waste rock dump require treatment? an ANP:AGP of only 1.1. Cost analyses of com­ ● plete blending suggested that this method might Longevity: Each type of coating is thin, and the be prohibitively expensive on a large scale. reactive bulk rock remains reactive. How long will these coatings prevent release of contaminants? ● Cost: What are the site characteristics that will 4.1.4 Passivation of Sulfidic Rock change the cost of these treatments? How do these In recent years, emerging technologies have been exam­ costs compare to conventional water treatment (e.g., ined that are designed to limit the release of acidic com­ lime precipitation)? ponents by forming a thin protective layer on the reactive Table 4-1 on pages 12 and 13 provides an overview of the rock surface. This “passivation” has the potential to reduce source control technologies covered in this section, tech­ or eliminate oxidation of the rock and thus reduce or elim­ nology selection factors, and limitations. inate release of contaminants from the rock. Each of the technologies examined utilizes liquids that can be applied to pit walls or reactive waste rock. The following three 4.2 Treatment of Contaminated Water technologies are examples that are being investigated: As discussed previously, waters draining from mine sites 1. Potassium permanganate: Pyritic surfaces are first can vary dramatically, and the methods used to treat those rinsed with a solution of lime, sodium hydroxide, waters will similarly vary. For this discussion, mine waters and magnesium oxide at a pH 12, followed can be put in three groups, although the distinction be­ by treatment with potassium permanganate. The tween the groups is sometimes not clear. manganese/iron/magnesium surface formed is ● Acidic water (pH 5.5): Most commonly, these resistant to further oxidation and substantially waters are contaminated as a result of pyrite oxida­ reduces the amount of acidic water draining from tion and contain elevated metals and sulfate. While the treated rock. any water with a pH 7 is acidic, the most prob­ ™ 2 2. Ecobond : MT (http://www.metalstt.com) uses a lematic waters are those with a pH 4 since metals phosphate-based solution to coat acid-generating rock loadings increase substantially. Because of the prob­ to form a stable, insoluble coating on the surface. The lems with acidic waters, extensive research has been technology forms stable iron phosphate complexes conducted on cost-effective methods of treatment. that resist hydrolysis and prevent further oxidation. Chemical neutralization consumption and sludge 3. Silica Micro Encapsulation (SME): KEECO has management are large factors for selecting a treat­ a patented process that treats acid-generating rock ment method. (Continued on page 14)

Engineering Issue 11 12 Table 4-1. Source Control Technologies for Hard-Rock Mine Waste

Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Capping and

Engineering Issue Cover waste rock or tailings with All soluble • Stability of waste or • Moisture retention • Availability of local • A zero-net-percolation Revegetation suitable growth medium and estab­ constituents cap to slope failure capacity of cap cap material, either cap has not been lish vegetation. (“Suitable” means in the solid and erosion material topsoil, benign demonstrated soil, waste rock, tailings, or blend waste • Availability of suit­ • Cap thickness waste rock or tail­ • Precipitation as snow of these materials that is non-acid ings, or mixture of able cap material • Potential for metals greatly increases net generating and contains heavy (i.e., non-acid gen­ these percolation metals below levels that are phyto­ uptake from cap erating, low met­ by plants (affects • Need to possibly • In acid-generating waste, toxic or that may cause ecological als, high moisture amend cap with or human health risk.) A “Store and ecological risk) reduced flow delays im­ retention) neutralizing agent pact, but may not reduce Release Cap” retains meteoric wa­ • Fire frequency or nutrients ter long enough for plants to uptake • Access to slopes contaminant load rate for seeding and • Fraction of precipi­ • Cap thickness (at and transpire the water, minimiz­ tation as snow • Highly engineered caps ing the “net percolation” (i.e., the grading least 1 m in semi­ (e.g., liners, capillary fl ux of water from the cap into the • Commonly per­ arid climate to breaks) have finite life waste) and associated rate that formed, with often sustain plants; may be thinner in wetter • Oxygen barrier to perma­ solutes in waste are flushed out. good but variable climates) nently stop ARD pro­ (No specific chemical reactions.) results duction is theoretically • Life expectancy of possible, but has not been cap demonstrated for long- term applications Wet Covers Storage of acid-generating rock All Can be utilized when Will generally reduce Highly variable, de­ • Requires in-perpetuity and tailings under water to prevent reactive rock can be oxidation rates of pending on the avail­ coverage of the reactive (or minimize) oxidation and release submerged in a per­ reactive rock ability of water and rock and management of of contaminants manent water body, site-specifi c the water body including a perma­ considerations • Previously oxidized rock nent tailings facility surfaces may still release pit lake or flooding problematic contaminants underground workings Hydrologic Drainage is controlled by direct­ All • Requires an un­ • Elimination of water • Highly variable • Effectiveness depends Controls ing water flow from reactive rock derstanding of the from the reactive depending on site­ on the ability to control surfaces, by diverting ground water hydrologic system surfaces can ef­ specifi c factors water, either by plugging fl ows, or by pumping ground water. surrounding the fectively stop acidic • Long-term pump­ or draining the adits A second option is to plug adits and reactive rock drainage ing has O/M costs, • This method, while it al­ shafts to allow water to fi ll under­ • Not commonly un­ • Effectiveness is de­ while an effective ways should be ground workings and cover reac­ dertaken due to dif­ pendent on the abil­ drainage system considered, is not often tive surfaces. Water and/or oxygen fi culties in obtaining ity to control water can be highly cost successful reaction with surfaces is minimized. a full understanding in the underground effective Rerouting surface water around • Plugging of underground of the hydrologic and surface water workings has resulted in surface or underground disturbanc­ system systems es is also sometimes an option. blowouts of the plugs in certain cases Table 4-1. Source Control Technologies for Hard-Rock Mine Waste (continued)

Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Acid Potential Waste rock or tailings that are Heavy metal • Access to all acid- • Ability to uniformly • Availability of • Surfi cial amendment of Neutralization net acid-generating are amended cations generating waste blend neutralizing local source of acid-generating waste with neutralizing agents (e.g., (e.g., Cu, rock targeted agents with acid- limestone or other does not stop ARD crushed calcite [CaCO3] or lime Cd, Pb, Zn), for blending in generating waste neutralizing agent production below the cap [CaO], alkaline industrial wastes), acidity, neutralizing agents • Crushing • Cost to mine, • Typically, it is not producing waste that will remain sulfate • Stability of slope neutralizing agents crush, and deliver economic to excavate permanently net neutralizing. (pore water during blending small enough to neutralizing agent and amend existing buried Amendments can be applied reduced ensure reaction (ideally waste surficially to existing waste and to ~1,500 - • For cap (e.g., 2 mm) 2 mm diameter for • Layering or sequential mixed (~0.5 – 2 m depth–typical 2,000 mg/L) amendment, blending) placement of neutralizing root-zone depth for cap only suffi cient access • Cost to spread and waste with acid- treatment) or added to new waste and slope to mix neutralizing generating waste often and mixed during emplacement. permit distribution and mixing of agent into cap does not stop acid release neutralizing • For existing waste, amendment into cost to excavate near-surface (~2 m and uniformly depth) amend Passivation Reduces or halts the oxidation Acidity and The effectiveness of Unknown. The Costs are highly Full passivation of waste of reactive surfaces, primarily soluble passivation remains costs need to be variable, depending rock dumps is difficult, pyrite. Application methods vary, constituents to be established. evaluated against the on the technology due to the problems with

Engineering Issue but usually involve coating rock in the solid Laboratory and pilot- probability of long- utilized and the need delivering fluids in such a surface with fluids and allowing waste scale treatments term treatment using for periodic treatment way that all surfaces are the specific passivation reaction to show promise for the more conventional contacted. Also unknown occur. These technologies are still various treatments, methods. is the longevity of these in a research and demonstration but full-scale treatments. Requires further mode. applications have not investigation. been undertaken. 13 ● Near-neutral water (pH 5.5–9): These waters are Methods for treatment of acidic drainage vary consider­ common at many non-acid – generating sites, partic­ ably, but most focus on increasing the pH to above pH 7, ularly those with high net neutralization in the waste which will subsequently reduce the solubility of a variety rock. Sulfate concentrations are generally less than of contaminants in the drainage water. This is especial­ 2,000 mg/L, but may contain elevated concentrations ly true for the divalent metals and aluminum, which are of certain metals (e.g., zinc, copper, or nickel), oxy- precipitated as hydroxides. The literature on treatment of anions, or arsenic, antimony and selenium, particu­ drainage from coal mines has examined the various types larly at the higher pH ranges. Common examples are of neutralizing agents in detail, and although differences drainage from carbonate-hosted waste rock dumps, exist between coal and hard-rock mine waters, treatment closed precious metals heaps, and pit lakes. of coal mine waters has been extensively examined in the past 25 years (http://www.wvu.edu/~agexten/landrec/ ● Alkaline water (pH 9): With few exceptions, ). Five chemicals that have commonly been these are commonly associated with process fl uids, chemtrt.htm used for treatment of acidic water are listed in Table 4-2. and the elevated pH is due to chemical reagent addi­ tion (e.g., sodium cyanide plus lime). The solubility Ammonia has also been utilized for treatment of coal of a variety of oxyanions can be enhanced at alkaline mine waters, but is uncommon for treatment of hard-rock pH. Over time, the pH of these waters is reduced mine waters and will not be considered further here. This when atmospheric carbon dioxide dissolves. leaves two general types of neutralization agents, the cal­ cium- and sodium-based systems. Of these, the calcium- 4.2.1 Treatment of Acidic Waters based systems are generally preferable to sodium due to the ability of calcium to remove sulfate as calcium sulfate Acidic water is generally considered the most problematic compounds (e.g., gypsum). Calcium will also ultimately mine-related drainage water, and it offers the greatest po­ precipitate as calcite when the water is equilibrated with tential for degradation of surface and ground water. While carbon dioxide in air if the pH is slightly elevated. Alterna­ prevention of acid drainage is a common goal for manage­ tively, while sodium-based neutralization agents are effec­ ment of acid-generating rock, treatment of acidic drainage tive in raising the pH, elevated sodium in irrigation water at many mine sites will be required far into the future. causes soil structure to collapse (sodic soils). Also, particu­ larly when handled in bulk, lime is generally less expensive than either sodium carbonate or sodium hydroxide.

Table 4-2. Neutralizing Reagents for Treatment of Acidic Water from Mines (Source: Modified from Skousen in http://www.wvu.edu/~agexten/landrec/chemtrt.htm.)

Conversion Common Name Chemical Name Formula Comments Factor*

Limestone Calcium CaCO3 1.0 Inexpensive chemical cost, but difficult to dissolve—tends carbonate to armor and reduce effectiveness. Utilization of only 30% of neutralizing capability.

Hydrated lime Calcium Ca(OH)2 0.74 Relatively inexpensive chemical cost and most utilized form hydroxide of lime as a slurry. Requires control to maintain suspension. Neutralizing efficiency of 90%. Lime (quicklime) Calcium oxide Ca0 0.56 Also commonly utilized, although more effort is required to maintain a suspension. Requires slaker to convert to hydrated lime. Neutralizing efficiency of 90%.

Soda ash Sodium Na2CO3 1.06 Dissolves rapidly; less caustic alternative to sodium carbonate hydroxide. Does not remove sulfate effectively. Increases sodium content of treated water. Neutralizing efficiency of 60%. Caustic soda Sodium NaOH 0.8 Does not remove sulfate effectively and increases sodium hydroxide content of treated water. Neutralizing efficiency of 100%. * The conversion factor is the relative amount of weight of each material (compared to limestone) to neutralize a given amount of acid. The estimated tons of acid/year can be multiplied by the conversion factor to get the tons of chemical needed for neutralization.

14 Engineering Issue 4.2.1.1 Conventional Physical/Chemical Treatment of Oxidation of soluble ferrous iron to ferric iron is required Acidic Water Using Lime for treatment of most mineral acidic waters and utilizes Use of lime (calcium hydroxide or calcium oxide) for neu­ atmospheric oxygen at an elevated pH 7. Ferric ox­ tralization is the accepted conventional water treatment ide rapidly precipitates at neutral or alkaline pH. Mixing for most hard-rock mine acidic waters, particularly when of the acidic water with lime slurries requires aeration to the acidity and/or flows are high. Not only does this treat­ ensure good contact with atmospheric oxygen. Most con­ ment raise the pH in a cost-effective manner, it also reduc­ ventional treatment systems utilize a stirred aeration basin es the sulfate concentrations to below 2,000 mg/L due to to accomplish this oxidation (see Figure 4-2). the relatively low solubility of gypsum (calcium sulfate). A relatively new method for oxidation uses the Rotating (See INAP, 2003 for summary of methods for treating Cylinder Treatment System (RCTS) to provide rapid oxy­ sulfate in water.) For this discussion, “conventional wa­ gen transfer to the solution and efficient utilization of the ter treatment” refers to fixed facilities of pipes, metering lime slurry (http://www.iwtechnologies.com). The RCTS pumps, reaction vessels, clarifiers, and solid management uses shallow trough-like cells to contain the impacted wa­ mechanical fi xtures (e.g., filter presses). Lime is mixed ters and rotating perforated cylinders for improved atmo­ with acidic water as a 10–15% slurry that is most com­ spheric oxygen transfer and improved agitation during monly generated on site from a storage tank of hydrated treatment of the water. lime using a slurry mixer. Following oxidation and neutralization, agitation of the Conventional acidic water treatment using alkaline sourc­ suspension and addition of flocculants allows the metal es is used for removal of Al, Fe, Cu, Cd, Pb, Zn, Ni, and oxide solids to settle out by growth of precipitants to suf­ Mn as metal hydroxides. The dissolved concentrations of ficiently large particles to form sludge. The sludge pro­ the oxyanions, including Cr, Se, Sb, Mo, As, and U, can duced is generally of low density and requires thickening also be substantially reduced by the co-precipitation with or filter presses to decrease the water content. Addition­ the metal hydroxide. ally, management of the sludge generally requires a de-

Turbulence box Bin vent filter High level sensor Re-order sensor Clear water Storage pump Tank Pneumatic Low level fill line Sump Slide gate Bin discharger valve Electrical panel

SCR Slurry mixer controller Flash mixer Acid water Volumetric pH probe screw feeder Blower Aerator Distribution Slurry mix pH probe trough tank Weir plates

Flash tank Aeration basin

Thickener or settling pond

Figure 4-2. Conventional water treatment utilizing lime. (Source: http://amd.osmre.gov/Cost.pdf)

Engineering Issue 15 termination of its contaminant leachability to decide ings increase, the fi xed costs become a smaller fraction of whether the sludge can be managed on-site or needs to be the total cost, and lime treatment is generally the most transported to an off-site hazardous waste management cost-effective method for treating large volumes of acidic facility. The Toxicity Characteristic Leaching Procedure drainage from mines. Comparison of costs of treatment (TCLP) is usually used to determine if a material is Re­ at different flows (using reagent costs in 1996) is avail­ source Conservation and Recovery Act (RCRA) – haz­ able at http://www.wvu.edu/~agexten/landrec/chemtrt. ardous due to its leaching characteristics. The Synthetic htm#Chemical. While the reagent costs change with time Precipitation Leaching Procedure (SPLP) is a better indi­ and location, as well as the implementation of a treatment cator of leaching behavior under natural environmental system at a specifi c location, this example provides an in­ conditions. State regulations also apply to how sludge is dication of the non-linear cost differences with differing managed, on site or off site. flows. Each treatment alternative needs to be evaluated relative to the total costs and intended characteristic of Because of the wide range of flows, the amount of lime the effl uent water. required, the length of time for each type of reaction, the method of settling or filtering out the solids from the wa­ Using an engineered system of conventional water treat­ ter, and the method for residual (sludge) management, ment requires proper road access, a power supply, stable site-specific information is required for the design of each land area, and manpower. In remote areas of the west­ system. Shakedown operations and modifications of orig­ ern U.S., access may be difficult and expensive during the inal design are often performed to meet target discharge winter months, and a conventional lime treatment system requirements and to optimize operations to reduce costs may not be appropriate. or volume of residual (sludge) produced. The costs for construction of active lime treatment facili­ Key Web Site References ties can be substantial due to the requirements of power, ● Overview of chemicals available to treat AMD: pumps, lime addition systems, tanks, and sludge manage­ http://www.leo.lehigh.edu/envirosci/enviroissue/ ment equipment. Several organizations have developed amd/links/chem1.html guidelines. An example of such guidelines is one devel­ ● AMD abatement cost-estimating tool developed co­ oped by the Office of Surface Mining (2000) (http://amd. operatively by the Pennsylvania Department of Envi­ osmre.gov/Cost.pdf). While this document is focused on ronmental Protection, the West Virginia Department costs for treatment of acidic drainage from coal mines, the of Environmental Protection, and the Office of Sur­ same approach can be used for estimating costs for treat­ face Mining (OSM) Reclamation and Enforcement: ment of acidic drainage from hard-rock mines. Because http://amd.osmre.gov/ the characteristics of water quality, flows, remoteness, and ● The MEND manual is a set of comprehensive work­ reagent costs, as well as other factors, can vary substantial­ ing references for the sampling and analyses, predic­ ly, it is difficult to provide a reliable estimate for treatment tion, prevention, control, treatment, and monitoring at a specific site until a careful engineering estimate is de­ of acidic drainage. The document provides informa­ veloped. However, estimates for treatment cost vary from tion on chemistry, engineering, economics, case stud­ less than $1/1,000 gallons to well over $10/1,000 gallons ies, and scientifi c data. http://www.nrcan.gc.ca/mms/ on an annual operating and maintenance basis. canmet-mtb/mmsl-lmsm/mend/mendmanual-e.htm While the newer designs for lime treatment systems are ● UK summary of active and passive treatment: increasingly automated, these systems still require fre­ http://www.parliament.uk/commons/lib/research/ quent monitoring and oversight due to the caking and rp99/rp99-010.pdf scaling problems common with the use of lime. Addition­ ● Britannia Mine Water Treatment Plant Feasibility ally, these active systems utilize pumps and mixing sys­ Study. An example of a feasibility study for a site- tems that require routine maintenance. Thus, operation specific conventional system: http://www.agf.gov. of a lime treatment plant has inherent fi xed construction bc.ca/clad/britannia/downloads/reports/tech_ and operation/maintenance costs that make these treat­ reports/WTP_feasibility.pdf. ment systems expensive on a cost per volume of water treated when the flows are low. However, as fl ows increase (e.g., 100 gal/min) or the acidity and metals load­

16 Engineering Issue ● Example of evaluating options for sludge manage­ Pennsylvania ment for a conventional water treatment system: Flow direction

http://www.agf.gov.bc.ca/clad/britannia/reports.html Rausch Cr. ● Detecting change in water quality from implementa­ tion of limestone treatment systems in a coal-mined watershed of Pennsylvania: http://www.mbcomp. 0 150 0 100 com/swatara/Cravotta.pdf Miles km ● Abandoned mine remediation clearinghouse for treatment of acidic drainage in Pennsylvania: N http://www.amrclearinghouse.org/Sub/ AMDtreatment/ZZTreatmentStrategies.htm Polishing Old stream ● National Lime Association Web site. A wealth of in­ Sludge pond 2 channel holding (now for overflow) formation about neutralizing acidic water with lime: ponds http://www.lime.org Polishing ● Army Corps of Engineers document “Engineering Thickener pond 1 and Design: Precipitation/Coagulation/Flocculation”: Press filter

http://www.usace.army.mil/inet/usace-docs/eng-man­ Aeration tanks uals/em1110-1-4012/toc.htm Point of Clarifiers lime addidtion 4.2.1.2 Physical/Chemical Treatment in Alkaline Ponds and Lagoons Diversion Heavy dashed lines show structure and Physical/chemical treatment in alkaline ponds and lagoons underground flow or pumping path spillway is very similar to conventional treatment as described in the preceding section. Using ponds and lagoons for aera­ Figure 4-3. Use of aeration ponds for polishing lime treatment pro­ tion, settling, and solids accumulation has the benefi ts of cess. (Source: http://www.facstaff.bucknell.edu/kirby/RCr.html) exploiting natural processes. Lime (calcium hydroxide) is of the wide range of flows, the amount of lime required, added using the same type of equipment that is used in the length of time for each type of reaction, the method conventional plants. of settling or filtering out the solids from the water, and Physical/chemical treatment in alkaline ponds and la­ the method for residual (sludge) management. Alkaline goons is used to remove metals, including Al, Fe, Cu, Cd, ponds and lagoons have been used to effectively remove Pb, Zn, Ni, Mn, and the oxyanions Cr, Se, Sb, Mo, As, metals, metalloids, and uranium from mine waters when and U. Depending on the water chemistry, the oxyanions designed to account for variations in flow and composi­ are reduced in dissolved concentrations by co-precipita­ tion. Removals at a treatment lagoon in Butte, Montana, tion with metal oxides and calcite. Formation of metal are presented in Table 4-3, and a photograph of the polish­ hydroxide precipitates and formation of calcium carbon­ ing pond is shown in Figure 4-4 on the next page. Shake­ ate with flux from the atmosphere result in solids settling down operations and modifications of original design are out in the ponds. While the primary neutralization of the often performed to meet target discharge concentrations acidic drainage is through lime addition, the ponds and and to optimize operations to reduce costs or the volume lagoons can improve the overall water treatment and met­ of residual (sludge) produced. als reduction by the photosynthetic activity in the water The construction cost for physical/chemical treatment in (see Figure 4-3). alkaline ponds and lagoons is usually similar to the addi­ A larger area of land is needed for physical/chemical treat­ tional chemical components of a conventional treatment. ment in alkaline ponds and lagoons than for conventional Due to land availability and status of land relative to the treatment plants. Pond or lagoon treatment systems are hydraulic profile of proposed system, site-specifi c factors often easier to construct if existing settling ponds, tailing can make ponds or lagoons less expensive than clarifi ­ ponds, or excavated areas are available for use. Site-specifi c ers and reaction vessels. Due to the relative larger size of information is critical for design of these systems because ponds and lagoons than most conventional treatments, Engineering Issue 17 fewer operator hours are required to account for system Key Web Site References and Pictures variations and to physically manage solids produced. ● Silver Bow Creek/Warm Springs Ponds One-Page Smaller systems can be designed with cleanout and sludge Summary: http://www.epa.gov/superfund/programs/ management at frequencies of a few years to decades. An recycle/success/1-pagers/bowcrk.htm additional advantage of using alkaline ponds and lagoons ● Pictures of a lime lagoon at Leviathan Mine, Cali­ is the buffering capacity of the lagoons, which corrects fornia, which has no biological component due to minor process upsets or variations. limited size of the pilot project. Filter bags are used to capture and manage the majority of the sludge, Table 4-3. Influent and Effluent Concentrations for Treatment Lagoon in Butte, Montana, for Year 2003 while a lined pond is used for settling and polishing: http://yosemite.epa.gov/r9/sfund/sphotos.nsf/0/ Untreated Treated % 75c4f97d7640242488256e98006656ab/$FILE/Le­ Basis Analyte Concentration Concentration Removed viathan_04%20p7-22.pdf (ppb) (ppb) Total Ag 5 5 Detection Limit 4.2.1.3 Low-Flow/Low-Acidity Chemical Treatment Total Al 155 34 78% Options Total As 35 7 80% While conventional lime treatment has distinct cost and Total Cd 15 0.3 98% treatment advantages, the costs of treating lower fl ows on Total Cr 11 10 Detection a per-gallon basis can potentially be reduced using alter­ Limit native neutralization methods in certain cases. Examples Total Cu 388 15 96% include the following: Total Fe 1,499 41 97% Automatic lime addition using an Aquafi x system: As Total Mn 2,478 72 97% discussed previously, addition of lime to acidic water in Total Pb 6 1 Detection a controlled and efficient manner requires lime addition Limit technology that increases the fixed costs and is often infea­ Total Zn 4,526 107 98% sible for small streams. Jenkins and Skousen (1993) have shown the utility of an Aquafix pebble quicklime (CaO) Using an engineered system of physical/chemical water water treatment system that utilizes a water wheel con­ treatment in ponds and lagoons requires proper road ac­ cept for coal mine drainage waters. The concept is that cess, a power supply, stable land area, and manpower. One these systems can be operated without intensive manage­ additional advantage of a lagoon system is the attractive­ ment, and the rate of addition of lime can be controlled ness and wildlife attributes of a wetland, although these by the flow rate of the acidic stream. For this system, the features need to be weighed against metals bioavailability amount of chemical utilized is controlled by a water wheel and insect breeding issues. attached to a screw feeder that dispenses lime directly into the flowing acidic drainage. This system was initially de­ veloped for small flows from coal mines of high acidity because calcium oxide is very reactive. Recently, however, water wheels have been attached to large bins or silos for high-flow/high-acidity situations. These systems have re­ ceived only limited applicability at hard-rock mine sites in the western U.S., although additional testing is warrant­ ed. Controlling the rate of application of the quicklime without operator attendance and problems with remote cold weather operation have somewhat limited the inter­ est for many mineral mine sites. These systems also may require settling basins and sludge management for the Figure 4-4. Final pond in the treatment lagoons in Butte, Montana, metals-laden precipitates (http://www.wvu.edu/~agexten/ used for polishing and robustness of system. landrec/chemtrt.htm).

18 Engineering Issue Open limestone channels: While limestone beds/chan­ metals. Limestone has the most potential as a pretreat­ nels have been used with some success in neutralizing ment method for passive microbial-based systems where a mildly contaminated coal mine acidic drainage, the rate decreased dissolution rate from armoring can be incorpo­ of release of alkalinity is difficult to control, and the lime­ rated into the design. stone tends to armor with aluminum and iron oxide coat­ Sodium hydroxide: Addition of solutions of 25% sodium ings. Open limestone channels are constructed simply by hydroxide to acidic water can be accomplished by either laying limestone rock in a channel and allowing the acidic gravity flow or small solar-powered pumps. This system solution to pass over the rock or by laying limestone di­ can be very inexpensive to construct, depending on the rectly in a channel of acidic drainage (Ziemkiewicz et al., site conditions, although the cost of sodium hydroxide is 1997: http://www.dep.state.pa.us/dep/deputate/minres/ higher than a similar amount of calcium-based neutraliza­ bamr/amd/science_of_amd.htm). Because of the armor­ tion agents. The sodium hydroxide solution is complete­ ing that occurs, this method has shown best treatment ly utilized and an effective neutralization agent. Because when the channel is sloped to allow rapid movement of 50% solutions will solidify under cold conditions, a 25% the water and scouring of the coatings on the limestone. solution is generally utilized and is available in bulk solu­ However, in mineral-mining applications, limestone tions. However, the volumes that one will need to use will channels have not been shown to be successful, and the require either frequent refilling or large storage capacity. applicability may be limited to iron-free, aluminum-free For example, a flow of 100 mL/min will utilize 52,500 waters that only contain metals that can be removed by liters per year (~13,900 gallons) of solution. Two other chemical precipitation at pH 7. Depending on the re­ disadvantages are the safety issues associated with using quirements, these systems can be lined or unlined. Settling sodium hydroxide, as well as the increase in sodium con­ basins may be used under certain conditions to collect centrations that remain in the treated water. precipitates. Limestone treatment is generally not effec­ tive for acidities exceeding 50 mg/L (http://www.osmre. Sodium carbonate: A less caustic alternative to sodium

gov/amdtcst.htm). A somewhat more effective method for hydroxide is the use of sodium carbonate (Na2CO3). So­ limestone treatment utilizes pulsed, fl uidized bed reactors dium carbonate briquettes are available and can be uti­ in which acidic water is injected in an upward manner at lized by simply diverting a small stream of the acidic wa­ high velocity into limestone columns. This method can ter through (or over) a bed of the briquettes and allowing improve the scouring of the limestone and increase the that solution to mix with the acidic water. Control of the release of alkalinity. Carbon dioxide (either from tanks or diversion can be managed with a weiring system. Sodi­

by utilization of CO2 released from the limestone) aids um carbonate tends to cement together and change the the process by reducing the rate of iron oxidation in the amount of surface available for dissolution. Temperature reactors (http://www2.nature.nps.gov/pubs/yir/yir2000/ changes can also affect the amount of delivered alkalinity. pages/07_new_horizons/07_02_reeder.html). Sodium increases in the treated water are also an issue. Anoxic limestone drains: An anoxic limestone drain (ALD) is similar to an open limestone channel, except the 4.2.2 Treatment of Neutral and Alkaline Waters limestone is buried under a cap and designed to exclude Contaminated neutral or alkaline mine drainage waters oxygen and reduce the amount of iron oxidation prod­ are present at sites that have sufficient neutralization (gen­ ucts that coat the limestone. This will tend to improve erally from calcite) in the rock such that any acid produc­ the release of alkalinity from the limestone. These systems tion is offset by the neutralization available. These acidic have been used to decrease the acidity of drainage waters waters are also commonly generated from precious met­ prior to aerobic wetlands or sulfate-reducing bioreactors als ore processing using cyanide for mill circuits or heap (SRBs). The downside to using these systems is that if the leach processing. limestone becomes armored, uncovering the limestone requires excavation of the cap. Because acidic drainage Neutral and alkaline drainage from mine sites is gener­ from hard-rock sites often contains appreciable amounts ally less of a water quality problem than acidic drainage of aluminum that coat the limestone, these systems are since the solubility of many of the problematic metals is not commonly utilized. Limestone dissolution can in­ low at neutral or alkaline pH. Neutral and alkaline drain­ crease the pH sufficiently to precipitate oxidized iron and age contaminants generally are most problematic for the aluminum, but does not effectively remove most heavy oxyanions of selenium, arsenic, and antimony since the

Engineering Issue 19 solubility of these constituents increases with higher pH. of time (months to years). Zero-valent iron systems have In addition, nitrate, sulfate, and other salts, as well as cy­ also been effectively applied as permeable reactive barri­ anide species, may be elevated at cyanidization facilities ers (PRBs) in subsurface systems for remediation of arse­ and exceed discharge requirements. nic-containing ground water at a mill tailings site (EPA, 2000). PRBs are described in Section 4.2.3.2. Arsenic can also (at least partially) be removed from mine waters by 4.2.2.1 Arsenic and Antimony sulfate-reducing bacterial systems as described below. Total arsenic concentrations in drainage water can vary from less than 10 µg/L to several mg/L. Antimony is gen­ 4.2.2.2 Heap Effl uent erally found at lower concentrations. Because these ele­ ments are closely related (group 5A in the Periodic Table), The use of heap leach technology for recovery of precious treatments for removal are similar and will be considered metals has evolved over the past 25 years and is commonly together. In general, methods to remove arsenic from wa­ employed for low-grade ore (typically 0.015–0.06 ounces ter also are effective for antimony. per ton-equivalent of gold) at many sites throughout the world. The tonnage of ore processed in this manner in Arsenic treatment technologies have received the greatest Nevada, for example, is estimated to be on the order of 2 focus in recent years, primarily due to the need for arsenic billion tons. In this process, ore is placed on high-density removal in drinking water. An extensive recent EPA ar­ polyethylene sheets and rinsed with dilute concentrations senic treatment review (EPA, 2002: http://cluin.org/con­ of sodium cyanide. In arid regions of the world, these sys­ taminantfocus/default.focus/sec/arsenic/cat/Treatment_ tems are operated in a zero-discharge mode: the amount Technologies/) and the U.S. Geological Survey Web site of water evaporating is greater than the rainfall, and ad­ (http://arsenic.cr.usgs.gov/) provide a more detailed dis­ ditional water is required to make up the difference of the cussion of the treatment options than is provided here. amount lost to evaporation and the amount of rainfall. The most common arsenic treatment systems are briefl y When precious metals recovery is completed, the process discussed below. for closure of the heaps begins. For specific application to mine-related waters, arsenic For arid sites, the most common method for initial re­ removal from large volumes of water (e.g., pit lakes, dis­ duction of water volume is to continue to recirculate the charge water from pit dewatering) most often utilizes iron water to the heap using enhanced evaporation methods: precipitation/co-precipitation methods. For these systems, water is sprayed into the air over the heaps and allowed to ferrous or ferric salts are added to the water and allowed to evaporate, subsequently increasing the concentration of precipitate. Arsenic, particularly in the 5 valence state, soluble constituents in the remaining water. The rate of sorbs strongly to the surface of the precipitates and is ef­ water that is recirculated will decrease over time from op­ fectively removed from the water. When arsenic in the erational flows of several hundred to several thousand gal­ 3 valence state is present in appreciable concentrations, lons/minute to residual flows that decrease to 0–50 gal­ a pre-oxidation step may be required since it sorbs less lons/minute. During this time, carbon dioxide dissolves strongly to iron oxides than in the 5 state. in the water and reduces the pH to between 8 and 9. This A recently developed method for arsenic treatment uti­ process also allows volatilization/oxidation of cyanide and lizes zero-valent iron (Su and Puls, 2001; Nikolaos et al., also enhances the activity of microorganisms that can con­ 2003). For this technology, arsenic-containing waters are vert the nitrogen in a variety of cyanide species to nitrate. passed over iron filings that generally have been mixed Because mercury is mobilized as a mercury–cyanide com­ into sand at a ratio of 10–20% iron. Iron oxidizes to iron plex, removal of the cyanide is also reasonably effective in oxide, and arsenic is sorbed to the iron oxide surface. Al­ reducing mercury concentrations in the drainage water. though the iron is ultimately mobilized (albeit slowly) The amount of water that drains from heaps will vary and the treatment system will need to be replaced, the depending on the site conditions, but it will also depend arsenic that is sorbed is generally not available. The iron on the amount of meteoric water, the type of cap (if any) oxide/arsenic residue is generally not hazardous, although that is placed on the top of the heap, and other site-spe­ its classification is dependent on the results of site-specifi c cific conditions that may be present. However, low-fl ow waste characterization testing. Depending on the design, drainage from heaps has been observed at most sites and these systems can remain effective for an extended period will continue for the foreseeable future. Discharge from

20 Engineering Issue heaps can be reliably eliminated only for sites in a highly Table 4-4. Heap Drainage Chemistry Profiles of Three Closed arid region or those that have a very effi cient store-and­ Heaps (Source: NDEP, 2004) release cap. Heap 1 Heap 2 Heap 3 Effl uent Effl uent Effl uent In higher-rainfall regions, where rainfall on the heap ex­ 6/23/98 4Q 95 5/02 ceeds the amount of water that evaporates, treatment and pH 7.79 8.17 9.6 discharge of excess water is required. Although many of TDS 3,032 11,200 5,670 the constituents in these fluids are the same as during clo­ sure, cyanide removal becomes more important and re­ nitrate 54 171 96 quires specialized treatment. sodium 340 3,880 1,640 chloride 160 1,130 3,200 The constituents present in residual cyanidization fl u- ids differ substantially from acid drainage sites. Drain­ WAD CN 3 0.11 14.3 age from three distinct closed heaps is described in Table sulfate 1,600 6,130 470 4-4. These waters contain elements that have enhanced antimony 0.023 – 0.003 solubility at higher pH, as well as residual cyanide com­ arsenic 0.08 0.543 0.209 ponents. The constituents that are of particular concern copper 0.007 0.028 0.515 include arsenic, antimony, selenium, nitrate, sodium, sul­ manganese 0.051 0.035 0.01 fate, cyanide species [both weak acid-dissociable (WAD) cyanide as well as total cyanide], mercury, and nickel. mercury 0.022 0.004 0.102 nickel 0.034 – 0.535 Effective treatment of heap effl uent requires consideration selenium 0.18 5.84 0.109 of all of the constituents present in the drainage water (Ta­ molybdenum 0.31 – 0.917 ble 4-4). While specific treatment methods are available for several of these constituents, or even groups of constit­ vanadium 0.002 – 0.642 uents, relatively few methods are available that can remove All units are mg/L, except pH all of these to surface water discharge requirements. Land application and French drains: For these meth­ Reduction in the volume of water by recirculation and ods, water is simply land-applied via irrigation systems or evaporation on the heaps is generally utilized. However, passively drained through perforated pipe. In both cases, the collection pond water volume is usually large, and the contaminants in the drainage water are released ei­ treatment is often required for the several millions of gal­ ther to the land surface or allowed to move downward lons typically left after recirculation of the water to the in the subsurface. Although this method is very inexpen­ heaps is discontinued. Since the volume is contained in sive, this form of water management carries risks from a pond, this water can often be treated in a single batch whatever contaminants exist in the water. For example, mode and can utilize intensive techniques (e.g., membrane the land application at Beal Mountain mine resulted in separation, ion exchange, or aggressive evaporation). a near-complete removal of all of the vegetation due to Because the water quality from these heaps is unlikely elevated concentrations of thiocyanate, a soil sterilant. to change substantially for years to decades due to the Elevated selenium and sodium have resulted in potential slow migration of meteoric water through the heaps, any plant uptake problems and changes in the soil structure treatment process will need to be either continuous or al­ for land applications from heap effluent from the Zort­ low accumulation of water for periodic batch treatments. man-Landusky mine in Montana. However, because of Thus, the more intensive management techniques be­ the very low expense of pond volume reduction, land ap­ come very costly on a per-gallon-treated basis, and pas­ plication is sometimes used. However, it can in some cases sive methods for water management (1–20 gal/min) are create serious problems. favored. However, few options are available, particularly Discharge of water to French drains: This method of for saline waters. disposal of contaminated water has been permitted in Current methods for residual heap drainage water treat­ Nevada for sites for which ground or surface water con­ ment include the following: tamination is unlikely. While the risk factors in certain situations in extremely arid areas are low, the release of

Engineering Issue 21 highly contaminated water (some of which meets hazard­ of Reclamation literature on desalinization: http://www. ous waste criteria) into the subsurface has been criticized, usbr.gov/pmts/water/reports.html. and it is unlikely to be permitted for new applications. Evaporation: Particularly for those sites that have high 4.2.3 Treatment of Mine Water with Microbial salinity, evaporative methods are one of the few options Processes available for long-term treatment of residual heap drain­ A variety of microorganisms can facilitate the removal age water. A recent analysis of alternatives of water man­ of metals, metalloids, and sulfate from mining-impact­ agement by consultants for a Nevada mine (see Heap 3 ed waters in both natural and engineered systems. The Effluent quality in Table 4-4) (Telesto Solutions, 2003) primary removal mechanism is the formation of oxide, indicated that the most cost-effective method was the use hydroxide, sulfides, or carbonate precipitates. Successful of evaporative ponds. In addition to the sodium load that removal of metals and metalloids from mining-impacted resulted from the addition of sodium cyanide, the source waters depends on providing appropriate environmental water was a geothermal water high in dissolved salts. As a conditions to promote the desired microbial activity in result, the number of treatment options was few. Bioreac­ conjunction with the appropriate chemistry. tors would not be effective for treatment of the high salin­ ity, freshwater rinsing would require very large volumes of Aerobic environments will promote the oxidation of re­ water (unavailable), and land application had similar is­ duced metals, particularly manganese and iron. After oxi­ sues with salts. The option of geothermal aquifer injection dation, manganese and iron will precipitate in neutral (or was seriously considered, but was found to be much more near-neutral) waters and potentially remove other con­ expensive than passive evaporation using surface ponds. taminants (e.g., arsenic) by co-precipitation. Evaporative processes are not completely passive, howev­ Anaerobic environments will promote the reduction of er, and require regular monitoring to ensure the integrity sulfate, nitrate, oxidized metals, and metalloids (e.g., sele­ of the pond liner and the piping system to deliver the nium, arsenic, and antimony). A byproduct of a number of water. Most heaps will have a soil cap to limit infi ltration anaerobic microorganisms is bicarbonate, which increases of meteoric water, and monitoring of this cap will be re­ the pH and promotes precipitation of metal hydroxides. quired to ensure that it retains the design characteristics. The production of bicarbonate also promotes the forma­ In addition, the salt loading in the heaps can be substan­ tion of metal carbonate precipitates (e.g., Zn, Mn, and tial, particularly when the source water has high salt load­ Pb). Biogenic sulfide (produced from sulfate reduction) ing (e.g., from a geothermal aquifer), and it will need to will promote the precipitation of metal sulfides (e.g., Cu, be managed on a year-to-decade time frame. Cd, Zn, Pb, Ni, and Fe) under a wide range of chemical conditions. Chromium (VI) and uranium (VI) can be re­ Biological treatment: Biological processes can also be duced by a number of microorganisms (fermenters, sulfate used for heap treatment, particularly when salt concen­ reducers, and iron reducers) under anaerobic conditions to trations are not excessive. SRBs (discussed below) can be Cr(III) and U(IV), respectively. Subsequently, Cr(OH) (s) successfully employed for sulfate and nitrate removal, as 3 and UO (s) are precipitated from solution. Selenate well as for treatment of selenium and arsenic. 2 (Se(VI)) can be reduced to selenite (Se(IV)), which is sub­ Membrane processes: Reverse osmosis and nanofi ltra­ sequently reduced to elemental selenium. Under sulfate- tion are examples of processes that can also be utilized for reducing conditions, As(V), Mo(VI), and Sb(V) can be treatment of heap effluent, although the costs for long- reduced and subsequently precipitated as a sulfi de mineral term treatment of low flows reduce the applicability of (As2S3, MoS2, Sb2S3). Some metals will also be removed by these methods that require intensive management and co-precipitation with aluminum or iron hydroxides. monitoring. Although one option is to accumulate a larg­ The design of microbial treatment schemes needs to er volume of water and follow this by periodic treatment consider: using various membrane processes, this technique has not been utilized extensively. The most extensive literature on 1. Identification of target compound(s) and desired applicable membrane processes is in the large-scale desali­ effl uent limits, nization technology. See, for example, the U.S. Bureau 2. Conditions for desired microbial activity,

22 Engineering Issue 3. Conditions for desired chemistry, and with temperature, although the overall rates of reaction 4. Mass transfer and kinetic constraints. (e.g., sulfate reduction) can be kept constant if the num­ ber of active bacteria increases proportionally. Most of the Additional issues that will affect the selection of any treat­ desired microbial processes have optimal rates at neutral ment process are solids management, operation and main­ pH. However, many microorganisms can adapt to lower tenance requirements, and cost. and higher pH values (5–9) or may be protected from As with other treatment technologies for mining-impact­ bulk solution–phase pH in microenvironments. Redox ed waters, identification of target contaminants and the conditions are important relative to the electron acceptor associated discharge requirements are necessary for selec­ used by an active consortium of bacteria. In general, the tion of a microbial system. While microbial systems can highest energy couple is used first, followed by those of treat a number of types of contaminants effectively, the decreasing energy. However, the presence of a microenvi­ microbial treatment options are usually constrained by ronment and microorganisms with different metabolisms the contaminant load in the water, as well as the require­ allows concurrent usage of multiple electron acceptors. ments for treatment. When flows are high, and conse­ Microorganisms alter the chemical environment to pro­ quently residence time is reduced, insuffi cient sulfi de is mote conditions conducive to desired precipitation or generated to precipitate the metals. co-precipitation reactions. The changes in chemical envi­ Microorganisms need an electron donor and acceptor cou­ ronment can include pH, redox, and reactant formation. ple for energy generation, a carbon source and nutrients for The theoretical predictions by chemical equilibrium pro­ cell synthesis, and appropriate environmental conditions. grams, such as PHREEQC (see http://wwwbrr.cr.usgs. Most microbial-based treatment systems require organic gov/projects/GWC_coupled/phreeqc), provide a useful material for the electron donor, which then also serves as estimate of the potential of precipitates to form, but the the carbon source. The organic material can be supplied added interactions of the microorganisms can alter the in a water-soluble form (e.g., molasses or ethanol) or in a expected distribution of precipitates formed. solid form (e.g., wood chips or leaf compost). Water-sol­ The rate of precipitation tends to be controlled by the rate uble organics have been used for active bioreactor systems of the microbial function of interest (e.g., sulfate reduc­ and ground water treatment systems. Solid-phase organics tion). One way the rate of sulfate reduction is controlled have been used in active and passive bioreactor systems, is by the rate-limiting step of the microbial community permeable reactive walls, and wetland systems. providing growth substrates for sulfate reduction. Models Potential electron acceptors used for energy acquisition developed to describe the rate of precipitation range from include oxygen, nitrate, sulfate, and carbon dioxide. Dis­ empirical constant-rate models to models that couple solved oxygen is typically insufficient for desired microbi­ microbial kinetics with a selected reactor confi guration. al reactions and must be added either actively or passive­ Mass transfer is also important in describing the overall ly. Sulfate is present at adequate concentrations in many rate of precipitation for biotreatment systems. Mass trans­ mining-impacted waters, particularly those where pyrite fer limitations are particularly important for biofi lm sys­ oxidation has occurred. Carbon dioxide is suffi cient for tems (any system with solid-phase growth media) and will fermentative reactions involving solid-phase organic mat­ be a function of linear velocity, media size, and biofi lm ter hydrolysis and production of organic acids and alco­ thickness. In biofilm systems, mass transfer can control hols, which are then available for sulfate reducers. the observed rate of reaction. Nutrient addition (particularly nitrogen) is typically re­ quired when water-soluble organic materials are used as 4.2.3.1 Sulfate-Reducing Systems the carbon source. Solid-phase organic substrates used are Sulfate-reducing systems promote the microbial-facilitat­ typically a combination of a number of materials (e.g., ed reduction of sulfate, production of sulfi de, generation manure, compost, or wood) and can be selected to in­ of alkalinity, and reduction of redox active metals, metal­ clude organic material containing suffi cient nitrogen. loids, or radionuclides. A carbon source, such as lactate or Mining-impacted waters exist with a range of tempera­ ethanol, is required to promote the growth of sulfate re­ tures, pH, and redox conditions, and microorganisms are ducers in these systems. Solid-phase organic material can sensitive to all of them. Microbial activity tends to decrease also be used to indirectly provide a carbon source for sul­

Engineering Issue 23 fate reducers from the actions of cellulolytic and ferment­ 4.2.3.1.1 Anaerobic Wetlands ing bacteria. Wide ranges of microbial species are able to An anaerobic wetland is a subsurface water body that sup­ catalyze sulfate reduction. (See INAP [2003] for a sum­ ports the growth of emergent plants, such as cattails and mary of water treatment methods designed specifi cally for reeds. The vegetation and sediment provide surfaces for removal of sulfate.) the growth of attached bacteria. Anaerobic removal pro­ The range of reactions promoted in a sulfate-reducing sys­ cesses control the treatment of metals and the neutraliza­ tem depends in part on the type of carbon source selected. tion of acid. The contaminated water is intercepted and The use of more complex organic compounds results in diverted through the wetland system (see Figure 4-6). A a greater diversity of microbial population in addition to minor aerobic component of this system is the surface sulfate reducers. The resulting number of reactions that vegetation, which allows the release of carbon dioxide and control sulfate reduction also increases in complexity. The hydrogen sulfide, and oxidation of iron on the surface. primary reactions of interest are shown in Figure 4-5 at Anaerobic wetlands utilize sulfate-reducing bacteria to the bottom of this page. immobilize metal cations (Fe, Cu, Cd, Pb, Zn), oxyani­ Sulfate-reducing systems may be implemented in ac­ ons (Cr, Se, Sb, Mo, As), and U. In addition, the produc­ tive or relatively passive treatment confi gurations. Rela­ tion of alkalinity allows for the neutralization of excess tively passive configurations include anaerobic wetlands, acid present in target mine waters. compost-based bioreactors, and PRBs. Relatively passive Large areas with a relatively flat topography are required systems with soluble carbon input include permeable re­ for wetland treatment systems. The area required is a active zones (PRZs) and rock-filled bioreactor ponds. Ac­ function of the mass loading of each target contaminant. tive systems include a number of patented confi gurations that may include partial sulfate removal as gypsum and The removal of metals as metal sulfides is typically based recovery of excess sulfide as elemental sulfur. Suspended on the expected rate of sulfate reduction (sulfi de produc­ reactor systems require the highest level of operation and tion). The rate of removal for metalloids and uranium is maintenance. A method of removing metal precipitates not as well established and may require bench- and pilot- and excess biomass must be included as part of the over­ scale testing. all system. Sulfi de precipitation is very effective in reduc­ ing a number of metals to low levels. Reduced metalloids Optional inlet Outlet zone manifold warm Water 2" to 3" also may be effectively removed. The overall effectiveness climates surface Vegetation gravel is dependent on the capture of precipitated metals and metalloids and the stability of the microbial community as a whole. Note that high-flow events, if not bypassed, may damage the microbial community and disperse precipitates downstream, where the precipitates can be Inlet zone Inlet manifold Membrane liner Treatment Outlet dissolved. 2" to 3" cold climates or impermeable zone 1/2" to manifold gravel soils 11/2" gravel

Figure 4-6. Schematic of anaerobic wetland with subsurface flow.

Bioreaction: sulfate organic carbon ⇒ sulfide alkalinity (bicarbonate)

2 → 2CH2 O (aq) SO 4 (aq) HS (aq) H 2HCO3 (aq) Chemical reaction: sulfide metal ⇒ metal-sulfide and carbonate metal ⇒ metal-carbonate

HS Me2 → MeS H and/or

Bioreaction: oxidized metalloid organic carbon ⇒ reduced metalloid (e.g., Selenate → Elemental selenium)

Chemical reaction: reduced metalloid precipitate formation

Figure 4-5. Examples of the multiple reactions that can occur under sulfate-reducing condition.

24 Engineering Issue The construction cost for anaerobic wetlands might be The use of solid-phase substrate in a packed bed system is lower compared to active treatment. In addition, the op­ affected by the precipitation of metals, which may reduce eration and maintenance effort and cost are proportion­ the hydraulic retention time and exclude fl ow through ally lower. Costs cannot be generalized on a per-mass ba­ portions of the reactive zone. Thus, the initial sizing must sis for target contaminants because of the effect of other take this and the replacement frequency into consider­ important factors such as flow, temperature, and pH. ation. Biofilm systems constructed of rock or plastic me­ dia may allow the release of precipitated metals, and thus The precipitation of metals modifies the pore structure the effluent from these reactors should be polished via within the wetland subsurface and may reduce the effec­ gravity settling or fi ltration. These fl ushable bioreactor tive hydraulic retention time; thus, the design hydraulic systems can allow continuous use, providing the precipi­ residence time should include a suitable safety factor. Low tated metal sulfides, calcite, and biomass are fl ushed into temperature will reduce the bacterial activity and hence a collection basin at a frequency that eliminates hydraulic the rates of sulfate, metalloid, and uranium reduction. A plugging in the bioreactor. larger wetland for colder climates will be required relative to more temperate areas. Highly variable flow may result in the flushing of collected precipitates; thus, subsequent Influent Effluent polishing ponds are required. Collection of precipitates Bioreactor and sediments, and loss of permeability, will lead to the Sludge periodic need to rebuild the entire system. A

B Recycle 4.2.3.1.2 Bioreactors SRBs can be designed in a number of confi gurations (see Bioreactor Settling Figure 4-7). Configuration A simply treats the infl uent tank Effluent acidic water and allows the precipitated sludge to settle in Influent Sludge the bioreactor, which will ultimately need to be removed, probably by flushing, and appropriately managed. Confi g­ uration B allows a more convenient settling of the sludge Figure 4-7. SRB configurations. A is a simple flow-through system in a settling basin, which is more easily removed and man­ that requires periodic removal of solids from main reactor. B is a aged from the bioreactor system. In this confi guration a modification that includes a separate tank or pond for precipita­ portion of the effluent from the settling basin is recycled tion and sludge collection. to the front of the bioreactor where SRBs reduce sulfate to The optimum pH for SRB systems is 7–7.5, and the ef­ sulfi de. The sulfide-containing water is then mixed with fectiveness of SRB systems can be substantially reduced the influent acidic mine drainage. The metals precipitate when the influent acidity is high. Experience at the Levia­ (mostly as sulfides) in the settling basin, and the pH is than bioreactor, which utilizes ethanol as a carbon source raised. The flow rate of discharge is the same as the fl ow to treat an infl uent fl ow of 40 L/min, has shown that the rate of the influent. The pumps used to recycle the water system is most effective when the infl uent pH is adjusted require a power source, although the ease of sludge man­ to above pH 4.5 or higher (Tsukamoto et al., 2004). This agement will usually outweigh the power costs. has been accomplished by the addition of a 25% sodium Solid-phase organic material can also be used to provide hydroxide solution, which can be added using a solar- a carbon source and a surface for bacterial attachment. driven pump. While ethanol can be added by a simple SRBs have been used to immobilize metal cations (Cu2 , gravity fl ow, the more viscous sodium hydroxide requires Cd2, Fe2, Pb2, Zn 2) as metal sulfides, oxyanions (Cr, a positive pumping system. Se, Sb, Mo, As), and U. They also are effective in reduc­ Alternatively, the bioreactor can be operated as a sulfi de­ ing sulfate concentrations. The effectiveness for removal generating system (Figure 4-7) in which a portion of the via sulfide precipitation is dependent on both the pH and bioreactor effluent is recycled back to the front of the biore­ sulfide concentration. For typical sulfi de concentrations actor. Ethanol is added to the bioreactor, and suffi cient sul­ 3 4 (10 10 M), the effective pH for removal of iron fate remains in the water to allow the SRB system to gener­ sulfide is above pH 6.5. ate sulfide and add alkalinity. The acidic drainage is then

Engineering Issue 25 mixed with the effluent from the bioreactor in a settling surface precipitation of aluminum and iron oxide coat­ pond, and the metal sulfide precipitates and is effective­ ings on the surface that limits the availability of the cal­ ly captured in this pond. This configuration allows better cium carbonate for neutralization. Many hard-rock acidic management of the sludge and maximizes the SRB activity drainage sites have high aluminum ( 30 mg/L) concen­ by keeping the pH close to optimal. However, this con­ trations, and even when iron oxidation is inhibited by figuration also requires pumping approximately 30 – 40 having anoxic conditions, aluminum coating alone will gal/min from the settling pond to the front of the bioreac­ reduce the effectiveness since precipitation only requires tor and requires an energy source of approximately 0.5 hp. a slight increase in pH to near pH 5 to result in armoring of the limestone. SRBs offer the advantage of a lower sludge management requirement since the sulfides are precipitated as metal sulfides or as sulfur. Bioreactors can also be managed 4.2.3.2 Permeable Reactive Barriers more effectively at remote locations, with visitations of A permeable reactive barrier (PRB) is a zone of reactive 1–2 times per month, rather than daily management, as is media emplaced in the flow path of contaminated ground usually the case with conventional lime/treatment facili­ water (see Figure 4-8). The reactive media promotes the ties. Site-specific criteria will determine which treatment removal of metals and radionuclides by precipitation, option provides the most cost-effective approach. sorption, or ion exchange. The contaminants are retained The cost of SRBs varies widely from site to site and is a in the barrier and eventually are removed by excavation. A function of both the system type and the size required to related subsurface technology is a permeable reactive zone treat the site-specific concentrations and types of contam­ (PRZ) that is created by the injection of a reactive solution inants. Simple, flushable lined systems to treat up to 50 in a series of wells that transect a ground water plume. L/min can be constructed for under $200,000. The cost of the carbon source (e.g., ethanol at $US2/gal) is gener­ ally a relatively small component ( 20%) of the cost of operating a bioreactor. The cost of adding base (gener­ ally sodium hydroxide or sodium carbonate) will vary, de­ pending on the acidity. If the acidity (or flow) of the water is sufficient that the cost of raising the pH dominates the Waste cost of treatment, lime treatment will, at some point, be­ come a more cost-effective and reliable option. Water table Plume Treated water

4.2.3.1.3 Alkalinity-Producing Systems GW flow

Alkalinity-producing systems (APSs) are an integration of Permeable reactive barrier ALD systems with anaerobic sulfate-reducing biosystems. Two configurations of APSs have been developed: the suc­ Figure 4-8. Schematic of PRB system. (Source: EPA/600/R-98/125) cessive APS (SAPS) and the reducing APS (RAPS). The The reactions promoted in a PRB depend on the reac­ SAPS consists of an ALD overlaid with organic material tive media selected and the target contaminant. The main (e.g., hay and manure); the RAPS consists of an ALD in­ types of reactive media used include organic material (to tegrated with organic material. Under certain conditions, promote biogenic sulfide production) and zero-valent these systems can help increase the pH of infl uent water iron. However, media that promote sorption or ion ex­ sufficiently to allow SRB systems to better thrive, as dis­ change can also be found. Reactive media types may be cussed previously. mixed to promote the removal of multiple contaminants APSs target acidity and metals that precipitate as hy­ by different reaction mechanisms. droxides or carbonates at slightly alkaline pHs. Relative ● Sulfate-reducing biozone reactions to active treatment, APSs are inexpensive and have low ● Zero-valent iron: operation and maintenance costs. However, while they oxidation/reduction: Fe(0) Cr(VI) Fe(III) Cr(III) have shown success in certain drainages from coal sites, the applicability in hard-rock mine sites is complicated by and precipitation: Cr(III) 3OH Cr(OH)3(s)

26 Engineering Issue ● Sorption: The precipitation of metals modifies the pore structure Me2 surface site ⇐⇒ Me2 -surface complex within the reactive zone and may reduce the hydraulic re­ tention time and exclude flow through portions of the re­ ● Ion exchange: Me2 R-Ca Ca2 Me-R active zone. Solutes may be released from the dissolution Sulfate-reducing biozones have been used to immobilize of solid-phase materials, ion exchange, or desorption. heavy metal cations (Cu, Cd, Pb, Zn), oxyanions (Cr, Se, Sb, Mo, As), and U. Zero-valent iron barriers can reduce Key Web Site References and immobilize redox active compounds that include Cr, ● EPA remediation technologies development forum: U, Mo, Sb, Se, and As. Media promoting sorption or ion http://www.rtdf.org/public/permbarr/default.htm exchange can be selected to target cations (Cu2, Cd2, Pb2, Zn2, UO 2) or anions (Cr, As, Mo, Se, Sb). ● EPA hazardous waste cleanup information: 2 http://clu-in.org/techfocus/default.focus/sec/ PRZs have been achieved by the injection of reactive com­ Permeable_Reactive_Barriers/cat/Overview/ pounds into the subsurface area to be treated. Organic ● University of Waterloo, Department of Earth Sci­ compounds, such as acetate, can promote biogenic sulfi de ences, Groundwater Geochemistry and Remediation, production or biological metal reduction. Inorganic com­ Permeable Reactive Barriers: http://www.science. pounds, such as sodium dithionite, can form Fe2 from uwaterloo.ca/research/ggr/PermeableReactiveBarri­ Fe3 on aquifer material. The Fe2 can then participate ers/PermeableReactiveBarriers.html in the reduction of Cr(VI) to Cr(III). ● An example of the use of a PRB for treatment of ura­ The most important implementation issue for PRBs and nium in ground water (Chapter 16) and acid reme­ PRZs is the ability to capture the contaminated ground diation in ground water (Chapter 17): http://www. water flow within the reactive zone. The second issue is image-train.net/products/proceedings_fi rst/ the ability to promote the desired chemical reactions, giv­ en the chemical composition of the target water. A third issue is the availability of cost-effective reactive media and 4.2.3.3 Other Bioremediation Systems the frequency of media replacement. A number of non–sulfate-reducing biosystems have been At a minimum, column experiments are required to de­ used for removal of contaminants from mine water. Aer­ termine the effectiveness of a specifi c reactive media con­ obic wetlands have been commonly used for coal mine figuration and a specific ground water. The microbial and drainage and, to a lesser extent, for hard-rock mine drain­ chemical complexity of processes in the PRB and PRZ age. New systems are constantly being developed. The precludes the use of a cookbook design protocol. The hy­ microbial oxidation of elemental sulfur to sulfide for met­ drologic and geologic properties of the subsurface also al sulfide precipitation is a recently developed proprietary must be adequately characterized to assess potential ef­ process, as is a patented process for the microbial oxida­ fectiveness. Uniform mixing and emplacement of the re­ tion of manganese. New proprietary systems require inde­ active media is another critical factor, as is the ability to pendent verification of effectiveness before selection. maintain an acceptable hydraulic conductivity through­ out the reactive zone. 4.2.3.3.1 Aerobic Wetlands The cost for PRBs and PRZs varies widely from site to An aerobic wetland is a shallow water body (less than 2 ft. site. The cost is a function of both the media type and the deep) with a free water surface that supports the growth barrier or zone size required to treat the site-specifi c con­ of emergent plants, such as cattails and reeds (see Figure centrations and types of contaminants. Organic materials 4-9). The vegetation and the sediment provide surfaces are the least expensive, zero-valent iron is more expen­ for the growth of attached bacteria. Aerobic removal pro­ sive, and the most expensive are specially designed sorp­ cesses control the treatment of metals. The contaminated tive or ion exchange materials. The frequency and cost of water is intercepted and diverted through the wetland replacement will also vary with media type and the level system. of contamination. Media has been demonstrated to have a life of about 7 years, but theoretically its life could be 10 years to several decades.

Engineering Issue 27 Table 4-5 on the following pages provides an overview Water table of water treatment technologies covered in this section, technology selection factors, and limitations.

Liner Soil 4.3 Mine Pit Lake Management Lakes are typically “windows to the ground water”— Figure 4-9. Schematic of a free water surface aerobic wetland. where the land surface drops below the water table, we The reactions promoted in an aerobic wetland are primar­ “see” the water table as the surface of the lake. Mine pit ily the oxidation of iron and manganese. The rate of abi­ lakes are special cases of this phenomenon, forming in otic oxidation is increased by the presence of bacteria. open pit mines that are excavated to below the water ta­ ble. In practice, excavation below a water table requires Bio-oxidation: Fe(II) oxygen ⇒ Fe(III) water dewatering to lower the water table, leaving the open pit and chemical precipitation Fe(III) 3OH Fe(OH) (s) 3 (or “void”) within the ground water cone of depression. Bio-oxidation: Mn(II) oxygen ⇒ Mn(IV) water With cessation of dewatering, ground water flows to the and chemical precipitation Mn(IV) O2 MnO (s) center of the cone of depression, forming a lake. Steady- 2 state mine pit lakes can have (1) throughfl ow to ground Aerobic wetlands for iron and manganese removal are water (in some cases, evaporation produces concentration most amenable to near-neutral and net-alkaline waters. of ground water), (2) outflow to surface water and ground Large areas with a relatively flat topography are required water, or (3) zero outflow (a “terminal” lake, where all in­ for wetland treatment systems. The area required is a func­ flows are balanced by evaporation). tion of the mass loading of both iron and manganese. The removal of manganese requires a larger area per unit- mass The ultimate quality of the pit lake is strongly affected of manganese removed than for iron. by the surrounding wall rock. Wall rock affects pit lake water quality primarily by leaching solutes released by the The construction cost for aerobic wetlands is relatively oxidation of sulfide minerals exposed in the pit. Further, low compared to active treatment. In addition, the opera­ dewatering of sulfide zones can pull air into surrounding tion and maintenance effort and cost are proportionally aquifers, potentially inducing regional oxidation in aqui­ lower. Costs cannot be generalized on a per-mass basis for fers and increasing, temporarily at least, solutes in ground iron or manganese because of the effect of other impor­ water. The depth of rapid oxidation may be limited to tant factors such as flow, temperature, and pH. a few meters into the face. Rock that is net neutralizing Low temperature will reduce bacterial activity and hence will produce a pit lake that is relatively benign since the the rates of iron and manganese oxidation. Ice covers will problematic divalent metals concentrations will be low. also limit the rate of oxygen transfer to the wetland. High­ Arsenic, antimony, and selenium, which are mobilized at ly variable flow may result in the resuspension of settled elevated pH, can be a concern under conditions where the iron and manganese precipitates. Most of the successful pH is alkaline and insufficient iron is present to cause co­ application of aerobic wetlands has been for coal mine precipitation of these constituents. drainages, not metals mine drainages. Pit mines are by definition in areas of elevated metals, and groundwaters often contain elevated trace metals or sul­ Key Web Site References fate. When ground water is a dominant source of infl ow, ● A general discussion of passive mine water treat­ the effect of ground water quality on pit lake quality tends ments: http://www.blm.gov/nstc/library/pdf/ to increase with increasing lake size. Ground water quality TN409.PDF. can change over time, particularly given the long times re­ quired to fill lakes. Where evaporation is large and ground ● The science of acid mine drainage and passive treat­ water outflow small, lakes will concentrate solutes and can ment: http://www.dep.state.pa.us/dep/deputate/ eventually becomes sources of ground water exceeding minres/bamr/amd/science_of_amd.htm. water quality standards for TDS or other solutes.

(Continued on page 32)

28 Engineering Issue Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effl uent

Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Conventional Lime or hydrated lime is mixed as Acidity. • Requires engi­ • Generally con­ The most cost-ef­ • Requires frequent Lime 10-15% slurry and added to acidic Most diva­ neered system to sidered the most fective method for monitoring and sludge Treatment water to raise the pH of the water lent metals. effi ciently utilize proven method for treating large flows management and precipitate metals as metal Al, As, Sb, lime, including pow­ acid drainage or highly contaminat­ • Arsenic treatment effec­ oxides and sulfate as gypsum sulfate (to er, pumps, tanks, treatment ed water. Less cost tive only with a high iron- 2,000 mg/L). mixers, and lime ad­ • Depends on types effective for small to-arsenic ratio dition systems of metal loading streams due to fixed • Can treat the most costs. concentrated acidic drainages Limestone Establish open ponds or channels Acidity, Al, • Acidic water is • Variable, depending • Relatively inex­ • High aluminum and iron Ponds and that can receive acidic water. The Fe, Mn. Par­ (generally) pas­ on the aluminum, pensive and low waters will armor the Open limestone neutralizes the acids and tial metal sively added to the iron, and acidity maintenance limestone and reduce Limestone allows precipitation of a variety of removal. limestone pond or • Armoring is a • Depends on the effectiveness Channels metals as metal oxides. channel and al­ problem availability of lime­ • Precipitated sludge may lowed to react • Usually low stone and con­ require management, • Turbulent systems maintenance struction costs depending on location and improve release of regulations alkalinity • May not treat certain di­ valent metals well (Cd, Cu, and Zn) Anoxic Intercept acidic water that pri­ Acidity, Al. • Care must be taken • Shown to be use­ Relatively inexpen­ • High aluminum-containing Limestone marily has ferrous iron and pass Some metal to maintain anoxic ful for coal acidic sive. Some main­ waters will armor lime­ Drains this water through limestone beds reduction is conditions drainage, but less tenance cost is stone and decrease the

Engineering Issue under anoxic conditions. This limits observed. • Generally need so for hard-rock required if a biologi­ rate of alkalinity addition the amount of oxidation of the iron sloping topography mine drainage and cal system is used • It is diffi cult to remove all and limits the amount of precipita­ and passive trans­ heavy metals to maintain anoxic of the oxygen, so some tion on the limestone. port of water • Decreased overall conditions. iron is oxidized and tends rate of reaction to armor the limestone • Longer residence • Unless sized appropri­ times provide better ately, these systems will neutralization and not respond well to large decrease in target fl uctuations in volume or analytes infl uent water quality 29 30

Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effl uent (continued)

Technology Technology Target Critical Feasibility Factors Important Engineering Issue Name Description Analytes Implementabilty Effectiveness Cost Limitations Anaerobic Intercept surface water fl ow and Fe, Zn, Cu, • Steepness of slope • Sensitive to low • Excavation • Relatively low flows Wetlands distribute through one or more sub­ Cd, Pb, As, • Suffi cient land area temperatures • Plants and support­ • Large land areas and flat surface water wetlands Cr, Mo, • pH 5 and moder­ ing soil topography Sb, Se, U, ate metal loading • Hydraulic • Periodic sediment remov­ sulfate, low structures al and wetland reestab­ levels of lishment required acidity • Oxidation and release of metals and sulfides is probable if the wetlands become dry • Difficult to control metal migration Sulfate- Collect flow with pumps or natural Fe, Zn, Cu, • Availability of in­ • pH 5 • Growth substrate • Best for water above pH Reducing hydraulic gradient and distribute Cd, Pb, As, expensive organic • Moderate metal • Bioreactor 5; effl uent metal concen­ Bioreactors through a vessel containing growth Cr, Mo, substrates loading • Additional tanks or tration may exceed dis­ substrate (manure, wood chips, Sb, Se, U, • Power availability • Method of retaining ponds for process charge limitations when other organic waste) and sulfate- sulfate, low for active systems metal precipitates modifi cations fl ows or contaminant reducing bacteria. SRBs reduce levels of • Accessibility for • Longevity is de­ concentrations are high sulfate, raise the pH, and precipi­ acidity system pendent on carbon • Systems with media that tate metals. maintenance source and the create small pores sizes • Suffi cient land area ability of SRB to (mm) are more prone to for passive systems maintain a pH suffi­ clogging by metal ciently high to sup­ precipitates port SRB activity • Longevity is dependent on carbon available to the microbial consortium Alcohol Alcohols (e.g., ethanol) and base See above See above Alcohol and base • Higher initial costs • Although these systems Amended added to lined impoundments con­ addition can be for construction, as are more adaptable to Sulfate- taining rocks, wood chips, or other controlled and allow well as the costs of variations in flow and Reducing physical support. Bacteria use the better treatment of alcohols, base, and contaminants, monitoring Bioreactor alcohols as reducing sources for varying flows and nutrients is required to maintain the sulfate. The system is designed to contaminant loads. • Allows substantially bioreactor operation manage sludge efficiently . Sludge manage­ improved longev­ • Requires a continuous ment and hydraulic ity of the bioreactor source of carbon, base, control are improved due to lack of plug­ and planned sludge compared to the more ging and a continu­ management passive SRB systems. ous carbon source Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effl uent (continued)

Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Alkalinity- Intercept surface water fl ow and Acidity, Al. • Steepness of slope • Flow • Excavation • Experience primarily Producing distribute through a series of Some metal • Suffi cient land area • Acid-loading rate • Limestone based on coal mine Systems shallow drains containing both reduction is • Metal-loading rate • Reducing organic • Improves water quality, limestone and reducing organic observed. material but may not meet strin­ material. Metals are precipitated as • Hydraulic structure gent discharge standards metal oxides and metal carbonates. • Periodic exchange of sub­ strate required, but time frame not well established Permeable Intercept contaminated ground wa­ See above • Stability of trench • Homogeneous • Reactive material • Uncertainty in PRB life Reactive ter plume with a permeable barrier wall during emplacement of • Excavation and – affects cost of Barriers constructed of reactive material. installation barrier material or dewatering during technology (“Reducing Water flows through and contami­ • Plume width injection of reactive excavation • Periodic replenishment of Reactive nants are retained. • Depth to ground solution • Soil and ground reactive media expected, Walls”) water and bottom • Column studies water disposal from but frequency not well of aquifer required to assess construction established potential • Thickness along • Concurrent iron reduc­ effectiveness fl ow line to achieve tion may mobilize metals • pH 5 and moder­ residence time sorbed to iron mineral ate metal loading surfaces • Sulfate reduction rates ~50 mg/L-d. Rate affects cost of technology. Aerobic Intercept contaminated surface Fe, Mn, As • Steepness of slope • Near-neutral pH re­ • Excavation • Periodic sediment and Engineering Issue Wetlands water and flow through one or • Suffi cient land area quired to maximize • Plants and support­ precipitate removal and more free water surface wetlands. oxidation reactions ing soil wetland reestablishment Iron and manganese oxidation form • High-fl ow variations • Hydraulic structures required species that are less soluble and may re-suspend • Experience primarily

tend to precipitate as Fe(OH)3 and metal precipitates based on coal mine as MnO2 , respectively. Arsenic can drainage be removed by co-precipitation with iron hydroxides. 31 Numerous studies of mine pit lakes indicate that they 4.3.1 Backfilling and Neutralization behave in accordance with well-understood processes of Backfilling pits completely with waste rock or tailings can limnology (e.g., Atkins et al., 1997). The fundamental preclude the formation of a pit lake and can also provide physical process is mixing, which is a balance between permanent stable disposal of sulfidic waste rock below a wind shear acting on the surface, which tends to increase water table. However, backfilling with reactive rock typi­ mixing, and stable density stratification caused by tem­ cally produces a plume of sulfate and other solutes re­ perature and salinity gradients, which tend to inhibit leased by partial oxidation caused by handling, particu­ mixing. As a result, pit lakes that mix annually (i.e., most larly if the pH of the backfilled material is not controlled. U.S. lakes) can be approximated as stirred reactors, where Backfilling is often eliminated based on cost, generally ground water inflow is mixed into the lake each year, and over $US1/tonne, depending on site conditions, but can in most cases, the water is oxygenated at least part of the greatly exceed this cost at challenging sites. year. A chemical mass balance on solutes needs to incor­ porate loads from inflow and outflow of ground water, Partial pit backfilling is an option that is becoming in­ precipitation, surface flows, and loss to precipitation and creasingly common in precious metals pits. Particularly adsorption. This analytical solution is typically used in for large pits, partial pit backfilling can be done as part of predictive lake models. a mine plan and reduces the haulage costs of waste rock out of the pit. Reactive rock placed appropriately in the Biological productivity in lakes is superimposed on the bottom of a pit during mining will then be fl ooded when physical stratification. In natural lakes, this is primarily mining is complete and effectively eliminate further oxi­ the use of light energy by phytoplankton to convert car­ dation of the rock that is placed below the water table. bon dioxide into cell mass and oxygen. Productivity in Handling of sulfide rock produces some oxidation: reac­ natural lakes is typically limited by nutrients, particularly tion of sulfide minerals with the oxygen in the pore space phosphate. Highly productive lakes can also become an­ of backfilled waste rock will produce ~500 mg/L sulfate oxic at depth as dissolved oxygen is consumed in reac­ in the fi rst flush of water, and any additional handling-in­ tions with organic detritus from the productive surface. duced oxidation adds to this baseline. In this case, lime or In mine pit lakes, where sulfate concentrations are elevat­ some other neutralization agent can be added to maintain ed, the presence of anoxic conditions induced by elevated a neutral pH as the acids are rinsed off the rock as the organic carbon will generally result in reduction of sulfate water table recovers. The effectiveness of this option was to produce alkalinity and hydrogen sulfi de. demonstrated by treatment of a large acidic pit lake using Finally, several studies of existing mine pit lakes demon­ lime at the Sleeper Mine in Nevada. strate that they generally respond as predicted by estab­ Treatment of acidic pit lakes can be achieved using di­ lished limnologic studies. Detailed measurements of sea­ rect addition of powdered lime (CaO), hydrated lime sonal profiles in mid-latitude pit lakes show that even in (Ca(OH)2) or limestone (CaCO3), and treatment costs steep-sided lakes with high walls, the lakes stratify from can be very low (if a local source is available, limestone surface warming during the summer, then completely crushed to 2 mm can typically be obtained for $US5 – mix in fall and spring. As important, observed physical 10/tonne, yielding lake neutralization costs of a few cents stratification and biological productivity in these pit lakes per cubic meter). However, the local type of limestone matched accurately with predictions using a numerical near hard-rock mining is usually not as reactive as other model (Atkins et al., 1997). Field-scale nutrient addition forms of process neutralization agents, which could in­ has demonstrated that pit lake productivity can be reli­ crease costs. Neutralization precipitates iron and typically ably increased through the addition of limiting nutrients removes most heavy metals by co-precipitation or adsorp­ (e.g., Martin et al., 2003). tion. However, a neutralized pit lake typically contains These fundamental characteristics of lakes manage­ below 3,000 mg/L sulfate. ment—the ability to isolate denser deep layers, to induce biological productivity and sulfate reduction, and to re­ liably simulate these phenomena with models—lay the foundation for remedial strategies that treat in-situ met­ als-contaminated pit lakes and even use pit lakes as reac­ tors to treat mine effluent from other facilities.

32 Engineering Issue 4.3.2 Bioremediation and Induced Stratifi cation of metallic sulfide (e.g., FeS, CdS, CuS, and ZnS). In addi­ Mine Pit Lakes tion, enhanced biological productivity increases biomass, Water in hard-rock mine pit lakes can in some cases be which can effectively remove metals such as zinc and cad­ acidic, and regardless of pH, they can contain concen­ mium, which adsorb and settle with detritus (Martin et trations of sulfate or metals that may be problematic. al., 2003). Ideally, the long-term fate of precipitated sol­ Pit lakes vary enormously in size, from a few acre-feet utes is burial in sediments in a chemically stable form. to over 400,000 acre-feet. Remediation requirements Two fundamentally different approaches are used to in­ include monitoring only (where water quality is good), troduce organic carbon to pit lakes: single or infrequent treatment (e.g., where sulfi dic wall ● Organic carbon addition: the direct addition of rock is eventually inundated by the lake and oxidation soluble organic carbon reagents, typically alcohols, ceases), or perpetual treatment (e.g., where sulfi dic wall sugars, or organic waste, to the lake (e.g., Castro et. rock remains above the lake, loading solutes in runoff or al., 1999) by direct sloughing). In-situ treatments include stratifi ca­ ● Nutrient addition: typically phosphate and nitrate, tion, which isolates deep lake water from oxygen at the which stimulate the growth of aquatic biota (algae, surface and potential exposure to terrestrial animals, and phytoplankton, and zooplankton) near the surface biotreatment technologies, which induce mineral forma­ of the lake, producing biologic detritus that induces tion, adsorption, and/or chemical reduction reactions reducing conditions at depth as it settles through the that remove metals from solution. These technologies, lake (Pederson et al., 2003; Poling et al., 2003) often combined, offer lower-cost options for closure and management of mine pit lakes. Both treatments can result in rapid production of bio­ mass, producing organic detritus that can adsorb and set­ In-situ bioremediation induces chemically reducing con­ tle out dissolved metals. ditions in lakes that remove target analytes by either transforming them to another form (e.g., acidity, sulfate) Where sulfide production is desired, anoxic conditions or inducing them to precipitate as insoluble minerals that must be created and maintained long enough to allow the settle out of solution (e.g., heavy metal sulfi des). Biore­ biologically induced reactions between organic carbon mediation is a relatively well-established alternative for and sulfate. This is where stratification is required—an­ treatment of mine pit lakes (Castro and Moore, 2000) oxic conditions generally require that a lake be stratifi ed and has been successfully demonstrated in microcosm (thermally and/or chemically) during at least part of a year (Frömmichen et al., 2004) and full-scale (Poling et al., so that a deep anoxic zone can form in isolation from the 2003) applications. Specific reactions include biologically atmosphere. Thermal stratification generally occurs each induced reduction of sulfate to sulfide in a lake (Castro et summer in temperate climates and can be a long-term al., 1999), which leads to precipitation of dissolved met­ natural condition in very cold or tropical climates. More als as sulfide (CdS, CuS, PbS) and reduction to a less sol­ stable stratification can also be induced by actively main­ uble reduced form [U(VI) to U(V), Sb(V) to Sb(III), or taining a layer of less dense water [e.g., warmer and/or less Cr(VI) to Cr(III)]. Chemical reactions involve reduction saline than the deep water (Poling et al., 2003)] on a lake. of a target analyte by organic carbon. Example reactions Direct carbon source addition has higher material costs, (using CHO to represent organic carbon source) include but the treatment is generally rapid (reactions completed reduction of sulfate to sulfide, which can also be used to over a few seasons) and may thus be best where infrequent neutralize acidity (Frömmichen et al., 2004): treatment is required. Nutrient addition has much lower 2 → 2CH2O (aq) SO 4 (aq) 2H H2S (aq) 2CO2 (aq) 2H 2O material costs, but it generally requires longer treatment times and a more detailed analysis of lake limnology, and and reduction of metals to a less soluble form: it may be more practical where long-term management 2CHO SbO3 HCO3 Sb(OH)3(S) is anticipated. Both methods have been demonstrated in full-scale applications. Target analytes are then removed from solution by being Finally, developing technologies such as metal-specifi c converted to a reduced form that precipitates as oxides microbes that precipitate arsenic and selenium as sulfi des

(e.g., UO2), hydroxides [e.g., Sb(OH)3, Cr(OH)3)], or in very low-volume sludges may offer potential for more

Engineering Issue 33 targeted treatment of metalloids. These are noted as pos­ Carbohydrate Addition sible future remedies for these often diffi cult-to-treat met­ Site Name and Location Gilt Edge Pit Lake, South Dakota, alloids, but are not addressed further here. USA. (Arcadis, Inc., not published) For nutrient addition, the effectiveness of inducing organ­ Experimental Design In-situ pit lake (volume 65 million gallons, dimictic lake). NaOH (125 ic carbon formation with nutrients can be estimated using tons, to increase pH), alcohol, and standard engineering relationships for lakes (Thomann sugar in three stages over a summer and Mueller, 1987; Martin et al., 2003). In practice, the (producing ~100 mg/L initial dissolved effectiveness of nutrient addition will be limited in part organic carbon in the lake). Duration of monitoring: 3 years. by the organic carbon production rate, and the effi ciency Results Cadmium, copper, lead, nickel, arse­ of sulfate reduction depends on the reaction rate, tem­ nic, selenium, and zinc decreased perature, detritus settling rate, and reactivity of organic from above to below treatment carbon in the sediment. Application of nutrient-addition objectives after treatment, including treatment should anticipate a site-specific pilot test, nu­ copper from 20 to 0.05 mg/L, cad­ mium from 0.2 to 0.02 mg/L, and zinc merical modeling to estimate dose rates, and several years from 5 to 0.9 mg/L. Monitoring for of active treatment. excess sulfide in the pit lake during treatment was identified as an impor­ For organic carbon addition, dosing depends on the stoi­ tant issue. chiometry between organic carbon and the desired target Site Name and Location Koyne/Plessa lignite field, Germany reactions, adding suffi cient organic carbon to remove ox­ (Frömmichen et al., 2004) ygen and then producing suffi cient sulfide to precipitate Experimental Design Laboratory microcosm. Ethanol, sug­ the heavy metals in solution. Direct reduction of specifi c ar industry byproduct (Carbocalk), elements to less soluble forms (e.g., U, Cr, Se, As, Sb) is and wheat straw dosed at 3.9 kg/m2 Carbocalk and 9.3 kg/m2 wheat straw. less widely described and may require pilot-scale demon­ Duration of monitoring: 1 year. stration. If the water quality in the pit is suffi ciently poor Results pH increased from 2.6 to 6.5, neutral­ that the microbial community cannot thrive, alternative, ization rate 6 to 15 equiv/m2-yr pre-biological treatments may be necessary.

For induced stratification, a supply of less dense water for Nutrient Addition maintaining a capping layer is generally required. This Site Name and Location Island Copper Mine Pit Lake (Poling can be warmer water (e.g., power plant cooling water) or et al., 2003) less saline water (e.g., fresh water over a saline lake). The Experimental Design Field-scale pit lake; water volume viability of maintaining an isolated deep layer of dense 241,000,000 m3 (and ~5 million m3 ARD water can be evaluated with a numerical limnologic mod­ added to deep layer); permanently el [e.g., CEQUAL/W2 (Cole and Buschek, 1995)] using stratified with seawater hypolim­ site-specific parameters for bathymetry, water salinities, nion; brackish epilimnion [a 5-m thick brackish layer is maintained over a and climate. more saline (seawater) hypolimnion]. The contaminant load was moderate, with a range of 5 – 10 mg/L heavy Performance and Cost Data metals and 500 – 2,000 mg/L sulfate. Examples are provided at right that demonstrate the tech­ Liquid nitrate and phosphate (N:P nology in practice for carbohydrate additions (commonly 6:1) added every 10 days to surface using a small boat. Duration of moni­ sugar industry byproducts) and nutrient additions (typi­ toring: 6 years. cally nitrate and phosphate). Results Treatment produced effective remov­ al of zinc, copper, and cadmium from the lake while maintaining accept­ able water quality in the epilimnion layer. Ongoing treatment is estimated at $100,000/yr and is treating be­ tween 4 and 6 million m3/yr of acidic infl ow.

34 Engineering Issue Nutrient Addition (continued) 5.0 CONCLUSION Site Name and Location Equity Silver Mine, British Columbia, Each mine disturbance that is the source of contami­ Canada (Martin et al., 2003) nated water requires careful consideration of site-specifi c Experimental Design Microcosm using “limnocorals” in characteristics prior to choosing a strategy to manage the a dimictic existing pit lake. Addition of 0.7, 1.4, and 14 mmole P/m2/week. water. The large majority of mine drainages will require Duration: 1 year. long-term treatment, on the order of decades and be­ Results High nutrient loading produced dra­ yond. Few walk-away options are available, and fi nancial matic increase in algal productivity in requirements for in-perpetuity treatment are a signifi cant the epilimnion and efficient removal component to the decision on which treatment option to of metal cations (e.g., zinc from use. Site characterization is critical and should address the 150 to 20 µg/L; copper from 3 to 0.1 µg/L, cadmium from 6 to 2 µg/L, and following questions: nickel from 15 to 5 µg/L). The removal ● mechanism is adsorption of metals to What is the potential for reducing the flow of the biogenic particles, which then settle water? out. ● What is the highest volume of water that will need Following several successful full-scale applications, in- to be treated during major events? situ bioremediation of mine pit lakes appears to be rela­ ● What is the water quality, and how does it vary tively well accepted by the scientific community, indus­ seasonally? try, and regulators. Successful carbohydrate treatments ● What are the regulatory discharge requirements? have been demonstrated using natural organic car­ bon (Frömmichen et al., 2004) and alcohols plus sugar Many types of rock will only go acidic after several years, (http://www.arcadis-us.com). Nutrient addition with in­ and the rate of acid generation will change over time, of­ duced stratification is providing ongoing treatment at the ten increasing for several years as the oxidizing bacteria Copper Island Mine (Poling et al., 2003). Limnologic become widespread. What level of data are required to models are mature and have demonstrated the ability to accurately predict these changes? reliably predict physical mixing and biological productiv­ Finally, each treatment technology presents different fi ­ ity in mine pit lakes. In-situ bioremediation of pit lakes nancial and treatability considerations that may require offers the potential in some cases for much lower cost pilot-scale testing in the field, in order to demonstrate treatment, particularly using nutrient addition. However, that it will indeed treat the mine water to acceptable dis­ this remains a research area, with site-specifi c conditions charge limits over the long term. The state and federal dramatically affecting implementability. Potential cost regulatory agencies, public interest organizations, and the savings thus need to be weighed against current uncer­ mining industry all are increasingly focused on issues re­ tainty and associated higher potential costs for research lated to mine water treatment. This emphasis is unlikely and characterization. to go away, since the long term treatment costs can be Table 4-6 on pages 36 and 37 provides an overview of pit very high. Confidence that a treatment option will actu­ lake treatment technologies covered in this section, tech­ ally do the job requires continual technical and fi nancial nology selection factors, and limitations. evaluation of each option, public release and dissemina­ tion of treatment data, and continued research on new methods for mine water treatment.

Engineering Issue 35 36

Table 4-6. Pit Lake Treatment Technologies for Hard-Rock Mining

Engineering Issue Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Induced Lake is stratified, using either a Primarily • Most lakes strat­ • Metal cation re­ • Materials can be • Inducing and maintain­ Stratifi cation low-salinity cap layer over saline heavy met­ ify naturally each moval is typically signifi cant cost ing stratification requires and Bio­ lake or warm-layer cool hypolimni­ als: Cd, Zn, summer, simplify­ effective • Carbohydrate (sugar dense deep water (saline remediation on. Organic carbon can be created Cu, Pb, Ni, ing isolation of the • Removal by adsorp­ or alcohols): ~$0.5 or cold) and a supply of by adding nutrients, and metals U. Possibly hypolimnion tion to detritus may – 1.0 per kg low-density water (fresh adsorb and settle with organic de­ metalloids • Salinity stratifi ca­ require several • Ammonium poly- and/or warm) for surface tritus. Alternatively, direct carbo­ As, Sb, Se. tion requires saline seasons sulfate (10-34-0) layer. Inducing reducing

hydrate addition can produce H2 S, Possibly lake and fresh-wa­ • Metalloid removal solution and urea conditions can mobilize precipitating metals as metallic-sul­ SO4 . ter source mechanisms are ammonium nitrate metals in sediments. fi de minerals. • Production and re­ not well known (28-0-0) solution • Several seasons of treat­

lease of excess H2 S • Long-term stability prices depend on ment may be required to the atmosphere of metals in sedi­ local availability • Carbohydrate addition is can be a health risk ments uncertain; patented • Access to lake is periodic re-treat­ • Sulfide production must required for reagent ment may be be closely controlled to addition required avoid health risk Backfi lling: Waste rock and/or tailings are used Eliminates Requires proximal • Reduces or elimi­ • Depends strongly on Sometimes difficult to ac­ Partial or to partially or completely fill the pit. surface ex­ source of waste rock nates further oxida­ the mine plan curately predict the water Complete Removes open surface water and posure to all or tailings for tion of rock below • Costs are low if quality that will result from access to humans and wildlife. Re­ analytes backfi lling water table partial backfill oc­ rinsing backfilled mate­ active backfill may require amend­ • Reduces the curs during mining. rial. Can degrade quality ment to reduce acidity or other water volume in the Backfi lling from in throughflowing ground solute release. pit—important in rock outside the water. arid areas pit at $1/tonne or higher. Accelerated Surface or ground water is used to All analytes Requires access to Can significantly During refilling, con­ • Rapid pit lake refilling Filling rapidly refill pit. This reduces the associated source of water. River improve water quality tinuous operating may force poor water time for sub-aerial wall rock oxida­ with wall diversion can allow over what would have ground water pumps back into the ground wa­ tion and may reduce the rinsing of rock oxida­ rapid filling, while existed by refilling by are typically the pri­ ter system reactive rock surfaces into the pit tion, i.e., sul­ ground water pump­ ground water recov­ mary cost • Appropriate monitoring is lake. fate, heavy ing is typically slower ery. However, a rapid required to fully under­ metals, and and more expensive. volume increase stand the impacts to aqui­ metalloids could force treat­ fer surrounding the pit ment earlier in time, increasing costs. Table 4-6. Pit Lake Treatment Technologies for Hard-Rock Mining (continued)

Technology Technology Target Critical Feasibility Factors Important Name Description Analytes Implementabilty Effectiveness Cost Limitations Neutralization Lime or other neutralizing agents Primarily Well-demonstrated Metals removal is Highly variable cost • Lime added to the surface are added to the pit lake. Adequate heavy met­ technology using similar to what is depending on lake may become coated, re­ mixing (natural turnover or multi­ als: Cd, Zn, lime addition from observed with con­ acidity, cost for de­ ducing efficiency level injection) is required to com­ Cu, Pb, and either floating barge ventional lime treat­ livered lime, and the • Oxidation of ferrous iron pletely mix in oxygen (to oxidize Fe) Ni. Possibly or amended inflow ment—very effective method used to add is necessary for effective to neutralize acidity throughout the metalloids water for acidity and metal the lime to the lake iron removal lake depth profile. As, Sb, Se, cations, less effective • Sulfate concentrations and sulfate. for oxyanion metal­ are below ~ 3,000 mg/L loids (e.g., As, Sb, Se) Engineering Issue 37 6.0 ACKNOWLEDGMENTS 7.0 ACRONYMS AND ABBREVIATIONS This Engineering Issues document was prepared for the ABA acid base accounting U.S. Environmental Protection Agency, Office of Re­ ACMER Australian Center for Mining search and Development, National Risk Management Environmental Research Research Laboratory by Science Applications Interna­ ADTI Acid Drainage Technology Initiative tional Corporation (SAIC) under Contract No. 68-C-02­ 067. Mr. Doug Grosse served as the EPA Work Assign­ AGP acid-generating potential ment Manager. Mr. Ed Bates acted as the EPA Technical ALD anoxic limestone drain Project Manager. Ms. Lisa Kulujian was SAIC’s Work As­ AMD acid mine drainage signment Manager and Mr. Kyle Cook served as SAIC’s ANP acid-neutralizing potential technical lead. The primary authors of this document ANSTO Australian Nuclear Science and Technology were as follows: Glenn C. Miller (lead author), University Organization of Nevada, Reno; Houston Kempton, Integral Consult­ ing, Inc., Boulder, Colorado; Linda Figueroa, Colorado APS alkalinity-producing system School of Mines; and John Pantano, Consultant, Butte, ARD acid rock drainage Montana. ASTM American Society for Testing and Materials Reference herein to any specifi c commercial products, BC British Columbia process, or service by trade name, trademark, manufac­ BLM U.S. Bureau of Land Management turer, or otherwise, does not necessarily constitute or im­ CN cyanide ply its endorsement, recommendation, or favoring by the DOE U.S. Department of Energy United States Government. The views and opinions of authors expressed herein do not necessarily state or refl ect EPA U.S. Environmental Protection Agency those of the United States Government, and shall not be ETSC U.S. EPA Engineering Technical Support used for advertising or product endorsement purposes. Center For additional information, contact the ORD Engineer­ ICARD International Conference on Acid Rock ing Technical Support Center (ETSC): Drainage INAP International Network for Acid Prevention David Reisman, Director U.S. EPA Engineering Technical Support Center MEND Mine Environmental Neutral Drainage NRMRL NAG net acid-generating (test) 26 W. Martin Luther King Drive MLK-489 NDEP Nevada Division of Environmental Cincinnati, OH 45268 Protection (513) 487-2588 NMA National Mining Association NNP net-neutralizing potential NRC National Research Council O/M operation/maintenance OSC on-screen coordinator OSM U.S. Office of Surface Mining PIRAMID Passive In-Situ Remediation of Acidic Mine/ Industrial Drainage PRB permeable reactive barrier PRZ permeable reactive zone RAMS Restoration of Abandoned Mine Sites RAPS reducing alkalinity-producing system RCRA Resource Conservation and Recovery Act

38 Engineering Issue RCTS Rotating Cylinder Treatment System 8.0 REFERENCES RPM remedial project manager Andrina, J., S. Miller, and A. Neale. “The design, con­ SAIC Science Applications International struction, instrumentation and performance of a full­ Corporation dcale overburden stockpile trial for mitigation of acid SAPS successive alkalinity-producing system rock drainage, Grasberg Mine, Papua Province, Indone­ SME Silica Micro Encapsulation sia.” In Proceedings of the 6th International Conference on Acid Rock Drainage, Cairns, Australia, 2003. SPLP Synthetic Precipitation Leaching Procedure SRB sulfate-reducing bioreactor ASTM. “Standard Test Methods for Analysis of Metal Bearing Ores and Related Materials by Combustion In­ SRK SRK Consulting Engineers and Scientists frared Absorption Spectrometry, Method E1915-01,” TCLP Toxicity Characteristic Leaching Procedure 2003. http://www.astm.org TDS total dissolved solids Atkins, D., J. H. Kempton, and T. Martin. “Limnologic Tonne metric ton conditions in three existing Nevada pit lakes: observations UNR University Nevada–Reno and modeling using CE-QUAL-W2.” In Proceedings of WAD weak acid dissociable the 4th International Conference on Acid Rock Drainage, Vancouver, BC, Canada, 1997. Barbour, L. Personal communication. Short course: “Un­ derstanding waste rock facilities in dry and wet climates.” In Proceedings of the 5th International Conference on Acid Rock Drainage, Denver, CO, Society for Mining, Metal­ lurgy and Exploration Inc., 2000. Bennett, J. W. Personal communication. Australian Nu­ clear Science and Technology Organization, Menai, New South Wales, Australia, 1998. BLM. Acid Rock Drainage Policy for Activities Authorized under 43 CFR 3802/3809, Instructional Memorandum no. 96-79. U.S. Bureau of Land Management, 1996. Brady, K., M. W. Smith, R. L. Beam, and C. A. Cravotta. “Effectiveness of the use of alkaline materials at surface coal mines in preventing or abating AMD: part 2. Mine site case studies.” In Proceedings of the 1990 Mining and Reclamation Conference, West Virginia University, Mor­ gantown, WV, 1990. Castro, J. M., and J. N. Moore. Pit lakes: their charac­ teristics and the potential for their remediation. Environ­ mental Geology 39 (11): 1254-1260 (2000). Castro, J. M., B. W. Wielinga, J. E. Gannon, and J. N. Moore. Stimulation of sulfate-reducing bacteria in lake water from a former open-pit mine through addition of organic wastes. Water Environmental Resources 71 (2): 218-223 (1999).

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40 Engineering Issue Morin, K. A., N. M. Hutt, and K. G. Ferguson. “Mea­ Ritchie, I. A. M. “Rates and mechanisms that govern pol­ sured rates of sulfide oxidation and acid neutralization lutant generation from pyritic wastes.” In Environmental in kinetic tests: statistical lessons from the database.” In Geochemistry of Sulfi de Oxidation, C. N. Alpers and D. W. Proceedings Sudbury ’95—Mining and the Environment, T. Blowes (editors), Washington, D.C.: American Chemical P. Hynes and M. L. Blanchette, eds., Canmet, Ottawa, Society, 1994. Canada, 1995. Romano, C. G., K. Ulrich Mayer, D. R. Jones, D. A. Morin, K. A., and N. Hutt. Control of Acidic Drainage Ellerbroek, and D. W. Blowes. Effectiveness of various in Layered Waste Rock at the Samatosum Minesite: Labora­ cover scenarios on the rate of sulfide oxidation of mine tory Studies and Field Monitoring. Draft MEND Report tailings. Journal of Hydrology (271): 171-187 (2003). 2.37.3. 1996. Sobek, A. A., W. A. Schuller, J. R. Freeman, and R. M. NDEP. Waste Rock Evaluation. Carson City, NV: Bureau Smith. Field and Laboratory Methods Applicable to Over­ of Mining Regulation and Reclamation, Nevada Division burdens and Mine Soils, EPA 600/2-78-054. Cincinnati, of Environmental Protection, 1990. OH: U.S. Environmental Protection Agency, 1978. NDEP. Quarterly Reports for the Sleeper Pit Lake. Car­ SRK Consulting. Draft Acid Rock Drainage Technical son City, NV: Bureau of Mining Regulation and Recla­ Guide, Volume 1. British Columbia Acid Mine Drain­ mation, Nevada Division of Environmental Protection, age Task Force Report. Prepared by Steffen Roberson and 1998-2004. Kirsten (B.C.) Inc. Vancouver, BC, Canada: SRK Con­ sulting, 1989. O’Kane, M, and S. L. Barbour. “Predicting fi eld perfor­ mance of lysimeters used to evaluate cover systems for Su, C., and R. W. Puls. Arsenate and arsenite removal by mine waste.” In Proceedings of the 6th International Con­ zero-valent iron: kinetics, redox transformation and im­ ference on Acid Rock Drainage, Cairns, Australia, 2003. plications for in situ groundwater remediation. Environ­ mental Science & Technology (35): 1487-1492 (2001). O’Kane Consultants. Evaluation of the Long-Term Per­ formance of Dry Covers: Final Report. OKC report 684­ Telesto Solutions. Heap Leach Closure Plan, Florida Can­ 02. March 2003. http://www.inap.com.au/completed_ yon Mine. A report presented to Florida Canyon Mine as research_projects.htm part of the permitting process. Nevada: Available from the Winnemucca District of the BLM, 2003. OSM. Methods for Estimating the Costs of Treatment of Mine Drainage. Prepared by Tetra Tech EM Inc. U.S. Thomann, R. V., and J. A. Mueller. Principles of Sur­ Office of Surface Mining, 2000. http://amd.osmre.gov/ face Water Quality Modeling and Control, New York, NY: Cost.pdf Harper & Row, Inc., 1987. Pedersen, T. F., J. Crusius, A. J. Martin, J. McNee, and P. Tsukamoto, T. K., H. Killian, and G. C. Miller. Column Whittle. “Pit lakes as mine-waste remediation cells: pluses experiments for microbiological treatment of acid mine and pitfalls.” Presented at the Annual Meeting of the Geo­ drainage; low temperature, low pH, and matrix investiga­ logical Society of America, Seattle, WA, November 2–5, tions. Water Research (38): 405-1418 (2004). 2003. http://gsa.confex.com/gsa/2003AM/fi nalprogram/ USGS. Value of Nation’s Mineral Production Still on the abstract_64346.htm Rise in 2005. United States Geological Survey, 2006. Poling, G. W., C. A. Pelletier, D. Muggli, M. Wen, J. http://www.usgs.gov/newsroom/article.asp?ID=1431. Gerits, C. Hanks, and K. Black. “Field studies of semi- Wilson, J. A., G. W. Wilson, and D. G. Fredlund. “Nu­ passive biogeochemical treatment of acid rock drainage at merical modeling of vertical and inclined waste rock lay­ the Island Copper Mine pit lake.” In Proceedings of the 6th ers.” In Proceedings of the 5th International Conference on International Conference on Acid Rock Drainage, Cairns, Acid Rock Drainage, Vol. 1, Denver, CO: Society for Min­ Australia, 2003. ing, Metallurgy and Exploration Inc., 2000a, 257-268.

Engineering Issue 41 Wilson, G. W., L. L. Newman, and K. D. Ferguson. “The co-disposal Of waste rock and tailings.” In Proceedings from the 5th International Conference on Acid Rock Drain­ age, Vol. 2, Denver, CO: Society for Mining, Metallurgy and Exploration Inc., 2000b, 789-796. Wilson, G. W., H. K. Plewes, D. Williams, and J. Robert­ son. “Concepts for co-mixing of tailings and waste rock.” In Proceedings of the 6th International Conference on Acid Rock Drainage, Cairns, Australia, 2003. Ziemkoewicz, P. F., J. G. Skousen, D. L. Brant, P. L. Stern­ er, and R. J. Lovett. Acid mine drainage treatment with armored limestone in open limestone channels. Journal of Environmental Quality (26): 1017-1024 (1997).).

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