Soil Horizon Development on a Mountaintop Surface Mine in Southern West Virginia

Soil Horizon Development on a Mountaintop Surface
Mine in Southern West Virginia

This article was prepared by K.A. Thomas, J.C. Sencindiver, J.G. Skousen, and J.M. Gorman . It appeared in the summer 2000 issue of "Green Lands" magazine.

Mountaintop surface mining for coal has been practiced in West Virginia for over two decades. Only recently has this practice been increasingly scrutinized by the public and regulatory agencies. Increased attention has focused on the environmental impacts of this mining process.

Even after reclamation, citizens and regulators have expressed concerns about soil and water quality and post-mining land use. Therefore, a study was initiated to evaluate the quality of soils developing on a reclaimed mountaintop removal mine in southern West Virginia.

Minesoils of four different ages (2, 7, 11, and 23 years) were described and sampled. Six pits were dug on each minesoil age class and three pits were dug on adjacent native soils.

O and A horizons were found in all native soils and A horizons were found in all minesoils. Thickness of A horizons varied from 10 cm on the 23-year-old site to 6 cm on the 2-year-old site. All native soils and one 23–year–old site minesoil had Bw horizons and were classified as Inceptisols (soils showing some development). All other minesoils were Entisols (showing little to no development).

When compared to native soils, the minesoils had much thinner sola (combined thickness of A, AC, and Bw horizons). However, all minesoils except those on the two-year-old site had thicker A horizons than the native soils. Seeding of grasses and legumes and extensive root establishment undoubtedly caused the increased thickness of A horizons on minesoils.

Aggregate stability tests showed more water-stable aggregates in native than in minesoils, but aggregation of the minesoils increased with age. Surface horizon bulk density tended to be higher in minesoils than in native soils. However, bulk density with depth was similar for all soils. Minesoil pH tended to be between 5 and 6, while native soil pH was between 4 and 5.

The process of mountaintop removal mining results in reclaimed landscapes that commonly differ from the original landscapes. Relief has generally been reduced and excess spoil is often placed in head-of-hollow or valley fills. The soils developing on these mined and filled areas differ from the original soil, but they have not been widely evaluated.

Minesoils are very young soils developing from mixtures of fragmented rock and fine earth material. The original soil profiles have been disrupted and often partially or totally replaced by earth materials from depths below the original profile. Studies have shown that upon exposure to the surface environment, the geologic materials placed at the surface experience accelerated weathering, thereby increasing soil development (Ciolkosz et al., 1985). Accelerated physical weathering of rocks caused by blasting and movement during both mining and reclamation, and the addition of organic materials during reclamation, increase the rate of soil development (Sencindiver and Ammons, In press).

Smith et al. (1971) studied soil genesis in 70- to 130-year-old mine sites in West Virginia. They found the minesoils to have deeper root zones, higher bulk densities, and weaker soil structure than native soils. The general conclusion drawn from Smith’s study and from other studies (Sencindiver and Ammons, In press; Schafer et al., 1980) was that minesoils were superior to native soils in some respects, yet inferior in others.

Few studies on soil development on mountaintop removal sites have been performed. Little information is known about long-term environmental changes on these sites. Therefore, we initiated a study to evaluate the quality of soils developing on a reclaimed mountaintop removal mine in southern West Virginia. The objective of this study was to document soil formation and to correlate minesoil property differences to age. This paper, a preliminary report of the study, compares development of soil horizons in native and different-aged minesoils. Further analysis on physical and chemical properties are ongoing.

In July 1999, minesoil pits were dug and soil samples were collected on a mountaintop removal site near Sharples, Logan County, West Virginia. The coal beds mined at this site were within the Kanawha formation, which is composed of approximately 50% sandstone and 50% shale, siltstone, and coal. There are several marine zones found throughout the formation (Cardwell et al., 1968). Most of the soils in the unmined area are moderately deep to very deep Inceptisols or Ultisols (Table 1) forming in residuum (soil formed in place by natural weathering) or colluvium (soils formed from materials transported downslope by gravity). General slope classes of the premined and the mined and reclaimed areas were gently sloping to very steep. However, the general relief of the reclaimed areas is less than the premined landscape.

Table 1. Description of 12 soil orders used for classifying soils.
Soil Order	Derivation	Description

Entisols	Recent	Little profile development showing few horizons.
Inceptisols	Beginning	Young soils showing development of weak B horizons.
Mollisols	Soft	Deep, rich soils of plains and grasslands.
Alfisols	Pedalf	Forest soils of humid, moderate climates with clay accumulation in the B horizon.
Ultisols	Ultimate	Forest soils of humid, hot climates. More acid than alfisols.
Oxisols	Oxides	Very deep, highly weathered soils of tropical areas.
Vertisols	Inverted	Soils containing swelling clays; deep crack when soil dry.
Aridisols	Arid	Dry soils developing in arid climates.
Spodosols	Podzol	Forest soils of humid and cold climates.
Histosols	Histos	Organic, peat and bog soils having >30% organic matter.
Andisols	Andesite	Soils formed from volcanic ejecta.
Gelisols	Gelatinous	Ice soils found in extremely cold climates.

Elevation of the native landscape where samples were collected ranged from 561 to 568 m (1845-1863 ft), and the reclaimed mined land elevations ranged from 442 to 525 m (1450-1720 ft). The average temperature during the summer months is 22.8^oC (73^o F), and in the winter 1.0^o C (34^o F). The annual precipitation is 112 cm (44 in), 55% of which falls between April and September. The major vegetation before mining was predominantly forest which consisted of northern red oak (Quercus rubra, L.), black oak (Q. velutina, Lam.), yellow poplar (Liriodendron tulipifera, L.), hickory (Carya sp.), scarlet oak (Q. coccinea, Muench.), white oak (Q. alba, L.) and American beech (Fagus grandifolia, Ehrh.) (Wolf, 1994).

Four different ages of reclaimed mined land with two slope classes each were sampled in 1999. These sites were reclaimed in 1976 (23 yrs), 1988 (11 yrs), 1992 (7 yrs), and 1997 (2 yrs). The slope classes consisted of one that was steep to very steep and a second that was nearly level to gently sloping. Vegetation on the 2- and 11-year-old minesoils was predominantly grasses and legumes, and the 7-year-old vegetation was a combination of grasses, legumes, and shrubs. The 23-year-old minesoil had predominantly forest cover of a few prominent trees with a sparse understory of grasses and legumes. Although several tree species were found on the site, the prominent species were black locust (Robinia pseudoacacia L.) and red maple (Acer rubrum L.) (Skousen et al., 1999).

Six replications of each of the minesoil slope classes and age combinations were sampled. One very deep and two moderately deep undisturbed native forest soils representing the major soil series in the county were sampled for comparison. The very deep soil developed in colluvium, and the moderately deep soils developed in residuum. Soil pits approximately 1 m wide x 2 m long x 1+ m deep were excavated at each sampling point. Each pedon was described using standard soil survey procedures (Soil Survey Division Staff, 1993). Bulk samples were collected from every horizon described.

Aggregate stability was determined using the wet sieve method developed by Kemper and Rosenau (1986). Soil clods were collected in triplicate from each subsurface horizon, coated with a saran resin, and analyzed for bulk density by a water-displacement method (Soil Survey Staff, 1996).

Surface horizons were normally too thin and too friable for clod sampling. Therefore, all surface horizons were sampled using a frame excavation bulk density procedure (Robert Grossman, personal communication). All bulk density values were corrected for rock fragments and reported as bulk density of the <2 mm fraction.

The pH was measured by a 1:1 soil to water suspension method using a standard pH probe on an Accumet 915 pH meter (Method 8C1, Soil Survey Staff, 1996).

Native soils have been formed during the past hundreds to thousands of years where physical and chemical weathering has acted on geologic materials. Minesoils also develop through physical and chemical weathering processes, but they are much younger.

Minesoils show signs of similar kinds of weathering but some physical and chemical characteristics are due more to mining and reclamation methods than natural factors. Human influences include blasting of rocks into small fragments, compaction due to grading, addition of organic materials, additions of lime and fertilizer, and the seeding of grasses and legumes, or the planting of trees.

Processes of soil formation resulted in similar horizon development in each of the native soils (Table 2) where each soil had an O, A, Bw, and C horizon (see descriptions of these horizons in Table 3). The A horizons had weak or moderate granular structure, while the Bw horizons had weak or moderate subangular blocky structure. The Bw horizons in native soils were thick enough to be classified as cambic, which allowed all of these to be classified as Inceptisols (Soil Survey Staff, 1998).

Sixteen of the 24 minesoil pits (6 pits x 4 ages) had O horizons and all (24 pits) had A horizons (Table 2). Most minesoils had AC horizons, which are transition horizons between the A and C horizons with no B horizon. Structure of the minesoil A horizons was predominantly weak or moderate granular with some subangular blocky. Structure of the AC horizons was predominantly weak subangular blocky with some granular. In general, structure was strongest in the 23-year-old minesoil and weakest in the 2-year-old minesoil.

We described Bw horizons in one 23-year-old and one 7-year-old profile. These horizons had weak subangular blocky structure. Since the structure of the AC and the Bw horizons was similar, the two horizons were separated primarily by color. The AC horizons had colors similar to the A and/or C horizons. The Bw horizons had colors with higher value and/or chroma than the A and C. The Bw in the 23-year-old minesoil fit criteria to be classified as an Inceptisol, but the 7-year-old Bw did not (Soil Survey Staff, 1998). Therefore, with the exception of the older minesoil, all other minesoils classified as Entisols.

Table 2. Percent of pits having specific horizons as described and classified by standard soil survey techniques.
Horizon	Native¹	1976	1988	1992	1997
O	100	100	33	33	100
A	100	100	100	100	100
AC	0	83	67	67	67
Bw	100	17	0	17	0
C	100	100	100	83	100
R	66	0	0	0	0
¹Three native pits were dug, while each minesoil age had 6 pits.

Table 3. Descriptions of horizons found in native and minesoils.
O Horizons	These horizons are comprised or organic horizons that form above the mineral soil. They result from litter derived from dead plants and animals.
A Horizons	These are the topmost mineral horizons. They generally contain enough partially decomposed organic matter to darken the soil color more than that of the lower horizons.
AC Horizons	These horizons contain properties that are similar to the upper A horizon and the lower C horizon. They are transition zones where weathering has not distinguished this zone to be classified as a B horizon.
B Horizons	These are subsurface horizons in which an accumulation of materials transported from above has taken place. In humid regions, the B horizons are the layers of accumulation of materials such as clays and iron and aluminum oxides. The Bw horizon designates a weakly developed B horizon, which shows a distinctive color or structure from the A or C horizons.
C Horizons	These are horizons that are partially weathered or unconsolidated bedrock. The C horizon is outside the normal zone of biological activities and is generally little afffected by the processes that formed the horizons above it.
R Horizons	These are found underlying consolidated rock and are normally considered the bedrock, which shows little to no effect of weathering.

As expected, solum (combined A, AC, and Bw horizons) development was considerably greater in native soils than in minesoils (Figure 1). However, minesoil solum thickness tended to increase with age. Our study and other minesoil studies indicate that minesoil development is more rapid in the first few years after reclamation.

Interestingly, the A horizons of the native soils were thinner than all minesoil A horizons, except the two-year-old site (Figure 2). We think this difference is primarily the result of human activities. Revegetation techniques normally disturb a 5- to 10-cm layer at the minesoil surface, and include some organic amendment. Also, it is highly probable that the native soils had experienced logging activities during the last 50 to 100 years.

If logging had occurred, then the O and A horizons would most likely have been removed by equipment and subsequent erosion, and they have been redeveloping without human influence. For the minesoils, A-horizon thickness tended to increase with age on both slope classes (Figure 2). The actual thicknesses described were very similar to A horizons described in studies on sites mined by methods other than mountaintop removal. Roberts et al. (1988b) found that a 4-cm-thick A horizon developed on a nontopsoiled minesoil and a 6-cm-thick horizon developed on a topsoiled minesoil in one year. When sewage sludge was added to the minesoil, a 1-yr-old A horizon was 11-12 cm thick. Studies of 23 to 29-yr-old minesoils (Ciolkosz et al., 1985; Thurman and Sencindiver, 1986) show that A horizons of 9 to 13 cm thick had developed.

One of the first characteristics recognized when these minesoils were described was the large amount of weakly consolidated fragments of primarily sandstone and shale. The average rock fragment content of all described horizons was approximately 60%.

Roberts et al. (1988a) documented that percent rock fragments in surface horizons decrease with time. However, this general trend was not documented in our study. Surface rock fragment content in our study varied as follows: 11yrs < 23 yrs < 2 yrs < 7 yrs. The 23-year-old minesoils had higher rock-fragment contents in the C horizons compared to all other minesoils.

Also, the 2-year-old minesoils had fewer rock fragments in their subsoils than any other minesoil. These rock-fragment differences are probably due to differences in mining (blasting) and reclamation techniques rather than weathering.

Soil properties vary in the degree of their vulnerability to external forces. One measure of a soil's vulnerability to erosion is aggregate stability (Kemper and Rosenau, 1986), which expresses the resistance of soil structural aggregates to breakdown when subjected to disruptive processes. Freezing and thawing, wetting and drying, additions of organic matter, secretions of microorganisms, earthworm activity, and presence of clay-size particles are some of the factors affecting aggregation in soils. Aggregates generally become more stable over time, and thus total aggregation generally increases, as processes of soil genesis develop soil horizons. The native soils, with an average of 63% in the surface horizon and 62% in the subsurface horizon, had higher water-stable aggregation than any of the minesoils (Figure 3). In minesoils, aggregation increased with age from a low of 12% in the subsurface horizon of the 2-year-old minesoil to a high of 54-56% in the surface horizon of the 11-year-old minesoil and the surface and subsurface horizons of the 23-year-old minesoil.

For the 2-, 7-, and 11-year-old minesoils, aggregation of the surface horizon was greater than in the subsurface horizon. These differences can be related to the time of soil development. As these soils age, aggregation should increase and the two horizons should become more similar as is indicated by the 23-year-old minesoil and the native soil.

On a site in northeastern West Virginia, Gorman and Sencindiver (1999) observed water-stable aggregation of 58% in the top 8 cm of the minesoil and 51% in the 8-16 cm depth in a 9-year-old minesoil. Also, they found that aggregation had increased over time from zero to 9 years.

Bulk density of the minesoils in the surface horizon was somewhat higher than that of the native soils (Figure 4). Bulk density of the minesoil A horizons ranged from a high of 1.1 Mg/m³ in the 11-year-old site to a low of 0.87 Mg/m³ in the 23-year-old site. Bulk density tended to increase with depth in all soils, but the values were similar for minesoils and native soils below the A horizon.

In general, minesoil pH tended to decrease with age, and native soils had lower pH values at all depths than the minesoils (Figure 5). For all soils, pH tended to increase with depth.

Although minesoils in this study are very young compared to native soils of the region, they show evidence of soil development. The data indicate that soil properties are changing with time, and that the minesoils are becoming better developed with increasing age. Thickness of A horizons, thickness of the solum (A and B horizons), and total aggregation have increased with age in the minesoils. Structure within the solum of some of the older minesoils was similar to comparable depths within the native soils. Minesoil bulk density was comparable to the native soils. Minesoil pH is somewhat higher than native soil pH because of the presence of alkaline shales or other high pH materials being placed at the surface during reclamation.

We extend our appreciation to Arch Coal, Inc. and the West Virginia Agricultural and Forestry Experiment Station for providing funding for this study.