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HARDROCK MINING ON FEDERAL LANDS Appendix D Research Needs The objective of the research discussed below is to minimize adverse environmental effects of mining on federal lands by filling gaps in knowledge about the long-term environmental impacts from hardrock mining. This appendix expands on the information provided in Chapter 3. Research is needed in a number of areas and is summarized in this appendix according to scientific discipline. The areas of highest priority are listed below and may encompass more than one of these disciplines. Pit Lakes: A number of significant research needs are related to the long-term environmental impacts of pit lakes. Making accurate long-term projections based on short-term data is challenging, and the concordance between predicted and actual outcomes has not been evaluated. Research on the chemistry, hydrology, and biology of pit lakes and their surroundings is needed to minimize the environmental impact of those that presently exist and to improve the design of those proposed for the future. Acid Drainage: Improved methods for predicting and preventing acid drainage are also a high-priority research need. Acid drainage is the source of many of the water quality problems associated with hardrock mining. Improved methods for prediction, prevention, and long-term treatment are needed to minimize the expenses related to acid drainage and to enhance the long-term protection of the environment. New Technologies: Research on the mining approaches of the future is the next most important research area. Mining methods such as bioleaching and in situ mining, which are being proposed for a number of new mining operations, have the potential to prevent pollution, yet the long-term environmental consequences of these methods have not been investigated.
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HARDROCK MINING ON FEDERAL LANDS Each topic area requires three types of research: modeling to accurately project the changes in environmental parameters forward in time and outward in space; monitoring to provide information on the averages, extremes, and trends of environmental indexes, and to help calibrate and verify the accuracy of the models; and sampling and testing to acquire baseline data in and around mines. It is impossible to predict which types of research will provide answers to the needs identified here or whether the critical problems have been identified. The scientific community needs support and flexibility to innovate and respond to new opportunities, as well as to resolve the well-identified problems. WATER QUALITY Water represents by far the most important interface between mining and the environment, and its quality, quantity, and distribution provide some of the most effective criteria for monitoring the state of the environment. There is an array of models and laboratory techniques used to predict water quality and quantity from the leaching of mine waste and rock at mine sites. These techniques and models include tests for acid-generation potential, leach tests, pit lake models with water quality and quantity components, and waste rock and tailings discharge models with water quality and quantity components. The results from these tests and models are used to determine if pit water or leachate from mine waste will pose a threat to aquatic life, wildlife, or human health. Predictions of long-term water quality related to acid drainage, especially in pit lakes, have a high degree of uncertainty. Without reliable forecasts of long-term water quality, it is difficult to design effective mine waste management techniques to protect against future deterioration of water quality. Uncertainty about long-term water quality and quantity predictions points to a number of research needs that could help increase the accuracy or define the appropriate use of these predictive tools. Pit Lake Water Quality The objective of pit lake models is to accurately predict the chemistry and hydrology of pit water as the lake forms, after water levels have stabilized and taking into account any long-term impacts of evaporation or other factors that may affect water quantity. The modeling results can indicate the factors that
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HARDROCK MINING ON FEDERAL LANDS control water quality and quantity and opportunities to adjust operation and closure plans so that water quality problem can be reduced. Pit lake models predict future water chemistry based on such factors as up-gradient groundwater concentrations, the amount and types of materials (including mineral phases), availability to weathering, leaching of materials, hydrological parameters, and oxidation and reduction reactions that may occur in the lake. It is currently not known how accurately pit lake models predict concentrations of contaminants in pit lakes and surrounding groundwaters. Thus, comprehensive comparisons of predicted and actual concentrations in pit lakes and groundwaters are needed to evaluate whether existing models can predict long-term water quality. For example, a number of pit lakes at uranium mines have filled, and although pit lake models were rarely used before the lakes filled, it would be possible to model retrospectively the pit water chemistry and compare modeled results to actual concentrations. The parts of the models that needed improving would be adjusted based on results from the comparison study. Pit lakes in Nevada that are currently filling should be monitored to evaluate how the chemistry changes with time. The use of a pit lake model without data from long-term leach tests increases the uncertainty of the predictions. Eary (in press) found that, for the toxic metals copper, cadmium, lead, and zinc, concentrations were not well represented by theoretical solubilities of known mineral phases, indicating the importance of empirical data based on adequate leach tests for predicting pit water quality. The Nevada Bureau of Mines has conducted an inventory of pit lake water quality in Nevada (Price et. al., 1995). This type of information can be used, in conjunction with knowledge of the geology and mineralization of the mines, to compare predicted and actual concentrations in pit lakes. Tempel et al. (in press) found that predicted arsenic concentrations in the North pit at the Getchell Mine in Nevada did not match actual concentrations. The lower measured concentrations in the pit water may be related to adsorption to aluminum hydroxide and clay mineral surfaces. Until more of these comparisons are conducted, the validity of pit water prediction models will remain uncertain. The possibility that backfilling of pits will become more common raises additional research questions, because the environmental consequences of backfilling have not been adequately investigated. Partial backfilling of pits with acid-generating wastes has led to temporary increases in metal concentrations and acidity of pit waters, and geochemical models used to predict pit lake chemistry have had to take these changes into account (e.g., at the Sleeper Mine in Nevada) (Mark Logsdon, personal communication, 1999). The impact of partial or complete backfilling on down-gradient groundwater should also be investigated.
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HARDROCK MINING ON FEDERAL LANDS Acid Drainage and Leaching of Mine Materials The prediction of both acid drainage and the leaching of mine waste and pit wall rock needs improving. Like pit lake models, there has been little effort to compare predicted and actual concentrations. Predicted concentrations of acid, metals and other constituents in waste rock, heap, and tailings discharge should be compared to measured concentrations to determine and improve the accuracy of these methods. Sample collection strategies for estimating both acid drainage and mine waste leachate need re-evaluation. Sulfide ore bodies, which generally have the greatest potential to generate acid, are known to be heterogeneous in their metal and sulfide content. Therefore, proper sampling of all geologic materials that will become exposed to the environment or become waste is essential. The recommended number of samples per rock type or waste unit varies widely, from one sample for every million tons to one sample per 20,000 tons of waste rock or 50 samples for each million tons (Schafer, 1993; Forest Service, 1992). Another approach is to allow the variability of the material itself to dictate how many samples are collected and analyzed, and then to quantify and evaluate the results using statistical methods (Runnells et al., 1997). Compositing of samples can underestimate the variability in acid-generation potential; therefore, it is important to composite materials only of similar lithology and mineralogy. Waste rock is much more heterogeneous than tailings, and compositing of samples may be more acceptable. In addition to sampling of solids, long-term monitoring strategies for leachate need to be improved. A better understanding is needed of when and how to sample effluents, especially under variable seasonal and meteorological conditions. Acid Drainage Acid drainage forms when oxygen and water come in contact with sulfide minerals and certain metal sulfates that form from the weathering of metal sulfides. Factors affecting acid generation include sulfide amounts and types, particle size of waste material, pH, oxygen availability and diffusion, temperature, storage (evaporative concentration of acid products), availability and type of neutralizing material, water saturation, amount of ferric iron present in the water, and the presence of iron and sulfur-oxidizing bacteria (EPA, 1994). Although many states require the use of acid-generation prediction testing as part of the permitting process, variability in one or more of the factors listed above over time and the type of testing performed can limit the reliability of the test and projection results.
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HARDROCK MINING ON FEDERAL LANDS Although acid drainage at ancient mine sites has continued for millennia, the study of acid generation and associated drainage is recent. In addition, the generation process is not well enough understood to determine definitively how long it will run and how high the concentration of acid and metals will reach. Accurately predicting acid drainage potential is a very important element of accurately projecting pit lake water quality. Because of the importance of acid drainage, all aspects of the acid drainage assessment process could benefit from additional research. Two general types of tests are used to predict acid-generation potential: static and kinetic. Static tests estimate the maximum acid-generation potential (AGP) and neutralization potential (NP) of a rock or waste material. Kinetic tests are conducted for six weeks or longer, use larger sample volumes, provide information on the acid production rate and drainage water quality, and are generally more reliable than static tests. Modified humidity cell- and column-type tests are currently the preferred kinetic tests (EPA, 1994). Special equipment is required, and the costs of the tests are higher than those of static tests. The opportunity to examine changes in pH and metal concentrations over time is available with the kinetic tests. There are a number of instances where acid drainage has occurred even though it was not predicted or expected. Examples include the Thompson Creek mine in Idaho, the LTV Steel Mining Company in Minnesota (EPA, 1994), the Newmont Rain facility in Nevada (EPA, 1994), and the Zortmann-Landusky mine in Montana (Federal Register, August 7, 1996, [61 FR 41182]). In some cases, waste rock or pit wall material was not initially acid generating, but waste material removed later was. In other cases, static or kinetic tests results showed that acid generation would not be a problem, but actual drainage from mine waste units was acidic. In still other cases, the acid-base accounting data showed the potential for acidification, but errors in construction of the piles or errors in interpreting and reporting the data led to on-the-ground problems. The effectiveness of the acid-generation prevention and source treatment methods is unknown. General research is needed into the effectiveness of the various treatments on various types of ores and waste. Research should focus on better understanding the acid-generation process and on the effectiveness of the prevention methods in the short and long term. Research is also necessary on comparison of modeled results, either by computer or in the laboratory, to actual field conditions at sites where the methods are in use.
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HARDROCK MINING ON FEDERAL LANDS Leaching of Mine Materials The leach tests most commonly used were developed primarily to predict contaminant concentration in leachate from solid and hazardous waste landfill. Substantially less effort has been devoted to determining their accuracy in mining situations. Leach tests used currently include the WET test, the toxicity characteristic leaching procedure (TCLP), the synthetic precipitation leaching procedure (SPLP), and humidity cell tests (EPA, 1978). The WET, TCLP, and SPLP tests are all short-term tests; the humidity cell tests can last for six weeks or longer. The leaching reagent (acetic acid) used in the TCLP may be appropriate for municipal landfills, but is not appropriate for mining wastes. The SPLP test uses a lower pH leach solution for waste materials east of the Mississippi River, because of acid rain from the use of high sulfur coals. The WET, TCLP, and SPLP tests all may underestimate leachate concentrations because they are conducted in time frames that do not allow equilibration of materials with the leachate solution. Alternating wetting and drying used in some humidity cell tests best approximates the conditions that are most favorable for the formation of acid drainage. In some cases, it may take months or years for stable pH and metal concentrations to be reached. If the tests are cut short before steady state concentrations are reached, predicted concentrations may underestimate actual concentrations. Finally, the fate and transport of leached metals and other constituents in the environment need additional research. For example, although cyanide and some cyanide-metal complexes are susceptible to photolytic degradation in surface water, not much is known about how they behave in down-gradient groundwater that eventually may be discharged to surface water or be used for drinking water or irrigation. HYDROLOGY Like long-term water quality predictions, the modeling of water quantity and hydrologic processes also contains uncertainties. It may not be known, for example, whether some pit lakes will have closed-basin or flow-through hydrologic features, or whether and under what circumstances they may turn over. Long-term ecological, water quantity, and water quality impacts of pit dewatering and discharge of pit water to streams are beginning to be investigated, but results of these studies are not yet available. Site-specific water balances, which in part were responsible for uncontrolled discharges from the mine at Summitville, Colorado, are not sufficiently understood. The flow of water through and interaction with unsaturated waste rock and
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HARDROCK MINING ON FEDERAL LANDS saturated fractured media is poorly characterized and affects the validity of models that predict the behavior of groundwater in these media. Water quantity affects water quality, and uncertainties in one area compound uncertainties in the other. The following research areas in hydrology warrant further investigation. Pit Lake Water Quantity The most important physical parameters for a pit lake model are evaporation, precipitation, groundwater flow, and transfer of gases into and in the lake. These processes affect lake level, whether or not the lake turns over, and the oxidation and reduction state of the pit water, all of which affect lake chemistry. There are many methods that can be used to estimate evaporation rates for the pit lake. Pan evaporation rates may overestimate or underestimate evaporation depending on the use of correction factors, the wind speed across the lake, and other factors (Bird, 1993). Wind speed, the amount of precipitation and surface runoff entering the lake, and the amount of metals and other constituents associated with run-on water will change from year to year depending on the amount of precipitation and other climatic factors. Long-term weather data may not be available to provide accurate maximum and minimum precipitation amounts. The original groundwater flow pattern around a pit may be altered by dewatering or by the presence of the lake, which presents large amounts of water for evaporation that were previously present as groundwater. Large precipitation events could cause the lake level to rise above that of the surrounding groundwater and discharge potentially contaminated pit lake water to groundwater (Macdonald et al., 1994). The long-term hydrologic status of pit lakes is affected by all the physical factors discussed above. Whether a lake is a flow-through system or a closed-basin lake with no outputs will affect the chemistry of the lake water. If evaporation is significantly greater than precipitation, the concentrations of metals and other constituents will increase over time through evapoconcentration. If, however, the lake flows through for all or part of the year, down-gradient groundwater concentrations may increase above pre-mining concentrations. Mine Area Dewatering and Discharge of Surplus Water Mining influences surface flows on and off site through alteration of runoff from several types of constructed surfaces (e.g., roads, waste piles, leach pads), through discharge of surplus water from pit or underground dewatering, and through use of water for processing. Groundwater
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HARDROCK MINING ON FEDERAL LANDS withdrawals for processing and pit dewatering may affect local and regional aquifers. Pits can influence local hydrology through interaction with the local aquifers, interception of runoff and precipitation, and lake surface evaporation. Mines may supplement stream flow with surplus water from pit dewatering, and then, after mining operations cease, diminish stream flow as the pits and dewatered aquifers are allowed to fill. The impacts of mine area dewatering include reduced flow in springs, which in arid areas provide habitat for terrestrial wildlife. The flow of streams may also be decreased, which can change the type of aquatic biota that can survive in that system. A decrease in streamflow can increase concentrations of contaminants in the stream, especially if mine pollution sources discharge directly or indirectly to surface water. The existence of a pit creates a permanent higher evaporation sink and can lower the water table off site, especially in arid areas such as Nevada. Modeling of groundwater withdrawal (Schaefer and Harrill 1995) has demonstrated the effects of pumping from deep and shallow aquifers on shallow basin-fill water tables. Other studies have shown reduction in local stream flow when water is withdrawn from nearby deep aquifers. Water balance models for different hydrogeological settings should be developed that address local and regional interrelationships among surface flow, pit lake hydrology, and hydraulic head of shallow and deep aquifers. These models should enable long-term predictions of the consequences of alteration of surface waters, and interruption, use, and withdrawal of groundwater by mining activities. The models should include consideration of transient events. Surface water discharge may alter the timing of high flow events, which may eliminate recruitment of riparian vegetation and eventually lead to senescence and death of existing vegetation. This can increase erosion of streambanks, which provide shelter and shade for fish. Long-term hydrologic, geomorphic, and water quality changes from riverine discharge of mine water should be investigated and modeled. Studies on riverine discharge of mine water to the Humboldt River in Nevada are under way at such federal agencies as the U.S. Geological Survey and the Fish and Wildlife Service, but similar studies elsewhere, and site-specific modeling, will facilitate long-term predictions of the response of riverine ecosystems to hydrological changes. Modeling Flow in Mine Dumps and Impoundments Waste rock dumps and tailings impoundments show strongly heterogeneous patterns of permeability, which current hydrologic models are incapable of reproducing. In the case of waste rock, end dumping from haul trucks produces inclined layering in the waste rock dump, with a pronounced
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HARDROCK MINING ON FEDERAL LANDS gradation from the finest material at the top of the dump to very coarse material at the bottom. Compaction of the upper surfaces of the dumps by movement of haul trucks and other vehicles tends also to produce horizontal layers that may be less permeable than other portions of the dump and may act as barriers to fluid flow. In the case of tailings impoundments, discharge of the tailings from spigots produces horizontal and vertical sedimentation of particles of different sizes. The coarsest materials are deposited near the points of discharge and the finest grained materials (slimes) are transported along a gentle slope down gradient from the points of discharge toward the center of the pond. As a result of this differential sedimentation, the horizontal permeabilities in a tailings impoundment tend to be much higher than the vertical permeabilities. These heterogeneities are not included in current hydrologic models and should be considered as improvements are made to these models. The flow of water through unsaturated waste rock and saturated fractured media is also not well understood and affects the validity of models that predict groundwater flow. Improved models are needed to more accurately predict the infiltration and flow of water into waste rock dumps and tailings impoundments from rainfall and snow melt. ECOLOGY Modern hardrock mining is creating potential biological habitats in pit lakes. The viability of these lakes as long-term habitat and food sources for aquatic biota and wildlife has not been evaluated. In addition, the long-term sublethal effects of cyanide and metals on aquatic biota and migratory birds have not been extensively studied. The following areas need further study: potential development of biological communities in pit lakes and impacts on aquatic biota and wildlife; sublethal effects of cyanide and metals on aquatic biota and migratory birds; establishment of water quality standards for pit lakes and waste leachate for protection of aquatic biota and wildlife; integrated ecologic-hydrologic studies and modeling to determine the consequences of mine water discharge on riverine ecosystems; and impacts of placer mining on stream ecology and aquatic biota. The viability of creating fish habitat in pit lakes turn in part on the long-term water quality predicted for the lakes. The hydrologic and water quality
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HARDROCK MINING ON FEDERAL LANDS condition of the lake may change if it later becomes a closed-basin lake or begins to turn over. The nature of the food chain in the lake, such as the type of algae and bacteria, and its ability to bioconcentrate or accumulate contaminants has not been studied. Certain birds may feed on zooplankton in the pit lake, while others may feed on fish. Food chain effects in general in pit lakes need to be studied to determine the viability of creating long-term biological habitats in these mine features. Sublethal (e.g., growth, reproduction, behavioral avoidance) effects of metals on salmonids and macroinvertebrates have been studied to some extent (see Appendix B, “Aquatic Biota”), but sublethal effects on wildlife, including migratory birds, have received less attention. The effects of cyanide and metal-cyanide complexes on aquatic life and wildlife have received less attention. If pit lake cyanide and metal concentrations meet chronic aquatic life criteria, aquatic biota will not die off immediately, but there may be a long-term effect for communities that live in the lake or feed on biota from the lake. Investigation into acceptable whole-body metal and cyanide burdens for consumption by migratory birds should also be investigated. Water quality standards for pit lakes and mine waste leachate have not been designed for protection of migratory birds. Research is needed to establish the scientific bases for standards for consumption of lake water by waterfowl and migratory birds. In addition, leachate standards have only been developed for a limited number of constituents, and are based only on human health concerns. Therefore, research is needed to establish the scientific bases of leachate standards for protection of aquatic life in cases where leachate from a mine waste deposit has the potential to discharge directly or indirectly (through groundwater) to surface water. The discharge of mine water to streams can increase loadings of contaminants, even if the concentrations are lower than those in the stream. The added influx of contaminants can be retained on stream sediments and periphyton, which serve as habitat and food sources for macroinvertebrates, which in turn are one of the main food sources for fish. In terminal basins, such as the Humboldt Sink in Nevada, discharge of mine water to streams that feed the sink will cause metals and other constituents to evapoconcentrate in the sink, which provides habitat for waterfowl and migratory birds. Finally, the impact of placer mining on stream ecology should be investigated. Placer (mechanized) or suction dredge mining disturbs streambed sediments, which provide habitat for macroinvertebrates and spawning habitat for salmonids. The long-term impact of placer mining is unknown, especially under low-flow conditions or when springtime high flows are not able to return bed sediment to its original characteristics. As discussed in Appendix B, studies on the impact of suction dredge and mechanized placer mining on macroinvertebrate and spawning habitat should be conducted.
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HARDROCK MINING ON FEDERAL LANDS CUMULATIVE IMPACTS A mine's environmental impacts are considered during the preparation of the environmental impact statement (EIS), but multiple, diverse demands on the resources of a region (e.g., a complex ecosystem or a watershed) can create interactions and interferences that may be beyond the purview of individual impact statements. If a proposed mine is near other mines or other human activities, there is a potential for cumulative impacts that extend beyond what an isolated mine may cause. Cumulative impacts are considered during the EIS process, but methodologies for accurately predicting and assessing these impacts are not well developed. Research is needed to improve our understanding of how mining participates in cumulative effects and to predict impacts from mining under different environmental circumstances. An example of cumulative impacts includes the impact of exploration combined with hunting, off-road vehicle use, fishing, and camping that may use access roads created for exploration. Also, groundwater withdrawal by several mines may deplete a regional aquifer that is shared by agriculture or urbanization. Each incremental withdrawal introduces potentially expanded impacts that need to be evaluated during the EIS process. In addition, the cumulative tapping of groundwater that feeds springs may affect the wildlife and riparian habitat. Research is needed on the cumulative impacts of past and current mining activity and the development of predictive models for use in evaluating proposed actions. Long-term monitoring should be designed to measure attributes that offer useful data on changes and sustainability of resources affected by mining activity and post-closure conditions. With respect to cumulative impacts, it is particularly important to develop long-term monitoring strategies for water quality and biological analyses that correspond with critical periods in the hydrograph. ALTERNATIVE MINING AND POLLUTION PREVENTION METHODS The potential environmental consequences, of new mining techniques should be assessed before they come into widespread practice. Some of these techniques have pollution prevention potential, but the net environmental impact of the practices discussed below generally has not been investigated. These techniques include bioleaching, deep underground mining, in situ mining, and the use of alternative lixiviants.
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HARDROCK MINING ON FEDERAL LANDS Alternative Mining Techniques In the precious metals industry, especially in Nevada, the near-surface oxidized ore is being exhausted, and the industry is moving into the deeper sulfide reserves. Because of their sulfide characteristics, these reserves are not as amenable to cyanide recovery as the oxide ores. This problem is being addressed mainly by finding ways of economically oxidizing the ore so that cyanide leaching can be used, and by identifying substances (lixiviants) that can effectively leach the precious metal from the sulfidic ores. Bioleaching is one way of oxidizing the ore. In this approach, the ore is treated with a low-pH solution into which iron- and/or sulfur-oxidizing bacteria are introduced. Typically, Thiobacillus ferrooxidans or thiooxidans (the same microbes responsible for acceleration of the acid drainage production rate) are issued and, not surprisingly, one of the by-products is sulfuric acid. After microbial oxidation, the ore is neutralized and put through the cyanidation process, either in a heap leach or a milling circuit. Although bioleaching is similar to leaching of copper ores with sulfuric acid or the production of acid drainage, the combined environmental impacts of acidic drainage and cyanide have not been examined. For example, the kinetics of formation and the stability of metal-cyanide complexes may be different under acidic, rather than basic, pH values. In addition, the long-term acid generation potential of neutralized and cyanidized sulfidic ore has not been investigated. The long-term environmental impacts of bioleaching and other new techniques for recovering precious metals from sulfide ore bodies need to be investigated. Much of the new mining in the United States may be deep underground mining (see Appendix A). Although there will be less surface disturbance, impacts on the quality and quantity of deep groundwater and other environmental factors have not been investigated. As with pit mining, these environmental impacts may be complicated by an increased emphasis on backfilling of the underground shafts and tunnels, which can affect the stability of the underground workings and drainage and groundwater chemistry. In situ mining has been conducted for decades in the uranium industry, but the techniques have only recently been applied to base metal mining, especially copper mining. This process involves injecting leaching agents directly into the ground and collecting the pregnant solutions in down-gradient wells. Permitting these types of facilities has been difficult, in part because of the uncertainties associated with groundwater quality and quantity. The use of alternative lixiviants has been investigated for years, but the long-term impacts of the alternatives on aquatic life, wildlife, and groundwater and surface water quality have not been sufficiently researched. Particular
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HARDROCK MINING ON FEDERAL LANDS attention should be given to identifying alternative lixiviants that are more environmentally acceptable than cyanide to leach precious metals; they would be valuable assets to industry and comforting changes to the public. Pollution Prevention and Treatment Some of the new technologies have the potential to prevent the amount of pollution generated by mining activities. However, additional research is needed to discover new and improved existing methods for cost-effective long-term treatment of acid drainage, other types of mine leachate, and other types of water quality problems. Some of the techniques being developed to clean up hazardous waste sites may also be applicable to mining. Examples include the use of iron curtains to treat groundwater contaminated with acid drainage (Shokes and Moller, 1999), bioremediation techniques, and the use of constructed wetlands to remove contamination from waste waters. Both the active and passive treatment systems require long-term maintenance and can be expensive to operate. A common method for treating in place to prevent acid drainage is to mix the acid-generating materials with a neutralizing material such as lime. The materials can be layered with lime or physically mixed either before or after disposal. The long-term effectiveness of this type of active treatment merits additional examination. Both active and passive treatment systems require long-term care (in some cases in perpetuity) and maintenance, and can be expensive to operate. The systems are difficult to standardize when each is treating a unique chemical mix of metals and salts. More research is necessary on treatment methods to determine whether there are better treatment options for contaminated water and to find improvements that can be made to help them function with less maintenance over the long term. Operators and regulators alike recognize that the most cost-effective control may be to prevent the drainage from occurring in the first place, because once drainage begins, there is very little that can be done except long-term treatment of the effluent. Environmentally acceptable alternatives, such as pre-treating incoming waters or controlling the activity of the acid-promoting biota, are worthwhile research goals. MINING TECHNOLOGIES FOR THE FUTURE Technologies of the modern mining industry are very different from those of the past. Mines now employ highly efficient, advanced production systems, coupled with innovative engineering designs to prevent or significantly mitigate
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HARDROCK MINING ON FEDERAL LANDS environmental disturbance. Even so, the remarkable advances in mining-related technologies, the better understanding of our environment, and the application of innovative engineering designs have not eliminated all detrimental impacts of mining, nor are they likely to do so. Mining technologies of the future will continue to improve operational efficiency, productivity, and safety while minimizing pollution and maximizing control through improved designs and efficient operation. It is in these areas where additional incentives exist to develop innovative, if not revolutionary, minerals extraction technologies. Future mine production systems will consist of people, machines, computers, sensors, and the communication links among them. New technologies will be developed that offer the potential for major discontinuous change in the way minerals and metals are discovered, mined, and processed. To extend the lives of U.S. mines and discover new mineral deposits, more and better technologies will be required as mineralized grades continue to decline, metallurgical characteristics of the ores become more complex, and environmental regulatory pressures build. Examples of new technologies are presented below (Beebe, 1995; Gentry, 1998). Exploration Advances in control mechanisms, mostly implemented with computers, and sensor development will enable minerals exploration to be far less intrusive to surface and subsurface environments, and include: satellite remote sensing techniques capable of penetrating high-density foliage, surface cover, and even short distances into the rock itself; advanced geophysics and geochemical techniques capable of detecting smaller and more subtle signatures of deeply buried deposits; borehole geophysics and other techniques to improve sampling, analysis, estimation of deposit reserves, and geotechnical assessments with a minimum of drilling into the subsurface; and manipulation of increasingly complex data bases for target and deposit modeling with sophisticated pattern recognition software that is both efficient and effective.
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HARDROCK MINING ON FEDERAL LANDS Mining Although surface mining is the most efficient method for mining minerals in use today, its economies of scale and labor productivity may be approaching limits. With respect to surface area disturbed, surface mines are less benign than their underground counterparts; however, underground mining is generally more expensive in terms of capital and operating costs, more labor intensive, and more dangerous. Also, some underground mines can result in land and hydrologic disturbances due to surface subsidence and groundwater withdrawals. In the future, there will be even greater pressures to increase mining efficiencies in terms of labor costs, productivity, capital investment, safety, and environmental sensitivity. Where these objectives cannot be achieved satisfactorily, social and environmental factors will tip the balance. New mining technologies may provide the key to achieving the above objectives (Beebe, 1995; Gentry, 1998). There will be greater use of: remote-controlled and autonomous robotic equipment, progressing toward “intelligent” mining systems that require minimal human intervention; geosensing techniques to navigate and control intelligent mining systems to obtain assay and geotechnical data in real time and to warn of potentially dangerous variations, such as faults, shear zones, or pockets of water or gas; improved mechanical and some form of nonexplosive rock fragmentation techniques fitted to intelligent mining machines; in situ mining of small or deeply buried deposits economically and in an environmentally safe manner; and more efficient, effective, and safe underground extraction systems that comply with growing environmental pressures and meet society 's demand for optimal life-cycle costing. Mineral Processing The tailings impoundments generated by mineral processing plants can preclude certain post-mining land uses or can release toxic constituents over time. The issue is to minimize the amount of tailings produced, find a suitable storage location for them, and subsequently stabilize and revegetate to the maximum extent possible.
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HARDROCK MINING ON FEDERAL LANDS It is frequently assumed that the relatively new advances in hydrometallurgical systems result in better waste minimization characteristics than the more traditional physical separations. This is true only if leaching agents can be entirely recycled or if discharge streams can be treated effectively to eliminate pollutants before discharge. Toxic leaching agents must be contained if subsequent environmental problems are to be avoided. Future mineral processing and extractive metallurgy technologies must improve efficiencies in energy consumption, liberation and process metallurgy (including mathematical modeling for process optimization augmented by better sensors and control mechanisms), and most importantly waste reduction and stable disposal techniques. From an environmental standpoint, future technological advances in process metallurgy must address issues pertaining to methods that: generate less waste or create waste forms that are more stable or benign; establish more efficient separations that avoid discarding valuable or troublesome constituents; avoid dissolution of particularly troublesome metals or fix them as inert compounds; promote selective solution or dissolution of minerals through pressure oxidation and bioleaching technologies; promote more efficient, selective, and timely oxidation, and stabilization of minerals using bio-oxidation techniques and associated bioremediation techniques for environmental mitigation; use genetically engineered microbial approaches to oxidation, dissolution, and immobilization of metals and associated toxic elements; use hybrid processing flowsheets combining physical, chemical, and microbiological methods in place of repetitive stages of the same methods; use alternative lixiviants for in situ mining and processing technologies that are efficient in mineral recovery and are environmentally benign (i.e., agents that dissolve desired metals while minimizing dissolution of impurities); efficiently eliminate cyanide and other troublesome processing chemicals safely and rapidly; improve recovery of by-products or co-products, even ones considered unconventional by today's standards; and
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HARDROCK MINING ON FEDERAL LANDS use waste treatments that minimize later dependence on time or natural processes to fix or eliminate troublesome constituents (Beebe, 1995; Gentry, 1998). These technological advances should eliminate, or at least significantly mitigate, many of the environmental concerns now associated with mining and related technologies; however, it is doubtful that these future technologies will provide utopian solutions to environmental concerns. For example, in situ mining will present enormous challenges relative to lixiviant selection; solution and dissolution of desired versus undesirable metals; lixiviant containment and control in the structurally complex geologic settings hosting most hardrock mineral deposits; and related impacts on the neighboring groundwater regime. Another example is the anticipated trend toward underground mining using automated, robotic mining systems. Such systems are likely to incorporate storage of significant amounts of waste rock and tailings underground in mined-out workings, but they will not eliminate surface disturbances or concerns about impacts to groundwater systems pertaining to waste isolation, containment, leakage, and impacts on hydrologic regimes. Surface subsidence will remain and will require equally sophisticated technologic systems to monitor and control. Mining and associated exploration and processing technologies will advance, some quite rapidly (e.g., hydrometallurgy and biotechnology). While these new technologies will resolve many environmental concerns, they no doubt will introduce other areas of concern not yet recognized or contemplated. These technologic advances will present new opportunities and challenges to all stakeholders in the domestic minerals industry. SUMMARY In conclusion, research related to the long-term environmental impacts of mining is needed in the areas of water chemistry, hydrology, ecology, and mine treatment and pollution prevention technology. A number of important research areas have been discussed in this appendix. Results from a well-coordinated and consistently funded research program could help improve current methods used to predict, prevent, and minimize the environmental impacts of mining and could also reduce costs associated with long-term maintenance at mine sites.
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Representative terms from entire chapter: