Isotopic composition of dissolved inorganic carbon in underground mine discharges of the northern appalachian basin

Abstract

This research examined the efficacy of using 13 C of dissolved inorganic carbon (DIC) to infer geochemical and microbiological processes in underground coal mine discharges. Groundwater samples from mines in two different coal seams (the Pittsburgh and Upper Freeport) and three different hydrogeologic settings (fully flooded, partially flooded, and free draining) were compared. Twenty mine discharges were sampled, fifteen from the Pittsburgh coal seam and five from the Upper Freeport. Sampling sites included mine portals and seeps. All sampling followed standard field protocols. Results were evaluated for possible relationships between δ 13 C DIC and 1) mine hydrogeologic setting and 2) geochemical properties of the individual coal seams and their overlying rock strata. Isotopic results were used in conjunction with inorganic geochemistry to test if different sources of carbon could be discriminated, e.g. carbonate minerals, organic carbon from coal and shale, organic carbon from the soil zone, or biogenic methane. Parameters describing hydrogeologic setting resulted in distinct statistical clusters separating fully-flooded, partially-flooded, and free-draining sites. Hydrogeochemical results show evidence of calcite dissolution, biogenic sulfate reduction, and CO 2 evasion in the sample set. However, no definitive correlation was observed between isotopic and geochemical results that would be useful for differentiating sources of dissolved inorganic carbon.