Authors: W. Lee Daniels, Associate Professor, and Barry Stewart and Dennis Dove, Graduate Research Assistants, Department of Crop and Soil Environmental Sciences, Virginia Tech
Publication Number 460-131, June 1996
This publication reviews problems associated with stabilization and revegetation of coal refuse disposal areas and suggests strategies for their successful long-term reclamation and closure. The primary focus is establishment of vegetation, but other refuse stabilization issues are discussed. The reader is encouraged to consult the papers and reports cited in the bibliography for specific details and technical data. The regulatory framework discussed in this paper is specific to Virginia, but it is similar to that of other coal mining states in the Appalachian coal region.
Modern coal cleaning technologies have allowed coal preparation facilities to become quite efficient at removing sulfur compounds, waste rock and low grade coals from run-of-mine coal. Up to 50 percent of the raw mined product may end up as refuse, particularly when the coal originates from longwall mining operations or is high in partings, rock, and impurities. The refuse materials vary from coarse fragments removed by physical screening to very fine materials removed by flotation and density separation processes.
The potential hazards of improperly reclaimed refuse include contamination of surface and groundwaters by acidic leachates and runoff, erosion and sedimentation into nearby water bodies, spontaneous combustion, and damage from landslides. While these problems were common on refuse piles constructed prior to the 1970s, modern regulations attempt to minimize the environmental impact of coal refuse disposal. Several, if not all, of the problems associated with coal refuse piles can be reduced significantly by the maintenance of a viable plant cover. A vigorous plant community can reduce water and oxygen movement down into the fill, thereby limiting the production of acidic leachates, while reducing sediment losses and stabilizing the fill surface. Establishment and maintenance of permanent vegetation on refuse, however, is complicated by physical, mineralogical, and chemical factors.
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Refuse disposal areas are generally constructed as large valley fills, with surface waters diverted around or through drains under the completed fill. These fills are commonly hundreds of acres in size and are perched in the headwaters of many watersheds. The refuse is compacted in place, and the entire fill must meet rigorous geotechnical stability standards. Many refuse disposal areas are constructed using a "zoned disposal" concept where refuse slurry generated in the fine coal cleaning circuit is impounded behind a compacted dam of coarse refuse. The face and sideslopes of the fills are generally constructed to a steep gradient to minimize the total disturbed area, and these steep slopes greatly complicate reclamation. Most fills are designed for a lifetime of tens of years. Therefore, many active fills were designed before current regulations were in force.
Once the fill is completed, regulations require that "the site shall be covered with a minimum of 4 feet of the best available non-toxic and non-combustible materials." Less than 4 feet of alternative materials may be used if chemical and physical analyses indicate its properties are conducive to establishing a permanent vegetative cover, and the applicant can prove that the standards for revegetation success can be met. Thick topsoiling is quite costly and may be impractical in areas where native soils are shallow. Extensive topsoiling also creates the problem of reclaiming the borrow areas.
Coal refuse disposal areas are required to meet the same standards for revegetation success following the 5-year bond liability period as surface mined sites. Topsoiling may be the only option available on some sites, due to toxic properties of the materials, but direct seeding appears to be a viable alternative for many refuse materials. Documented field trials have generally been required to evaluate the suitability of refuse materials as a plant growth medium, since reliable laboratory testing methods correlated with plant growth response have not been available. It is our belief, however, that many coal refuse piles can be successfully direct seeded by following the procedures outlined in this paper without long-term dedicated on-site experimental trials.
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Many factors influence the reclamation potential of a given coal waste pile, including the geologic source of the refuse, the prep plant processes, and local site conditions. The following sections summarize properties and conditions known to influence refuse pile reclamation and surrounding environmental quality and relates them to reclamation planning.
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Coal refuse is usually composed of rock fragments derived from interseam shale or siltstone partings and waste rock materials from above or below the seam. The refuse shares many properties with the associated coal seam. For example, some coal seams are inherently high in sulfur (i.e. the Pittsburgh seam of Northern Appalachia ), some are low in sulfur (the Pocahontas seam of the South Central Appalachian Basin), and some are variable. Southwest Virginia coal seams and associated strata are generally low in sulfur compared to other Appalachian states. As a result, Virginia coal refuse tends to be comparatively low in sulfur and associated potential acidity (see Table 1 ).
The processes utilized in the prep plant also influence the physical and chemical properties of the refuse stream. Some prep plants re-combine coarse and fine refuse fractions before disposal, while others dispose of these fractions separately or in zoned fills. Our work has focused on the reclamation of coarse refuse and re-combined refuse materials, and not upon slurry impoundments. The approach to reclaiming slurry materials would be similar to that described here, once the surface has stabilized.
The content and reactivity of pyritic sulfur exert a dominant influence on refuse chemical properties over time. The efficiency of a preparation plant at removing sulfur from the marketed coal and the degree to which the sulfide fragments are fractured and reduced in size influence the reactivity and potential acidity of the final refuse product. Numerous reagents and additives such as cationic surfactants, fuel oil, and strong bases are used in various separation processes and also end up in the refuse stream to some extent. The influence of these additives on the revegetation potential of fresh refuse has not been studied.
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Older piles, which pre-date the enactment of SMCRA and VDMLR regulations, often have steeply sloping sides which remain uncompacted. The surfaces of these abandoned piles tend to slide downward, exposing fresh refuse with hard rains. For successful revegetation, these slopes must be regraded to stable angles. No amount of vegetative cover will stabilize materials with fundamental slope instability problems. Fine refuse particles washed from recently exposed surfaces present problems of acidification and sedimentation in nearby streams. This is predominantly a problem with abandoned piles, constructed prior to enactment of modern reclamation law, that are not under permit.
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The pyrite reaction rate is dependent not only on the oxygen supply and microbial catalysis, but also on the size and morphology of pyrite particles. Two types of pyrite are commonly found in coal. Framboidal (fine) pyrite forms concurrently with the coal, while coarse-grained pyrite is a secondary product of coal formation and is usually found in former plant structures and joints in the coal. Framboidal pyrite particles (2-15 µ) have a high surface area and will oxidize rapidly. Coarse-grained pyrite is much less reactive. In some refuse materials, a large amount of the total sulfur is contained in relatively unreactive organic forms or as sulfate, which is one of the reaction products of the oxidation processes discussed above. These two forms are not generally considered to be acid producing. Thus, the total-S content of refuse is not as reliable a predictor of acid producing potential as is pyritic-S content.
Freshly exposed pyritic refuse often has a near-neutral pH. After oxidation, pH values can drop dramatically, and many pyritic coal refuse materials have a very low (2.0 to 3.5) pH once they weather. After complete oxidation of sulfides and subsequent leaching of acid salts, the pH often rises into the low 4's but is strongly buffered in that range by aluminum and other metals. The pH of a particular refuse material will depend not only on its pyrite content, but also on the length of exposure time and its acid-neutralizing capacity. Most coal refuse materials in the Appalachians contain an excess of oxidizable sulfur compared to neutralizing carbonates and are therefore net acid producing over time. The average fresh refuse material in Virginia requires 10 tons of CaCO3 per 1000 tons of raw refuse to neutralize the acidity present, assuming complete reaction of pyrite and carbonates via the regular acid-base accounting technique ( Table 1 ). The potential acidity of refuse materials in West Virginia and Kentucky is often much higher, sometimes exceeding 50 tons of lime requirement per 1000 tons.
The rate of pyrite oxidation and acid production is generally highest in the oxygenated surface layer, which is also the zone utilized by plant roots. A rapid drop in pH releases plant-toxic concentrations of acid-soluble metal ions into soil solution and reduces the availability of many plant nutrients. When the pH falls below 4.5, root growth of many plant species ceases. Another problem caused by pyrite oxidation is the production of sulfate salts, which may accumulate to toxic levels in the root zone. These salts are generally water-soluble and accumulate on coal wastes during dry periods as water is lost by surface evaporation. The whitish surface coating seen on refuse and coal piles during dry weather is evidence of this process (see Figure 2 ).
Heavy metals such as copper, nickel, cobalt, and zinc are often associated with pyrite and other sulfide minerals. Our studies have indicated that soil solutions leaching from coal refuse materials may contain much higher concentrations of Cu, Zn, and Ni than previously assumed. Elevated levels of heavy metals in soil solution can be toxic to plant roots and microbes and may pose a water quality hazard.
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Pyrite oxidation is catalyzed by Thiobacillus ferooxidans, and the ubiquitous bacteria are capable of functioning in very low oxygen (< 1.0% partial pressure) environments. Therefore, as long as acid water is allowed to percolate through a refuse fill, pyrite oxidation will occur deep within the pile, regardless of surface revegetation and stabilization efforts. The net-leaching environment of the Appalachians assures that acid mine drainage is inevitable for any coal refuse pile that contains net acid-forming materials. Due to the total mass of the pyrite in many refuse piles and the relatively slow rate of water movement through them, it is reasonable to expect that acid mine drainage will be emitted for decades, if not longer.
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Modern refuse piles are generally lower in coal than older piles, due to improved coal separation technologies, and are compacted in place to limit air and water penetration. The thick topsoil requirement for refuse pile reclamation is also intended to further limit oxygen movement into the fill, although our results indicate that significant S oxidation occurs in refuse even under 4 feet of topsoil cover. Reports of combustion of modern refuse fills are very rare. When they do occur, they are generally the result of arson or accidental ignition.
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Soil P does not leach from the rooting zone in the same fashion as N; however, P is readily converted into soil mineral forms which are not available to plants. Soil P held in organic forms is protected against these losses, so the establishment and turnover of an organic matter pool in the reclaimed "refuse soil" is also critical for long term P-fertility. Organic amendments such as sewage sludge supply large amounts of N and P in addition to their beneficial effects on the soil physical environment and should be considered for use on refuse piles when available (see Figure 3 ). For additional discussion of N and P behavior in mine soils see VCE Publication 460-121.
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Plant roots are able to extract nearly all available water which is retained in the rooting zone of refuse (usually the upper 24 inches) if potential acidity has been neutralized. There are a number of ways to increase moisture retention in coal refuse. The addition of organic amendments, heavy mulching, and the natural process of soil organic matter accumulation over time will all improve the water supplying ability of coal refuse. We have frequently observed that the addition of only several inches of topsoil or similar finer spoil materials to an otherwise barren coal refuse material is all that is necessary to promote plant growth, in cases where potential acidity has been neutralized. This occurs because the cover material improves water retention and supply. In older piles where weathering has taken place, the upper surface may contain very fine particles similar in texture to silt or clay; such materials will have a higher moisture retention than coarse, fresh refuse. When revegetating older piles where soil cover is expensive or limited, weathered surface materials should be segregated prior to regrading and then re-applied to the pile as final cover.
Virginia mining regulations require coal refuse to be "spread in layers no more than 2 feet in thickness; and compacted to attain 90 percent maximum dry density to prevent spontaneous combustion and to provide the strength required for the stability of the coal processing waste bank." Excessive compaction has been identified as a major factor limiting reclamation success throughout the USA and will cause similar problems in coal refuse materials by limiting the available rooting depth. Whenever possible (i.e. on near-level or mildly sloping surfaces, where surface stability is not a major concern), the final lift or surface of the refuse pile should be left as loose as possible to enhance its potential to support plant growth.
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Although many different companies and researchers have evaluated strategies for establishing plant cover on coal refuse, a unified study to incorporate a variety of potentially important factors (such as refuse properties, weathering with time and reclamation strategies) has not been reported. Often, experiments are conducted and abandoned before the long-term effectiveness of a single or multiple treatments can be determined. Also, many studies are site specific; remedies are developed for localized conditions, and no effort is made to correlate results with refuse character or general reclamation principles.
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Moisture stress, induced by high coarse fragment contents, salts, and high surface heat, is the primary growth limiting factor in most fresh coal refuse. As the materials weather, acidity becomes a major problem in some refuse; but acidity can be controlled to a large extent by liming. Many coal refuse materials can be successfully direct seeded once their potential acidity has been neutralized through appropriate liming practices (See Figure 5 and Figure 6 ). Reagents and chemicals used in mineral processing may also limit plant growth in fresh wastes, but little is known about their effects. Once the coal refuse weathers and leaches for several years, and its physical and chemical properties stabilize, it becomes easier to utilize as a plant growth medium. Many of the older abandoned piles in the Appalachians are invaded by native pioneer vegetation after this stabilization occurs. Care should be taken not to disturb this fragile surface zone on older piles (if possible) during reclamation.
The use of a reduced thickness of soil cover (< 4 feet) to reclaim coal refuse has been successful in several experiments in Virginia and other states. Even thin (< 1 ft.) layers can provide enough water-holding capacity and suitable rooting environment for establishment of both grasses and legumes on moderately acidic wastes. Thicker covers may be necessary for long-term legume vigor on highly acidic refuse. The use of lime at the refuse/soil contact is essential when thin topsoil covers are employed; lime application rates should be based on the potential acidity of the underlying material. Where high surface temperature and low water supply are major problems, topsoiling also appears to be the best alternative for establishing a permanent vegetative cover. Direct seeding appears feasible for refuse with low to moderate levels of acidity, particularly when heavy agricultural lime, mulch, and other organic treatments like sewage sludge are employed. Topsoiling plus liming is the best alternative for highly acidic materials.
Revegetation strategies should establish a quick annual cover to rapidly provide shade and a natural mulch for perennials. Any plant materials used on coal refuse must be capable of withstanding extreme short- and long-term changes in soil and site conditions. The importance of overcoming the heat and water holding limitations of bare refuse cannot be overemphasized. The combination of liming, fertilization, surface treatments, and seeding mix must be designed to rapidly establish an annual cover that will shade the surface and thereby improve soil moisture and temperature conditions. The initial cover crop also takes up and holds essential plant nutrients against leaching and runoff and then returns these nutrients to the soil as it decomposes. The permanent perennial species then germinate and establish in the favorable micro-climate provided by the cover crop. Once the perennial species are well established (usually by the second year) and plant/soil nutrient cycles have become established, the chances for long-term reclamation success (and bond release) are greatly improved. Over the years we have observed many vigorous stands of annual cover crops on direct seeded coal refuse materials. However, diverse self-sustaining stands of perennial grasses and legumes after multiple seasons are much more difficult to achieve.
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Particle size distribution should be determined by sieve analysis. Any refuse that is less than 20 percent fines (< 2mm) will be difficult to reclaim regardless of acidity levels and should be topsoiled. It is possible to increase the water holding capacity of coarse refuse with additions of organic amendments and fine textured soils as discussed later. Compaction is also a major factor in limiting water holding in refuse materials. Therefore, for direct seeding options, the surface 18 inches of refuse should be left uncompacted or should be ripped before seeding.
Potential acidity should be determined by a qualified laboratory using either the conventional acid-base accounting method or the hydrogen peroxide oxidation technique. These two techniques give somewhat different estimates of the liming requirement for refuse materials (see Table 1 ), with the peroxide oxidation technique being more conservative. Potential acidity or acid base accounting (ABA) results are typically reported in net tons of lime required per 1000 tons of spoil or refuse tested. Given that one acre of refuse to a depth of six inches weighs approximately 1000 tons, these figures equate to a field liming estimate in tons per acre. Simple measurements of pH are not valid for estimating refuse potential acidity since they do not account for unoxidized pyritic sulfur and/or the native lime content in the sample, and the chemical reactions in the weathering refuse will cause the pH to change with time.
The ABA lime requirements should be considered as a bare minimum lime application; additional quantities may be applied to help ensure success. Many experts in the field of acid mine drainage control advocate the use of two to four times the amount of lime prescribed by the ABA technique to insure that the treated zone of acid-forming material is permanently stabilized. Studies have shown that in some cases the rate of pyrite oxidation is so fast and the levels of iron plus acidity generated in solution are so high that a large excess of reactive lime is necessary to prevent the alkaline side of the balance from being overwhelmed.
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Where possible, it is advisable to allow fresh refuse to lie exposed for a period of 6 months or more before seeding. During this time, refuse samples representative of areas to be seeded should be collected and analyzed for potential acidity as discussed earlier. Dependent on this analysis, agricultural lime or other suitable liming materials should be applied and incorporated 2 to 3 months before planting. It is possible to reduce the potential acidity of highly acidic materials (as discussed in Table 2 ) by repeated addition of lime over an extended period. Should this method be used, it is recommended that no more than 25 tons/acre of lime be applied at any one time. Single applications using higher rates have been shown to form FeCO3 concretions around larger sized lime particles rendering the lime ineffective unless the lime is thoroughly incorporated to a depth of six inches or more. Similar problems have been noted when coarse textured liming materials have been utilized.
Sloping areas are of particular concern in site preparation. Often lateral water flow through a pile will result in an acid seep or "hot spot" along the slope. These areas often appear chalky white during dry weather and may exhibit a pH below 3.0. These "hot spots" should be pinpointed and treated heavily with lime where possible to prevent future problems in plant establishment.
Immediately prior to seeding, sloping areas should be prepared for seeding. The conventional approach is to track the slope with a dozer or other suitable equipment. Tracking should be done in a manner that leaves narrow "track" depressions across the face of the slope. In practice, these "tracks" retain water, seed, and mulch during rains and are usually the first areas to show plant growth. However, a large body of revegetation literature clearly indicates that rough graded slopes are much superior to "tracked" slopes for the prevention of short-term runoff and the establishment of vegetation. This is particularly true of sites where forest establishment is required (See VCE Publication 460-123. ). Tracked slopes are also more compact than rough-graded slopes. In situations where surface stability is not a major concern, we strongly recommend only rough grading be applied to coal refuse disposal surfaces. This recommendation is contrary to common practice, however, since the current "mind set" of equipment operators, reclamation planners, and mine inspectors tends to favor the smooth-grading-tracking-compaction approach.
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As a base rate of fertilizer for direct seeding, 100 lb/acre (lb/a) of N, 350 lb/a of P (as P2O5) and 100 lb/a of K (as K2O) is recommended. To attain this high P level it may be necessary to supplement conventional fertilizers (e.g. 10-20-10) with a high P fertilizer like superphosphate. These rates are suggested when the seed mixture to be used contains legumes (clovers, trefoil, etc.) and assume adequate establishment of legumes for continuing N availability in succeeding years as discussed earlier.
When legumes are seeded, the appropriate inoculant should be added at time of seeding (see VCE Publication 460-122. ). Care should be taken to keep the pH of the hydroseeder slurry buffered above 4.0 with lime. The inoculant should be added to the hydroseeder tank immediately before seeding, since the inoculant bacteria will perish if left in the high salt environment of the hydroseeder slurry for more than a few minutes. If only grasses are to be used, then the N rate should be adjusted upward to 150 lb/a, but the grasses will need additional N fertilizer in successive years in the absence of legumes.
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Powell River Project direct seeding field trials, which were established using the above criteria, have been successful for five growing seasons and beyond. Species mixtures and seeding rates detailed in Table 3 appear to be suitable for direct seeding of refuse and for use with topsoil covers. These recommendations were based on the conditions at our various research sites, and addition or deletion of species should be considered depending upon your local site conditions and seed availability. Each mixture contains species adapted to a variety of site conditions which is intended to overcome local mine soil variability problems and make the mixes usable on a variety of sites. Spring seedings should be done after March 15 and before May 15 for optimal results ( Table 4 ). Fall seeding is recommended between September 15 and November 15 for best results. Environmental conditions during the summer and winter are generally unfavorable for successful establishment of mixed perennial vegetation, and annual covers should only be seeded during these periods.
Commercially available wood fiber or paper mulches at conventional application rates perform satisfactorily for their intended use: establishment of grasses on topsoil. However, they are inadequate under the extreme environmental stresses on refuse piles. Our recommendation is that paper mulches be used at higher rates (> 2000 lbs/ac) in the hydroseeder tank mix, or in conjunction with straw mulch on refuse. Field trials indicate that using straw and wood fiber/paper mulches together greatly improves plant establishment and long term vigor, particularly on hot, south-facing fills.
A technique which has proven successful in our work is as follows: when loading the hydroseeder include paper mulch to achieve 1000 to 1500 lb/a along with the desired amount of seed and fertilizer. Spray this mixture in such a manner that it covers twice the normal area usually covered with l tankful (or half the normal rate). Next, using a mechanical straw blower or manual spreader, cover the area just sprayed with straw. Good coverage is achieved with 2500 lb/a of straw. Respray this area again with the mulch/seed/fertilizer mixture and in the same manner as indicated above.
By using this seeding method, several factors critical to successful establishment are ensured. The shade provided by the mulch reduces water loss from the seedbed and shields young seedlings from the high temperatures common to these areas. The first tankful provides good seed/soil contact which is necessary for good germination. The use of straw mulch over this initial tankful provides shade which reduces water loss and lowers surface temperatures. The addition of the final tankful adds more seed and water which may infiltrate the straw mulch, while the paper mulch tacks the straw mulch in place by forming a mat-like surface. While this technique adds to the cost and time involved, we feel that it is justified in terms of long-term establishment success, particularly on hot droughty sites.
In summary, any direct seeding should be done with heavy mulch, applications of at least 350 lb/a of P2O5,and normal rates of N and K as discussed previously. Many direct seeding alternatives may be impossible due to the difficulty of working amendments on steep fill faces. In these cases, some combination of lime and topsoil will be the only viable alternative.
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While various treatments have been shown to slow the rate of the acid-producing pyrite weathering reactions, eventually the reactions will continue to completion. The mass of sulfur within most disposal areas far exceeds the neutralization potential of any surface applied treatments. Thus, unless water is completely excluded from the fill, even moderately sulfidic refuse materials should be expected to discharge acidic leachates and long-term water treatment strategies should be planned. For net-acid producing refuse piles, these discharges will generally continue well beyond the 5-year bond liability period. For such piles, the leachates will have to be neutralized with caustic additions and/or acid treatment wetlands.
Acid-treatment wetlands are not currently accepted by regulatory authorities as a "walk-away" solution to acid leachate water quality problems. Where sufficient land area is available, however, wetland treatment systems have proven to be a more cost-effective means of treating acid water than alkaline chemical systems. Lack of sufficient land area in the right location has proven to be a major barrier to use of acid treatment wetlands. Proper placement and design in the landscape can allow refuse fills to utilize acid treatment wetland systems as a cost-effective means of leachate water treatment. Design requirements of acid treatment wetlands are reviewed in VCE Publication 460-132.
The only technology that is known to be effective in eliminating the acid leachate potential at refuse disposal sites is the bulk-blending of alkaline materials with the refuse as it is placed in the fill. Ground agricultural limestone serves this purpose well, but may be required at mixture ratios of up to 5 percent. This would add a considerable cost to refuse disposal.
Current research is evaluating the potential to use alkaline fly ash as a lime substitute for this purpose. However, not all fly ash materials are alkaline, and the net water quality impacts of blending ash and other coal combustion by-products, such as scrubber sludges, with acid-forming refuse materials must be carefully considered. Details on such uses of coal combustion by-products in mined land reclamation are given in VCE Publication 460-133.
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The long-term acid generation potential of a refuse pile must be taken into account during reclamation and closure planning. It is likely that permit approval for refuse disposal areas in the future will depend on the permittee's ability to prove that the site will not pose a long-term acid mine drainage hazard. Currently, bulk blending of lime or other alkaline materials is the only viable long term approach to permanent control of acid mine drainage in-situ.
Finally, we believe that the current practice of designating reclaimed coal refuse piles to support "conventional" post-mining land uses such as wildlife enhancement or unmanaged forest uses does not recognize the particularly fragile nature of their surface plant/soil communities. These areas need to be strenuously protected from surface disturbance and erosion once they are reclaimed. Perhaps a special "environmental protection zone" land use category should be developed. The need for augmentation seed and fertilizer, within certain guidelines, could be recognized within this land-use category.
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The research that allowed us to reach this level of understanding was supported by the Powell River Project, the Virginia Center for Innovative Technology, and the U.S.D.I. Bureau of Mines Mineral Inst. program, grant # G1164151. Primary sponsors of the Powell River Project program are Penn Virginia Coal Corporation, Norfolk Southern Foundation, other southwestern Virginia firms, Virginia Tech, and the Commonwealth of Virginia.
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Buttermore, W.H., E.J. Simcoe, and M.A. Maloy. 1978. Characterization of Coal Refuse. Technical Report No. 159, Coal Research Bureau, West Virginia University, Morgantown, WV.
Daniels, W. L., and D. C. Dove. 1987. Revegetation strategies for coal refuse areas. p. F 1-13 In: Proc. Eighth Annual WV Surface Mine Drainage Task Force Symposium, Morgantown, WV, 4/17-4/19, 1987.
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Daniels, W.L., K.C. Haering, B.R. Stewart, R.V. Ruark and D.C. Dove. 1990. New technologies for the stabilization and reclamation of coal refuse materials. p. 1-20 In: Proceedings, 1990 Powell River Project Symposium. Powell River Project, Virginia Tech, Blacksburg.
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Stewart, B.R. and W.L. Daniels. 1992. Physical and chemical properties of coal refuse from southwest Virginia. Journal of Environmental Quality 21:635-642.
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