CONCENTRATION AND POTENTIAL MOBILIZATION OF SELENIUM IN PENNSYLVANIAN COAL-BEARING STRATA IN THE APPALACHIAN BASIN, USA

Natural sources of Se to the food web include rock, soils, and dissolved components in both surface and ground water systems. Current coal mining methods of mountaintop removal surface mining and longwall underground mining remove a higher fraction of the coal, and disturb a larger volume of rock, than older methods of contour surface mining and room and pillar underground mining. It is unclear whether large-scale anthropogenic land disturbance, such as coal mining and highway construction, enhances the potential for Se mobilization into the food web. To examine this question, claystone, shale, siltstone, and sandstone samples (N=236) were collected from three cores in Middle Pennsylvanian coal-bearing strata in southern West Virginia. These samples have whole-rock Se concentrations that range from below the detection limit (10 ppb) in 33 percent of the samples, to a maximum of 1800 ppb (excluding two high outlier values), with a median of 200 ppb. Samples (N=26) of roof, parting, and floor rock adjacent to coal beds from one of the cores have Se concentrations that range from below detection to 3300 ppb with a median of 660 ppb. The USGS COALQUAL database lists 737 Middle Pennsylvanian coal bed samples from the central Appalachian basin. These samples have a Se concentration range from below detection (0.1 ppm) for less than 1 percent of the samples, to a maximum of 21 ppm (excluding two high outlier values), with a median value of 5.0 ppm (whole-coal).

Leaching studies on 185 rock samples from the three cores suggest that Se may be easily mobilized by water from recently exposed mineral grain and organic matter surfaces. Se mobilization does not correlate with total Se concentration in the rock as one third of the lowest and almost half of the highest Se concentrations in leachates were derived from rock samples with Se concentrations below detection. Apart from overall concentration, Se mode of occurrence may be a significant factor in determining its mobility. These preliminary results suggest that land disturbance may contribute to relative increases in Se concentrations in surface waters. However, further work is needed to quantify increased Se mobilization, if any, from large-scale land disturbance in the Appalachian region.