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Home , Acid mine drainage

United States Department of Agriculture Forest Service Treating Mine Technology & Development Drainage From Program 7100 Engineering Abandoned Mines in October 1998 9871-2821-MTDC Remote Areas

John J. Metesh Hydrogeologist

Terrie Jarrell Project Leader

Steve Oravetz Program Leader

USDA Forest Service Technology and Development Program Missoula, Montana

7E72G71— Acid Mine Drainage Study

October 1998

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 (voice or TDD). USDA is an equal opportunity provider and employer. 1 Contents

Overview ______3

Chemistry ______4

Characterizing Mine Sites ______5

Solutions ______6

Specific Passive Treatments ______7

Alkaline Amendments ______7

Wetlands ______7

Anoxic Drains ______8

Alkalinity-Producing Systems ______8

Limestone Ponds ______8

Reverse Alkalinity-Producing Systems ______9

Open Limestone Channels ______9

Diversion Wells ______9

Limestone Sand Treatment ______10

Permeable Reactive Wall ______10

Passive In-Situ Reduction of AMD ______11

Mine Flooding ______11

Direct Reduction of Flow ______11

Planning to Control AMD at Current or Proposed Mines ______14

Conclusions ______15

References ______16

2 Overview

etween 20,000 and 50,000 mines the body and surrounding country years, making active treatment generate Acid Mine Drainage rock, and the enhanced transmissivity prohibitively expensive. These problems B(AMD) on USDA Forest Service of the rock near the openings. Each of have been considered so intractable lands, affecting approximately 14,000 these factors, along with preexisting that they have only recently been miles of (Benner and others conditions, such as groundwater addressed. 1997). The majority of the AMD comes recharge rates, will determine the from inactive or abandoned mines. quality and quantity (if any) of the Passive systems can treat AMD to Consequently, there is limited funding discharge after . The stability of varying degrees at lower capital and for treatment and surface and the workings after mining is an operating costs than conventional groundwater resources are continually additional consideration. Few adits treatment plants, and they can be contaminated (Skousen and others remain open for more than a few years. installed at abandoned mines and in 1996b). The stability of deeper workings remote locations (Eger and Wagner depends on rock quality and the mining 1995). Because they take advantage of Underground mining often requires methods used. natural processes to improve draining (passive removal) or pumping contaminated water conditions (Hedin (active removal) of groundwater so ore Any type of abandoned mine can be and others 1994), they should not can be removed. Continued dewatering hard to treat. Abandoned underground require significant operation and of the ore body and surrounding mines may have cave-ins and flooding, maintenance. However, they will still material creates the conditions for AMD restricted access, and unreliable mine need some attention. production: a continuous supply of maps. Abandoned surface mines may water, , and exposed sulfide have huge volumes of spoil, often of This report summarizes literature about minerals. In addition, groundwater flow unknown composition and hydrology many different types of mines and AMD near the underground openings is (Ziemkiewicz and Skousen 1996). problems. We have used the summary altered. The volume of an aquifer Rehandling and mixing alkalinity into the to determine remediation steps that offer affected depends on the dimensions of backfill of surface mines and is the greatest chance of success for the mine (its shape, depth, volume, and generally prohibitively expensive. Sites remote, inactive mines that produce so forth), the premining transmissivity of can generate AMD for more than 100 flows of less than 20 gallons per minute.

3 Chemistry

cidic drainage is a natural weathering in an extremely short period Alkalinity and acidity are not mutually process that becomes of geologic time. In addition, rock is exclusive terms. When water contains Aaccelerated and intensified by pulverized in the mining process. This both mineral acidity and alkalinity, a mining. When rock is exposed to allows more surface area to come into comparison of the two determines weathering, it will release minerals as it contact with water, and increases the whether the water is net alkaline comes into equilibrium with its chance that the water will pick up (alkalinity greater than acidity) or net environment (Brick 1998). Any (Brick 1998). acidic (acidity greater than alkalinity) dissolved leached from the rock (Hedin and others 1994). Net alkaline will hydrolyze when it comes into can add to the problem by water contains enough alkalinity to contact with water, and will produce significantly speeding up the reaction neutralize the mineral acidity associated acid (Brick 1998). However, the primary time. If there were no bacteria in the with metals such as dissolved and metal related to AMD is iron. Combined system, it would take close to 15 years . As these metals react to with sulfate and/or sulfide, iron can for ferric iron to produce acid (Brick become more neutral, the proton acidity cause real problems. 1998). However, the presence of that is produced is also neutralized. bacteria will shorten the reaction time , an iron sulfide found in both down to 8 minutes (Brick 1998). However, if the mine rock contains and metal mines, has the potential to more acid-generating minerals than produce more acid than any other When mine water has a pH greater than alkaline materials, the alkaline materials mineral. It can produce 16 units of acid 4.5, it can neutralize acid and is said to will eventually be used up, and the for each unit of pyrite (Brick 1998). contain alkalinity (Hedin and others water acidity will increase (Durkin and 1994). Alkalinity can result from Herrmann 1996). This process can last In a natural environment, the process in hydroxyl (OH), carbonate, silicate, for weeks, months, or centuries until the which rock becomes exposed to borate, organic ligands, phosphate, and minerals completely oxidize and the weathering and releases minerals is ammonia. The primary source of alka- rock comes into equilibrium (Durkin and very slow (DeLuca 1997). Mining linity in water is dissolved carbonate, Herrmann 1996). At that point, the water accelerates this process by exposing a which can exist in the or will regain a more neutral pH. tremendous amount of rock to carbonate form (Hedin and others 1994). Both can neutralize proton acidity.

4 Characterizing Mine Sites

he first step in the remediation concerns as they appear and evaluate . The site should be process is to characterize the their significance. Since some site looked at in the spring when the flushing Tsite in terms of premining specifics (such as slope, water is the greatest, in midsummer when conditions, the nature of mine chemistry, and land available for staining is most apparent, and at first development, and its present or treatment) may rule out certain types of snowfall when hot spots can be potential environmental effect on land treatment, the investigator should also observed. Signs to look for include downstream from the site (Robertson consider the treatment options and staining or precipitates, dead fish or 1996). Characterizing a mine site can assess which options may be employed vegetation, melting snow, or steaming be divided into three steps: at the site (Robertson 1996). vents.

• Planning Instead of concentrating on the Initial water analyses should include pH, Define the potential concerns for downstream environmental impacts, the conductivity, alkalinity, iron, manganese, the site and the problems to site investigator should carefully assess and acidity measurements (Hedin and consider. the acid-generating conditions of the others 1994). If an anoxic limestone potential source material (Robertson drain is being considered, the acidified Determine the information 1996). Once the source material has sample should also be analyzed for needed. been remediated, the downstream ferric iron and aluminum. A field impacts may be able to take care of measurement of dissolved oxygen (DO) • Investigation themselves. should be made. Tailings samples Establish a set of techniques for should be tested for low paste-pH and investigating the site. If at all possible, the investigator should high conductivity. Paste-pH helps determine the background metal levels determine the pH a soil or rock will Evaluate existing information to before mining. This can be done by generate as it weathers. Materials with determine the additional looking at a similar drainage that has not low paste-pH will generate acid. information needed. been mined. Knowing background levels is important because the water should Since both the flow rate and the • Evaluation usually be cleaned up to its premining chemical composition of a discharge Quantify potential issues to standard, if feasible. In general, when can vary seasonally and in response to determine the real problems. background metal levels exceed clean storm events, the discharge flow rates Evaluate alternative control water standards, the regulatory and water quality should be measured measures to solve the problems. agencies will take that into account, and in different seasons and under repre- will require the background levels to be sentative weather conditions. This will Conduct a cost/benefit evaluation the standard for that site. allow the passive treatment system to be to decide the best remediation designed for operation during all weather measure for the cost. The investigator should look at the site conditions (Hedin and others 1994). material, any surface pools, any seeps Throughout the investigation, the at the toe of the wastes, and at the investigator must recognize new

5 Solutions

assive treatment systems offer a Given the right site conditions, the Each passive treatment has an area of low-cost, low-maintenance passive system can be designed to application. It is difficult to achieve water Psolution to AMD problems. While meet water quality standards in most quality standards by passive treatment they will not always provide effluent that cases. In general, systems that are not with any one system (Ziemkiewicz and meets water quality regulations, they 100% effective are improperly designed, others 1997). However, coupling will improve the water quality, which is are undersized, or both (Hedin and systems could produce the desired the main goal of treating AMD from others 1994). Operational problems can results (Ziemkiewicz and others 1997). abandoned mines. be attributed to inadequate design, Four main components create acid mine unrealistic expectations, pests, drainage: water, oxygen, bacteria, and Active treatment of mine drainage inadequate construction methods, . Methods to control AMD often requires precipitating metal weather, or other natural problems target one of these variables (Marcus contaminants from the water and (Hedin and others 1994). If properly 1997). If one variable can be removed neutralizing acidity (Hedin and others designed and constructed, a passive or reduced, the problem is reduced. This 1994). Passive treatment differs from treatment system can be operated with can often be achieved by manipulating active treatment by distinguishing a minimum amount of attention and the site hydrology or by adding an between these two objectives. It is money. alkaline agent. Although it is possible to possible to passively remove iron remove the bacteria, they are extremely contaminants from mine water, but have The cost of a passive treatment system resilient, and will come back in a very little effect on the mine water acidity depends primarily on the amount of land short time (Brick 1998). Repeated (Hedin and others 1994). Alternatively, it needed for the system. Because applications of bactericide and/or is possible to passively add neutralizing passive systems rely on processes that application of a time-release bactericide capacity to acidic mine water without are slower than conventional treatment, may be able to remove the bacteria, but decreasing metal concentrations. they require longer retention times and this treatment is still in the testing phase Depending on the site, meeting only one larger areas to achieve similar results (Ziemkiewicz and Skousen 1996). of these objectives may be necessary. (Hedin and others 1994).

6 Specified Passive Treatments

Alkaline Limestone is the most commonly used moisture and sets up as hard as a rock. Amendments alkaline amendment. It is generally the It is widely used as a stabilization and least expensive and it has a very high barrier material. For waters that have a net acidity neutralization capability. It can be used greater than zero, the treatment design in almost any application. Its only The third amendment is phosphate, needs to incorporate systems that add downfall is that it has no cementing which works almost as well as alkalinity (Hedin and others 1994). While properties, preventing it from being limestone. Unless there is a source of several different types of alkaline used as a barrier. phosphate near the mine site, it can be amendments can reduce mine water a very expensive amendment. If there is acidity, a couple of them are most suited Kiln dust is another very good a local source of phosphate, it may be for low-flow, remote mine sites amendment. It mainly consists of more cost-effective than limestone.

(Ziemkiewicz and Skousen 1996). unreacted limestone, but it can absorb

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Vegetation is optional. Wetlands 1-2 in. water Wetlands can reduce AMD through 12-24 in. organic matter physical, chemical, and biological processes. The physical processes, yyyyy 6-12 in. limestone such as and sedimentation, are important in removing particulate Figure 2—Basic anaerobic wetland design (from Skousen and others 1996a). metals. The chemical and biological processes remove dissolved metals wetlands generally contain a layer of manure mixed with hay. Other types of (Eger and others 1996). Wetlands limestone on the bottom. When mine substrate material include mushroom mainly remove metals through water contains dissolved oxygen (DO), compost, log yard waste, and peat adsorption to organic substrates, and ferric iron, or aluminum, or is net acidic, moss (Skousen 1997). through bacteria (Skousen and others construction of an anaerobic wetland is 1996b). Because plant uptake accounts recommended (Hedin and others 1994). Two problems must be recognized when for only about 10% of the metal removal, The limestone in an anaerobic wetland using wetlands for remediation. Due to plants are considered optional. will raise the pH and decrease the seasonal variation, the acid removal required to treat AMD. If rate is not consistent (Hedin and others Acid mine drainage should not be routed DO, ferric iron, or aluminum is present, 1994). No design corrections are into a natural wetland for treatment. the limestone can become armored, available now to solve this problem. Instead, a wetland should be limiting its effectiveness. In such cases, Also, the rates of acid reduction/metal constructed to meet the needs of AMD the limestone needs to be placed so removal will decrease over time (Eger treatment. A natural wetland may or that it is anaerobic to prevent armoring. and Wagner 1995) as the substrate may not meet those needs, and may becomes filled with metals. However, if cause more problems than it solves. In The processes going on at the bottom the input flows are low and periodic addition, it is illegal to intentionally of the wetland are what makes the maintenance is performed, wetlands pollute a wetland. wetland so effective at treating mine can provide long-term treatment of mine water. Both types of artificial wetland drainage (Eger and others 1994). Two types of wetlands are used for have a constructed substrate that will AMD treatment: aerobic (Figure 1) and pull out most of the minerals and elevate For wetlands to remove metals, the anaerobic (without oxygen, Figure 2). the pH (Eger 1994). The two best mine water needs to be held for 20 to 40 The aerobic wetlands collect water and materials to use for the substrate are hours (Cohen 1996). If the mine water provide residence time. Anaerobic fairly fresh municipal compost or cow pH is below 5, the residence time should be 40 hours. If the pH is nearly neutral, the mine water needs Vegetation is optional. approximately 20 hours of residence 1-3 in. water time. The final design and construction 12-36 in. organic matter decisions will be based on the flow rate to be treated, the loading rates of the metals, and the space available Figure 1—Basic aerobic wetland design (from Skousen and others 1996a). (Skousen and others 1996b). 7 Anoxic Limestone the ALD to work properly. The DO, ferric occur. If possible, the water should be iron, and aluminum should be under aerated as soon as it leaves the ALD Drains 1mg/L; if they are not, the risk of and be directed to an aerobic pond or premature failure increases (Hedin and wetland. Anoxic Limestone Drains (ALD’s) are others 1994). trenches of buried limestone into which mine water is diverted (Figure 3, In theory, an ALD that will last for 30 24-48 in. soil Skousen and others 1996b). Often the years requires 30 tons of baseball-sized limestone is covered in plastic and limestone for each gallon per minute of buried to limit the amount of oxygen in flow (Hedin and others 1994). Because the system (Hedin and others 1994). the oldest operating ALD’s are only 7 yyyyy ALD’s also allow carbon dioxide to build Limestone years old, no one knows if the limestone Impermeable barrier, trench up in the system. This is a benefit generally 20-40 mil plastic will actually last for 30 years. yyyyy because higher levels of carbon dioxide Regardless, the ALD must be large Figure 3—Basic limestone trench design allow more alkalinity to be added to the enough to detain the mine water for (from Skousen and others 1996a). AMD (Hedin and others 1994). The about 14 hours to allow the reactions to

AMD must meet specific parameters for

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Alkalinity-Producing In designing an APS system, the sizing allows the mine water to use up its Systems calculations can be based upon the pH alkalinity buffer and remove some of the and buffering capacity of the AMD metals. When the water goes through (Kepler and McCleary 1994). The initial the APS, the water is further neutralized Alkalinity-Producing Systems (APS) are component of an APS system is and more of the metals are removed. a combination of an ALD and an generally a settling pond. The pond anaerobic wetland. The limitations that dissolved oxygen places on ALD design can be eliminated in an APS by combining open water and a substrate 60-96 in. water Inflow with a high organic content overlying a 12-24 in. organic matter limestone treatment zone (Kepler and 12-24 in. limestone Outflow McCleary 1994). The APS design Effluent pipe provides a relatively sound assurance yyyyimpermeable barrier that the AMD contacting the limestone Figure 4—Basic alkalinity-producing system design (from Skousen and others 1996a).

will be anoxic (Figure 4).

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Limestone Ponds it comes from a seep or spring limestone (Figure 5). The pond’s size (Skousen and others 1996a). Limestone and design are based on the topography Limestone ponds provide a solution for is placed in the bottom of the pond and of the area and the water that emanates places that have room to treat AMD as the water flows up through the from the ground. The pond should retain the water for 1 to 2 days to enable the limestone to dissolve and to keep the Outflow 36-72 in. water seep and the limestone underwater. The

yyyyyy pond should be built to hold 3 to 10 feet 12-36 in. limestone of water, to contain 1 to 3 feet of limestone at the bottom, and to keep the Seep inflow seep and limestone under water

yyyyyy (Skousen 1997). Figure 5—Basic limestone pond design (from Skousen and others 1996a).

8 Reverse Alkalinity- and removed as they pass through the to 6 feet of water covers the organic organic matter. Iron and sulfate are matter and limestone to maintain Producing Systems reduced to more benign forms, and the anaerobic conditions. This system oxygen content is decreased. About 3 works well for moderate to low flows. The applications for Reverse Alkalinity- Producing Systems (RAPS) are similar to those for limestone ponds (Figure 6). Outflow 36-72 in. water

If the water contains oxygen, a pond yyyyyy can be constructed at the seep’s 12-24 in. limestone upwelling. Organic matter may be 6-12 in. Organic matter layered in the bottom of the pond, and Seep inflow overlain by limestone (Skousen and Figureyyyyyy 6—Basic reverse alkalinity-producing system design (from Skousen and others 1996a).

others 1996b). The metals are filtered

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Open Limestone an AMD treatment opportunity where no other passive system is appropriate Channels (Ziemkiewicz and others 1997). Since settlement basins will remove metals, Limestone will become armored when they should be incorporated into the yyyyyyyy Limestone exposed to oxygen in mine water, but design. several studies have shown that it still yyyyyyyy adds alkalinity to the water although at a The most successful channels have reduced rate. As a result, open slopes steeper than 20% and use yyyyyyyy limestone channels can be effective if coarse limestone. Both the slope of the designed properly (Figure 7). channel and the size of the limestone Figure 7—Basic open limestone channel design. Limestone can be placed along sides can minimize the settling of metals in and in the bottom of culverts, diversions, Open limestone channels are suspension, preventing the channel ditches, or channels (from Skousen particularly useful in steep terrain where from becoming plugged (Ziemkiewicz and others 1996a).

long channels are possible. They offer and others 1997).

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8-12 inch Diversion Wells mine portal with a hydraulic head of at diameter least 8 feet (the height of well). The Sand-sized The diversion well is a simple device water flows down the pipe, exits the limestone yyyy developed for treating stream acidity pipe near the bottom of the well, then caused by acid rain in Norway and flows up through the limestone in the Sweden (Skousen 1997). It has been well. The flow must be rapid enough to

yyyy adapted for AMD treatment in the agitate the bed of limestone particles. Eastern United States. A typical The acid water dissolves the limestone, diversion well consists of a cylinder or generating alkalinity. The metal that 8 feet vertical tank of metal or concrete, 5 to 6 precipitates out of the water is flushed feet in diameter and 6.5 to 8 feet deep, through the system by water flowing out the top of the well. The churning action filled with sand-sized limestone and Flow sunk into the ground by a stream also helps dissolve limestone and (Figure 8). A large pipe, 8 to 12 inches ensures that fresh limestone surfaces Flow in diameter, is placed vertically in the are always exposed. center of the well with its end slightly 6 feet above the bottom. Water is fed into the pipe from an upstream dam or deep Figure 8—Basic diversion well design (from Skousen 1997).

9 Limestone be redistributed downstream, Because of limestone grain size and neutralizing acid as the limestone stream redistribution, the river would Sand Treatment moves along the streambed. The probably need to be treated three times limestone particles can become a year to maintain water quality suitable Sand-sized limestone may be directly armored, but the agitation and scouring for fish populations. dumped into streams at various in the streambed should keep fresh

locations in the watershed. The sand will surfaces available for reaction.

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The reactive material that makes up the tions. Designs for a permeable reactive Permeable wall can either remove the wall are still in their initial phase, but it Reactive Wall contaminants in the water or decrease has been determined that a substrate them. The material for the wall must comprised of municipal compost, leaf One way to treat acidic groundwater is satisfy five criteria. First the material compost, and wood chips mixed with to build a permeable reactive wall within must be sufficiently reactive to reduce pea gravel will provide an effective the contaminated aquifer (Benner and sulfate concentrations. The material also catalyst for cleaning groundwater. At this others 1997). The wall needs to be must be permeable enough to point, no one is sure exactly how long placed so that all of the groundwater in accommodate the groundwater flux the reactive materials will maintain their the aquifer will eventually pass through rates at the sites. The material must effectiveness. it (Figure 9). This requirement limits use sustain its permeability and reactivity of the wall to areas where the aquifer is over a long time period. Finally, the relatively small or is confined (Benner material must be readily available and and others 1997). affordable with respect to site condi-

15 feet Ground surface

Water table

Bedrock

Figure 9—Basic permeable reactive wall design (from Benner and others 1997).

10 Passive In-Situ Reduction of AMD

he passive in-situ methods for nearly neutral. This has been exposed surface within the treating AMD focus on: demonstrated in ongoing work by the workings. Applying either technique T Montana Bureau of Mines and Geology requires detailed knowledge of the • Increasing pH to precipitate metals at several mines near Butte, Montana. rock and grout material (for • Using biological reduction agents Similar conditions have been observed example, see Fotieva and Sammal to precipitate metals. at several small, flooded shafts and 1987). The success of this type of adits throughout Montana. grouting appears to be greatest These treatments enhance natural when it is injected ahead of the processes to reduce maintenance and Inducing flooding in some mines will water that flows into the mine. cost. An alternative is to create require that adits be plugged. However, conditions that eliminate one or more of recognition of the geochemical process • Grouting (Outside) the components of AMD generation may reduce the degree of plugging There have been several recent within the source area: remove the needed. In mines where the final attempts at drilling and grouting supply of oxygen, prevent water from hydrostatic head on the plug may be too from the ground surface into the entering the workings, and/or prevent high, relief holes at the appropriate unmined area just outside the the oxidation of sulfide minerals. Two elevations may reduce the final head, opening of a new mine adit. general methods to eliminate the but would yield good quality water. Success is generally very limited. components of AMD generation are to Recent investigations by the flood the underground workings or to Montana Bureau of Mines and directly reduce groundwater flow Geology in the Butte area indicate through the AMD source rock. the need for precise placement of Direct Reduction drill holes. The area or volume of of Flow influence by the underground opening can be quite limited and One method of AMD prevention is require a precision of a few feet. Mine Flooding through the use of barriers. They can be Accurate information on the depth placed to prevent the movement of and position of the opening is Flooding underground workings greatly water and oxygen into areas containing needed and can be difficult to reduces sulfide oxidation. Subsequent acid-producing rock. In general, these obtain. Several methods of mine rock-water chemical reactions may methods can reduce AMD, but not water control using this approach reduce the oxidation- entirely control it (Ziemkiewicz and are described by Szentirmai enough to precipitate dissolved metals Skousen 1996). Barriers can be used in (1985), USDI Bureau of from the mine waters. Metals such as two different ways: grout barriers can be Reclamation, (1987), DuBois , , and are transported used to plug rock fractures within a (1984), Ewert (1992), Fotieva and as dissolved oxide or sulfate complexes mine, and soil barriers can be used to Sammal (1987), and Jacenkow and in acid waters. In a flooded mine, encapsulate acid-producing tailings. others (1984). dissolved oxygen that is consumed Both methods redirect groundwater flow cannot be replaced by atmospheric away from acidic material. • Adit Plugging oxygen. As this process continues, the This method has been applied oxidation-reduction potential decreases Several methods are used in mining and under a variety of conditions with and sulfides begin to precipitate. A construction to reduce, divert, or mixed success. Several adit plugs reducing agent such as sulfur is needed eliminate groundwater flow to openings. have failed catastrophically a few to promote precipitation of sulfides. The The more common methods include: years after placement. Site- sulfur can exist as free sulfur (Lindsay specific, detailed information is 1979) or as pyrite (Guilbert and Park • Grouting (Inside) needed on the rock quality, the 1986), which is relatively soluble and is predicted final hydrostatic generally abundant in sulfide ore bodies. Grouting fractures to reduce flow is often used in active mines where pressure, and the effects water Organic carbon, such as mine timbers quality may have on the plug and other wood debris, is another pumping costs and water-treatment costs can be reduced if grouting is material. Several methods, reducing agent, although the supply is including the one presented by limited. at least partially successful. The grout can be injected into the Cogan and Kintzer (1987), have been developed for active and Inundating the exposed sulfides that country rock outside the ore body or can be applied as a lining on the inactive mines. The possibility of generate acid can reverse the process unwanted side effects remains. As and produce waters in which the pH is

11 discussed, flooding underground groundwater flow to underground scheme (Figure 10) has been workings often causes conditions workings. constructed to determine the best that lead to precipitation of metal- methods to consider for a given mine. sulfides and an increase in pH, • Interception/Diversion both desirable effects. However, if This method has been more The mine type axis of Figure 10 refers the hydraulic conductivity of the ore commonly proposed for coal mines to the general location of the surface body and country rock are where the coal bed lies flat and the opening with respect to the workings. sufficiently high, acidic water can groundwater quality in the The adit can be the lowest point in the leak out into other adjacent areas overburden is good. Horizontal mine with all workings above the adit, at before reducing conditions can be wells are installed in the the highest point (rare), or at some point achieved. This makes water overburden to reduce flow into the in between with workings above and treatment difficult and may lead to mine and divert water away from below the adit (common). Mines that other conditions such as slope the workings. This sometimes used a shaft for access may be partially failure. At this time, the developing occurs naturally in hardrock mines flooded or completely flooded. consensus is that plugging coal of similar shape. mines is at best a temporary Accurately classifying a given site and solution (Ziemkiewicz 1996). The size and shape of the mine play an selecting a remedy will require important role in selecting a method to information on the geologic and • Backfilling control groundwater flow. The geologic mineralogic controls on ore placement Backfilling the mine may also be a and hydrogeologic nature of the ore and weathering, mining and exploration viable option. Backfilling can reduce body and surrounding country rock also methods, and controls on groundwater the acid loading substantially by determines how groundwater flow is flow (fracture density, aperture, and reducing the groundwater flow and affected by the opening and any orientation). Each method of adit infiltration, and dissolved oxygen subsequent attempt at control. A discharge control comes with its own (DO) and acid concentration simplifying assumption is that the size, set of limitations. These limitations (Skousen and others 1996b). shape, and geologic history of the ore should be well understood for the condi- Encouraging rapid runoff and body control the size, shape, and tions of each site. No single method is a discouraging recharge into zones mining method used to extract the ore. cure-all. A combination of methods will of acidic backfill will also help With this in mind, a simple classification usually give the best results. reduce the total acid load from the site. Control • Control Methods In some mines, the ore body extended to or was near the ground Plugging surface. Mining such ore bodies necessitated shallow workings Recharge Control confined to vertically oriented pay Interception/Division zones. The majority of groundwater Grouting (inside) now entering such workings is from surface recharge originating from a Grouting (outside) relatively small area around the Flooding mine. The closer the mine is to the groundwater recharge area, the smaller the recharge area is for the Shaft (full) V >> H (shallow) mine. Reducing or eliminating Relative shape Shaft (part) V >> H (deep) infiltration in the recharge area can greatly reduce or eliminate adit Adit (base) H >> V (shallow) discharge. Ackman and Jones H >> (deep) Adit (top) Mine type (1991) describe techniques used to Adit (mid) reduce stream flow loss to underground openings. Similar methods could be used to identify Figure 10—The mine type and the shape of the mine are considered when selecting one or high-recharge areas of more methods to control adit discharge. The vertical extent of the workings may be much greater than their horizontal extent (V>>H) or their horizontal extent may be much greater than their vertical extent (H>>V). Shafts may be fully flooded (full) or partially flooded (part). 12 Consider a mine that has a discharging recharge area for the mine may be quite The recharge area would likely be adit at the base of the workings (adit, large and difficult to control. For this smaller since the workings are closer to base), located near the bottom of the type of mine, the best approach may be the surface. Grouting from the outside drainage (deep), with several levels of grouting fractures within the mine, an the mine or recharge control may be the workings that extend up into the ore adit plug, or, depending on the quality preferred control method. body (V>>H). Grouting from the outside and quantity of the water, a combination of the mine would require a lot of deep of the two methods. Once adit discharge has been drilling, which is expensive and difficult controlled to the degree possible, the to apply with accuracy. Because of the The approach would be different if the remaining discharge will need to be mine’s vertical extent, the workings same mine was near the top of the treated. The flowchart below (Figure 11) intercept a large part of the drainage (adit, base; shallow; V>>H). will help you choose the appropriate groundwater flow. The groundwater passive treatment system.

Determine Flow Rate Analyze Water Chemistry

Net Alkaline Water Net Acid Water

Determine DO, Fe3+ , and Al Content

DO <1mg/L DO 1-5 mg/L DO >5 mg/L Al <25 mg/L 3+ 3+ Fe3+ acceptable Fe unacceptable Fe unacceptable

Low Flow High Flow Aerobic or (<100 L/min) (>100 L/min) Anoxic Anaerobic Limestone Wetland or Drain APS

Strip DO, 3+ Precipitate Fe Open Limestone Net Net Channel Alkaline Acid Water Water pH>4.5 pH<4.5

Settling Settling Pond Pond Settling Pond

Aerobic Anaerobic Wetland Wetland, or APS Does Water Meet Effluent Requirements?

Yes No

Discharge Add Additional Treatment

Figure 11—Flow chart for selecting the proper passive treatment design (from Hedin and others 1994). 13 Planning to Control AMD at Current or Proposed Mines

ome measures can be taken at Geochemical analyses of overburden different composition than it did before operating mines to prevent AMD should take place during the planning mining. This could cause water to move Sfrom becoming a problem. The stage of a mine. Before mining, through the site differently than it had Surface Mine Control and Reclamation overburden strata need to be classified previously (Ziemkiewicz and Skousen Act requires operators and regulators to according to Acid-Base Accounting 1996). Knowing where the water will go predict the amount of acid mine (ABA). This testing process determines helps determine where the acid- drainage that may occur on a potential if the material is net alkaline or net producing material should not be mine site before disturbance acidic (Meek 1996). These tests will placed. (Ziemkiewicz and Skousen 1996). determine whether the overburden will produce acid once it has been broken By knowing the hydrology after mining, Three important factors in determining up and subjected to weathering. the acid-producing overburden can be and preventing AMD are: geochemical placed in the backfill so that it will analysis of overburden, the site’s The hydrology of the site after mining is remain relatively dry. If that is not hydrology after mining, and the method also important. Because the overburden possible, the material can be blended of overburden placement in the backfill is broken into smaller rocks during with acid-neutralizing rock to develop an during reclamation (Ziemkiewicz and mining, the reclaimed ground will have a inert rock mass (Ziemkiewicz and Skousen 1996). Skousen 1996).

14 Conclusions

cid mine drainage affects many solution because mine sites vary so of them—to create nearly pristine water. sites and can be an incredibly much and because site characteristics Even if the acid mine drainage problem Adifficult problem to solve. determine what can and cannot be may not completely disappear, reducing Unfortunately, there is no simple done. It may be possible to use any of it will go a long way toward solving it. the above systems—or a combination

15 References

Ackman,T.E.; Jones, J.R. 1991. Eger, P.; Wagner, J.R. 1995. Sulfate Kepler, D.A.; McCleary, E.C. 1994. Methods to identify and reduce potential reduction for the treatment of acid mine Successive alkalinity-producing surface stream water losses into drainage: long term solution or short systems (SAPS) for the treatment of abandoned underground mines. term fix? Conference on mining and the acidic mine drainage. In: International Environmental Geology and Water environment. Sudbury, Ontario, Canada. Land Reclamation and Mine Drainage Sciences. 17(3): 227-232. May/June. Conference and the Third International Eger, P.; Wagner, J.R.; Kassa, Z.; Conference on the Abatement of Acidic Benner, S.G.; Blowes, D.W.; Ptacek, Melchert, G.D. 1994. Metal removal in Drainage. Vol. 1: Mine drainage. Spec. C.J. 1997. A full-scale porous reactive wetland treatment systems. In: Publ. SP 06A-94.U.S. Department of the wall for prevention of acid mine International Land Reclamation and Interior, Bureau of Mines. drainage. Groundwater Monitoring and Mine Drainage Conference and the Remediation. Third International Conference on the Lindsay, W.L. 1979. Chemical equilibria Abatement of Acidic Drainage. Vol. 1: in soils. New York: John Wiley and Brick, C. 1998. Unpublished notes from Mine drainage. Spec. Publ. SP 06A-94. Sons. 449 p. Geology 431-Geochemistry. Missoula, U.S. Department of the Interior, Bureau MT: University of Montana. of Mines. Marcus, Jerrold J. 1997. Mining environmental handbook: effects of Cogan, J.; Kintzer, F.C. 1987. Tunnel Eger, P.; Wagner, J.R.; Melchert, G.D. mining on the environment and plug design at Tyee Lake. Bulletin of the 1996. The use of overland flow wetland American environmental controls on Association of Engineering Geologists. treatment systems to remove mining. San Mateo, CA: Imperial College 24(1): 27-42. February. from neutral mine drainage. Annual Press. 785 p. meeting of the American Society for Cohen, R.R.H. 1996. The technology Surface Mining and Reclamation. Meek, F.A. 1996. Evaluation of acid and operation of passive mine drainage Knoxville, TN. prevention techniques used in surface treatment systems. In: Managing mining. In: Skousen, J.G.; Ziemkiewicz, environmental problems at inactive and Ewert, F. K.1992. The individual P.F. Compilers. Acid mine drainage: abandoned metals mine sites. Sem. groutability of rocks. International Water control and treatment. Morgantown, WV: Publ. EPA/625/R-95/007. Environmental Power and Dam Construction. 44(1): 23- West University and the Protection Agency 30. January. National Mine Land Reclamation Center: 121-128. DeLuca, T.H. 1997. Unpublished notes Fotieva, N.N.; Sammal, A.S. 1987. from Forestry 410-Soil Morphology. Calculation of pressure tunnel linings Robertson, A. MacG. 1996. The Missoula, MT: University of Montana. with consideration of consolidation importance of site characterization for grouting of the rocks. Hydrotechnical remediation of abandoned mine lands. DuBois, H.L.E. 1984. High pressure Construction. 21(1): 6-8, July. Translated In: Managing environmental problems at grouting in deep mines. In: Water in from Gidrotekhnicheskoe Stroitel’stvo. inactive and abandoned metals mine mining and underground works (El Agua (1): 17-19, January 1987. sites. Sem. Publ. EPA/625/R-95/007. en la Mineria y Trabajos Subterraneos). Environmental Protection Agency. Vol. I. SIAMOS 78: 391-407. Guilbert, A.M.; Park, C.F. 1986. The geology of ore deposits. New York: W.H. Skousen, J.G. 1997. Overview of Durkin, T.V.; Herrmann, J.G. 1996. Freeman and Company. 985 p. passive systems for treating acid mine Focusing on the problem of mining drainage. Green Lands. 27(4). wastes: an introduction to acid mine Hedin, R.S.; Narin, R.W.; Kleinmann, drainage. In: Managing environmental R.L.P. 1994. Passive treatment of coal Skousen, J.G.; Faulkner, B.; Sterner, P. problems at inactive and . Inf. Circ. 9389. U.S. 1996a. A passive treatment system and metals mine sites. Sem. Publ. EPA/625/ Department of the Interior, Bureau of improvement of water quality. In: R-95/007. Environmental Protection Mines. Skousen, J.G.; Ziemkiewicz, P.F. Agency. Compilers. Acid mine drainage: control Jacenkow, B.; Balcerzak, A.; Gizynski, and treatment. Morgantown, WV: West Eger, P. 1994. Wetland treatment for W.; Gnatowski, M.; Pietak, A.1984. Virginia University and the National trace metal removal from mine drainage: Application of new chemical grouts in Mine Land Reclamation Center: 331- the importance of aerobic and strengthening and sealing of soils. In: 344. anaerobic processes. Water Science Water in mining and underground works Technology. 29(4). Great Britain. (El Agua en la Mineria y Trabajos Subterraneos). Vol. 1. SIAMOS 78: 429- 439. 16 treating acid mine drainage. In: Institute for Mining Development Ziemkiewicz, P.; Skousen, J. 1996. Skousen, J.G.; Ziemkiewicz, P.F. Budapest: 411-423. Overview of acid mine drainage at Compilers. Acid mine drainage: control source: control strategies. In: Skousen, and treatment. Morgantown, WV: West U.S. Department of the Interior, Bureau J.G.; Ziemkiewicz, P.F. Compilers. Acid Virginia University and the National of Reclamation Engineering and mine drainage: control and treatment. Mine Land Reclamation Center: 249- Research Center. 1987. Cement grout Morgantown, WV: 260. flow behavior in fractured rock. Rep. University and the National Mine Land REC-ERC-87-7. Springfield, VA: Reclamation Center: 69-78. Szentirmai, L.1985. Combined system National Technical Information Service. of sealing a mine under heavy karstic B87-235420. 51 p. Ziemkiewicz, P.F.; Skousen, J.G.; Brant, water hazard. In: Mine water: D.L.; Sterner, P.L.; Lovett, R.J. 1997. proceedings of the Second International Ziemkiewicz, P.F. 1996. Post nasal drip. Acid mine drainage treatment with Congress, Granada, Spain, September http://www.info-mine.com/list_archives/ armored limestone in open limestone 1985. Budapest, Hungary: Central enviromine_technical/ channels. Journal of Environmental Quality (26): 1017-1024.

17 About the Author…

John J. Metesh is an associate research hydrogeologist and associate professor for the Montana Bureau of Mines and Geology at Montana Tech. He received a bachelor’s degree in geology from Montana State University and a master’s degree in geological engineering (hydrogeology) from the Montana College of Mineral Science and Technology. He is certified as a professional geologist by the State of Wisconsin.

Terrie Jarrell is an aquatics engineer for the Coeur d’Alene River Ranger District of the Idaho Panhandle National Forests. During 1997 and 1998 she attended the University of Montana and worked part-time for the Missoula Technology and Development Center. She received a bachelor’s degree in environmental engineering from the Montana College of Mineral Sciences and Technology in 1992 and a master’s degree in watershed management from the University of Montana in 1998. She received a professional engineering license from the State of Idaho in 1996.

Steve Oravetz graduated from the University of Washington in civil engineering and is now licensed as a Professional Civil Engineer. He began his career on the Wenatchee National Forest in 1980. He became Chief Engineer for the Northeastern Research Station in 1993. Steve has worked as a city building inspector and as a consulting Civil Engineer. In 1996, he became Engineering Program Leader at MTDC.

Library Card

Metesh, John J.; Jarrell, Terrie; Oravetz, Steve. 1998. Treating acid mine drainage from abandoned mines in remote areas. Tech. Rep. 9871-2821-MTDC. Missoula, MT: U.S. Department of Agriculture, Forest Service, Missoula Technology and Development Center. 22 p.

Describes passive treatments for acid mine drainage suitable for mines abandoned in remote areas. A flow chart shows how treatments could be combined to reduce specific acid mine drainage problems. Although these treatments may not eliminate acid mine drainage, they should reduce it.

Keywords: mined land; passive treatments; remediation

Additional single copies of this document may be ordered from:

USDA Forest Service Missoula Technology and Development Center Building 1, Fort Missoula Missoula, MT 59804-7294 Phone: (406) 329-3900 Fax: (406) 329-3719 IBM: pubs/wo,mtdc E-mail: pubs/[email protected]

An electronic copy of this document is available on the Forest Service’s FSWeb intranet at:

http://fsweb.mtdc.wo.fs.fed.us

For further technical information, contact Steve Oravetz at:

Phone: (406)329-1037 Fax: (406)329-3719 IBM: soravetz/wo,mtdc E-Mail: soravetz/[email protected]

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