Geochemical and geohydrological characteristics of bedrock and spol from two methods of mining at a reclaimed surface coal mine, Clarion County, PA, USA

Abstract

Two methods of mining caused subtle differences in geochemical and geohydrological characteristics of spoil at a reclaimed surface coal mine in western Pennsylvania. A dragline was used in the southern area of the mine, and bulldozers and front-end loaders were used in the northern area. Mining methods used in the intervening, middle area are uncertain. In general, overburden at the mine consisted of sideritic gray shale and siltstone. Calcareous zones were laterally discontinuous. However, a 1.2-m thick stratum of pyritic shale above the mined coal was laterally continuous and had total sulfur (S) concentrations >2.5 weight percent (wt %). Regardless of mining methods, pyritic material in backfill is inverted relative to its stratigraphic sequence in bedrock. Where bulldozers and front-end loaders were used, the pyritic shale was selectively handled and buried in compacted layers above the water table, and only low-S ( <0.2 wt%) material was buried near the pit floor. Where the dragline was used, high-S ( ≥.5 wt %) material was placed near the surface, but above intermediate-S material. In the middle area, where middle mining methods are uncertain, high-S material was randomly distributed, ear the surface and on the pit floor, within the zone of water-table fluctuation. In the northern and middle areas, mass-weighted average S in spoil was comparable to that in premining bedrock. In contrast, average neutralization potential of spoil was about one-third of that of premining bedrock, possibly because of preferential weathering of carbonates in shallow bedrock (premining) or spoil. Despite differences in mining methods, hydraulic conductivities for spoil were similar among the northern, middle, and southern areas, ranging from 10 -8.2 to 10 -3.0 meters per second (m/s), with median hydraulic conductivities from 10 -3.8 to 10 -3.6 m/s. Hydraulic conductivities for spoil were not always greater than those for underlying bedrock.