Powell River Project
Reclamation Guidelines
for Surface-Mined Land in Southwest Virginia

Creation and Management of Productive Mine Soils

Authors: W. Lee Daniels, Associate Professor of Crop and Soil Environmental Sciences, Virginia Tech; and Carl E. Zipper, Assistant Professor, Crop and Soil Environmental Sciences, and Director, Powell River Project.

Publication Number 460-121, posted May 1999

Table of Contents

Introduction Controlled Overburden Placement
Geology and Spoil Characteristics
Mine Soil Properties Important to Plant Growth Summary
References and Literature Cited
Sampling and Analyzing Mine Soil Properties Powell River Project "Reclamation Guidelines"

Introduction

Since the 1940's, surface mining for coal in Southwest Virginia has disturbed over 80,000 acres of land. Mining before the implementation of the federal Surface Mining Control and Reclamation Act (SMCRA) of 1977 and the resultant Virginia Permanent Regulatory Program of 1981 generated thousands of acres of surface-mined benches in Southwest Virginia, many with potential for more intensive future land uses. While the majority of these benches are relatively flat, use of these areas has been hindered by a number of problems, including poor soil productivity.

Mined lands reclaimed in Virginia since 1981 have been returned to Approximate Original Contour (AOC) and generally have much more carefully constructed surfaces than older "pre-law" mining sites (Fig. 1). The vast majority of surface mines in Virginia today employ some form of controlled overburden placement techniques and utilize topsoil substitutes derived from blasted mine spoil materials. This occurs because natural soils tend to be thin, rocky, acidic, and infertile over much of the Southwest Virginia coalfields, often making it impractical to salvage and re-spread topsoil on surface mined areas. The plant species used in active reclamation must therefore be grown in "mine spoils" composed of freshly blasted overburden materials. The properties of these mine spoils are directly controlled by the physical and geochemical properties of the rock strata from which they are derived. Once these "mine spoils" are placed at a reclaimed surface, and are utilized to support plant growth, we can consider them to be "mine soils." These two terms (mine spoil vs. mine soil) will be used throughout this publication, but it is important to remember that spoils are blasted rock materials while soils actually support plant growth, accumulate organic matter, and cycle nutrients over time. More information on basic soil science is available in Brady and Weil (1995) and Nagle et al. (1996).

In this publication, we will consider mine soil properties and management from two different perspectives: (1) the evaluation of mine soils on older established mined lands; and (2) the creation of new mine soils on active mining areas through the selection and careful placement of spoil materials to generate productive topsoil substitutes. We use similar criteria and properties to evaluate both situations, although the two situations are very different from the reclamation manager's perspective. Older existing mine soils (Fig. 2) must be evaluated and managed "as is" in the field, and the manipulation of their properties (particularly physical) may not be economically feasible. On an active mine, however, the reclamationist has the opportunity to custom-build a set of mine soil physical and chemical properties specifically suited to the intended postmining land use. Therefore, the first part of this publication deals with the description and understanding of mine soil properties as they exist after mining, while the second part deals with the evaluation of geological, chemical, and physical properties of the various overburden rock strata commonly encountered in the Virginia coalfields, and with the conversion of these mine spoils into productive mine soils through the process of controlled overburden placement.

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Geology and Spoil Characteristics

The strata disturbed by surface mining in Southwest Virginia are dominantly Pennsylvanian-aged sedimentary rocks derived from erosion of the Blue Ridge and Ridge and Valley Provinces hundreds of millions of years ago. The sediments were deposited in the margins of a shallow basin which supported lush swamp vegetation that accumulated in thick deposits, eventually forming coal beds. The rocks found between the numerous coal seams in Southwest Virginia are dominantly sandstones, siltstones, and shales. This general classification is based upon the dominant grain size present in the rock. The individual sediment grains are cemented by iron, calcium carbonate, or silica, which "glue" the grains together into hard rocks. Iron sulfide, also known as pyrite (FeS2), is also commonly found in the coal seams and occasionally in the rocks around the coal seams. As discussed later, the balance of acid-forming pyrites to base-forming carbonates is very important to spoil quality and potential environmental impacts. During mining, the rock strata above (overburden) and between (interburden) the coal seams are blasted into spoils which typically range from 10 to 25% fines (< 2 mm or 0.1 in.). Rocks that originally resided immediately below the soil layer tend to be reddish brown in color due to long term weathering and oxidation reactions, while deeper unweathered rocks are typically gray to bluish gray (Fig. 2).

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Mine Soil Properties Important to Plant Growth

Chemical Properties

pH
Soil pH is a measure of active soil acidity and is the most commonly used and misused indicator of mine soil/spoil quality. The pH of a given mine soil can change rapidly as the rock fragments weather and oxidize. Pyritic minerals (FeS2), when present, oxidize to sulfuric acid and drastically lower pH, while carbonate (Ca/MgCO3) bearing minerals and rocks tend to increase pH as they weather and dissolve. Unweathered (unoxidized) mine soils that contain significant amounts of pyritic-S in excess of their neutralizers (carbonates) will rapidly drop in pH upon exposure to water and oxygen. We have measured declines in pH from 8.0 to 3.0 in a matter of months in highly reactive acid-forming materials. The reaction of weathered and leached mine soils is generally not subject to the rapid pH changes typical of fresh spoils; pH is therefore a relatively good indicator of quality in a weathered mine soil. In assessing soil pH, care must be taken to sample a number of different locations because the pH of mine soils may change drastically within several feet. (If you are not familiar with soil sampling procedures, consult with the local Virginia Cooperative Extension office). Attempts to revegetate mine soils with pH values below 4.0 should be avoided entirely, while mine soils with pH's between 4.0 and 5.5 will require significant addition of lime for optimal growth. A mine soil pH range of 6.0 to 7.5 is ideal for forages and other agronomic or horticultural uses. When the soil pH drops below 5.5, reduced legume and forage growth occurs due to metal (Al, Fe, Mn) toxicities, phosphorus fixation, and reduced populations of nitrogen fixing bacteria. However, some species of trees (such as pines) and other native plants can do quite well in soils with pH between 4.0 and 5.5. A mine soil pH that is too high may also inhibit growth due to micronutrient deficiencies (Mn, Zn, Fe) and P-fixation by carbonates. This is particularly true for most forest species, which are adapted to lower pH soils.

Soluble Salts
Salts are inorganic chemical compounds that dissolve easily in water. Many of the sedimentary rock strata in Southwest Virginia weather to produce spoils that are relatively high in salts until leached by rainfall. These salts often include the sulfates of sodium, calcium, magnesium, and potassium. The oxidation of pyritic spoils (FeS2) produces large quantities of soluble salts in addition to acidity, and it may take many years for such spoils to leach and stabilize. High levels of soluble salts are toxic to plants and also inhibit nutrient and water uptake. Mine soils with water-extractable soluble salt levels greater than 4 mmhos/cm (5,000 ppm on a soil test report) should be avoided, and salt sensitive plant species may be affected at 1 mmho/cm (1250 ppm) or less. Soils with high levels of soluble salts tend to be sparsely vegetated or barren, may develop whitish powdery deposits on the surface during dry weather, and may be associated with very low pH.

Soil Fertility (N, P, K, Ca, Mg)
All newly created mine soils, and many older ones, will require significant fertilizer element applications for the establishment and maintenance of any plant community. Mine spoils are essentially devoid of nitrogen (N) initially, so the total amount of N required to sustain plant growth over time must come from initial fertilization and subsequent symbiotic microbial N-fixation by legumes. Usually less than 150 lb per acre of N are added as fertilizer, and much of this may be subject to leaching and gaseous losses. The vast majority of N needed to supply plant/soil community needs must therefore come from N-fixation and subsequent mineralization of organically combined N. Therefore, maintenance of a vigorous legume component within the plant community is critical for reclamation success. Most mine soils do not contain native populations of the essential N-fixing Rhizobium bacteria that enable legumes to capture atmospheric N, so care must be taken to carefully inoculate all legume seed used in new plantings.

Since N is primarily combined in organic matter in soils, the addition of organic amendments to the soil can greatly enhance total soil N and its availability over time. Sewage sludge has been shown to be an effective mine soil amendment in numerous studies, but it may not always be available in sufficient quantities for use on remote sites. Local and state regulations, and community attitudes frequently complicate the use of sewage wastes on disturbed lands. Sawdust and bark mulch are also helpful in increasing the initial mine soil organic matter content but are generally low in N content. Therefore, use of these materials as soil amendments will also require heavy fertilization with N. Recently, yardwaste composts and other fully stabilized organic materials have become increasingly available in most parts of Virginia and could play an important role in mined land reclamation.

The maintenance of plant available phosphorus (P) in mine soils over time is hindered by two factors: (1) Fresh mine spoils are generally low in readily plant available (water soluble) P; (2) as mine soils weather and oxidize they become enriched in Fe-oxides that adsorb water soluble P which is then "fixed" into unavailable forms. The tendency of mine soils to fix P increases over time (Fig. 3). Because organic bound P is not subject to P-fixation, it is critical to establish and build an organic-P reservoir in the soil to supply long term plant needs through P-mineralization. Large fertilizer applications of P during reclamation will insure that sufficient P will be available over the first several years to support plant growth and to build the organic-P pool. Some P will also become available to the plant community as native calcium phosphates in the rocks decompose, but this P is not sufficient to meet the needs of a vigorous plant community. Many plant species, particularly those that are mycorrhizal (e.g. Sericea lespedeza), are able to draw P from difficultly available sources.

With adequate fertilization and liming, the fertility needs of newly established vegetative covers can be easily met on almost any mined site. However, while vegetation establishment is certainly an important first step, SMCRA requires that a vigorous plant community persist for at least five years. This can be exceedingly difficult when fertilizer and lime augmentation is not allowed by the designated land use practices. Initial fertilization effects will usually last for the first two growing seasons, assuming good initial establishment. After that time, steady decreases in standing biomass and ground cover are common, even on the best of mine soils (Fig. 4).

Assuming adequate initial mine soil conditions, the long term productivity of the plant/soil system is dependent upon several major factors: (1) the accumulation of soil organic matter and N; (2) maintaining N-fixing legumes in the stand; and (3) the establishment of an organic-P pool and the avoidance of P-fixation. Both of these are in turn highly dependent on the introduction and function of microbial communities over time. On most newly reclaimed soils, it is likely that N will first limit plant growth due to greater plant needs, and that P fixation will become a problem in later seasons as the mine soils weather. However, if plant production in the first few years is limited by N, the transfer of fertilizer P into organic forms will be limited, thus increasing P-fixation losses. Similarly, low soil P levels may also hinder N accumulation since symbiotic N-fixing bacteria have a high P demand. Therefore, the development of a mine soil organic matter pool is essential to long term fertility; mine soil N and P must be managed together, not as independent factors.

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Physical Properties

Rock Content
The majority of plant available water in soils is held in soil pores formed by particles less than 2 mm (0.1 in.) in size. Any particles larger than 2 mm are referred to as "coarse fragments." Soils high in coarse fragments have larger pores which cannot hold enough plant available water against leaching to sustain vigorous growth over the summer months. Mine soil coarse fragment contents vary (< 30- > 70%) due to differences in rock hardness, blasting techniques, and spoil handling. The rock content in the surface of a reclaimed bench or outslope will decrease over time due to weathering of rock fragments to soil sized particles. Topsoil materials, when they can be salvaged, are typically much lower in rock content than spoils and therefore have better water retention characteristics.

Soil Texture
Soil-sized particles are smaller than 2 mm and are responsible for the majority of water and nutrient holding capacity in mine soils. The relative amounts of sand (2 - .05 mm), silt (.05 - .002 mm), and clay (< .002 mm) sized particles determine soil texture. Mine soils with sandy textures cannot hold as much water or nutrients as finer textured soils like loams and silts. The finer textured soils (e.g., silts) have a tendency to form surface crusts, often contain high levels of soluble salts, and have a poor "tilth" or consistence. The particle size distribution of the soils with loamy textures is generally ideal. The particle size distribution of mine soils is directly inherited from their parent rocks or spoils as discussed later, and it is typically sandy loam to loam in Southwest Virginia. Silt loam textures are also common where spoils are dominated by siltstones.

Bulk Density, Compaction and Available Rooting Depth
The bulk density of productive natural soils generally ranges from 1.1 to 1.5 g/cm3. Many mine soils in Southwest Virginia are highly compacted (bulk density > 1.6 g/cm3) within several feet of the surface due to heavy machinery traffic (Fig. 5). Soil compaction directly limits plant growth, as most species are unable to extend roots effectively through high bulk-density mine soils. In a study of older mine soils (5 to 20 years) in the Powell River Project watershed (Daniels & Amos, 1981), we found that compaction was the major soil factor limiting long-term revegetation success. Virginia Tech foresters have found that compaction limits tree growth and survival on reclaimed mine sites (VCE Publication 460-123). Severely compacted (bulk density > 1.7 g/cc) mine soils, particularly those with less than two feet of effective rooting depth, simply cannot hold enough plant-available water to sustain vigorous plant communities through protracted drought. Compacted zones may also perch water tables during wet weather conditions, causing saturation and anaerobic conditions within the rooting zone. Compacted zones result from the repeated traffic of rubber tired loaders and haulers, and bulldozers to a lesser extent.

Three to four feet of loose non-compacted soil material is required to hold enough water to sustain plants through prolonged droughts. Shallow intact bedrock, the presence of large boulders in the soil, and heavy compaction commonly limit rooting depth in mine soils. The only definitive way to determine rooting depth in an older mine soil is to dig backhoe pits and actually measure it. The presence of rock outcrops or extreme stoniness can be used as a general indicator of rooting volume. On older mined lands, the depth of spoil above bedrock can often be discerned by looking at the point where the bench meets the outslope. The soil depth at this point may not reflect that of the entire area, however. The presence of wet, swampy areas on a bench usually indicates shallow rock or compacted zones. Overall, when poor plant growth is encountered in soils with otherwise suitable physical and chemical properties, insufficient rooting depth is probably the cause.

Slope and Topography
Mine soils with slopes greater than 15% are generally unsuitable for intensive land uses such as vegetable or crop production, but they may be suitable for grazing and reforestation. Broad flat benches and fills with slopes less that 2% often have seasonal wetness problems. Many benches with an overall gentle slope contain areas of extreme rockiness, pits, hummocks, and ditches. Many of these features can only be discerned by walking over the area. The average slope of most reclaimed modern mines (post-1977) is quite a bit steeper than the older benches, but the newer land forms are considerably smoother and more uniform in final grade.

Stability
On older mined lands (Fig. 2), bench areas directly above intact bedrock are usually fairly stable, but may be subject to slumping, especially when near the edge of the outslope. Tension cracks running roughly parallel to the outslope indicate that an area is unstable and likely to slump. Areas perched above outslopes with slopes greater than 30 degrees should also be avoided, even if tension cracks are not present. In general, landforms created since 1977 are more stable than those created earlier since spoils are no longer allowed to be placed over the outslope. However, many AOC backfills in the region are still extremely steep.

Mine Spoil/Soil Color
The color of a mine spoil or weathered mine soil can tell us much about its weathering history, chemical properties, and physical make up. Bright red and brown colors in spoils and soils generally indicate that the material has been oxidized and leached to some degree. These materials tend to be lower in pH and free salts, less fertile, low in pyrites, and more susceptible to physical weathering than darker colored materials. Gray colors in rocks, spoils and soils usually indicate a lack of oxidation and leaching and these materials tend to be higher in pH and fertility. Very dark gray and black rocks, spoils, and mine soils contain significant amounts of organic materials and are often quite acidic. Dark colored spoils are also difficult to revegetate during the summer months because they absorb a great deal of solar energy and become quite hot.

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Sampling and Analiyzing Mine Soil Properties

Most mine soils vary tremendously in all of the parameters discussed above from point to point on any mined site. A fair amount of accuracy in sampling and characterizing the mine soils of a given area can be achieved, however, by carefully surveying the area, and then sampling and evaluating each area of contrasting vegetation, slope, rock type and stoniness. All of these field soil properties are certainly not important to all potential land uses, but together they will indicate overall site quality for a variety of uses. Mine soil samples should be taken from uniform areas of rock type wherever possible, and we recommend a minimum sampling intensity of one composite soil sample per 10 acres of mined land.

Once an appropriate mine soil sample has been taken, it should be submitted to a qualified soil testing laboratory for chemical analysis. It is important to indicate that the sample is from a mined land area rather than from a natural soil. This will alert the laboratory that the sample may not react as expected to some of the tests; at the Virginia Tech Extension Soil Testing Laboratory, proper labeling will ensure that appropriate lime and fertilizer recommendations are made. At a minimum, the sample should be analyzed for pH, soluble salts, and extractable nutrients (P, Ca, Mg, K). Mine soils contain large amounts of readily oxidizable iron and manganese which interfere with the determination of organic matter by wet oxidative techniques, so do not request this test.

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Mine Soil pH and Liming Recommendations

If the soil pH of a weathered mine soil is less than 4.0 and the soluble salts are high, you can be reasonably certain that the soil contains reactive pyrites, and acid-base accounting should also be performed as discussed in the next section to determine the appropriate liming rate. If the mine soil has been exposed at the land surface for a sufficient period of time for pyrite oxidation effects to become apparent (if sulfides are present -- up to 2 years), you can assume that the pH, soluble salt, and liming recommendation values given on your soil test are reasonably accurate. If the mine soil material is less than several years old, the full extent of pyrite oxidation might not be evident (e.g. low pH), and acid-base accounting should be performed to accurately estimate liming needs. In general, mine soils are fairly coarse textured and low in buffering capacity, so applications of agricultural lime at 1 to 5 tons per acre (depending on initial pH) are typically sufficient to achieve a target pH of 6.5 for typical hayland/pasture forages. Forest species, particularly pines, thrive at lower pH levels, however, and liming above pH 5.5 should be avoided (see VCE Publication 460-123).

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Extractable Nutrients and Fertilizer Recommendations

Unfortunately, extractable levels of P, Ca, Mg, and K for most soil tests have never been correlated with actual plant uptake and performance on Southwest Virginia mine soils. This fact, coupled with the extreme variability in rock type and mineral solubility found in mine soils, makes it very difficult to make accurate fertilizer recommendations. When P, Ca, and Mg levels are in the low range for a given soil test, you can assume that the spoil material has been extensively weathered and leached and that lime and fertilizer applications will likely be needed for optimal plant production. On the other hand, very high values of P, Ca, and Mg should not be interpreted as being plant-available since many soil testing extracts (particularly those employing acids) actually dissolve these elements out of solid mineral phases that are not plant-available over the short term. Potassium (K) is found in very high levels in fresh mine spoils but is also subject to long-term leaching losses. For these reasons, we do not believe that fertilizer recommendations can be "fine tuned" to a given site and its vegetative cover based on conventional soil testing approaches. We recommend that at least 250 pounds per acre of P205 and 100 pounds per acre of K20 be applied to new seedings or as supplements to existing forage stands. The N application rate for new seedings should not exceed 150 lbs per acre to avoid suppression of legumes, but at least 75 lbs per acre is required to support the establishment of annual and perennial grasses. Established forage stands with a vigorous legume component will require little if any N fertilization. These recommendations are given as a general minimum for conventional establishment of erosion control mixtures or the maintenance of hayland and pasture covers. Establishing vegetative cover for a forest land use is covered in VCE Publication 460-123.

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Overburden Analysis for Topsoil Substitute Selection

The first step in this process is to obtain accurate overburden analysis samples prior to mining. Each major strata should be sampled from drill cuttings at several locations, and care should be taken to avoid contaminating the samples with drilling greases. Each overburden sample should be characterized for rock type, texture, and thickness of host strata. Ground samples of each strata should be analyzed by a competent laboratory for pH, acid-base accounting, P, K, Ca, Mg, and soluble salts.

From a chemical standpoint, the acid-base accounting which balances acidity (from pyrites) against total alkalinity (from carbonates) is probably the best indicator of spoil quality. Greater detail on acid-base accounting fundamentals is given in VCE Publication 460-131. Conventional acid-base accounting measures the total-S content of a sample and assumes that it will all oxidize to form sulfuric acid. Similarly, the total neutralization capacity of carbonates in the sample is measured and assumed to be fully reactive. In the absence of carbonates, and assuming complete reaction, 1% total pyritic sulfur in a spoil will generate enough acidity to require 32 tons of agricultural lime addition per 1000 tons of spoil to achieve pH 7.0 conditions. Since the average mine soil weighs approximately 1000 tons per acre in its upper 6 inches of soil depth, the "potential acidity" value corresponds to the predicted per-acre liming requirement for a given spoil or mine soil. Negative values for potential acidity indicate a lime demand for a given material while positive values indicate that the sample contains an excess of neutralizers, typically from carbonates. The conventional acid-base accounting method described here is subject to a number of potential sources of error which are beyond the scope of this discussion, but it is a fairly reliable and readily available analysis which generates consistent liming prescriptions. Overburden strata which are net alkaline should weather to produce mine soils with a pH > 5.5 which are quite suitable for plant growth following N, P, and K fertilization. Overburden strata with net acidity levels below negative 2 to 3 parts per thousand should be avoided; strata with net acidities below negative 5 parts per thousand are required by law to be handled as potentially toxic materials.

As mentioned earlier, pH is a measure of active soil acidity and is often a poor predictor of spoil quality. While pH can be used as a rough indicator, there is no substitute for accurate acid-base accounting. Spoils high (> 5000 ppm or 4 mmhos/cm) in soluble salts should be avoided as well. A salty spoil that does not contain pyritic materials will rapidly lose its salts to leaching in the field, however, and might be suitable as topsoil substitute material once this occurs. The discussion of extractable nutrients and fertilizer recommendations presented earlier also applies to pre-mining overburden analysis. In particular, extractable P, Ca, Mg, and K levels from fresh, unweathered spoils are typically quite high and cannot be assumed to be plant available, and therefore they should not be used as criteria for selecting topsoil substitute materials.

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Controlled Overburden Placement

Use of Natural Topsoils vs. Topsoil Substitutes

Certain geologic strata can be designated for use as topsoil substitutes when it can be shown through pre-mining sampling and testing that their physical and chemical properties surpass those of the native soil (where present) for potential plant growth. However, if sufficient quantities of productive natural soils are present on a site, they should be used as the surface medium whenever possible. While the majority of soils in these Appalachian landscapes are shallow and infertile, occasional deeper soil bodies do occur, particularly in coves and on stable ridgetops. If it is practical to isolate and store sufficient quantities of these materials so that final surface coverage of the site will exceed 24 in., there are great advantages to the use of natural soils. The organic matter and microbial populations of natural soils are invaluable to the revegetation process. Regardless of the quality of the natural soil, however, it must be present in large enough quantities to allow sufficient thickness on the final reclamation surface. Quite frequently, only 6 to 18 in. of natural soil are spread over highly compacted spoil materials, leading to rooting restrictions and problems of seasonal wetness. It is also important that the true topsoil (A plus E horizons) be separated from the underlying soil and rock horizons during stripping, and then either spread immediately or stored properly to maintain the viability of aerobic microbial populations. Due to the thin nature of most of the surface soil horizons in southwestern Virginia's coalfields, when natural topsoils are employed in reclamation they are usually a mixture of all soil horizons above hard bedrock. Thus, the positive influences of their organic matter and microbial content are considerably reduced. Some mining operators do find it beneficial, however, to mix whatever true topsoil becomes available with their designated topsoil substitutes during final reclamation grading. The benefits of this practice include inoculation with beneficial soil microbes, a source of slow-release (organic) N and P, and physical properties that are more favorable as a seedbed than raw spoil.

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Selecting Geologic Materials for Use as Topsoil Substitutes

The rock beds in the SW Virginia coal fields lie essentially flat with a gentle dip to the NW. The coal seams are separated by varying thicknesses of sandstones and siltstones, with a minor component of shales as discussed earlier. Commonly, multiple seams of coal are mined, with all strata above the lower seam being blasted into spoils and handled in some fashion during mining. In order for these hard rock spoils to be successfully employed as topsoil substitutes, the optimal strata must be identified before mining, so that the mining plan can be designed to place the proper strata at the final reclaimed surface. Also, any potentially toxic strata must be identified and then isolated away from the surface and local ground-water. Before mining, the thickness and variability of the various overburden strata are determined by exploratory drilling. All major strata are tested unless previous experience indicates that one particular stratum is a superior topsoil substitute material.

In general, the following criteria (see Table 1), in order of importance, are essential to evaluate the suitability of a given strata for use as a topsoil substitute: (1) Acid-base accounting; (2) Rock type; (3) Extractable nutrients; (4) pH; (5) Soluble salts; (6) Degree of weathering and oxidation before mining. Ideally, a stratum or multiple strata are isolated which will be non-acid forming over time, and therefore high in pH and low in soluble salts. Rocks that are low in acid-forming pyrites (FeS2) and high in carbonate cementing agents are ideal. Rock type is important since spoils derived entirely from sandstones tend to be very coarse and droughty, while those derived entirely from fine siltstones and shales tend to form hard surface crusts and impede water percolation. We have found that mixtures of rock types are superior to those composed of all siltstone or sandstone, but the differences attributable to rock type diminish with time (see Fig. 4). As mentioned earlier, extractable nutrient tests are fairly unreliable when used with fresh spoils or young mine soils, but they can be used to compare among potential substitutes in a relative sense.

Table 1. Topsoil substitute selection criteria for Southwest Virginia. Criteria are listed in order of importance.

Potential Acidity
Determined by acid-base accounting by competent lab. Should be net neutral or better (0 to positive values). Materials with values less than -5 tons per thousand CaCO3 requirement must be avoided.

Rock Type
Mixtures of sandstones and siltstones/shales are superior to unmixed spoils. Avoid pure fine siltstone and shales when possible.

Extractable
Useful for general comparisons among candidate materials, but cannot be literally interpreted in terms of plant availability or for fertilizer recommendations.

pH
Not reliable as a predictor of the long term soil quality for unweathered mine spoils. Values less than 4 indicate that pyrite oxidation has occurred.

Soluble Salts
Avoid when > 4 mmhos/cm (or 5000 ppm). Many pasture legumes and other salt sensitive crops will be affected at much lower values (< 3000 ppm). Generally not a problem as long as acid forming materials are avoided.

Weathering
Brown oxidized strata will blast to finer spoils with higher water and nutrient holding capacities. However, oxidized strata will be lower in pH and extractable nutrients (P, Ca, Mg, K).

Thickness
Any designated topsoil substitute strata must be present in sufficient thickness and location within the overburden section to be economically isolated and hauled by the active mining operation.

In weathered near-surface strata, materials with pH below 5 should generally be avoided. Quite often, these leached and oxidized strata are preferred by the mining operators for use as topsoil substitutes because they blast into a finer, less rocky spoil which is easily handled and spread. These brownish-red oxidized materials are usually high in Fe-oxides, however, which can be detrimental to long-term P availability due to their P-fixing capacity (see Fig. 3). Another advantage to using the finer, pre-///weathered strata is that they will hold more plant-available water due to their lower rock content. Brown, oxidized, sandy spoils may be a good topsoil substitute choice for establishing pine forest vegetation (VCE Publication 460-123), but other spoil types will often be superior for hayland-pasture and other land uses. The majority of unweathered strata blast into spoils that contain 20% to 40 % soil sized (< 2mm) fragments and will supply sufficient plant-available water as long as they are placed at the final surface with sufficient uncompacted depth.

Perhaps the most important criteria for selecting a topsoil substitute is whether or not the designated strata can be isolated and handled within the mining plan without excessive cost to the operator. If an ideal stratum is thinner than the usual blasting lift thickness, or placed inopportunely within the geologic column, its use may be impossible. Quite often, two or more adjacent strata within the same blasting lift will be identified as the substitute materials and then handled and spread together. In this fashion, strata with dissimilar physical and chemical properties can be mixed together into a composite with properties more favorable than those of individual strata.

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Mine Soil Construction to Ensure Reclamation Success

Before the enactment of SMCRA, little thought was given to selective handling of overburden. The final surface generated for revegetation was usually a rough graded mixture of all strata present in the overburden column, leading to extreme heterogeneity in mine soil properties on these older benches. It is not uncommon in these areas to find pH 3 and pH 8 mine soils directly adjacent to one another. This spoil variability, combined with problems of severe compaction, makes it difficult to develop uniform management strategies for many of these older benches. The primary objective of modern controlled overburden placement techniques is to place a designated topsoil substitute of controlled geologic origin at the final reclamation surface. This final lift of spoil must be placed with sufficient thickness (at least 3 to 4 ft.) to support vigorous plant growth over time, and must not be excessively compacted Fig. 5).

Continuous on-site coordination and supervision by the mining operator or job foreman is necessary to insure that the designated strata are correctly isolated and hauled to the final reclamation surface. First, the reclamation area is filled with non-selected spoil to a level 4 to 5 feet below the planned final surface, and rough graded. Then, the entire area is end-dumped with the appropriate spoil in closely spaced piles. The spoil may remain in this configuration indefinitely before the final reclamation grading is performed in one operation (Fig. 6). Grading should be delayed until just before seeding whenever possible to prevent surface crusts from forming. This will also minimize surface runoff and erosion. Grading wet spoils promotes compaction and should be avoided whenever possible. By following these procedures, thick, uniform, uncompacted mine soils can be produced with few direct costs to the operator (other than those involved with coordination and supervision) since all of the materials must be handled and moved regardless of placement location. Throughout the process it is important to maintain alternate spoil dumpsites so that spoils unsuitable for topsoil substitute use are eliminated from the final surface. The key factor is control of overburden handling and movement for the sake of improved reclamation success.

Careful coordination of the entire surface mining operation is required for a designated "topsoil substitute" strata to actually become a part of the final reclaimed surface. Failure to actively control the placement of overburden materials results in variable soil properties because a number of different rock strata comprise the final surface. While this may not be a problem when all of the strata are suitable for plant growth, patches of highly acidic or high salt spoils at the surface may decrease the percentage of vegetative cover enough to prevent bond release. Severe compaction of otherwise suitable spoils will also greatly decrease the density of vegetation.

In summary, we recommend that the following overburden selection and placement procedures be followed:

  1. Select the best substitute overburden strata, basing your judgment on the following parameters: Acid-base balance, pH, soluble salt content, rock type and overall thickness. Other parameters such as Ca, Mg, K and P should enter into the decision but are not as critical.

  2. Coordinate the surface mining operations so that the designated strata are separated and hauled to the proper areas for final grading. Exclude excessively stony (> 80%) spoils.

  3. Using only the designated spoil, end dump the entire final surface with enough spoil to insure a minimum thickness of 4 feet over any adverse underlying materials.

  4. Grade the final surface with a bulldozer, leaving at least a 2% grade for drainage of surface water. Exclude all machinery from the area after grading to avoid excessive compaction.

  5. Seed and/or mulch the site immediately when possible to avoid the formation of surface crusts and begin the development of a soil nutrient and organic matter pool.

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Summary

Existing mine soils tend to be quite variable in the field, but they can be managed effectively once their chemical and physical properties have been correctly determined. Compaction, low water holding capacity, and associated rooting restrictions are the major factors limiting the productivity of mine soils in this region. High levels of potential acidity seriously restrict the productivity of some mine soils, but this problem is much more limited in extent than mine soil compaction. Soil testing procedures are useful for comparing overburden materials for use as mine soils, but they cannot be interpreted with the same degree of accuracy as for natural soils.

Productive topsoil substitutes can be generated from hard rock overburden in Southwest Virginia, but care must be taken in spoil selection and placement. It is particularly important to reclamation success that controlled overburden placement techniques be used to generate at least 4 ft. of loose spoil at the final surface for seeding. The accumulation of soil organic matter and organically complexed N and P over time, maintenance of N-fixing legumes in the vegetation, and the minimization of P fixation by soil Fe-oxides are important factors controlling the long term productivity of mine soils.

Mine soils carefully constructed from topsoil substitute overburden materials can be more productive than many of the Appalachian region's natural soils.

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References and Literature Cited

Barnhisel, R.I., R.G. Darmody, and W.L. Daniels (eds.). 1998. Reclamation of Drastically Disturbed Lands. American Soc. Of Agron., Madison WI. (Forthcoming).

Brady, N.C. and R.R. Weil. 1996. The Nature and Properties of Soils. 11th Edition. Prentice Hall, Upper Saddle River, New Jersey. 740 pp.

Daniels, W. L. and D. F. Amos. 1981. Mapping, characterization and genesis of mine soils on a reclamation research area in Wise County, Virginia. p. 261-275 In: Proc. 1981 Symp. on Surface Mining Hydrology, Sedimentology and Reclamation, Univ. of Ky., Lexington, KY.

Daniels, W. L. and D. F. Amos. l982. Chemical characteristics of some SW Virginia minesoils. p. 377-381 In: Proc. 1982 Symp. on Surface Mining Hydrology, Sedimentology and Reclamation, Univ. of Ky., Lexington, KY

Daniels, W. L. and D. F. Amos. 1985. Generating productive topsoil substitutes from hard rock overburden in the southern Appalachians. Environ. Geochem. and Health 7:8-15.

Daniels, W.L. and C.E. Zipper. 1994. Improving coal surface mine reclamation in the central Appalachian region. p. 187-218 In: J. Cairns (ed), Rehabilitating Damaged Ecosystems (2nd Ed.), Lewis Publishers, Boca Raton.

Daniels, W. L., C. J. Everett and J. A. Roberts. 1984. Factors governing plant uptake of Mn from SW Virginia mine soil materials. p. 421-462 In: Proc. 1984 Symp. on Surface Mining Hydrology, Sedimentology and Reclamation. Univ. of Ky., Lexington, KY.

Dove, D. C., D. D. Wolf and W. L. Daniels. 1984. Dry matter and nutrient loss from legume litter grown on minesoils. p. 197-202 In: Proc. 1984 Symp. on Surface Mining Hydrology, Sedimentology and Reclamation. Univ. of Ky., Lexington, KY.

Haering, K.C., W.L. Daniels, J.L. Torbert, and J.A. Burger. 1990. The Effects of Controlled Overburden Placement on Topsoil Substitute Quality and Bond Release: Final Report. USDI-OSMRE Coop. Agree. #HQ51-GR87- 10022, Office of Surface Mining Reclamation and Enforcement Technical Library, Wash. D.C., 86 p.

Haering, K.C., W.L. Daniels and J.A. Roberts. 1993. Changes in mine soil properties resulting from overburden weathering. J. of Environ. Quality 22:194-200.

Howard, J. L., D. F. Amos, and W. L. Daniels. 1988. Phosphorus and potassium relationships in southwestern Virginia mine spoils. J. Environ. Quality. 17(4) 695-671.

Li, R.S. and W.L. Daniels. 1994. Nitrogen accumulation and form over time in young mine soils. J. Environ. Quality. 23:166-172.

Mustafa, G., T. A. Dillaha, S. C. Sarin, W. L. Daniels and S. Mostaghimi. 1990. Revegetation of reclaimed mine soils under weather uncertainty: A stochastic dynamic optimization approach. Resource Mgt. and Optim. 8:15-30.

Nagle, S.M., G.E. Evanylo, W.L. Daniels, Douglas Beegle, and V.A. Groover. 1996. Chesapeake Bay Region Nutrient Management Training Manual. CSES Dept., Virginia Tech, Blacksburg, VA. 200 pp.

Roberts, J. A., W. L. Daniels, J. C. Bell and D. C. Martens. 1988. Tall fescue production and nutrient status on southwest Virginia mine soils. J. Environ. Quality. 17:55-62.

Roberts, J. A., W. L. Daniels, J. C. Bell and J. A. Burger. 1988. Early stages of mine soil genesis in a SW Virginia mine spoil lithosequence. Soil Sci. Soc. Am. J. 52:716-723.

Roberts, J. A., W. L. Daniels, J. C. Bell and J. A. Burger. 1988. Early stages of mine soil genesis as affected by topsoiling and organic amendments. Soil Sci. Soc. Am. J. 52:730-738.

Sencindiver, J., Dollhopf, D. and W.L. Daniels. 1990. Minesoil morphology and genesis. p. 79-85 In: Skousen, et al., (eds.), Proc. of the 1990 Mining and Reclamation Conf., Annual meeting of the Am. Soc. Surface Min. and Rec., West Virginia Univ., Morgantown.

Torbert, J.L., J. A. Burger and W. L. Daniels. 1990. Pine growth variation associated with overburden rock type on a reclaimed surface mine in Virginia. J. Environ. Quality. 19(1):88-92.

Zipper, C. E., and W. L. Daniels. 1988. Institutional constraints to Production of surface mined lands suitable for development in central Appalachia. p. 319-325 In: Mine Drainage and Surface Mine Reclamation, Vol II: Mine Reclamation, Abandoned Mine Lands and Policy Issues. Information Circ. 9184, U.S. Bureau of Mines, Pittsburgh.

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Powell River Project "Reclamation Guidelines"

Other "Reclamation Guidelines" chapters completed at the time of this chapter's publication are listed below:

J. Skousen and C.E. Zipper. 1996. Revegetation Species and Practices. VCE Publication 460-122.

J.A. Burger and J.L. Torbert. 1992. Restoring Forests on Surface Mined Land. VCE Publication 460-123.

P.T. Bromley and C.T. Cushwa. 1990. Wildlife and Fish Habitat on Reclaimed Surface-Mined Lands. VCE Publication 460-125.

J.R. Hall III. 1992. Establishment and Maintenance of Quality Turfgrass on Surface Mined Land. VCE Publication 460-127.

H.J. Gerken and C. Baker. 1990. Beef Production from Forages Grown on Reclaimed Surface Mined Land. VCE Publication 460-128.

Atkinson, R., C. Zipper, W.L. Daniels, and J. Cairns Jr. 1997. Constructing Wetlands During Reclamation to Improve Wildlife Habitat. VCE Publication 460-129.

Zipper, C, and S. Winter. 1997. Stabilizing Reclaimed Mines to Support Buildings and Development. VCE Publication 460-130.

W.L. Daniels, B.R. Stewart, and D. Dove. 1995. Reclamation of Coal Refuse Disposal Areas. VCE Publication 460-131.

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