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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:231-237 (2008)
DOI: 10.2136/sssaj2007.0047
© 2008 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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SOIL & WATER MANAGEMENT & CONSERVATION

Method for Determining Coal Carbon in the Reclaimed Minesoils Contaminated with Coal

David A. N. Ussiri* and Rattan Lal

Carbon Management and Sequestration Center, School of Environment and Natural Resources, Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210

* Corresponding author (ussiri.1{at}osu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Minesoils are anthropic soils developed on land disturbed by mining activities. Minesoils developed on the reclaimed surface-mined sites for coal are contaminated with coal particles resulting from mining and reclamation activities. Therefore, the total organic carbon (TOC) pool in these sites is a mixture of coal and plant-derived recent soil organic carbon (SOC). Accurate estimates of SOC pools and C sequestration rates in the reclaimed minesoils (RMS) is limited by the lack of a standard and cost-effective method for determination of coal C concentrations in the RMS. The chemi-thermal method, based on the oxidative resistance of coal, was developed and validated with radiocarbon analysis using selected artificial soil–coal mixtures and minesoil samples. Radiocarbon analysis of RMS samples indicated that minesoils from the top 10-cm depth developing from topsoil applied during reclamation was coal C free. The contribution of coal C and the radiocarbon age of TOC increased with increasing soil depth. The coal C fraction accounted for 0 to 92% of TOC in the RMS samples. The coal C fraction was highly correlated with {delta}13C (r2 = 0.84), suggesting that stable isotope composition could estimate the coal C concentration in RMS samples. Analysis of coal and artificial soil and coal mixtures indicated that chemi-thermal treatment was effective in removing recent SOC with minimum effect on coal. Analysis of RMS samples indicated that both radiocarbon activity and the chemi-thermal method were effective in estimating coal C concentration in RMS of southeast Ohio. The coal C concentrations for both methods were highly correlated (r2 = 0.95), suggesting that the chemi-thermal method was as effective as radiocarbon activity measurement in estimating coal C concentration in these soils.

Abbreviations: OC, organic carbon • OM, organic matter • RMS, reclaimed minesoils • SIC, soil inorganic carbon • SOC, soil organic carbon • SOM, soil organic matter • TOC, total organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Minesoils are soils formed on landscapes altered by human activities. They exhibit soil profile characteristics and physical, chemical, and biological conditions that reflect anthropogenic perturbations rather than natural soil-forming processes (McSweeney and Jansen, 1984). They are also referred to as spoils, drastically disturbed soils, or anthropogenic soils (Sencindiver and Ammons, 2000). Minesoils are pedogenically young soils that are developing from heterogeneous mixtures of fragmented and pulverized rock and sediment material. The original soil profile is disrupted to a depth of at least 1 m, and sometimes partially or completely replaced by materials from depths below 1 m (Soil Survey Staff, 1994). Minesoils developing on surface-mined lands are characterized by having more rock fragments than the native soils, with rock fragment content often ranging from 32 to 67% (Ashby et al., 1984; Ciolkosz et al., 1985; Thurman and Sencindiver, 1986).

Minesoils formed from reclamation of surface-mined sites for coal are characterized by the presence of intrinsically stable forms of organic C (OC) derived from coal (Schafer et al., 1980; Stroo and Jencks, 1982; Insam and Domsch, 1988; Roberts et al., 1988; Rumpel et al., 1998b). This geogenic C (i.e., OC subjected to geological processes) is the result of coal particles incorporated with spoil during overburden removal, coal mining, and reclamation operations and coal dust particles dispersed during coal mining and reclamation processes. With establishment of vegetation, accumulation and incorporation of soil organic matter (SOM) derived from plant materials and microorganisms lead to mixing of recently formed SOM with coal C of the parent substrate. In addition, inorganic carbonates may also occur in significant concentrations, depending on the parent material type. Therefore, total C in reclaimed minesoils (RMS) is usually a mixture of: (i) soil inorganic C (SIC) originating from the parent material, (ii) coal C or geogenic C from mining and reclamation operations, and (iii) plant-derived recent soil organic C (SOC).

In these sites, organic matter (OM) derived from plant material occurs in intimate mixture with coal from the overburden material. Because of the dark color of humus formed by decomposition of plant litter and fine-sized coal particles, coal may not be distinguished from humus materials by morphological observations. Differentiation of coal and recent C is crucial for quantification of the accumulation of recent SOC during soil genesis, evaluation of C sequestration potential and ecosystem function, and assessment of the degree of humification in these sites.

Research efforts to quantify the contribution of coal-derived C to the total organic C (TOC) concentration in RMS are few. Rumpel et al. (1998b) examined the types of C present in minesoil landscapes of the Lusatia mining district in Germany and showed, after quantification of lignite C content by 14C activity measurement, that OC of the subsoil of the minesoils was almost exclusively composed of lignite C, while up to 50% of all C originated from lignite in the A horizon. In eastern Germany, areas heavily contaminated by fly ash, soot, and lignite dust, lignite-derived material accounted for up to 40% of OC of a forest floor (Rumpel et al., 1998a) and up to 80% of the soil C of an agricultural soil (Schmidt et al., 1996).

Carbon derived from coal and recent plant OC can be quantitatively estimated by radiocarbon (14C) activity (Rumpel et al., 1998b, 2000, 2003; Fettweis et al., 2005; Chabbi et al., 2006). The utility of this method is limited, however, for routine soil analysis due to the high analytical cost and limited availability of the facilities needed for the analysis. Therefore, the objectives of this study were to: (i) quantify coal- and plant-derived C in selected minesoils and artificial soil–coal mixtures; and (ii) validate the chemi-thermal oxidation technique for coal C quantification in RMS of Ohio.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Sampling and Sample Preparation
Soil samples were collected from Mount Carmel (39°44'30'' N and 81°48'02'' W). The site was surface mined for bituminous coal, reclaimed with topsoil application in 1978, and maintained under pasture. The mining process involved land clearing, scraping and storing topsoil, and removing the overburden for access to coal seams. Reclamation involved restoration of the original physiography by grading the overburden material, spreading the stored topsoil on top of the graded mine spoil (20–30-cm depth), and establishing vegetative cover. Soils of the experimental site are predominantly Lowell silty clay loam (fine, mixed, mesic Typic Hapludalfs), mainly derived from sandstone, siltstone, and shale parent materials (NRCS, 1998). Samples were collected from summit, shoulder (midslope), and footslope at 0- to 5-, 5- to 10-, 10- to 20-, 20- to 30-, 30- to 40-, and 40- to 50-cm depths. Samples were air dried, ground, and sieved (2 mm). Ground samples were ball milled to pass through a 250-µm sieve. A bituminous coal sample was obtained from the active mining site nearby.

Carbonate-free soil (a fine, mixed, mesic Aeric Ochraqualf) was collected from southwestern Ohio (Ohio Agricultural Research and Development Center, Western Branch Research Farm) near South Charleston, a site with no history of coal mining activities. Soil samples were air dried, ground, and sieved to pass a 250-µm sieve. Coal was crushed and then ground by mortar and pestle and sieved to pass a 250-µm sieve. Different concentrations of coal and soil mixtures were prepared by homogenously mixing known quantities of coal and soil manually. Mixtures were used for testing the recovery of coal C by the chemi-thermal treatment.

Sample Pretreatment and Elemental and Stable Carbon Isotope Analysis
Samples were acid washed to remove carbonates before submission for radiocarbon dating. Briefly, 5 g of finely ground (<250 µm) soil was treated with 50 mL of 1 mol L–1 HCl and the acid–soil mixture was allowed to react with frequent stirring for a 24-h period. Soils were washed with deionized (DI) water five times, oven dried at 60°C for 72 h, and finely ground by mortar and pestle. Both C and N concentrations were determined by the dry combustion method (1100°C) using an elemental analyzer (Euro EA, EuroVector Instruments & Software, Milan, Italy) interfaced with an isotope ratio mass spectrometer (IsoPrime, GV Instruments, Manchester, UK) used for the analysis of stable C isotopes. The isotopic signature was expressed in the delta notation ({delta}13C) relative to international standard Pee Dee belemnite (PDB) (Craig, 1957):

Formula
where 13C/12Csample is the stable isotope ratio of the sample and 13C/12Creference is the stable isotope ratio of the PDB standard. The {delta}13C measurements are reproducible at ±0.3{per thousand}.

Radiocarbon Activity Measurement
The 14C activity of the soil was measured at Woods Hole Oceanographic Institution using accelerator mass spectroscopy (AMS). Due to high analytical costs, only eight minesoil samples and two soil–coal mixtures were analyzed. For this analysis, soil samples containing 250 to 500 µg of C were ignited at 900°C to produce CO2. The CO2 produced was reduced to graphite by H2 over an Fe catalyst at 600°C. The graphite was pressed into a target holder and analyzed using AMS. Oxalic acid (National Institute of Standards and Technology [NIST] standard reference material [SRM]-4990, {delta}13C = –19.0{per thousand}) and graphite were used as standard and process blanks, respectively. Blanks were used for correction of 14C from contamination introduced during chemical preparation, collection, and handling of the samples. Radiocarbon activity measurements are reported as {Delta}14C ({per thousand}). The 14C activity was corrected for 13C isotopic fractionation as described by Stuiver and Polach (1977).

Calculation of Soil Organic Matter Age and Radiocarbon Activity
Coal in the Appalachian region was formed during Pennsylvanian and Permian periods about 245 to 320 million yr ago (Crowell, 2002) and free of 14C activity (i.e., "dead carbon") due to the 14C half-life of 5730 yr (Rumpel et al., 1998b, 2000). The 14C activity of the atmospheric CO2 has not been stable during the past 50 yr (Manning et al., 1990). In the late 1950s to early 1960s, 14C activity of the atmospheric CO2 increased dramatically due to nuclear bomb testing, and it has been decreasing progressively since (Rafter and Stout, 1970). The 14C activity of plant materials after correction for isotopic fractionation is the same as the atmospheric 14CO2 that the plant acquired during photosynthesis and incorporated in its tissue. Recent SOM in the RMS is a mixture of OM of different ages that accumulated during minesoil development. Due to changes in atmospheric 14CO2, the 14C content of recent SOM depends on the period when 14CO2 was assimilated by the stand and entered into the soil. Therefore, the 14C activity of recent SOM is a function of the mean age of the respective organic material, which may differ with soil depth and is influenced by stand variability.

The 14C activities of process blank, standard, and samples were used for the calculation of the fraction modern (Fm) from this expression:

Formula
where Fm is a measurement of the deviation of the 14C/12C ratio of a sample from "modern" and {Delta}14Csample, {Delta}14Cblank, and {Delta}14Cstandard represent the 14C/12C ratios of the sample, blank, and standard (the modern reference), respectively. Modern is defined as 95% of the radiocarbon activity in 1950 of the oxalic acid (NIST SRM-4990) normalized to {delta}13CVPDB = –19{per thousand} with respect to PDB (Olsson, 1970; Stuiver and Polach, 1977).

The radiocarbon age of the sample was calculated from the Fm by the expression

Formula
The conventional 14C age is reported in years before present (BP), which means years before 1950, because the activity of the oxalic acid standard reference is defined for that year.

Recent SOC and coal C concentrations of the sample were calculated from Fm and TOC as

Formula

Formula
where TOC is total organic C concentration of the sample (g kg–1).

Chemi-thermal Method
A method for quantification of coal C based on its resistance to chemical extraction and thermal oxidation was developed. The method involves pretreatments to remove SIC and recent SOC fractions. Coal C is quantified by an elemental C analysis of the residual soil sample. The summary of the procedure in a schematic flow diagram is presented in Fig. 1 . The detailed procedure is as follows.


Figure 1
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  		Fig. 1. Flow diagram showing the determination of different C fractions in the reclaimed mined soils (OC = organic carbon).

 
Two grams of dry, finely ground soil samples (250 µm) were placed in a centrifuge tube and treated with 20 mL of 1 mol L–1 HCl. Samples were allowed to react for 30 min with intermittent shaking, centrifuged for 20 min (3000 x g), and the supernatant was decanted and discarded. This step was repeated twice, with continuous shaking followed by centrifuging and decanting of the supernatant. The acid treatment removed all carbonate (inorganic) C. Residual acid was neutralized by addition of 20 mL of 0.5 mol L–1 NaOH, shaking for 1 h, and centrifuging. Recent SOC was extracted with 0.5 mol L–1 NaOH at a 1:10 soil/extractant (w/v) ratio. Samples were shaken on a mechanical shaker for 15 h, centrifuged, and the supernatant was discarded. Extraction was repeated two times, with 1-h shaking each. The residual soil was rinsed twice with DI water to remove released SOM and excess NaOH. Twenty milliliters of 60% HNO3 was added and shaken for 15 min. Samples were centrifuged and the supernatant was discarded. To the residual soil, 20 mL of 10% HF was added and allowed to react for 4 h with intermittent stirring. The HF treatment demineralizes the sample and releases mineral-bound OM. Samples were centrifuged and again extracted twice with 20 mL of 0.5 mol L–1 NaOH for 30 min each. Residual soils were rinsed with DI water six times to remove all NaOH, oven dried at 60°C for 48 h, and ground. These steps removed all inorganic C and most of the recent SOC (see below).

To remove highly recalcitrant SOC (i.e., humin), 1 g of the extracted sample was weighed into a crucible and placed in a muffle furnace at 340 ± 5°C for 3 h. This thermal oxidation step removed recalcitrant C that was resistant to NaOH extraction. The remaining C that was resistant to chemical extraction and thermal oxidation is defined as coal C. This was quantified by the dry combustion method using an elemental CN analyzer.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Chemical Properties of Minesoils and Artificial Coal and Soil Mixtures
The pH values of the minesoils ranged from 6.2 in the upper (0- to 5-cm) layer to 8.2 in the subsoil (Table 1 ). Soil pH increased with increasing soil depth (Table 1). The highest pH values of the subsoil could be attributed to the presence of carbonates in the spoil material. The organic C content of the minesoils ranged from 3.6 to 155 g C kg–1 soil, and N concentration ranged from 0.5 to 17 g N kg–1 soil (Table 1). Both C and N concentrations of the coal used for the coal–soil artificial mixtures are listed in Table 1. Analysis of bituminous coal samples from various mine sites of southeast Ohio indicated that the C and N concentrations of bituminous coal range from 500 to 650 and 9 to 12 g kg–1 coal, respectively, and C/N ratios range from 49 to 55 (data not shown). The C/N ratio of the minesoils ranged from 6.8 to 28.8 (Table 1), and was larger in the subsoil (20–50-cm depth) than in the surface soils. The C concentration and C/N values of the 20- to 50-cm depth suggest that the SOM of these layers are contaminated with coal that is highly N depleted. The C/N ratio of the top 0- to 20-cm depth (Table 1) indicates that C and N present in this depth is solely from recent OM contents, as corroborated by radiocarbon and chemi-thermal data (see below). The results are in agreement with mining and reclamation techniques used for this site, where up to 30 cm of the premining stored topsoil (free of coal) was applied on top of graded spoil material contaminated with coal. Artificial mixtures of soil and coal exhibited an increase in OC concentration and C/N ratios proportional to the concentration of added coal (Table 1).


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  		Table 1. Characteristics of minesoils, coal, and artificial mixtures investigated for coal C concentration.

 
Radiocarbon Analysis
Analysis of soil and artificial mixtures of coal and soil indicated that recent SOC in the soil was modern, and the mixtures of coal and this soil produced an average 14C age of 1380 and 5880 yr BP for 22.8 and 60.2% coal C, respectively (Table 2 ). For the mixtures M1 and M3, an increase in average 14C age was proportional to the increase in coal C concentration. Coal C concentrations recovered by 14C activity analysis accounted for 80 and 95% of added coal C in the artificial mixtures of soil and coal containing 22.8 and 55% of TOC as coal C, respectively (Table 2). These results, although limited, indicate that both radiocarbon age and 14C activity may be useful for determining the contribution of dead C derived from coal in the minesoils.


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  		Table 2. Radiocarbon age and vegetation- and coal-derived C of the artificial soil and coal mixtures.

 
The mean radiocarbon age of the minesoils ranged from modern to 20,800 yr BP (Table 3 ). The average radiocarbon age of the top 10 cm was modern, indicating that the activity of 14C for these soils was greater than the standard and mean organic matter age younger than 55 yr BP. The oldest 14C ages were recorded in the 30- to 50-cm depths (>16,500 yr). Similar OM ages have been reported for Lusatian mining district reclaimed sites contaminated with lignite (Rumpel et al., 2003). The apparent 14C age recorded from the subsoils is much older than expected for the SOM derived from the recent plant material. The maximum 14C age of SOM and humic substances was estimated to be between 2000 and 5000 yr (Becker-Heidmann and Scharpenseel, 1992; Arai et al., 1996). The old ages of SOM detected by 14C dating in the 30- to 50-cm depth of the RMS are due to the presence of Pennsylvanian-aged coal-derived C (Crowell, 2002), which is 14C activity free (Rumpel et al., 1998b, 2000; Fettweis et al., 2005). The 14C age of soils containing coal C may be useful in determining the contribution of C free of 14C activity ("dead carbon") derived from coal.


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  		Table 3. Radiocarbon age and vegetation- and coal-derived C contribution to total organic C in the reclaimed minesoils (RMS).

 
Isotopic Composition of Minesoils and Mixtures
The {delta}13C of the coal was –24.24 ± 0.63 (Table 2). This range is within the ranges for coal reported by Whiticar (1996). Soils used for the artificial coal and soil mixtures were collected from an agricultural site under long-term corn (Zea mays L.); therefore it is enriched in the 13C isotope. Overall, {delta}13C decreased with increasing coal concentration in the mixture (Table 4 ). The {delta}13C of minesoils ranged from –26.64 to –23.83 (Table 3). The lowest {delta}13C was observed in the top 5- to 10-cm soil depth, and the highest value was from the subsoil (30–50-cm depth). The enrichment of 13C with increasing depth has been associated with increasing age and degree of OM decomposition (Balesdent et al., 1993) or preferential sorption of 13C-enriched compounds in the subsoil (Kaiser et al., 2001). The enrichment of 13C in the subsoil was accompanied by an increase in mean radiocarbon age of SOM.


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  		Table 4. Effects of chemi-thermal treatment on artificial mixtures of coal and soil.

 
Coal and Recent Soil Organic Carbon Concentration
Coal C concentration of the minesoils calculated by 14C activity is presented in Table 3. Coal C accounted for 0 to 92% of TOC in the RMS (Table 3). As expected, no coal C was detected in the top 10-cm depth (Samples RMS3 and RMS4), indicating that all of the TOC in the top 10 cm originated from recent plant materials. The top 10 cm of soils at this site developed from the applied topsoil, which was scraped off and stored before mining, thereby minimizing chances for coal contamination. In the 10- to 20-cm depth, however, about 12.5% of the TOC was from coal (Table 3). In the 20- to 30-cm depth, coal C accounted for 38 and 75% of the TOC for the summit and shoulder, respectively. In the 30- to 50-cm depth, coal C ranged from 87 to 92% of TOC. Soil OC contributions of <100%, as observed in the soil depths below 10 cm, indicate that OC derived from plant litter and from coal are mixed in the lower soil depths.

For the minesoils analyzed, recent SOC concentration decreased while coal C concentration increased with increasing soil depth. In addition, the coal C concentration in the mineral horizons varied with elevation position. These variations are the result of the reclamation technique used at this site. The Ohio Mining and Reclamation Act of 1972 and the Federal Surface Mining and Reclamation Act of 1977 mandated the application of topsoil or suitable media for plant growth of about 30 cm on top of the spoil material. The topsoil that was scraped off before mining is generally free of coal contamination and contains low OC due to mineralization and leaching losses during storage, while the OC in the spoil is exclusively coal. Spatial and depth variations in coal C concentration observed at this site could be due to physiographic variations and the uneven surface created during grading. Coal C as a fraction of total C and {delta}13C of the minesoil SOM were strongly correlated (r2 = 0.84, P < 0.01). High correlation between the coal C fraction and {delta}13C indicated that the presence of a coal C fraction influenced the stable isotope composition of SOM. The slope and intercept of this relationship may vary, however, depending on site-specific factors that govern the values of the end members.

Chemi-thermal Method Estimates of Coal Carbon Concentration
Table 4 summarizes an overview of the effectiveness of removal of recent SOC from the artificially made mixtures of soil and coal with different concentrations of coal. Chemical extraction and thermal oxidation had little effect on coal C (Table 4). Mass balance indicated that about 98.5% of coal C was recovered after chemi-thermal treatment. This observation suggested that the coal sample tested was resistant to chemical treatments and low-temperature oxidation applied in this method. Chemi-thermal treatment of coal-free soils (Samples S01 and S02) indicated that 5.1 and 4.7% of SOC remained unextracted (Table 4). The amount of OC removed from the mixtures ranged from 87 to 99% of recent SOC (Table 4). The effectiveness of the treatment decreased with increasing coal C in the mixture (r2 = 0.66, P ≤ 0.05). Chemi-thermal-treated artificial mixtures of soil and coal were more depleted in stable 13C isotopes than the untreated samples (Table 4). This suggests that chemi-thermal treatment removed mainly recent SOC, which was enriched in {delta}13C. This is supported by the fact that the {delta}13C of post-treatment mixtures falls within the isotopic range of the coal sample.

Coal C concentration of the RMS determined by the chemi-thermal method ranged from 1.5 to 25.4 g kg–1 of soil (Table 5 ). Similar to radiocarbon analysis, coal C concentration was higher in the subsoil (30–50-cm depth) than in the surface layers (0–10 cm, Tables 3 and 5). Except RMS3 and RMS4, all other post-treatment minesoil samples were enriched in {delta}13C compared with pretreatment samples, and post-treatment {delta}13C fell within the range of coal (Tables 3, 4, and 5), suggesting that the residual C in the treated samples is dominated by the coal C fraction. Radiocarbon analysis indicated that Samples RMS3 and RMS4 from the top 0- to 5- and 5- to 10-cm depths are coal free, with radiocarbon age greater than modern (i.e., 14C activity of the sample is greater than 95% of measured net standard oxalic acid 14C activity [Stuiver and Polach, 1977]); however, the chemi-thermal method detected 3.3 and 7.9% residual OC (Table 5) in these samples. The stable isotope composition of the residual OC of these samples did not differ from the untreated sample, and remained within the range of OM of plant origin, indicating that residual OC of the top 0- to 10-cm depth at this site is of recent OM origin (Table 5). Chemical extraction of recent OM from mineral soils is controlled by multiple factors, including reaction conditions such as pH and temperature, and soil properties, namely mineralogy and OM concentration and quality (Mikutta et al., 2005). Studies involving black C determination in soils have indicated that chemical oxidative techniques are not able to completely remove OM from soils (Schmidt and Noack, 2000; Schmidt et al., 2001; Simpson and Hatcher, 2004) due to OM stabilization by association with the mineral matrix and the highly recalcitrant nature of a small fraction of SOM. In this procedure, demineralization using HF treatment liberates mineral-protected OC and makes it accessible to thermal treatment, thereby increasing the efficiency of the chemi-thermal treatment. Several studies have indicated that aliphatic C makes a large portion of highly recalcitrant OM resistant to oxidation treatments (Righi et al., 1995; Schulten et al., 1996; Cuypers et al., 2002).


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  		Table 5. Effects of chemi-thermal treatment on total organic C of the reclaimed minesoils.

 
Generally, the reclamation techniques used influence the distribution of coal C in RMS. Minesoils from the sites reclaimed with topsoil application tend to have an upper 10- to 20-cm soil depth with very low or no coal contamination. This was observed for summit, shoulder, and footslope sampling locations (Table 4; data for summit and footslope not shown). Overall, coal C contributed 3.3 to 91.2% of TOC in the RMS at these sites. Thus, coal C was the dominant fraction of TOC in the subsoil (30–50-cm depth), accounting for >70% of TOC in RMS. This can be attributed to the dominance of spoil in the subsoil. Overall, spoils contain a larger coal C fraction than the topsoil.

Relationship between Carbon-14 Activity Analysis and Chemi-thermal Method
Comparison of coal C concentrations obtained by chemi-thermal analysis and 14C activity analysis indicated that chemi-thermal treatment was as effective as radiocarbon analysis in quantifying coal C at this site. Coal C calculated by 14C activity and chemi-thermal analysis were highly correlated (Fig. 2 , r2 = 0.95), suggesting that about 95% of the residual OC determined by the chemi-thermal method can be explained by 14C activity. This relationship is probably not site specific. The high correlation of coal C determined by the two methods suggests that the chemi-thermal method could be as effective as radiocarbon analysis in predicting coal C concentrations in coal-contaminated minesoils.


Figure 2
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  		Fig. 2. Relationship between coal C determined by radiocarbon activity calculations and residual C after chemi-thermal treatment of the minesoil samples.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Radiocarbon measurement is a useful tool for quantification of the coal C contribution to TOC in reclaimed minesoils contaminated with coal. Analysis of coal and soil mixtures indicated that the radiocarbon age increased with increasing coal C concentration. Radiocarbon analysis indicated that the coal C contribution to SOC increased with increasing soil depth, accounting for up to 92% of SOC in the subsoil of the reclaimed minesoils. The 14C age of SOM increased with increasing depth due to the presence of coal C. The coal C contribution to TOC strongly influenced the stable isotope composition of SOM. Residual C determined after chemi-thermal treatment of samples was highly correlated with the coal C concentration calculated by radiocarbon activity (r2 = 0.95, P < 0.01). Therefore, both radiocarbon activity and the chemi-thermal method were effective in estimating the coal C concentration in reclaimed minesoils of southeast Ohio.


   ACKNOWLEDGMENTS
 
This project was funded by the Department of Energy (DOE). We thank American Electric Power for providing the sites for this study. Radiocarbon analyses were conducted at National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institute, Woods Hole, MA.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication February 2, 2007.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
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