Nevin Strock

Department of Environmental Protection, Harrisburg, PA 17105

Mine reclamation practices can influence postmining water quality and the extent to which acid mine drainage is generated. Research suggests mine site rock geochemistry and lithology have more influence on postmining drainage chemistry than does reclamation (diPretoro and Rauch, 1987). Nevertheless, a discussion of reclamation and its relationship to acid mine drainage is warranted. The practices of alkaline addition and special handling of rock overburden during mining and reclamation are discussed in Chapters 13 and 14, respectively. This chapter discusses reclamation and revegetation and the relationship to acid mine drainage. The information presented is based primarily on research and experience with reclamation and revegetation of coal mined lands in Pennsylvania.

Establishing vegetation on coal mined land is required in accordance with state and federal coal mining laws and regulations. Revegetation is an important step in the overall reclamation process. Vegetation aids in stabilizing the soil surface from erosion and controlling siltation. From the standpoint of preventing acid mine drainage, vegetation is beneficial for reducing the amount of water and atmospheric oxygen entering the mine soil/spoil environment.

The Federal Surface Mining Control and Reclamation Act of 1977 (Section 515(b)(20)) requires regions of the country where annual average precipitation is 26 inches (66 centimeters) or less to maintain a minimum period of liability for revegetation success of 10 years on coal mined lands. For regions where annual average precipitation is greater than 26 inches (66 centimeters), the minimum period of liability for revegetation success under the Act is 5 years. The longer liability period for assuring revegetation success reflects the more difficult conditions in arid and semi-arid climates for revegetating mined lands. Problems of revegetating coal mined areas in the arid and semi-arid regions of western United States differ drastically from those in the humid regions of eastern United States (Grimm and Hill, 1974). Pennsylvania’s annual precipitation generally ranges between 34 and 52 inches (86 and 132 centimeters) (Dailey, 1971). The arid and semi-arid regions of the country experience extremely adverse conditions for vegetation establishment on mined land in comparison to Pennsylvania with its humid climate. However, even with Pennsylvania’s humid climate, vegetation on mined land may not be successful if plant rooting is limited or restricted by acid and toxic materials.

Vegetation is a factor in the hydrologic cycle and affects both surface and groundwater. The extent to which the soil/spoil surface is exposed or unvegetated affects evapotranspiration. The rate of evapotranspiration increases as the vegetative cover increases. Evapotranspiration losses may range from 15 inches (38.1 centimeters) per year for barren rocky areas to 35 inches (89 centimeters) per year for heavily forested areas (Wisler and Brater, 1963). The type of vegetation, stage of vegetative growth and density of vegetation affects the amount of water loss through evapotranspiration. Transpiration is limited by the growing season and by available soil moisture.

The grading and vegetation of the surface can have a substantial impact on the quantity of mine drainage generated. For example, average annual precipitation in the West Branch of the Susquehanna River watershed is approximately 40 inches (102 cm). Of this 40 inches (102 cm), an average of 17 inches (43.18 cm) is lost annually through evapotranspiration and 8 inches (20 cm) is lost through overland runoff (Taylor et al., 1983). This leaves an average of 15 inches (38 cm) per year to infiltrate into the groundwater flow system, eventually to discharge directly as baseflow to streams or via springs and seepage zones.

The practical reality of this is that in Pennsylvania and other humid areas where precipitation exceeds evapotranspiration, virtually all mine sites will receive groundwater recharge and generate drainage - acidic or alkaline. That there may be no obvious springs or seeps does not imply that there is no drainage from the site. To illustrate what 15 inches (38 cm) of infiltration per year means in terms of the quantity of mine drainage which can be generated, each acre of spoil surface would produce an average flow rate of 0.75 gallons per minute (gpm) (2.84 L/min). A 100-acre surface mine, then, would yield 75 gpm (284 L/min) of groundwater flow.

Where vegetation is poor, precipitation can rapidly enter mine spoil leaving less opportunity for evapotranspiration and uptake by plants. Further, the sparse vegetation consumes less water, allowing more to infiltrate below the rooting zone. A site with little or poor vegetation would generate much less evapotranspiration and therefore, much higher infiltration into the groundwater system - - ultimately increasing the volume of mine drainage generated by the site.

Hawkins (1995, and Chapter 17, this report) noted that many Pennsylvania remining operations showed reduced pollution loading rates, not so much from changes in mine drainage chemistry but rather due to decreased groundwater recharge. This reduction was due to two factors: the regrading of abandoned mine lands and improved vegetation. Regrading can be particularly important where there is no positive surface drainage. Surface runoff is directed to abandoned pits and surface depressions, ultimately to infiltrate into the subsurface and increase the availability of groundwater in the spoil. Again, using the average runoff rates reported by Taylor, an unreclaimed, internally-drained site may divert approximately 8 inches (20 cm) of precipitation per year (0.4 gpm/acre) (3.74 L per min/ha) into the subsurface. Clearly, proper regrading and revegetation is one effective but frequently overlooked means of minimizing AMD production.

Vegetation and associated microbiological activity also influence the composition of the mine soil/spoil environment. A well vegetated and biologically active mine soil/spoil increases carbon dioxide levels in the mine soil/spoil and reduces oxygen flux into the mine soil/spoil. The oxygen content of mine soil/spoil is frequently inversely related to the carbon dioxide content (i.e. oxygen levels decline as carbon dioxide increases). Oxygen concentrations have been shown to be rate limiting in the oxidation of pyrite in laboratory studies (Erickson, Kleinmann, and Campion, 1982). Oxygen consumption by plants, soil biota and organic material decay provide potential means of limiting oxygen availability for pyrite oxidation and production of acid mine drainage. In addition, high concentrations of carbon dioxide increase the solubility of carbonate minerals. In theory, more alkalinity is generated in groundwater having high carbon dioxide content, all other factors being constant. Gas composition in mine soil/spoil has been reported by a few investigators (Erickson, 1985; Guo, Parizek, and Rose, 1994; and Jaynes, Rogowski, Pionke, and Jacoby, 1983).

Plant species may be used as indicators of mine spoil/overburden chemistry. On disturbed mine land with exposed acidic spoil/overburden, the plant species which volunteer and become established are species which are capable of surviving under acidic conditions.

Observations and field studies by McKee and associates (1982) on several Pennsylvania bituminous strip mine sites found that sites with little or no vegetative cover had mine spoil pH values below 4.5 and were relatively high in soluble aluminum, whereas mine sites with relatively adequate vegetative cover had mine spoil pH values of 4.5 or above and very minimal, if any, soluble aluminum. Greenhouse studies by McKee and associates identified the effects of both low pH and aluminum on plants which they found volunteering or naturally invading Pennsylvania bituminous mine spoils. These studies found volunteer plant species such as poverty grass (Danthonia spp.), deertongue grass (Panicum clandestinum), dewberry (Rubus flagellaris) and fleeceflower (Polygonum cuspidatum) survived at mine spoil 3.3 pH and 17 ppm water soluble aluminum while goldenrod (Solidago spp.), and wild carrot (Daucus carota) did not survive at mine spoil 4.2 pH and 0.5 ppm water soluble aluminum (McKee, Raelson, Berti, and Peiffer, 1982).

Abandoned mine land may remain sparsely vegetated for several years. A study of 20 abandoned mine spoil banks (ranging from 2 to 35 years in age) in central Pennsylvania found extremely acidic spoils with a pH of 3.5 or less with no vegetation and some areas remaining barren or unvegetated after 20 years or more. The study also evaluated the rate of development of vegetative cover on the spoil banks and found spoil banks nearly barren of naturally invading vegetation until about 4 years old. Spoil banks 10 years of age or less did not exceed 50 percent vegetative cover with one bank 29 years of age having only 15 percent vegetative cover (Bramble and Ashley, 1955).

Where vegetation is sparse or nonexistent, the surface of the mine spoil may provide further indications of the mine spoil chemistry. Extremely acid conditions can dissolve many minerals resulting in saline mine spoil conditions. "Greasy/wettish spots" on the mine spoil surface caused by the hygroscopic nature of free acid produced by pyrite oxidation are field indicators of acid saline conditions (Singh, Grube, Smith, and Keefer, 1982). White or yellowish coating or crusting on the mine spoil surface result from evaporated salts when evapotranspiration exceeds rainfall and salts migrate to the surface of the mine spoil. High salt concentrations in the mine spoil inhibit plant growth. The saline conditions increase the osmotic pressure that plants must overcome to extract moisture and render water unavailable to the plants. In addition, essential nutrients may be unavailable to plants under saline spoil conditions. Struthers and Vimmerstedt (1965) concluded that revegetation of mine spoil was generally more successful in the spring months as plant establishment and growth are improved by precipitation infiltrating the mine spoil and leaching the salts from the surface layer.

Species of plants vary widely in tolerance to acid and toxic conditions. Plant physiologists have found that only below pH of 3.0 are plants harmed directly by acidity alone (Arnon and Johnson, 1942). Indirect effects of acidity are the cause of limited plant growth under acid conditions above 3.0 pH. Indirect effects of acidity on plants result from increased solubility and availability of metals such as aluminum and manganese to plants under acid conditions. Aluminum and manganese are known to cause plant toxicity problems on acidic mine spoils.

Aluminum is a major growth limiting factor for plants in many acid soils below 5.0 pH and can be limiting at pH values as high as 5.5 (Foy, Chaney, and White, 1978). Concentrations of soluble aluminum that could be toxic to plants are found in mine spoils that have a pH of 5.5 or below (Berg and Vogel, 1973). Aluminum inhibits root growth and interferes with the uptake of phosphorus, an essential plant nutrient (Foy, Chaney, and White, 1978).

Manganese toxicity in plants is a problem in mine spoils below pH 5.5 where parent materials are high in total manganese (Foy, Chaney, and White, 1978). Decreasing soil pH as well as reducing soil aeration by compaction increases solubility and availability of manganese to plants (National Academy of Science, 1973).

The tolerance of plant species to acidic and toxic mine soil/spoil conditions varies between species and within species. Grasses such as deertongue grass (Panicum clandestinum) and switchgrass (Panicum virgatum) tolerate more acidic conditions than tall fescue (Festuca arundinacea) and perennial ryegrass (Lolium perenne) (Mckee and Harper, 1985). Kenland and Pennscott varieties of red clover (Trifolium pratense) are more adapted to acid mine spoil conditions than Chesapeake and Mammoth varieties of red clover (Bennett, Armiger, and Jones, 1976). Berg and Vogel (1973) reported sericea lespedeza (Lespedeza cuneata) usually had manganese toxicity symptons on mine spoils with 5.0 or lower pH while kobe lespedeza (Lespedeza striata), birdsfoot trefoil (Lotus corniculatus) and black locust (Robinia pseudoacacia) seldom developed manganese toxicity symptons on mine spoils above 4.4 pH. McCormick and Steiner (1978) reported hybrid poplar (Populus maximowiczii x trichocarpa clone NE-388) was very sensitive to low concentrations of aluminum while species of oak (Quercus spp.), pine (Pinus spp.) and birch (Betula spp.) were tolerant of much higher concentrations of aluminum. Tall fescue (Festuca arundinancea) is moderately tolerant of saline soils while red clover (Trifolium pratense) is relatively sensitive to salt conditions (USDA 1954).

Many factors in addition to chemical properties of mine spoil or overburden affect plant species establishment. Physical properties of the spoil or overburden, degree and aspect or direction of slope, and biological conditions are additional factors which may also affect plant species establishment. As an example, Hedin’s (1988) studies of volunteer vegetation on 20 abandoned bituminous surface mines in northwestern Pennsylvania (varying in age from 12 to 41 years since abandonment) found sparsely vegetated spoils with few trees and the ground cover dominated by lichens (primarily Cladonia) and mosses (primarily Polytrichum). Chemicals produced by the lichens plant Cladonia have been shown to reduce tree seed germination and also to inhibit growth of mycorrhizae fungi that are important symbionts or companions of successful colonization and establishment of woody plants on acid spoils (Hedin 1988).

Removal of topsoil prior to mining and replacement of the topsoil as the final cover following coal mining is a requirement of state and federal coal mining laws and regulations. The removal and replacement of topsoil is one of the most beneficial methods for assuring establishment of vegetation (Grim and Hill, 1974). In addition to the benefits of topsoiling for improving vegetation success and restoring premining soil productivity, topsoil also serves to retain water or limit the movement of water; decreases the influx of atmospheric oxygen into the underlying mine spoil; and increases neutralization capacity. These effects of topsoiling on water movement, oxygen influx and developing alkalinity are beneficial for limiting the generation of acid mine drainage.

Topsoil affects the infiltration rate of water. Generally a final cover of topsoil on a mine backfill will have a significantly less infiltration rate of water than a final cover consisting of mine spoil. Rogowski’s (1977) research on mine spoil from lower and middle Kittanning coal seams in central Pennsylvania found the infiltration rate of water into mine spoil to be much higher (i.e. greater than 189 cm/hr) than into mine spoil with a final cover of topsoil (i.e. less than 30 cm/hr).

Compaction of topsoil or other final cover material on a mine backfill could occur during reclamation activities resulting in slower rates of water infiltration. However, compaction of the final cover to reduce infiltration rates should be avoided because of the adverse affects on establishing and maintaining vegetation. Compacted soils impede plant root growth. Soil bulk density is commonly used as an index of compaction. In general, bulk densities suitable for plant growth range from 1.3 to 1.8 g/cm3 (Rogowski and Weinrich, 1983; Pearson, 1965; and Bowen, 1981).

A final cover of topsoil on a mine backfill contributes towards reducing the influx of atmospheric oxygen and decreasing the oxygen concentrations within the underlying mine spoil. Rogwoski’s (1977) research indicated oxygen concentrations within the mine spoil profile decreased when the mine spoil was covered with topsoil. The effects of topsoil on oxygen concentrations within underlying mine spoil becomes more significant considering that mine spoil is typically comprised of coarse rock fragments resulting in substantial pore or void space which provides pathways for oxygen transport. Pennsylvania mine spoils have been found to have from 40 to 60 percent coarse rock fragments (i.e. greater than 2 mm in diameter) in the upper portion of the mine spoil profile (Ciolkosz, Cunningham, Petersen, and Crone, 1979). A discussion of the role of oxygen in pyrite oxidation and generation of acid mine drainage is found in Chapter One.

Caruccio (1968) found in his work in evaluation factors affecting acid mine drainage that a soil cover plays an important role in preventing acid mine drainage. He concluded that the most critical factor determining the presence or absence of acid mine drainage is calcium carbonate. Caruccio also found soil cover to be extremely important in developing alkalinity as high carbon dioxide levels found in soil air contribute towards increasing neutralization capacity.

Replacing topsoil and establishing vegetation following mining are very important reclamation practices in the surface mining of coal. Abandoned coal mined lands may remain sparsely vegetated for several years when dependent upon naturally invading vegetation, especially where the top material is acidic or has high concentrations of aluminum or manganese. In addition to the benefits of topsoil for restoring premining soil productivity and of a good vegetative cover for controlling erosion and siltation, topsoil and vegetation aids in reducing water movement to the underlying mine spoil, decreasing oxygen concentrations, and increasing the capacity for carbonate dissolution. These effects on water movement, oxygen concentrations and neutralization capacity aid in reducing or preventing acid mine drainage.

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