CHAPTER 16.5 Mitigating Acid Rock Drainage


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CHAPTER 16.5 Mitigating Acid Rock Drainage

  1. 1. CHAPTER 16.5 Mitigating Acid Rock Drainage Rens VerburgINTRODUCTION to select the option that has the most desirable combinationSustainable mining requires the prevention, mitigation, man- of attributes (e.g., protectiveness, regulatory acceptance, com-agement, and control of mining impacts on the environment. munity approval, cost). Mitigation measures implemented asAcid rock drainage (ARD) continues to be one of the most part of an effective control strategy should require minimalserious and visible environmental issues facing the mining active intervention and management.industry because it is often the transport medium for a range Proper mine characterization, drainage-quality prediction,of pollutants, which may affect on-site and off-site water and mine waste management can prevent ARD formation in mostresources, and associated human and ecological receptors. cases and minimize ARD formation in all cases. Prevention ofThe impacts of ARD on near and distant water resources and ARD must commence at exploration and continue throughoutreceptors can also be long term and persist after mine clo- the mine life cycle. Ongoing ARD planning and management issure. Therefore, ARD prevention, mitigation, and treatment critical to the successful prevention of ARD.are important components of overall mine water management Stopping ARD formation, once initiated, may be chal-over the entire life of a mining operation. lenging because it is a process that, left unimpeded, will con- This chapter addresses the prediction, prevention, and tinue (and may accelerate) until one or more of the reactantstreatment of ARD. A comprehensive approach to ARD man- (sulfide minerals, oxygen, water) are exhausted or excludedagement reduces the environmental risks and subsequent from reaction. The ARD formation process can continue tocosts for the mining industry and governments, reduces produce impacted drainage for decades or centuries after min-adverse environmental impacts, and promotes public support ing has ceased, as is illustrated by the portal in Spain shown infor mining. The extent and particular elements of the ARD Figure 16.5-2, which dates from the Roman approach that should be implemented at a certain The cost of ARD remediation at orphaned mines in Northoperation will vary based on many site-specific factors, not America alone has been estimated in the tens of billions oflimited to the project’s potential to generate ARD. U.S. dollars. Individual mines can face postclosure liabilities Acid rock drainage is formed by the natural oxidation of of tens to hundreds of million dollars for ARD remediationsulfide minerals when exposed to air and water. Activities that and treatment if the sulfide oxidation process is not properlyinvolve the excavation of rock with sulfide minerals, such as managed during the mine’s life.metal and coal mining, accelerate the process. ARD results This chapter draws heavily from and follows the generalfrom a series of reactions and stages that typically proceed from structure of the Global Acid Rock Drainage Guide (GARDnear-neutral to more acidic pH conditions. When sufficient base Guide), a state-of-practice summary of the best practicesminerals are present to neutralize the ARD, neutral mine drain- and technologies. It was developed under the auspices of theage or saline drainage may result from the oxidation process. International Network for Acid Prevention (INAP) to assistNeutral mine drainage (NMD) is characterized by elevated ARD stakeholders, such as mine operators, regulators, com-metals in solution at approximately neutral pH, whereas saline munities, and consultants, with addressing issues related todrainage (SD) contains high levels of sulfate at neutral pH with- sulfide mineral oxidation (INAP 2009). Readers are encour-out significant dissolved metal concentrations. Figure 16.5-1 aged to make use of the GARD Guide and its references forillustrates the various types of drainage schematically. further detail on the subjects covered in this chapter. A risk-based planning and design approach forms thebasis for prevention and mitigation. This approach is applied FORMATION OF ACID ROCK DRAINAGEthroughout the mine life cycle but primarily in the assessment The process of sulfide oxidation and formation of ARD isand design phases. The risk-based process aims to quantify the very complex and involves a multitude of chemical and bio-long-term impacts of alternatives and to use this knowledge logical processes that can vary significantly depending on Rens Verburg, Principal Geochemist, Golder Associates, Inc., Redmond, Washington, USA 1721
  2. 2. 1722 SME Mining Engineering Handbook Typical Relation to Drainage pH Saline Drainage Neutral Mine Drainage Acid Rock Drainage pH 2 3 4 5 6 7 8 9 10 Typical Drainage Characteristics Acid Rock Drainage Neutral Mine Drainage Saline Drainage • Acidic pH • Near neutral to alkaline pH • Neutral to alkaline pH • Moderate to elevated metals • Low to moderate metals. • Low metals. May have • Elevated sulfate May have elevated zinc, moderate iron. • Treat for acid neutralization cadmium, manganese, • Moderate sulfate, and metal and sulfate removal. antimony, arsenic, or magnesium, and calcium. selenium. • Treat for sulfate and • Low to moderate sulfate sometimes metal removal. • Treat for metal and sometimes sulfate removal. Source: INAP 2009. Figure 16.5-1 Types of drainage produced by sulfide oxidation Fe(II) + S22– [1a] [1a] + O2 + H2O [1] FeS2(s) + O2 + H2O SO42– + Fe(II) + H+ Fast + O2 [3] [2] + FeS2(s) Slow Fe(III)@Fe(OH)3(s) + H+ [4] Source: Stumm and Morgan 1981. Figure 16.5-3 Model for the oxidation of pyrite majority of ARD (Stumm and Morgan 1981). The reactions shown are schematic and may not represent the exact mecha- nisms or reaction stoichiometry, but the illustration is a useful visual aid for understanding sulfide oxidation. The chemical reaction representing pyrite oxidation (Reaction 1 in Figure 16.5-3) requires three basic ingredi-Figure 16.5-2 Roman portal with acid rock drainage—Spain ents: pyrite, oxygen, and water. This reaction can occur either abiotically or biotically (i.e., mediated through microorgan- isms). In the latter case, bacteria such as Acidithiobacillusenvironmental, geological, and climate conditions (Nordstrom ferrooxidans, which derive their metabolic energy from oxi-and Alpers 1999). Sulfide minerals in ore deposits are formed dizing ferrous to ferric iron, can accelerate the oxidation reac-under reducing conditions in the absence of oxygen. When tion rate by many orders of magnitude relative to abiotic ratesexposed to atmospheric oxygen or oxygenated waters due to (Nordstrom 2003). In addition to direct oxidation, pyrite canmining, mineral processing, excavation, or other earth-moving also be dissolved and then oxidized (Reaction 1a).processes, sulfide minerals can become unstable and oxidize. Under the majority of circumstances, atmospheric oxy- Figure 16.5-3 presents a simplified model that describes gen acts as the oxidant. However, aqueous ferric iron can oxi-the oxidation of pyrite, the sulfide mineral responsible for the dize pyrite as well according to Reaction 2. This reaction is
  3. 3. Mitigating Acid Rock Drainage 1723 Reactions in Stages I and II 9 FeS2 + 7⁄2O2 + H2O @ Fe2+ + 2SO42– + 2H+ [1] e.g., Carbonates Fe2+ + ¼O2 + H+ @ Fe3+ + ½H2O [3] 8 Fe2+ + ¼O2 + 5⁄2H2O @ (Fe(OH)3 + 2H+ [4] pH in Microenvironment Around Minerals 7 pH Plateaus Resulting from Minerals 6 Buffering at Various pH Values e.g., Gibbsite Stage I 5 4 e.g., Ferrihydrite Stage II 3 e.g., Aluminosilicates Stage III 2 1 Reactions in Stage III Lag Time Fe2+ + ¼O2 + H+ @ Fe3+ + ½H2O [3] 0 FeS2 + 14Fe3+ + 8O2 @ 15Fe2+ + 2SO42– + 16H+ [2] Time Source: INAP 2009. Figure 16.5-4 Stages in the formation of ARDconsiderably faster (two to three orders of magnitude) than the enargite) generate acid when they react with oxygen and water.reaction with oxygen, and generates substantially more acid- Sulfides with metal/sulfur ratios equal to 1 (e.g., sphalerite,ity per mole of pyrite oxidized. However, this reaction is lim- galena, chalcopyrite) tend not to produce acidity when oxygenited to conditions in which significant amounts of dissolved is the oxidant. Therefore, the acid generation potential of anferric iron occur (i.e., acidic conditions: pH 4.5 and lower). ore deposit or mine waste generally depends on the amount ofOxidation of ferrous iron by oxygen (Reaction 3) is required iron sulfide present. However, when aqueous ferric iron is theto generate and replenish ferric iron, and acidic conditions are oxidant, all sulfides are capable of generating acidity.required for the latter to remain in solution and participate in Neutralization reactions also play a key role in determin-the ARD production process. As indicated by this reaction, ing the compositional characteristics of drainage originatingoxygen is needed to generate ferric iron from ferrous iron. from sulfide oxidation. As for sulfide minerals, the reactiv-Also, the bacteria that may catalyze this reaction (primarily ity, and accordingly the effectiveness with which neutraliz-members of the Acidithiobacillus genus) demand oxygen for ing minerals are able to buffer any acid being generated, canaerobic cellular respiration. Therefore, some nominal amount vary widely. Most carbonate minerals are capable of dissolv-of oxygen is needed for this process to be effective, even when ing rapidly, making them effective acid consumers. However,catalyzed by bacteria, although the oxygen requirement is hydrolysis of dissolved Fe or Mn following dissolution ofconsiderably less than for abiotic oxidation. their respective carbonates and subsequent precipitation of a A process of environmental importance related to ARD secondary mineral may generate acidity. Although generallygeneration pertains to the fate of ferrous iron resulting from more common than carbonate phases, aluminosilicate min-Reaction 1. Ferrous iron can be removed from solution under erals tend to be less reactive, and their buffering may onlyslightly acidic to alkaline conditions through oxidation and succeed in stabilizing the pH when rather acidic conditionssubsequent hydrolysis and the formation of a relatively insol- have been achieved. Calcium–magnesium silicates have beenuble iron (hydr)oxide (Reaction 4). When Reactions 1 and 4 known to buffer mine effluents at neutral pH when sulfide oxi-are combined, as is generally the case when conditions are not dation rates were very low (Jambor 2003).acidic (i.e., pH > 4.5), oxidation of pyrite produces twice the The combination of acid generation and acid neutraliza-amount of acidity relative to Reaction 1 as follows: tion reactions typically leads to a stepwise development of ARD (Figure 16.5-4). Over time, pH decreases along a series FeS 2 + 15 4O 2 + 7 2H 2 O = Fe ]OHg3 + 2SO 2− + 4H + 4 of pH plateaus governed by the buffering of a range of min- eral assemblages. The lag time to acid generation is a verywhich is the overall reaction most commonly used to describe important consideration in ARD prevention. It is far morepyrite oxidation. effective (and generally far less costly in the long term) to Although pyrite is by far the dominant sulfide responsible control ARD generation during its early stages. The lag timefor the generation of acidity, different ore deposits contain also has significant ramifications for interpretation of testdifferent types of sulfide minerals. Not all of these sulfide results. Because the first stage of ARD generation may lastminerals generate acidity when being oxidized. As a gen- for a very long time, even for materials that will eventually beeral rule, iron sulfides (pyrite, marcasite, pyrrhotite), sulfides highly acid generating, it is critical to recognize the stage ofwith molar metal/sulfur ratios less than 1, and sulfosalts (e.g., oxidation when predicting ARD potential. The early results
  4. 4. 1724 SME Mining Engineering Handbook Exploration Assessment Design Construction Operation Closure Postclosure Corporate Regulatory and Community Context Framework Environmental, Social, and Economic Setting ARD Risk Mine Environmental Management Source: INAP 2009. Figure 16.5-5 Conceptual ARD management frameworkof geochemical testing, therefore, may not be representative Many mining companies have established clear corporateof long-term environmental stability and associated discharge guidelines that represent the company’s view of the prioritiesquality. However, early test results provide valuable data to to be addressed and their interpretation of generally acceptedassess future conditions such as consumption rates of avail- best practice related to ARD. Caution is needed to ensure thatable neutralizing minerals. all specifics of the country’s regulations are met, as corporate A common corollary of sulfide oxidation is metal leaching ARD guidelines cannot be a substitute for country regulations.(ML), leading to the frequent use of the acronyms ARD/ML Mining companies operate within the constraints of aor ML/ARD to describe the nature of acidic mine discharges “social license” that, ideally, is based on a broad consensusmore accurately. Major and trace metals in ARD, NMD, and from all stakeholders. This consensus tends to cover a broadSD originate from the oxidizing sulfides and dissolving acid- range of social, economic, environmental, and governanceconsuming minerals. In the case of ARD, Fe and Al are usually elements (sustainable development). ARD plays an importantthe principal major dissolved metals, while trace metals such part in the mine’s social license because it tends to be one ofas Cu, Pb, Zn, Cd, Mn, Co, and Ni can also achieve elevated the more visible environmental consequences of mining.concentrations. In mine discharges with a more circumneutral The costs of closure and postclosure management of ARDcharacter, trace metal concentrations tend to be lower as the are increasingly recognized as a fundamental component ofresult of formation of secondary mineral phases and increased all proposed and operating mining operations. Some form ofsorption. However, certain parameters remain in solution as financial assurance is now required in many jurisdictions.the pH increases, in particular the metalloids As, Se, and Sb,as well as other trace metals (e.g., Cd, Cr, Mn, Mo, and Zn). Characterization The generation, release, transport, and attenuation of ARDFRAMEWORK FOR ACID ROCK DRAINAGE are intricate processes governed by a combination of physi-MANAGEMENT cal, chemical, and biological factors. Whether ARD becomesThe issues and approaches to ARD prevention and manage- an environmental concern depends largely on the charac-ment are the same around the world. However, the specific teristics of the sources, pathways, and receptors involved.techniques used for ARD prediction, interpretation of ARD Characterization of these aspects is therefore crucial to thetest results, and ARD management may differ depending on prediction, prevention, and management of ARD.the local, regional, or country context and are adapted to cli- Environmental characterization programs are designed tomate, topography, and other site conditions. collect sufficient data to answer the following questions: Therefore, despite the global similarities of ARD issues, 1. Is ARD likely to occur?there is no “one size fits all” approach to address ARD man- 2. What are the sources of ARD?agement. The setting of each mine is unique and requires a 3. How much ARD will be generated and when?carefully considered assessment to find a management strat- 4. What are the significant pathways that transport contami-egy within the broader corporate, regulatory, and community nants to the receiving environment?framework that applies to the project in question. The site- 5. What human and ecological receptors have contact withspecific setting comprises the social, economic, and environ- the receiving environment?mental situation within which the mine is located; the frame- 6. What level of risk would the anticipated concentrationswork comprises the applicable corporate, regulatory norms and of contaminants in the receiving environment pose to thestandards and community-specific requirements and expecta- receptors?tions. This framework applies over the complete life cycle of 7. What can be done to prevent or mitigate/manage ARD?the mine and is illustrated conceptually in Figure 16.5-5. All mining companies, regardless of size, need to comply The geologic and mineralogical characteristics of thewith the national legislation and regulations pertaining to ARD ore body and host rock are the principal controls on thefor the countries within which they operate. It is considered type of drainage that will be generated as a result of mining.good corporate practice to adhere to global ARD guidance as Subsequently, the site climatic and hydrologic/hydrogeologicwell; in many cases, such adherence is a condition of funding. characteristics define how mine drainage and its constituents
  5. 5. Mitigating Acid Rock Drainage 1725 Mine Phase—Increasing Knowledge of Site Characteristics Mine Planning, Feasibility, and Design Construction and Exploration (Development) Commissioning Operation Decommissioning Postclosure Source Ore-body Laboratory testing of Ongoing Ongoing Ongoing water Long-term water characterization waste and ore materials laboratory laboratory quality monitoring quality monitoring (static and kinetic) testing and field testing (if necessary) Collection and analysis Field testing of Instrumentation of of water samples from waste and ore waste facilities existing historic sources materials Collection and analysis of water samples from Conceptual Site Model Component sources Pathway Exploration Hydrogeologic Hydrogeologic, Ongoing Ongoing Ongoing drilling may characteriztion— hydrologic, and hydrogeologic, hydrogeologic, hydrogeologic, characterize groundwater occurrence, water quality hydrologic, and hydrologic, and hydrologic, and groundwater direction, and rate of flow monitoring water quality water quality water quality occurrence monitoring monitoring monitoring Hydrologic (if necessary) characterization— surface water flow Baseline surface water and groundwater quality Baseline soil and sediment quality Receptor Receptor identification Receptor and Ongoing receptor Ongoing receptor Ongoing receptor habitat monitoring and habitat and habitat and habitat Baseline characterization monitoring monitoring monitoring (receptors and habitat (if necessary) including vegetation metals surveys) Ore-body Peak Ongoing Characterization and Monitoring information Characterization supports site Effort and source characterizationSource: INAP 2009.Figure 16.5-6 Overview of ARD characterization program by mine phaseare transported through the receiving environment to recep- receptors within the watershed boundary requires expertise intors. To evaluate these issues, expertise from multiple disci- field biology and public health.plines is required, including geology, mineralogy, hydrology, Over the mine life, the focus of the ARD characteriza-hydrogeology, geochemistry, (micro)biology, meteorology, tion program evolves from establishing baseline conditions toand engineering. predicting drainage release and transport and to monitoring of The geologic characteristics of mineral deposits exert the environmental conditions, receptors, and impacts. Despiteimportant and predictable controls on the environmental inherent differences at mine sites (e.g., based on commoditysignature of mineralized areas (Plumlee 1999). Therefore, a type, climate, mine phase, regulatory framework), the generalpreliminary assessment of the ARD potential should be made approach to site characterization is similar.based on a review of geologic data collected during explora- Figures 16.5-6 and 16.5-7 present the chronology of antion. Baseline characterization of metal concentrations in vari- ARD characterization program and identify the data collec-ous environmental media (i.e., water, soils, vegetation, animal tion activities typically executed during each mine phase.tissue) may also provide an indication of ARD potential and The bulk of the characterization effort occurs prior to min-serves to document the potential for naturally elevated metal ing during the mine planning, assessment, and design (some-concentrations. times collectively referred to as the development phase). In During mine development and operation, the initial addition, potential environmental impacts are identified andassessment of ARD potential is refined through detailed char- appropriate prevention and mitigation measures, intended toacterization data on the environmental stability of the waste minimize environmental impacts and human and ecologicaland ore materials. The magnitude and location of mine dis- risk, are incorporated. During the commissioning/constructioncharges to the environment also are identified during mine and operation phases, a transition from site characterization todevelopment. Meteorological, hydrological, and hydro- monitoring occurs, which is continued throughout the decom-geological investigations are conducted to characterize the missioning/closure and postclosure phases. Ongoing monitor-amount and direction of water movement within the mine ing helps refine the understanding of the site, which allows forwatershed(s) to evaluate transport pathways for constituents adjustment of remedial measures, in turn resulting in reducedof interest. Identification of potential human and ecological closure costs and improved risk management.
  6. 6. 1726 SME Mining Engineering Handbook Mine Phase—Increasing Knowledge of Source Material Characterization Mine Planning, Postclosure (Pre-) Feasibility Construction and (Care and Exploration and Design Commissioning Operation Decommissioning Maintenance) Waste Rock Drill core Laboratory testing of Ongoing laboratory Ongoing laboratory Collection and Collection and descriptions and drill core samples— testing of drill core testing* analysis of runoff analysis of runoff assay data sample selection or development and seepage and seepage (petrology and targets waste* rock samples* Ongoing field leach samples from waste samples from mineralogy) testing rock facility waste rock facility Field leach testing (if necessary) Block model (barrels, test pads) Collection and (quantity of ore analysis of runoff and waste) and seepage samples from waste Waste or Facility Type—Potential ARD/NMD/SD Sources Review of any rock facility historical data Tailings Laboratory testing of pilot Ongoing laboratory Ongoing laboratory Collection and Collection and plant tailings* testing of pilot-plant testing of tailings analysis of analysis of tailings* discharge* supernatant and supernatant and Analysis of pilot testing seepage samples seepage samples supernatant Collection and from tailings from tailings analysis of storage facility storage facility supernatant and (if necessary) seepage samples from tailings storage facility Ore Laboratory testing of Ongoing laboratory If ore stockpiles drill core samples* testing* exist, collection and analysis of runoff and seepage samples Pit Laboratory testing of Field scale leach Collection and Collection and drill core samples— testing (e.g., wall analysis of pit analysis of pit sample selection targets washing) water and pit water samples pit walls* inflow(s) water (if necessary) Collection and samples analysis of water samples (i.e., runoff, sumps) Underground Laboratory testing of Collection and Collection and Collection and Workings drill core samples— analysis of water analysis of mine analysis of mine sample selection targets samples (i.e., sumps, pool water samples pool water samples mine walls* dewatering wells) (if necessary) *Typical laboratory testing components: particle size, whole rock analysis, mineralogy, acid–base accounting, static and kinetic leach testing.Source: INAP 2009.Figure 16.5-7 ARD characterization program for individual source materials by mine phasePrediction processing method, regulations, and stakeholders. PredictionOne of the main objectives of site characterization is predic- programs therefore need to be tailored to the mine in question.tion of ARD potential and drainage chemistry. Because pre- Also, the objectives of a prediction program can be variable.diction is directly linked to mine planning, in particular with For instance, they can include definition of water treatmentregard to water and mine waste management, the character- requirements, selection of mitigation methods, assessment ofization effort needs to be phased in step with overall project water quality impact, or determination of reclamation bondplanning. Early characterization tends to be generic and gen- amounts.erally avoids presumptions about the future engineering/mine Predictions of drainage quality are made in a qualitativedesign, whereas later characterization and modeling must con- and quantitative sense. Qualitative predictions are focused onsider and be integrated with the specifics of engineering/mine assessing whether acidic conditions might develop in minedesign. Iteration may be required as evaluation of the ARD wastes, with the corresponding release of metals and acidity topotential may result in the realization that a reassessment of mine drainage. Where qualitative predictions indicate a highthe overall mine plan is needed. Integration of the character- probability of ARD generation, attention turns to review ofization and prediction effort into the mine operation is a key alternatives to prevent ARD, and the prediction program is refo-element for successful ARD management. cused to assist in the design and evaluation of these alternatives. Accurate prediction of future mine discharges requires Significant advances in the understanding of ARDan understanding of the sampling, testing, and analytical pro- have been made over the last several decades, with parallelcedures used, consideration of the future physical and geo- advances in mine water quality prediction and use of preven-chemical conditions, and the identity, location, and reactivity tion techniques. However, quantitative mine water qualityof the contributing minerals. All mine sites are unique for rea- prediction can be challenging because of the wide array of thesons related to geology, geochemistry, climate, commodity, reactions involved and potentially very long time periods over
  7. 7. Mitigating Acid Rock Drainage 1727which these reactions take place. Despite these uncertainties, weathering products that result from sulfide oxidation. Suchquantitative predictions that have been developed using real- methods involve the following:istic assumptions (while recognizing associated limitations) • Minimizing oxygen supplyhave proven to be of significant value for identification of • Minimizing water infiltration and leachingARD management options and assessment of potential envi- • Reducing, removing, or isolating sulfide mineralsronmental impacts. • Controlling pore water solution pH Prediction of mine water quality generally is based on • Controlling bacteria and biogeochemical processesone or more of the following: Factors influencing selection of the above methods • Test leachability of waste materials in the laboratory include the following: • Test leachability of waste materials under field conditions • Geological, hydrological, chemical, and mineralogical • Geochemistry of source materials and the potential of characterization of waste materials source materials to produce ARD • Geochemical and other modeling • Type and physical characteristics of the source, including water flow and oxygen transport Analog operating or historic sites are also valuable in • Mine development stage (more options are available atARD prediction, especially those that have been thoroughly early stages)characterized and monitored. The development of geo- • Phase of oxidation (more options are available at earlyenvironmental models is one of the more prominent examples stages when pH is still near neutral and oxidation prod-of the “analog” methodology. Geo-environmental models, ucts have not significantly accumulated)which are constructs that interpret the environmental char- • Time period for which the control measure is required toacteristics of an ore deposit in a geologic context, provide a be effectivevery useful way to interpret and summarize the environmental • Site conditions (i.e., location, topography and availablesignatures of mining and mineral deposits in a systematic geo- mining voids, climate, geology, hydrology and hydro-logic framework; they can be applied to anticipate potential geology, availability of materials and vegetation)environmental problems at future mines, operating mines, and • Water quality criteria for dischargeorphan sites (Plumlee et al. 1999). A generic overall approach • Risk acceptance by company and other stakeholdersfor ARD prediction is illustrated in Figure 16.5-8. More than one measure, or a combination of measures, may bePrevention and Mitigation required to achieve the desired objective.The fundamental principle of ARD prevention is to apply a Typical objectives for ARD control are to satisfy envi-planning and design process to prevent, inhibit, retard, or stop ronmental criteria using the most cost-effective technique.the hydrological, chemical, physical, or microbiological pro- Technology selection should consider predictions for dischargecesses that result in the impacts to water resources. Prevention water chemistry, advantages and disadvantages of treatmentshould occur at, or as close to, the point where the deterioration options, risk to receptors, and the regulatory context relatedin water quality originates (i.e., source reduction), or through to mine discharges. Figure 16.5-9 provides a generic overviewimplementation of measures to prevent or retard the transport of the most common ARD prevention and mitigation measuresof the ARD to the water resource (i.e., recycling, treatment, available during the various stages of the mine life cycle.and/or secure disposal). This principle is universally applica-ble but methods of implementation are site specific. Treatment Prevention is a proactive strategy that obviates the need The objectives of mine drainage treatment are varied.for the reactive approach to mitigation. For an existing case Recovery and reuse of mine water within the mining opera-of ARD that is adversely impacting the environment, miti- tions may be desirable or required for processing of ores andgation will usually be the initial course of action. Despite minerals, conveyance of materials, operational use (dust sup-this initial action, subsequent preventive measures are often pression, mine cooling, irrigation of rehabilitated land), andconsidered with the objective of reducing future contami- so forth. Mine drainage treatment, in this case, is aimed atnant loadings, thus reducing the ongoing need for mitigation modifying the water quality, so that it is fit for the intended usecontrols. Integration of the prevention and mitigation effort on or off the mine site.into the mine operation is a key element for successful ARD A more public objective of mine water treatment is themanagement. protection of human and ecological health in cases where Prior to identification or evaluation of prevention and mit- people or ecological receptors may come in contact with theigation measures, the strategic objectives must be identified. impacted mine water through indirect or direct use of on-siteThat process should consider assessment of the following: and off-site water resources. Water treatment then aims to remove the pollutants contained in mine drainage to prevent • Quantifiable risks to ecological systems, human health, or mitigate environmental impacts. and other receptors In the majority of jurisdictions, any discharge of mine • Site-specific discharge water quality criteria drainage into a public stream or aquifer must be approved by • Capital, operating, and maintenance costs of mitigation the relevant regulatory authorities; regulatory requirements or preventive measures stipulate a certain mine water discharge quality or associated • Logistics of long-term operations and maintenance discharge pollutant loads. Although discharge quality stan- • Required longevity and anticipated failure modes dards may not be available for many developing countries in Prevention is the key to avoiding costly mitigation. The which mining occurs, internationally acceptable environmen-primary objective is to apply methods that minimize sulfide tal quality standards generally still apply as stipulated by proj-reaction rates, metal leaching, and the subsequent migration of ect financiers and mining company policies.
  8. 8. 1728 SME Mining Engineering Handbook Minimum Objective of Typical Project Phase ML/ARD Program Stage ML/ARD Program Activities ML/ARD Program Development of • Compile and review Initial Exploration/Site Conceptual Geological Prescreening historical data. Reconnaissance Model for the Site • Develop logging manual. • Perform diamond drilling and store cores. • Log cores. • Analyze cores for total Exploration elements. • Obtain geological report. • Interpret geological information. Phase 1 • Arrange site visit by project Advanced Exploration/ Initial Assessment of (Initial Geochemical geochemist. Detailed Site Investigation Potential ML/ARD Issues Characterization) • Develop conceptual geochemical model. • Compare site with analogs. • Design static testing. • Perform static testing. • Sample site water (existing facilities, groundwater, surface water). • Interpret ML/ARD potential. Development of Mine and • List mine facilities (including Prefeasibility (Initial Phase 2 Waste Management infrastructure). Mine, Waste, Water, (Detailed Geochemical Plans to Address • Identify data characterization and Closure Planning) Characterization) ML/ARD Potential needs by facility. • Design characterization plan. Feasibility Studies, Design • Execute testing (detailed static and kinetic). Mine Planning, Redesign of Mine Data Needed • Interpret test data. and Waste to Refine • Define waste management Management Source Term criteria. Plans • Perform block modeling. Feasibility/Permitting Continue Phase 2 program. Assessment of Development of • (Detailed Initial Mine, Define geometry of facilities. Project Effects Facility Source • Waste, Water, and Develop mine waste schedule. with Proposed Term • Closure Planning) and Interpret climatological data. Mine Plan • Effects Assessment • Select modeling methods. • Execute modeling. • Couple water and load Refinement of balance. Source • Evaluate uncertainty and risk. Term Reevaluation of Downstream Water • Interpret baseline water Project Effects Quality Modeling quality. Decommissioning, Postclosure • Develop downstream Construction, Operation, hydrological and hydrogeological modeling. • Select water quality modeling method. • Execute modeling. • Evaluate uncertainty and risk. • Design verification monitoring. Project Implementation Assessment and • Execute monitoring plan. Verification (Construction, Mining, Modification of Mine Plan • Evaluate results. Monitoring Closure)Source: INAP 2009.Figure 16.5-8 Generic overview of ARD prediction approach
  9. 9. Mitigating Acid Rock Drainage 1729 Exploration Characterization Assessment Prediction Design Planning for Avoidance Construction Surface Water Control Works Groundwater Control Waste Rock Tailings Open Pit Underground Workings Special Handling Desulfurization Segregation Compaction Encapsulation Amendment Layering Dewatering Operation Blending Remining Backfilling Passivation Selective Mining and Avoidance Hydrodynamic Controls Appropriate Siting of Facilities Co-Disposal In-Pit Disposal Permafrost and Freezing Bactericides Alkaline Materials Organics Dry Cover Seals Decommissioning Water Cover Flooding Monitoring, Maintenance, Inspection Postclosure Where Long-Term Collection and Treatment Are Required Source: INAP 2009. Figure 16.5-9 Generic overview of ARD prevention and mitigation measures The approach to selection of a mine drainage treat- prepared. Mine drainage treatment must always be evaluatedment method is premised on a thorough understanding of and implemented within the context of the integrated minethe integrated mine water system and circuits and the spe- water system. Treatment will have an impact on the flow andcific objective(s) to be achieved. The approach adopted for quality profile in the water system; hence, a treatment systemmine drainage treatment will be influenced by a number of is selected based on mine water flow, water quality, cost, andconsiderations. water use(s) and receptors. Prior to selecting the treatment process, a clear statement Characterization of the mine drainage in terms of flowand understanding of the objectives of treatment should be and chemical characteristics should include consideration
  10. 10. 1730 SME Mining Engineering Handbook Drainage Treatment Technology Categories Specific Target Neutralization Metals Removal Desalination Pollutant Treatment Lime/Limestone Precipitation/ Biological Sulfate Cyanide Removal Process Hydroxide Removal • Chemical Oxidation • Biological Oxidation • Complexation Sodium-Based Alkalies Precipitation/ Precipitation Processes (NaOH, Na2CO3) Carbonates Such as Ettringite Radioactive Nuclides • Precipitation • Ion Exchange Ammonia Precipitation/ Membrane-Based Sulfides Processes Arsenic Removal • Oxidation Reduction Biological Sulfate Wetlands, Ion-Exchange • Precipitation Reduction Oxidation Ponds Processes • Adsorption Wetlands, Other Wetlands, Passive Molybdenum Removal Anoxic Drains Technologies Treatment Processes • Iron Adsorption Other Other Technologies TechnologiesSource: INAP 2009.Figure 16.5-10 Generic overview of ARD treatment alternativesof temporal and seasonal changes. Flow data are especially • Sources of mine drainage feeding the treatment facilityimportant, as this information is required to size any treat- • Nature and location of treated water receptorsment system properly. Of particular importance are extreme Various ARD treatment alternatives are presented inprecipitation and snowmelt events that require adequate siz- Figure of collection ponds and related piping and ditches. Thekey chemical properties of mine drainage relate to acidity/ Monitoringalkalinity, sulfate content, salinity, metal content, and the Monitoring is the process of routinely, systematically, andpresence of specific compounds associated with specific min- purposefully gathering information for use in managementing operations, such as cyanide, ammonia, nitrate, arsenic, decision making. Mine site monitoring aims to identify andselenium, molybdenum, and radionuclides. There are also a characterize any environmental changes from mining activi-number of mine drainage constituents (e.g., hardness, sulfate, ties to assess conditions on the site and potential impacts tosilica), which may not be of regulatory or environmental con- receptors both on- and off-site. Monitoring consists of bothcern in all jurisdictions but that could affect the selection of observation (e.g., recording information about the environ-the preferred water treatment technology. Handling and dis- ment) and investigation (e.g., studies such as toxicity testsposal of treatment plant waste and residues such as sludges where environmental conditions are controlled). Monitoringand brines and their chemical characteristics must also be fac- is critical in decision making related to ARD management,tored into any treatment decisions. for instance through assessing the effectiveness of mitigation A mine drainage treatment facility must have the flexibil- measures and the subsequent implementation of adjustmentsity to deal with increasing/decreasing water flows, changing to mitigation measures as required.water qualities, and regulatory requirements over the life of Development of an ARD monitoring program starts withmine. This may dictate phased implementation and modular a review of the mine plan, the geographical location, and thedesign and construction. Additionally, the postclosure phase geological setting. The mine plan provides information onmay place specific constraints on the continued operation and the location and magnitude of surface and subsurface distur-maintenance of a treatment facility. bances, ore processing and milling procedures, waste disposal A variety of practical considerations related to mine site areas, effluent discharge locations, groundwater withdrawals,features will influence the construction, operation, and main- and surface water diversions. This information is used to iden-tenance of a mine drainage treatment facility: tify potential sources of ARD, potential pathways for release • Mine layout and topography of ARD to the receiving environment, receptors that may • Space be impacted by these releases, and potential mitigation that • Climate may be required. Because the spatial extent of a monitoring
  11. 11. Mitigating Acid Rock Drainage 1731 Problem Definition Conceptual Site Model (CSM) • Physical Setting • Regulatory and Legal Registry ARD/ML Pathway Receptor • Community Requirements Source Dynamic System Model (DSM) • Corporate Requirements • Quantitative Representation • Financial Considerations of CSM Goals and Objectives Define Monitoring Objectives Audit (Internal/External) Characterization and Prediction • Characterize Current Conditions Continuous Feedback • Assess ARD/ML Potential • Detect Onset of ARD/ML • Meeting Objectives? Design for ARD Prevention/Mitigation • Predict Onset of ARD/ML • New Objectives? • Assess Effects/Impacts of ARD/ML • Assess Engineered ARD/ML Controls ARD Management Plan • Materials Definition • Risk Assessment and Management • Management Strategy Design Monitoring Program • Adequate Data Collection? Appropriate Locations? • Integration with Mine Plan • Data Requirements to Meet Objectives • Appropriate Frequency? • Operational Controls (SOPs, KPIs, QA/QC) • Sampling Locations and Media • Appropriate Methods? • Roles, Responsibilities, and Accountability • Sampling Frequency • Appropriate Analytes? • Data Management, Analysis, and Reporting • Sampling Methods (SOPs) • • Parameters/Analytes to Be Measured • Laboratory Performance? • Quality Assurance/Quality Control Performance Assessment and Monitoring • Reconciliation with Goals, Objectives, and KPIs • Assumption Validation Implement Monitoring Program • Learnings • Data Collection • Implementation of SOPs • Accountability • Data Management • Data Security and Integrity • Auditing and Management Review • Risk Assessment and Management Data Analysis and Interpretation • Appropriate analyses? No Yes • Validate or Update CSM/DSM • Timely Analyses? Goals Satisfied?Source: INAP 2009. Source: INAP 2009.Figure 16.5-11 Development of an ARD monitoring program Figure 16.5-12 Flow chart for ARD performance assessment and management reviewprogram must include all these components, a watershed consideration of the physical setting, regulatory and legalapproach to ARD monitoring (including groundwater) is often registry, community and corporate requirements, and finan-required. Monitoring occurs at all stages of project develop- cial considerations. Characterization and prediction programsment, from preoperational through postclosure. However, identify the potential magnitude of the ARD issue and pro-over the life of a mine, the objectives, components, and inten- vide the basis for the selection and design of appropriate ARDsity of the monitoring activities will change. The development prevention and mitigation technologies. The design processand components of a generic ARD monitoring program are includes an iterative series of steps in which ARD controlpresented in Figure 16.5-11. technologies are assessed and then combined into a robust system of management and controls (i.e., the ARD manage-Management and Performance Assessment ment plan) for the specific site. The initial mine design mayThe management of ARD and the assessment of its perfor- be used to develop the ARD management plan needed for anmance are usually described within the site environmental environmental assessment. The final design is usually devel-management plan or in a site-specific ARD management oped in parallel with project permitting.plan. The ARD management plan represents the integration The ARD management plan identifies the materials andof the concepts and technologies described earlier in this mine wastes that require special management. Risk assess-chapter. It also references the engineering design processes ment and management are included in the plan to refine strat-and operational management systems employed by mining egies and implementation steps. To be effective, the ARDcompanies. management plan must be fully integrated with the mine plan. The need for a formal ARD management plan is usu- Operational controls such as standard operating proceduresally triggered by the results of an ARD characterization and (SOPs), key performance indicators (KPIs), and quality assur-prediction program or the results of site monitoring. The ance/quality control (QA/QC) programs are established todevelopment, implementation, assessment, and continuous guide its implementation. The ARD management plan identi-improvement of an ARD management plan are ongoing pro- fies roles, responsibilities, and accountabilities for mine oper-cesses throughout the life of a mine, which will typically fol- ating staff. Data management, analysis, and reporting schemeslow the sequence of steps illustrated in Figure 16.5-12. are included to track progress of the plan. As shown in Figure 16.5-12, the development of an In the next step, monitoring is conducted to compareARD management plan starts with establishment of clear field performance against the design goals and objectives ofgoals and objectives, such as preventing ARD or achieving the management plan. Assumptions made in the characteriza-compliance with specific water quality criteria. This includes tion and prediction programs and design of the prevention/