Isotopic evidence of enhanced carbonate dissolution at a coal mine drainage site in Allegheny County, Pennsylvania, USA

摘要

Stable isotopes were used to determine the sources and fate of dissolved inorganic C (DIC) in the circumneutral pH drainage from an abandoned bituminous coal mine in western Pennsylvania. The C isotope signatures of DIC (δ~(13)C_(DIC)) were intermediate between local carbonate and organic C sources, but were higher than those of contemporaneous Pennsylvanian age groundwaters in the region. This suggests a significant contribution of C enriched in ~(13)C due to enhanced carbonate dissolution associated with the release of H_2SO_4 from pyrite oxidation. The Sr isotopic signature of the drainage was similar to other regional mine waters associated with the same coal seam and reflected contributions from limestone dissolution and cation exchange with clay minerals. The relatively high δ~(34)S_(SO4) and δ~(18)O_(SO4) isotopic signatures of the mine drainage and the presence of presumptive SO_4-reducing bacteria suggest that SO_4 reduction activity also contributes C depleted in ~(13)C isotope to the total DIC pool. With distance downstream from the mine portal, C isotope signatures in the drainage increased, accompanied by decreased total DIC concentrations and increased pH. These data are consistent with H_2SO_4 dissolution of carbonate rocks, enhanced by cation exchange, and C release to the atmosphere via CO_2 outgassing.