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Mining and Acid Mine Drainage


INTRODUCTION

BEST MANAGEMENT PRACTICES

ACTIVE MINING MEASURES
Surface Stabilization
Dust Control
Mulching
Riprap
Sodding
Surface Roughening
Temporary Gravel Construction Access
Temporary and Permanent Seeding
Topsoiling
Runoff Control and Conveyance Measures
Grass-Lined Channel
Hardened Channel
Paved Flume (Chute)
Runoff Diversion
Temporary Slope Drain
Outlet Protection
Level Spreader
Outlet Stabilization Structure
Sediment Traps and Barriers
Brush Barrier
Check Dam
Grade Stabilization Structure
Sediment Basin/Rock Dam
Sediment Fence/Straw Bale Barrier
Sediment Trap
Temporary Block and Gravel Drop Inlet Protection
Temporary Excavated Drop Inlet Protection
Temporary Fabric Drop Inlet Protection
Temporary Sod Drop Inlet Protection
Vegetated Filter Strip
Stream Protection
Check Dam
Grade Stabilization Structure
Streambank Stabilization
Temporary Stream Crossing
ACID MINE DRAINAGE MEASURES
Anoxic Limestone Drains
Chemical Treatment
Covering
Reclamation
Runoff Diversion
Wetlands, Constructed
Wetlands, Natural and Restored


LINKS

REFERENCES

INTRODUCTION

Mining activities and inactive mine sites can generate a variety of pollutants, including some of the most environmentally detrimental compounds of any discharging activity. In Appalachia alone, there are around 66,500 documented sources of active and inactive coal mines which have polluted an estimated 10,500 miles of streams. In the eastern U.S., there are around 4000 active or abandoned coal piles and impoundments totaling 3 X 109 tons of refuse. Midwestern mines generate runoff that affects over 5000 miles of streams and rivers (Cohen and Gorman, 1991). Sources can be divided into those requiring sediment and erosion controls (largely the active mining process), and those, both active and inactive sites, that produce acid mine drainage, which requires more involved treatment.


BEST MANAGEMENT PRACTICES

ACTIVE MINING MEASURES

Successful control of erosion and sedimentation from mining activities should involve a system of BMPs which targets each stage of the erosion process. The most efficient approach involves minimizing the potential sources of sediment from the outset. This means limiting the extent and duration of land disturbance to the minimum needed, and protecting surfaces once they are exposed. The second stage of the BMP system involves controlling the amount of runoff and its ability to carry sediment by diverting incoming flows and impeding internally generated flows. The third stage involves retaining sediment which is picked up on the project site through the use of sediment-capturing devices. On most sites successful erosion and sedimentation control requires a combination of structural and vegetative practices. All of these stages are better performed using advance planning and good scheduling. The following is a collection of BMPs for erosion and sediment control for active mining activities. Sources of material for this section include the North Carolina Nonpoint Source Management Program report (NCDEHNR, 1989), Erosion and Sediment Control Planning and Design Manual (Smolen et al., 1988), and other references as noted.


ACTIVE MINING MEASURES-SURFACE STABILIZATION

Dust Control: Watering, mulching, sprigging, or applying geotextile materials to a construction area to prevent soil loss as dust. Redeposited dust can become a source of sediment in runoff. Control measures should be applied routinely and thoroughly in drier seasons and climates for effective dust control.

Mulching: A protective blanket of straw or other plant residue, gravel, or synthetic material applied to the soil surface to minimize raindrop impact energy and runoff, foster vegetative establishment, reduce evaporation, insulate the soil, and suppress weed growth. Mulch provides immediate protection, and straw mulch is also typically used as a matrix for spreading plant seed. Organic mulches such as straw, wood chips, and shredded bark have been found to be the most effective. Straw typically requires some kind of tacking, such as liquid emulsions or netting. Netting may also be needed to hold mulch in place on slopes. Mats made from a wide variety of organic and synthetic materials are useful in establishing grass in channels and waterways, and they promote seedling growth (Smolen et al., 1988). Mulching assists in the first, source reduction, and second, conveyance, stages of a BMP system.

Riprap: A layer of stone designed to protect and stabilize areas subject to erosion, slopes subject to seepage, or areas with poor soil structure. Riprap is used on slopes where vegetation cannot be established, channel slopes and bottoms, stormwater structure inlets and outlets, slope drains, streambanks, and shorelines. It should be a well-graded mixture of stone sizes, and should be underlain by a filter blanket of gravel, sand and gravel, or synthetic material to prevent soil movement into or through the riprap (Smolen et al., 1988). Riprap can assist in all stages of a BMP system.

Sodding: Permanent stabilization of exposed areas by laying a continuous cover of grass sod. Sod is useful for providing immediate cover in steep critical areas and in areas unsuitable for seed, such as flowways and around inlets. Sod must be rolled over after placement to ensure contact, and then watered. Sodded waterways and steep slopes may require netting and pegging or stapling (Smolen et al., 1988). Sodding assists in the first, source reduction, and second, conveyance, stages of a BMP system.

Surface Roughening: Roughening a bare, sloped soil surface with horizontal grooves or benches running across the slope. Grooves can be large-scale, such as stair-step grading with small benches or terraces, or small-scale, such as grooving with disks, tillers, or other machinery, or with heavy tracked machinery which should be reserved for sandy, noncompressible soils. Roughening aids the establishment of vegetative cover, improves water infiltration, and decreases runoff velocity, assisting in the first, source reduction, and second, pollutant transport, stages of a BMP system (Smolen et al., 1988).

Temporary Gravel Construction Access: A graveled area or pad located at points where vehicles enter and leave a construction site, this BMP provides a buffer area where vehicles can drop their mud and sediment to avoid transporting it onto public roads, to control erosion from surface runoff, and to help control dust (Smolen et al., 1988). This measure assists in the third, pollutant capture stage of a BMP system.

Temporary and Permanent Seeding: Temporary seeding involves planting rapid-growing annual grasses, small grains, or legumes to provide initial, temporary stabilization to minimize runoff, erosion, and sediment yield on disturbed soils that will not be brought to final grade for more than approximately one month. Seeding is facilitated by fertilizing and surface roughening. Broadcast seeds must be covered by raking or chain dragging, while hydroseed mixtures are spread in a mulch matrix (Smolen et al., 1988). Permanent seeding involves establishment of perennial vegetative cover with seed on disturbed areas. Disturbed soils typically require amendment with lime, fertilizer, and roughening. Seeding should be done together with mulching. Mixtures are typically most effective, and species vary with preferences, site conditions, climate, and season (Smolen et al., 1988). Temporary and permanent seeding assist in the first, source reduction stage of a BMP system.

Topsoiling: Preserving and subsequently using the upper, biologically active layer of soil to enhance final site stabilization with vegetation. Topsoiling should not be conducted on steep slopes. Stockpiled soil should be contained with sediment barriers, and temporarily seeded for stability. Surfaces which will receive topsoil should be roughened just prior to spreading the soil to improve bonding. Spread topsoil should be lightly compacted to ensure good contact with the subsoil. Topsoil can act as a mulch, promoting final vegetation establishment, increasing water infiltration, and anchoring more erosive subsoils, assisting in the first, source reduction, and second, pollutant transport, stages of a BMP system (Smolen et al., 1988).


ACTIVE MINING MEASURES-RUNOFF CONTROL AND CONVEYANCE

Grass-Lined Channel: A swale vegetated with grass which is dry except following storms and serves to convey specified concentrated stormwater runoff volumes, without resulting in erosion, to disposal locations. Typical uses include roadside swales, outlets for runoff diversions, site stormwater routing, and drainage of low areas. Channels should conform to the natural drainage patterns. Channels are not meant to collect sediment, as it will reduce their conveyance capacity. Lining with geotextile or other material is required if design flows are to exceed 2 feet per second. Channel vegetation should be allowed to establish before flows are introduced (Smolen et al., 1988). Channels assist in the second, conveyance, stage of a BMP system.

Hardened Channels: Channels with erosion-resistant linings of riprap, paving, or other structural material designed for the conveyance and safe disposal of excess water without erosion. Hardened channels replace grass-lined channels where conditions are unsuitable for the latter, such as steep slopes, prolonged flows, potential for traffic damage, erodible soils, or design velocity over 5 feet per second (Smolen et al., 1988). Channels assist in the second, conveyance, stage of a BMP system.

Paved Flume: A small concrete-lined channel to convey water down a relatively steep slope without causing erosion. Flumes serve as stable, permanent elements of a stormwater system receiving drainage from above a relatively steep slope, typically conveyed by diversions, channels, or natural drainageways. Setting the flume well into the ground is important, particularly on fill slopes. Some means of energy dissipation should be provided at the outlet, and an inlet bypass route should be available for extreme flows (Smolen et al., 1988). Flumes assist in the second, conveyance, stage of a BMP system.

Runoff Diversions: Structures that channel upslope runoff away from erosion source areas, divert sediment-laden runoff to appropriate traps or stable outlets, or capture runoff before it leaves the site, diverting it to locations where it can be used or released without erosion or flood damage. Diversions include graded surfaces to redirect sheetflow, diversion dikes or berms which force sheetflow around a protected area, and stormwater conveyances (swales, channels, gutters, drains, sewers) which intercept, collect and redirect runoff (USEPA, 1992). Diversions can be either temporary or permanent in nature. Temporary diversions include excavation of a channel along with placement of the spoil in a dike on the downgradient side of the channel, and placement of gravel in a ridge below an excavated swale. Permanent diversions are used to divide a site into specific drainage areas, should be sized to capture and carry a specific magnitude of design storm, and should be constructed of more permanent materials. A water bar is a specific kind of runoff diversion that is constructed diagonally at intervals across a linear sloping surface such as a road or right-of-way that is subject to erosion. Water bars are meant to interrupt the accumulation of erosive volumes of water through their periodic placement down the slope, and divert the resulting segments of flow into adjacent undisturbed areas for dissipation (Smolen et al., 1988). Runoff diversions assist in the second, conveyance, stage of a BMP system.

Temporary Slope Drain: Flexible tubing or conduit extending temporarily from the top to the bottom of a cut or fill slope for the purpose of conveying concentrated runoff down the slope face without causing erosion. They are generally used in conjunction with diversions to convey runoff down a slope until permanent water disposal measures can be installed (Smolen et al., 1988). Temporary slope drains assist in the second, conveyance, stage of a BMP system.


ACTIVE MINING MEASURES-OUTLET PROTECTION

Level Spreader: An outlet designed to convert concentrated runoff to sheet flow and disperse it uniformly across a slope without causing erosion. This structure is particularly well-suited for returning natural sheet flows to exiting drainage that has been altered by development, especially for returning sheet flows to receiving ecosystems such as wetlands where dispersed flow may be important for maintain pre-existing hydrologic regimes. The outlet's receiving area must be uniformly sloped and not susceptible to erosion. Particular care must be taken to construct the outlet lip completely level in a stable, undisturbed soil to avoid formation of an outlet channel and subsequent erosion. Erosion-resistant matting of some kind may be necessary across the outlet lip depending on expected flows. Alternative designs to minimize such channeling include hardened structures, stiff grass hedges, and segmenting discharge flows into a number of smaller, adjacent spreaders. The level spreader is often used as an outlet for runoff diversions (Smolen et al., 1988). Level spreaders assist in the second, conveyance, stage of a BMP system.

Outlet Stabilization Structure: A structure designed to control erosion at the outlet of a channel or conduit by reducing flow velocity and dissipating flow energy. This should be used where the discharge velocity of a structure exceeds the tolerances of the receiving channel or area. Designs will vary based on discharge specifics and tailwater conditions. A riprap-lined apron is the most commonly used practice for this purpose because of its relatively low cost and ease of installation. Riprap stilling basins or plunge pools should be considered in lieu of aprons where overfalls exit at the ends of pipes or where high flows would require excessive apron length (Smolen et al., 1988). Outlet stabilization structures assist in the second, conveyance, stage of a BMP system.


ACTIVE MINING MEASURES-SEDIMENT TRAPS AND BARRIERS

Brush barriers: Temporary sediment barriers constructed of limbs, weeds, vines, root mat, soil, rock, or other cleared materials piled together to form a berm, and located across or at the toe of a slope susceptible to sheet and rill erosion.

Check Dam: A small dam constructed across a drainageway to reduce channel erosion by restricting flow velocity. Check dams should not be used in live streams. They can serve as emergency or temporary measures in small eroding channels that will be filled or permanently stabilized at a later date. They can also serve as permanent measures that will sediment in over time in gullies, which is a more common usage in range and agricultural settings. In permanent usage, when the impounded area is filled, a relatively level surface or delta is formed over which the water flows at a noneroding gradient. the water then cascades over the dam through a spillway onto a hardened apron. By constructing a series of check dams along the gully, a stream channel of comparatively steep slope or gradient is replaced by a stair-stepped channel consisting of a succession of gently slopes with "cushioned" cascades in between (Gray and Leiser, 1982). For temporary usage, consider the alternatives of protecting the channel bottom with materials such as riprap, geotextile, biodegradable, or other matting, or other linings in combination with vegetation before selecting check dams (Smolen et al., 1988). Dams can be nonporous, such as those constructed from concrete, sheet steel, or wet masonry, or they can be porous, using available materials such as straw bales, rock, brush, wire netting, boards, and posts. Porous dams release part of the flow through the structure, decreasing the head of flow over the spillway and the dynamic and hydrostatic forces against the dam. Nonporous dams are durable, permanent, and more expensive while porous dams are simpler, more economical to construct, and temporary. For construction details on a number of temporary check dam types, see Gray and Leiser (1982).

Grade Stabilization Structure: A structure designed to reduce channel grade in natural or constructed watercourses to prevent erosion of a channel that results from excessive grade in the channel bed or artificially increased channel flows. This practice can prevent headcutting or stabilize gully erosion. Grade stabilization structures may be vertical drop structures, concrete or riprap chutes, gabions, or pipe drop structures. Permanent ponds or lakes may be part of a grade stabilization system. Concrete chutes are often used as outlets for large water impoundments where flows exceed 100 cfs and the drop is greater than 10 ft. Where flows exceed 100 cfs but the drop is less than 10 ft., a vertical drop weir constructed of reinforced concrete or sheet piling with concrete aprons is generally recommended. Small flows allow the use of prefabricated metal drop spillways or pipe overfall structures. Designs can be complex and usually require detailed site investigations. Design of large structures (100 cfs) requires a qualified engineer. The National Engineering Handbook (Drop Spillways, Section 11, and Chute Spillways, Section 14) prepared by the USDA Natural Resources Conservation Service gives detailed information useful in the design of grade stabilization structures (Smolen et al., 1988).

Sediment Basin/Rock Dam: An earthen or rock embankment located to capture sediment from runoff and retain it on the construction site, for use where other on-site erosion control measures are not adequate to prevent off-site sedimentation. Sediment basins are more permanent in nature than sediment traps, and can be designed as permanent features of a development. Basins are most commonly used at the outlets of diversions, channels, slope drains, or other runoff conveyances that discharge sediment-laden water. Earthen basins should use barrel and riser discharge structures, while rock dams can be designed to discharge over the top of the embankment, where a crest should be constructed as the low point. Smaller gravel should line the inside face of the rock dam (Smolen et al., 1988). Sediment basins and rock dams assist in the third, capture, stage of a BMP system.

Sediment Fence (Silt Fence)/ Straw Bale Barrier: A temporary sediment barrier consisting of filter fabric buried at the bottom, stretched, and supported by posts, or straw bales staked into the ground, designed to retain sediment from small disturbed areas by reducing the velocity of sheet flows. Because silt fences and straw bales can cause temporary ponding, sufficient storage area and overflow outlets should be provided. Ends must be well-anchored (Smolen et al., 1988; USEPA, 1993). Sediment fences and straw bale barriers assist in the third, capture, stage of a BMP system. These fences are some of the most common, visible controls used and most often mis-used or poorly used.

Sediment Trap: A small, temporary ponding basin formed by an embankment or excavation to capture sediment from runoff. Traps are most commonly used at the outlets of diversions, channels, slope drains, or other runoff conveyances that discharge sediment-laden water. It is important to consider provisions to protect the embankment from failure from runoff events that exceed the design capacity. Plan for nonerosive emergency bypass areas. Make traps readily accessible for periodic maintenance. High length-to-width ratios minimize the potential for short-circuiting. The pond outlet should be a stone section designed as the low point (Smolen et al., 1988). Sediment traps assist in the third, capture, stage of a BMP system.

Temporary Block and Gravel Inlet Protection: A temporary sediment control barrier formed around a storm drain inlet by the use of standard concrete block and gravel, to filter sediment from stormwater entering the inlet prior to stabilization of the contributing area soils, while allowing use of the inlet for stormwater conveyance. The height of the barrier should allow overflow into the inlet and not let overflow bypass the inlet to unprotected lower areas. An alternative design eliminates the blocks and involves only a gravel doughnut around the inlet. This practice can be used in combination with other temporary inlet protection devices, such as excavation and fabric (Smolen et al., 1988). Inlet protection structures assist in the third, capture, stage of a BMP system.

Temporary Excavated Drop Inlet Protection: A temporary excavated area around a storm drain drop inlet or curb inlet designed to trap sediment prior to discharge into the inlet. This practice allows use of the permanent inlet early in the development prior to stabilization of the contributing area soils. Frequent maintenance is required. This practice can be used in combination with other temporary inlet protection devices, such as fabric and block and gravel (Smolen et al., 1988). Inlet protection structures assist in the third, capture, stage of a BMP system.

Temporary Fabric Drop Inlet Protection: A temporary fabric barrier placed around a drop inlet to help prevent sediment from entering storm drains during construction operations, while allowing use of the inlet for stormwater conveyance. The height of the barrier should allow overflow into the drop inlet and not let overflow bypass the inlet to unprotected lower areas. This practice can be used in combination with other temporary inlet protection devices, such as excavation and block and gravel (Smolen et al., 1988). Inlet protection structures assist in the third, capture, stage of a BMP system.

Temporary Sod Drop Inlet Protection: A permanent grass sod sediment filter area around a storm drain drop inlet for use once the contributing area soils are stabilized. This area is well-suited for lawns adjacent to large buildings (Smolen et al., 1988). Inlet protection structures assist in the third, capture, stage of a BMP system.

Vegetated Filter Strip (VFS): A low-gradient vegetated area that filters solids from overland sheet flow. VFSs can be natural or planted, should have relatively flat slopes, and should be vegetated with dense-culmed, herbaceous, erosion-resistant plant species. The main factors influencing removal efficiency are the vegetation type and condition, soil infiltration rate, and flow depth and travel time, which are affected by size of contributing area, and slope and length of strip. Channelized flows decrease the effectiveness of VFSs. VFSs are often used as buffers bordering on construction areas. Level spreaders are often used to distribute runoff evenly across the VFS (Dillaha, 1989; USEPA, 1993).


ACTIVE MINING MEASURES-STREAM PROTECTION

Check Dam: A small porous or nonporous dam constructed across a drainageway to reduce channel erosion by restricting flow velocity. Check dams should not be used in live streams. They can serve as emergency or temporary measures in small eroding channels that will be filled or permanently stabilized at a later date. They can also serve as permanent measures that will sediment in over time in gullies, which is a more common usage in range and agricultural settings. In permanent usage, when the impounded area is filled, a relatively level surface or delta is formed over which the water flows at a noneroding gradient. the water then cascades over the dam through a spillway onto a hardened apron. By constructing a series of check dams along the gully, a stream channel of comparatively steep slope or gradient is replaced by a stair-stepped channel consisting of a succession of gently slopes with "cushioned" cascades in between (Gray and Leiser, 1982). For temporary usage, consider the alternatives of protecting the channel bottom with materials such as riprap, geotextile, biodegradable, or other matting, or other linings in combination with vegetation before selecting check dams (Smolen et al., 1988). Dams can be nonporous, such as those constructed from concrete, sheet steel, or wet masonry, or they can be porous, using available materials such as straw bales, rock, brush, wire netting, boards, and posts. Porous dams release part of the flow through the structure, decreasing the head of flow over the spillway and the dynamic and hydrostatic forces against the dam. Nonporous dams are durable, permanent, and more expensive while porous dams are simpler, more economical to construct, and temporary. For construction details on a number of temporary check dam types, see Gray and Leiser (1982).

Grade Stabilization Structure: A structure designed to reduce channel grade in natural or constructed watercourses to prevent erosion of a channel that results from excessive grade in the channel bed or artificially increased channel flows. This practice can prevent headcutting or stabilize gully erosion. Grade stabilization structures may be vertical drop structures, concrete or riprap chutes, gabions, or pipe drop structures. Permanent ponds or lakes may be part of a grade stabilization system. Concrete chutes are often used as outlets for large water impoundments where flows exceed 100 cfs and the drop is greater than 10 ft. Where flows exceed 100 cfs but the drop is less than 10 ft., a vertical drop weir constructed of reinforced concrete or sheet piling with concrete aprons is generally recommended. Small flows allow the use of prefabricated metal drop spillways or pipe overfall structures. Designs can be complex and usually require detailed site investigations. Design of large structures (100 cfs) requires a qualified engineer. The National Engineering Handbook (Drop Spillways, Section 11, and Chute Spillways, Section 14) prepared by the USDA Natural Resources Conservation Service gives detailed information useful in the design of grade stabilization structures (Smolen et al., 1988).

Streambank Stabilization: For a discussion of different technologies used to stabilize streambanks from erosion due to excess runoff or artificially increased flows, please click here.

Temporary Stream Crossing: A bridge, ford, or temporary structure installed across a stream or water course for short-term use by construction vehicles or heavy equipment, intended to keep sediment out of the stream and avoid damage to the streambed. Stream crossings should be avoided if at all possible, since they are a direct source of water pollution, they can cause flooding, and they are expensive to construct. While bridges are the most expensive method, they are the most preferred, as they cause the least disturbance to streambeds, banks, and surrounding floodplain, they provide the least obstruction to flow, and have the least erosion potential. Culvert crossings are the most common form of crossing, but can cause the most damage to the stream environment, cause the most flow blockage, and therefore can result in the most erosion. Fords involve making cuts in the banks and placing stone over filter cloth in the stream. They are often used in steep areas subject to flash flooding where normal flow is shallow (<3 inches deep) or intermittent. Fords should only be used where crossings are infrequent and banks are low. Temporary crossings may overtop during peak storm events, unlike permanent crossings. Fill in the floodplain should be kept to a minimum to reduce erosion potential and avoid upstream flooding. Choose crossing sites where erosion potential is low. Try to locate temporary crossings where permanent crossings will occur. Where appropriate, install in-stream sediment traps immediately below stream crossings to reduce downstream sedimentation. Temporary stream crossings, and bridge designs in particular, should be undertaken by a qualified engineer (Smolen et al., 1988).



ACID MINE DRAINAGE MEASURES

Anoxic Limestone Drains: Acid mine drainage (AMD) is characteristically low in pH and high in certain metals, such as iron and manganese. Constructed aerobic wetlands can provide a major removal mechanism for metals through oxidation/reduction and hydrolysis reactions producing insoluble precipitates. However, while such reactions effectively remove metals from drainage water, they can further lower the pH in poorly buffered wetlands. Such wetlands require a source of alkalinity. If drainage water is anoxic and iron and aluminum are in reduced form, an option for producing alkalinity is to route this AMD, maintaining anaerobic conditions, through an anoxic limestone drain (ALD) prior to aerobic wetland treatment. An ALD is simply a quantity of high quality limestone, sealed in plastic to maintain anaerobic conditions, typically buried in a trench, over which the drainage water is passed. The limestone reacts with the free protons to impart bicarbonate buffering capacity to the AMD. The anaerobic conditions are needed to keep metal hydroxides from forming immediately, armoring and clogging the limestone, and sealing it from further reaction. Precipitates can instead form upon aeration in a settling pond or wetland system.

If, on the other hand, the drainage water contains large amounts of dissolved oxygen, ferric iron, and aluminum, it can be run first through an anaerobic wetland to exhaust the DO and reduce the metals. Such an anaerobic wetland can be generated by introducing large amounts of organic matter, such as compost, into a constructed wetland. In combination with a limestone supply, such a system can produce alkaline water for treatment by an aerobic settling pond/wetland. The discharge from these systems may require some degree of active chemical treatment to meet receiving water standards. Thus, anoxic limestone drains are often necessary as the first practice in a passive BMP system for treating AMD (Brodie et al., 1993; Skousen et al., 1994).

Chemical Treatment: Among nonpoint sources, a process for ameliorating acid mine drainage involves capturing the discharge and treating to neutralize, remove metals from, and soften the water. Neutralization is typically performed with lime, which also precipitates iron and aluminum carbonates, followed by further metals removal through any of a number of processes including aeration and hydrogen sulfide introduction. These treatments produce a sludge that must be stored. Soda ash can be used to lower calcium and magnesium levels (Powell, 1988; Cohen and Gorman, 1991). Chemical treatment is most effectively used as part of a system to address acid mine drainage, which would also include a means of diverting incoming water from making contact with tailings or exposed formations, and measures to mitigate the severity of such contamination sources. Such measures might include covering, runoff diversion, sumps and pumping systems, and reclamation.

Covering: The partial or total physical enclosure of stockpiled or stored material, loading/unloading areas, or processing operations, this BMP is applicable to mining sources such as tailings piles and surface impoundments used for waste storage and disposal. Drainage from a covering is captured and directed around potential contamination areas. Coverings are used, in the form of clay caps, plastic sheeting, and plastic foam, to exclude water from coal mine tailings to prevent acid mine drainage (Powell, 1988). This measure is useful for mitigating metals pollution and acidity from mining operations (USEPA, 1992). Covering is most effective as part of a system of BMPs which also addresses interception of runoff prior to contact with potential sources of contamination, as well as BMPs which address treatment of contaminated discharge from such sources.

Reclamation: Erosion and pollution from mine tailings can be minimized through land reclamation. Tailings can be modified and/or isolated from the surrounding environment. Modification includes leaching, amendment applications, and biological treatment. Isolation involves separation of tailings from potential receiving waters and can include construction of barriers and depth isolation (Cohen and Gorman, 1991).

Runoff Diversion: Structures that channel runoff away from pollutant source areas include graded surfaces to redirect sheetflow, diversion dikes which force sheetflow around a protected area, and stormwater conveyances (swales, channels, gutters, drains, sewers) which intercept, collect and redirect runoff. Diversion features are useful in mining settings to prevent contamination with metals and high acidity (USEPA, 1992). Sumps and pumping systems can be used to remove water from mine tailings areas (Powell, 1988).

Wetlands, Constructed: Interest has steadily increased in the United States over the last two decades in the use of natural physical, biological, and chemical aquatic processes for the treatment of polluted waters. This interest has been driven by growing recognition of the natural treatment functions performed by wetlands and aquatic plants, by the escalating costs of conventional treatment methods, and by a growing appreciation for the potential ancillary benefits provided by such systems. Aquatic treatment systems have been divided into natural wetlands, constructed wetlands, and aquatic plant systems (USEPA, 1988). Of the three types, constructed wetlands have received significant application in treatment of mine drainage. Constructed wetlands are a subset of created wetlands designed and developed specifically for water treatment (Fields, 1993). They have been further defined as:

engineered systems designed to simulate natural wetlands to exploit the water purification functional value for human use and benefits. Constructed wetlands consist of former upland environments that have been modified to create poorly drained soils and wetlands flora and fauna for the primary purpose of contaminant or pollutant removal from wastewaters or runoff (Hammer, 1992).
Constructed wetlands as defined here are not typically intended to replace all of the functions of natural wetlands, but to serve as do other water quality BMPs to minimize point source and nonpoint source pollution prior to its entry into streams, natural wetlands, and other receiving waters. Constructed wetlands which are meant to provide habitat, water quantity, aesthetic and other functions as well as water quality functions (termed created, restored, or mitigation wetlands (Hammer, 1994) typically call for different design considerations than those used solely for water quality improvement, and such systems are not addressed here. In fact, debate continues over the advisability of intentionally combining primary pollution control and habitat functions in the same constructed facilities. Nonetheless, constructed wetlands can provide many of the water quality improvement functions of natural wetlands with the advantage of control over location, design, and management to optimize those functions. While costs can vary significantly, constructed wetlands have successfully provided these functions at lower cost than conventional wastewater treatment options (USEPA, 1988).

Constructed wetlands vary widely in their pollutant removal capabilities, but can effectively remove a number of contaminants (Bastian and Hammer, 1993; Bingham, 1994; Brix, 1993; Corbitt and Bowen, 1994; USEPA, 1993). Among the most important removal processes are the purely physical processes of sedimentation via reduced velocities and filtration by hydrophytic vegetation. These processes account for the strong removal rates for suspended solids, the particulate fraction of organic matter (particulate BOD), and sediment-attached nutrients and metals. Oils and greases are effectively removed through impoundment, photodegradation, and microbial action. Similarly, pathogens show good removal rates in constructed wetlands via sedimentation and filtration, natural die-off, and UV degradation. Dissolved constituents such as soluble organic matter, ammonia and ortho-phosphorus tend to have lower removal rates. Soluble organic matter is largely degraded aerobically by bacteria in the water column, plant-attached algal and bacterial associations, and microbes at the sediment surface. Ammonia is removed largely through microbial nitrification(aerobic)-denitrification(anaerobic), plant uptake, and volatilization, while nitrate is removed largely through denitrification and plant uptake. In both cases, denitrification is typically the primary removal mechanism. The microbial degradation processes are relatively slow, particularly the anaerobic steps, and require longer residence times, a factor which contributes to the more variable performance of constructed wetlands systems for these dissolved constituents. Phosphorus is removed mainly through soil sorption processes which are slow and vary based on soil composition, and through plant assimilation and subsequent burial in the litter compartment. Consequently, phosphorus removal rates are variable and typically trail behind those of nitrogen. Metals are removed largely through adsorption and complexation with organic matter. Removal rates for metals are variable, but are consistently high for lead, which is often associated with particulate matter.

The use of constructed wetlands for treatment of acid mine drainage (AMD) is a "passive" technology, and provides a potential alternative to the conventional, "active" methods of chemical treatment with alkaline reagents which are costly and must be continued indefinitely (Brodie et al., 1993; Skouson et al., 1994). However, wetlands treatment of AMD is still an emerging technology, so long-term treatment effectiveness data do not yet exist and there are no widely accepted design criteria. There have been both successful and unsuccessful attempts to date, and the characteristics of AMD appear to present greater design challenges than more conventional applications face. Nonetheless, technical understanding of AMD wetland treatment issues is improving. The two major AMD contaminants from operations that encounter pyrite, which includes coal mines, are characteristically acidity and metals, usually iron (Fe) and manganese (Mn). Significant metals removal can take place through physical/chemical cation exchange and complexation with organic matter, both of which occur in the substrate. This physical filtering function is ultimately finite as saturation of all available sites will occur. On the other hand, oxidation/reduction reactions yielding precipitation occur in wetlands and can provide a major sink for metals.

A system of constructed wetland BMPs is most effective and often necessary for optimizing chemical precipitation reactions in AMD. For example, if the constructed wetland is not sufficiently buffered, metal precipitation reactions can increase the acidity of the discharge. Such wetlands require a source of alkalinity. If drainage water is anoxic and iron and aluminum are in reduced form, an option for producing alkalinity is to route this AMD, maintaining anaerobic conditions, through an anoxic limestone drain prior to aerobic wetland treatment. If the drainage water contains large amounts of dissolved oxygen, ferric iron, and aluminum, then it can instead be run through an anaerobic wetland to exhaust the D.O. and reduce metals. Such an anaerobic wetland can be generated by introducing large amounts of organic matter, such as compost. In combination with a limestone supply, such a system can produce alkaline water for treatment by an aerobic settling pond/wetland. The discharge from these systems may require some degree of active chemical treatment to meet receiving water standards. Thus, wetlands constructed at or near the source of acid mine drainage discharges can effectively concentrate and immobilize metals and raise pH at a fraction of the cost of conventional treatment of acid mine drainage. Systems designed with an anaerobic component that maximizes sulfate reduction produce the byproducts of hydrogen sulfide and carbonate alkalinity given an alkalinity source (limestone), which precipitate metals as sulfides and raise pH, respectively (Hedin et al., 1989).

Some base and precious metal mining operations can produce almost every type of heavy metal contamination in hazardous concentrations in acid drainage, and can present a more severe problem than most coal mine drainages (Wildeman and Laudon, 1989). In these cases it appears that optimizing the ability of wetlands to precipitate and neutralize metals using redox potentials and through the generation of ammonia and bicarbonate may be the most effective long-term metals storage approach. Organic matter consumption throughout wetlands will generate ammonia and bicarbonate, which will raise pH and cause hydroxide precipitation. Anaerobic zone bacteria will reduce undesirable sulfate to hydrogen sulfide, which precipitates metals as sulfides. Thus, wetland design to facilitate these functions should: optimize organic substrates which foster bacteria that raise pH; optimize anaerobic zones that foster sulfide-producing bacteria; and optimize plant biomass production.

Two key considerations in deciding whether to construct wetlands for AMD are the wetland area needed for a particular flow and chemistry, and resulting cost relative to conventional chemical treatment. The U.S. Bureau of Mines proposed a rule of thumb in 1986 based on empirical survey information, and the TVA subsequently provided recommendations based on water chemistry and empirical observations of functioning systems. These guidelines are summarized in Weider et al. (1989).

Wetlands, Natural And Restored: The many water quality improvement functions and values of wetlands are now widely recognized. At the same time, concern has grown over the possible harmful effects of toxic pollutant accumulation and the potential for long-term degradation of wetlands from altered nutrient and hydraulic loading that can occur with the use of wetlands for water treatment. Because of these concerns, the use of natural wetlands as treatment systems is restricted by federal law (Fields, 1993). Most natural wetlands are considered "waters of the United States" and are entitled under the CWA to protection from degradation by NPS pollution. Natural wetlands do function within the watershed to improve water quality, and protection or restoration of wetlands to maintain or enhance water quality are acceptable practices. However, NPS pollutants should not be intentionally diverted to wetlands for primary treatment. Wetlands must be part of an integrated landscape approach to NPS control, and cannot be expected to compensate for insufficient use of BMPs within the upgradient contributing area. Restored wetlands are subject to the same restrictions as unmodified natural wetlands. Wetlands created from upland habitat for the purpose of mitigating the loss of other wetlands as required by regulatory agencies are generally also subject to the same restrictions as natural wetlands. Constructed wetlands (see WETLANDS, CONSTRUCTED above), which have been defined as a subset of created wetlands that are designed and developed specifically for water treatment (Fields, 1993), clearly are not intended for the same protections as natural wetlands, and can serve as valuable BMP options.

LINKS

Land Reclamation - West Virginia University
Office of Surface Mining - Department of the Interior

REFERENCES

Bastian, R.K., and D.A. Hammer, 1993. The Use of Constructed Wetlands for Wastewater Treatment and Recycling. Pages 59-68. In G.A. Moshiri (ed.), Constructed Wetlands for Water Quality Improvement, CRC Press, Boca Raton, FL.

Bingham, D.R., 1994. Wetlands for Stormwater Treatment. Pages 243-262. In D.M. Kent (ed.), Applied Wetlands Science and Technology. Lewis Publishers, CRC Press, Boca Raton, FL. 436pp.

Brix, H., 1993. Wastewater Treatment in Constructed Wetlands: System Design, Removal Processes, and Treatment Performance. Pages 9-22. In G.A. Moshiri (ed.), Constructed Wetlands for Water Quality Improvement, CRC Press, Boca Raton, FL.

Brodie, G.A., C.R. Britt, T.M. Tomaszewski, and H.N. Taylor, 1993. Anoxic Limestone Drains to Enhance Performance of Aerobic Acid Drainage Treamtnet Wetlands: Experiences of the Tennessee Valley Authority. Pages 129-138. In G.A. Moshiri (ed.), Constructed Wetlands for Water Quality Improvement, CRC Press, Boca Raton, FL.

Cohen, R.R.H., and J. Gorman, 1991. Mining-Related Nonpoint Source Pollution. Water Environment & Technology, 3(6):55-59.

Corbitt, R.A., and P.T. Bowen, 1994. Constructed Wetlands for Wastewater Treatment. Pages 221-241. In D.M. Kent (ed.), Applied Wetlands Science and Technology. Lewis Publishers, CRC Press, Boca Raton, FL. 436pp.

Fields, S., 1993. Regulations and Policies Relating to the Use of Wetlands for Nonpoint Source Pollution Control. Pages 151-158. In R.K. Olson (ed.), Created and Natural Wetlands for Controlling Nonpoint Source Pollution, C.K. Smoley, CRC Press, Boca Raton, FL.

Gray, D.H., and A.T. Leiser, 1982. Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold Company, New York, NY.

Hammer, D.A., 1992. Designing Constructed Wetlands Systems to Treat Agricultural Nonpoint Source Pollution. Ecological Engineering, 1:49-82.

Hammer, D.A., 1994. Guidelines for Design, Construction and Operation of Constructed Wetlands for Livestock Wastewater Treatment. Pages 155-181. In P.J. DuBowy and R.P. Reaves (eds.), Constructed Wetlands for Animal Waste Management: Proceedings of Workshop. Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN. 188pp.

Hedin, R.S., R. Hammack, and D. Hyman, 1989. Potential Importance of Sulfate Reduction Processes in Wetlands Constructed to Treat Mine Drainage. Pages 508-514. In D.A. Hammer (ed.), Constructed Wetlands for Wastewater Treatment: Municipal, Industrial,and Agricultural. Lewis Publishers, Chelsea, MI.

NCDEHNR, 1989. North Carolina Nonpoint Source Management Program, Report no. 89-02, April 1989. North Carolina Department of Environment, Health, and Natural Resources, Raleigh, NC.

Powell, J.D., 1988. Origin and Influence of Coal Mine Drainage on Streams of the United States. Environ. Geol. Water Sci., 11(2):141-152.

Skousen, J., A. Sexstone, K. Garbutt, and J. Sencindiver, 1994. Acid Mine Drainage Treatment With Wetlands and Anoxic Limestone Drains. Pages 263-281. In D.M. Kent (ed.), Applied Wetlands Science and Technology. Lewis Publishers, CRC Press, Boca Raton, FL. 436pp.

Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, A.L. Lanier, W.W. Woodhouse, and S.W. Broome, 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission, NC Dept. of Natural Resources and Community Development, Raleigh, NC.

USEPA, 1988. Design Manual: Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewateqr Treatment. EPA/625/1-88/022. U.S. Environmental Protection Agency, Office Of Research and Development, Washington, DC. 83pp.

USEPA, 1992. Storm Water Management For Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-006. U.S. Environmental Protection Agency, Office Of Water, Washington, DC.

USEPA, 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution In Coastal Waters. EPA-840-B-92-002, January 1993. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

Weider, R.K., G. Tchobanoglous, and R.W. Tuttle, 1989. Preliminary Considerations Regarding Constructed Wetlands for Wastewater Treatment. Pages 297-305. In D.A. Hammer (ed.), Constructed Wetlands for Wastewater Treatment: Municipal, Industrial,and Agricultural. Lewis Publishers, Chelsea, MI.

Wildeman, T.R., and L.S. Laudon, 1989. Use of Wetlands for Treating of Environmental Problems in Mining: Non-Coal-Mining Applications. Pages 221-231. In D.A. Hammer (ed.), Constructed Wetlands for Wastewater Treatment: Municipal, Industrial,and Agricultural. Lewis Publishers, Chelsea, MI.