This article was prepared by Jeff Skousen, Professor of Soil Science and Land Reclamation Specialist and John Foreman. It appeared in the winter 2000 issue of "Green Lands" magazine.
In 1999, the Acid Drainage Technology Initiative published "A Handbook of Technologies for Avoidance and Remediation of Acid Mine Drainage." The Handbook is comprised of five chapters: 1. Introduction, 2. Alkaline Addition and Overburden/Refuse Reclamation, 3. Engineered Structural Techniques, 4. Active Treatment Technologies, and 5. Passive System Technologies. The Handbook can be obtained by contacting Kelley Schillingburg (formerly Wolfe) at WVU (304-293-2867 x5444) or by the internet via www.wvu.edu/~agexten/landrec/land.htm . This article is a condensed version of Chapter 3 of the Handbook.
Diverting surface water above a mined site to decrease the amount of water entering the mined area is highly recommended in acid-producing areas. This technique can control water volume and direction and minimize the effects of acid mine drainage (AMD) on receiving streams. Surface diversion of runoff involves construction of drainage ditches to move surface water quickly off the site before infiltration or to limit its movement into the backfill. The diversion is accomplished either by ditching on the uphill side of surface mines or by providing new channels or impervious channels of existing surface streams to convey water across the disturbed area.
A strategy for alkaline loading can be accomplished by diverting surface water into beds of alkaline material (slag or other lime materials) to pick up alkalinity and allowing the alkaline water to flow into spoils or underground mine pools. Alkaline loading of water upgradient of mined areas or before it enters the backfill buffers the effects of subsequent contact with acid water.
Soil Covers and Plastic Liners
Covers are constructed from natural or man-made materials that retard or divert the movement of water and oxygen into areas containing acid-producing rock. Soil covers can achieve substantial reductions in water flow through piles, but generally do not control AMD completely. Plastic liners are rarely used in mining because covering large volumes of waste with a liner is usually too expensive. However, this method may be appropriate in settings where isolation of small pods of acid-producing material is possible.
At the Upshur Mining Complex in West Virginia, Meek (1994) reported covering a 20-ha spoil pile with a 39-mil PVC liner. This treatment reduced acid loads by 70%. In Illinois, a soil cover system was developed for a coal refuse pile that was a graded and compacted, and overlain by a compacted clay liner and a protective soil cover (Gentile et al. 1997). Infiltration was reduced by about 18% initially compared to an unreclaimed refuse surface. Over time as the material in the cover consolidates and becomes vegetated, the cover should reduce infiltration by 80% or more.
Removing water, one of the principal reactants in pyrite oxidation, should theoretically stop the production of AMD. Without water to move reaction products from the surfaces of pyrite, no contamination of waters should occur. While this can be done in a laboratory setting, complete removal of water in nature is nearly impossible. However, reducing the amount of water contacting pyritic material and containing water that is in contact with acid-producing materials may reduce the impacts of AMD to off-site water bodies and streams. Removing water before it contacts pyritic material by pumping may also be done. A series of peripheral deep wells were installed to dewater abandoned deep mines adjacent to the Crouch Mining Dahlquandy surface mine in Britain to allow the water to be discharged without treatment (Norton 1987). This mine is one of the largest open cast mines in the UK and the drainage from surrounding deep mines required chemical treatment before discharge. The water quality from pumped wells was alkaline and metal concentrations were low enough to allow water to be discharged to receiving streams without treatment. Another benefit was the improved working conditions in this pit with respect to water management.
Draining water away from pyritic materials as rapidly as possible may keep water from reacting and forming acid products. Water may move around mine tailings or compacted refuse of low permeability if placed within high-permeability overburden within a backfill. If the permeability contrast between the tailings and the surrounding material is large, groundwater will flow around the tailings mass rather than through it, and metal leaching will be minimized. This phenomenon is indicated at the Falconbridge Fault Lake tailings site (St-Arnaud 1994).
Drains can be installed in various places to move water from one area to another. Chimney drains collect water from a backfill or valley fill and convey the water through a long, high, column of coarse sandstone. Highwall drains collect water from the highwall along permeable channels of sandstone or limestone. French drains are small, distinct constructed channels in disturbed rock, usually along the pavement, where water is directed to flow out of the fill. Blanket or bottom drains are constructed by forming a coarse, permeable rock covering along the entire fill bottom to allow water to move out of the fill through the base. All of these methods are reliable methods for moving water from spoil, refuse, and fills. However, in one case a blanket drain in a hydrologically isolated surface mine in WV did not reduce AMD formation (Geidel and Caruccio 1984).
Water diversion away from acid-producing materials or dewatering is not always the best approach when these materials can be rapidly and permanently inundated, thereby minimizing oxidation of acid-forming materials. Inundation is only suggested where a water table may be re-established to cover the materials (such as below drainage deep mines) and has not been recommended for surface mined lands or above drainage deep mines in the mountainous Appalachian region. Complete inundation has been successful in other areas where acid-producing materials are submerged in lakes or other permanent impoundments.
Constructing impoundments to inundate isolated areas of surface mines has been used to minimize or eliminate AMD. Saturation of acid-producing spoils may not always improve pH, but there is usually some reduction in metal concentrations. However, the drainage often has a less deleterious effect on downstream water quality than that from unreclaimed areas. The creation of an impoundment in the final cut of a surface mine forms recreation areas, aids in recharging the water table in the local area, and can eliminate or greatly reduce the amount of pollution from AMD and silt. The impoundment can also be designed so the body of water will completely flood any deep mine workings or auger mining holes, thereby limiting pyrite oxidation.
Orava et al. (1997) reported that in-pit disposal of 3 million tons of spoil from an open pit gold mine inhibited acid generation. The elevated water table flooded the tailings and caused an improved water quality in the pit. Flooding of inactive zinc and lead mine workings at the Eagle Mine in Colorado submerged about 80% of the mined ore body. Water pH has increased from 3 to 6, and Zn concentrations have decreased from 350 mg/L to 50 mg/L (Neukirchner and Hinrichs 1997). Perry et al. (1997) found that acid-producing materials placed in a seasonal water table produced AMD in the eastern coal fields of the U.S. Fluctuating water tables or partial flooding produced worse drainage than that predicted for either "dry" or submerged placement.
The disposal of acid-producing materials in an impoundment to limit its exposure to air and water has been used for decades with mixed results. Many coal slurry impoundments have been constructed for disposal of pyritic materials near coal preparation plants. In most cases, acid products are stored in the slurry impoundment and are released as water drains from the impoundment. Many coal refuse disposal sites must treat the drainage with chemicals before discharge.
Disposal of sulfide tailings under a water cover, such as a lake or fjord, has been used extensively. Wet covers also include flooding of above ground tailings ponds. Fraser and Robertson (1994) found tailings submerged in four freshwater lakes were not reactive and dissolved metal concentrations in the lake water were very low. Pedersen et al. (1994 and 1997) concluded that little Cd, Pb or Cu were being released from submerged tailings in Anderson Lake in north-central Manitoba, Canada (See also Amyot and Vezina 1997, Arnesen et al. 1997, and Dave and Vivyurka 1994).
Inundation of an underground mine can be an effective method of decreasing AMD by depriving pyrite of oxygen. In addition, if overlying rocks contain carbonate minerals, flooding can provide additional alkalinity by increasing the volume of alkaline strata in contact with mine water. On the other hand, if the mine walls contain readily soluble oxidation products of sulfides, inundation will cause a temporary increase in acid concentrations that should decline over time. If the water table fluctuates and the mine does not remain inundated, oxidation of pyrite can cause continued water pollution and the temporary increase in acid concentrations becomes an increase that does not decline over time (Younger 1997).
Underground Mine Sealing
Throughout most of the Appalachian coal fields, abandoned underground mines are the principal source of AMD pollution. This is based on the sheer number and areal extent of abandoned underground mine workings where most older mines were developed up-dip to promote gravity drainage. The AMD problems associated with older underground mines can be aggravated by inadequate barrier pillars between mines, inadequate outcrop barriers, and hydraulic interconnection of adjacent mine complexes.
While AMD problems associated with some of these older mines can be addressed by remining the abandoned mine complex, this option is not economical for most abandoned mine complexes in the Appalachian coal fields (Skousen et al. 1997). Mine sealing can minimize the AMD pollution associated with abandoned underground mines. The primary factor affecting the selection, design and construction of underground mine seals is the anticipated hydraulic pressure that the seal will have to withstand when sealing is completed.
A dry mine seal is a wall across a mine entrance where water does not drain from the entrance. A wet mine seal is a wall across a draining mine entrance that allows water flow through the seal but prevents air from entering the mine. Production of AMD can be inhibited to the extent that the seal raises the water level in the mine and inundates the workings. Although complete blockage of adits at the down-dip side of underground mines has been attempted in order to prevent drainage and to raise the water level in the mine, this procedure has commonly led to breakout of the water, sometimes explosively, either at the seal or at nearby locations. The placement and construction of mine seals, therefore, must be carefully planned and executed.
Surface access seals (or dry seals) are installed in entries where little or no hydrostatic pressure will be exerted on the seals. The primary functions of these seals are to eliminate access to the mine and to decrease AMD production by limiting movement of air and water into the deep mine. Dry seals are typically constructed of concrete block, masonry, or concrete-flyash mixtures, and are often backfilled from the front side of the seal (Figure 1 and Picture 1). The lack of hydraulic head allows these seals to be simple in construction and low cost. These seals have good long term effectiveness due to the lack of hydraulic head (pressure) on the seals.
Air trap seals (wet seals) are installed in mine entries where mine discharges flow from the mine. These seals were installed extensively in the 1930's through the Works Project Administration (WPA) mine sealing efforts. Wet seals almost always were constructed with concrete blocks (Figure 2) and either holes were left or pipes were inserted into the block wall to allow drainage. Unfortunately, the long term effectiveness of these old wet seals was generally poor. Failures occurred when debris and sediment clogged the hole or pipe, thereby increasing the head of the impounded water and resulting in collapsing or leaking seals. A study done by the US Bureau of Mines (Borek et al. 1991) showed that 14 mine seals installed in 1967 were all intact and only a few leaked. Water quality from the mines actually improved over time.
Due to the leaking or collapse of many wet seals, hydraulic mine seals are being constructed in most current wet sealing situations. This type of mine seal serves as a structural bulkhead and acts as a water tight dam capable of withstanding the maximum hydrostatic head that may develop as a result of flooding the mine complex (Figure 3 and Picture 2). Cracks and fissures from surrounding rock, which may allow water migration around the seal, are also treated to restrict water conduction. Water movement around seals can be minimized by pressure grouting of adjacent strata, increasing the mine seal thickness, and installing additional seals.
Accessibility of the mine and seal location are important design considerations. Dry or wet seals placed at easily-reached portal entrances are considerably cheaper than portals with poor access. In many cases, mine seals can be remotely placed through drill holes (Figure 4). Foreman et al. (1969) constructed 69 remotely-placed, deep mine seals at Moraine State Park in Pennsylvania to abate discharges from 22 abandoned mine complexes draining to Lake Arthur. Maksimovic and Maynard (1982) assessed the seals and concluded that alkalinity and Fe in water from the mines increased, and acid loads decreased. Overall, they found mine sealing improved the water quality of Lake Arthur (see also Foreman et al. 1970). Foreman et al. (1979) designed and constructed a large clay dike and 35 hydraulic drift seals at Lake Hope State Park in Ohio. Nichols (1983) found water pH from the mines had increased nearly two units and a viable bass fishery has been reestablished in Lake Hope. Measurements in 1998 showed water quality from the mines is still around pH 6.0. Mine seals placed in slope and shaft mines also have been successful (see Foreman and Foreman 1985 and 1992).
Coal barrier pillars are intact blocks of coal left unmined to provide hydraulic barriers for water management as well as to provide roof support. Coal barrier pillars can be classified as peripheral barriers (intact coal at the edge of the deep mine works) or internal barriers (intact coal located in the interior of the mine works). Typically, peripheral barriers at abandoned mine sites are accessible if the barriers are situated above regional drainage, but peripheral barriers below drainage and internal barriers at abandoned mine sites are usually unventilated and often flooded making them unsafe.
Foreman et al. (1979) constructed a large clay dike and 35 hydraulic drift seals at Lake Hope State Park in Ohio. The clay dike was constructed to replace the outcrop coal barrier along the edge of the mine. The dike allowed the mine complex to achieve full inundation when coupled with the construction of the mine seals. Foreman and Foreman (1985) constructed a concrete mine seal in the breached barrier of the active Island Creek Coal Company Providence #1 deep mine in Kentucky. The miners had inadvertently mined through the barrier into the adjacent, abandoned Hall-Luton #9 deep mine which was fully flooded at that time. The breach flooded 60% of the active Providence mine causing temporary closure. The MSHA-approved mine sealing project was totally successful and allowed the mine to be reopened and returned to full productivity (other examples are in Foreman and Glenn 1980, Foreman et al. 1977, and Foreman et al. 1982).
Alkaline slurry trench barriers were constructed at the Stewart Run and Nutter Run sites in West Virginia to cause inundation of adjacent underground workings. The barriers were placed during surface remining activities. Several problems occurred during barrier construction so that complete inundation of the underground works did not occur, but the water quality has improved steadily since the barriers were placed in 1978 (Moore 1989, Skousen et al. 1999).
Grout Curtains and Walls
Grouts can be used to separate acid-producing rock and groundwater. Injection of grout curtains may significantly reduce the volume of groundwater moving through spoil and thereby greatly reduce the amount of AMD coming from a site. In one sense, grouting to form curtains or walls is analogous to underground water diversion. Gabr et al. (1994) characterized the groundwater flow of an acid-producing reclaimed site where a 1.5-m thick wall was installed by pumping a mixture of Class F fly ash and portland cement grout into vertical boreholes near the highwall. After two years, the grout wall reduced groundwater inflow from the highwall to the spoil by 80%, resulting in one of two seeps completely drying up and substantially reducing the flow of the other seep.
Foreman et al. (1973) constructed a grout curtain along Interstate 80 in Pennsylvania to abate seepage through an inadequate coal barrier, which was causing slippage of the road-cut highwall and AMD pollution to the Clarion River. The grout curtain was placed by drilling injection holes along the top of the road-cut. After grouting, no further discharges or slippages were observed.
Underground Mine Filling and Injection
Due to the miles of passages in underground mines where coal has been removed, huge volumes of void space are available for mine pools to develop, and this water is often acidic. Filling the mine voids completely or creating barriers inside the mine to break up interconnected underground pools may be used to control flow and improve drainage quality. Materials to fill underground mines must be cheap and readily available, so waste products such as steel slags and fly ash are generally used in these situations.
A mixture of Class F fly ash and cement kiln dust was injected down a series of holes into the Longridge Mine in West Virginia. Limestone barriers were placed into three areas and grout materials were injected behind the barriers, thereby creating three unconnected segments in the mine. Preliminary investigations show that the injection has resulted in a 90% reduction in flow from the mine. At the Frazee Mine in Maryland, a Class F fly ash, along with FBC and FGD ashes were injected to improve drainage quality. The mine was larger than originally anticipated and the entire mine void was not filled. Slight improvements in water quality have been noted 18 months after injection. At the Mettiki mine in Maryland, a mixture of FGD ash, coal refuse and calcium-generated AMD sludge has been injected into underground mine works. Metal concentrations have dropped substantially and water pH has increased from 3 to 4.5.
Water management techniques for controlling AMD include water diversion, soil covers and plastic liners, dewatering, inundation, underground mine sealing, barriers, grout curtains and walls, and underground mine filling by injection. Each method is suited for specific situations and good success can be realized when adequate planning, design, and construction are practiced. Water diversion is one of the easiest and cheapest methods for reducing the amount of water in contact with acid-producing materials. Special care and planning are essential in designing and constructing mine seals and when using grouting techniques for underground mine filling or barrier construction.
Picture 1. A concrete block dry seal placed at an abandoned mine land reclamation project in West Virginia. (Photo courtesy of the West Virginia Division of Environmental Protection, Office of Abandoned Mine Lands and Reclamation).
Picture 2. A hydraulic wet seal was constructed with a gravel bulkhead for water and subsidence control at the Douglas Abandoned Mine Land Reclamation Project.
The authors gratefully acknowledge the work of members of the Acid Drainage Technology Initiative and especially Art Rose, Gwen Geidel, Robert Evans, and Bill Hellier. Thanks are also extended to Paul Ziemkiewicz and Courtney Black, National Mine Land Reclamation Center at West Virginia University, for information about grouting projects.
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