Using stable isotopes (S, O) of sulfate to track local contamination of the Madison karst aquifer, Montana, from abandoned coal mine drainage

The Stockett–Centerville area, south of Great Falls, Montana, was extensively mined for coal in the late 1800s through the early 1950s. After closure, the underground mine portals were sealed and the workings became partially flooded. The coal seams are rich in pyrite, and many of the mines have horizontal drains that are point sources of acid mine drainage (AMD) which flow into dry streambeds and soak into the ground. The Mississippian Madison Limestone is located stratigraphically below the coal and is an important source of drinking water. Twenty-six domestic water wells completed in the Madison near the former coal mines were sampled for water chemistry and stable-isotope analysis of water and dissolved SO 4 2 - . Madison aquifer groundwater plots close to the meteoric water line and reveals no clear evidence of mixing with water from the mine drains, the latter being slightly enriched in 18O by evaporation. In contrast, dissolved SO 4 2 - in the wells has isotopic compositions that suggest mixing from three end-member sources: (1) regional Madison aquifer groundwater with moderate SO 4 2 - concentrations (110–150 mg/L) that is isotopically enriched (δ34S = 20‰; δ18O = 14‰); (2) AMD water with very high SO 4 2 - concentrations (>1.9 g/L) that is isotopically depleted (δ34S = −15‰; δ18O = −11‰); and (3) local recharge water with low SO 4 2 - concentrations (<80 mg/L) and intermediate SO4-isotope characteristics (δ34S = −10‰; δ18O = −6‰). The percentage of SO 4 2 - derived from AMD is estimated to range from 0% to nearly 80%. Wells with the highest proportion of AMD SO 4 2 - have higher SO 4 2 - concentrations, but otherwise show no mining-associated water quality problems, such as low pH or elevated concentrations of trace metals, underscoring the ability of the Madison aquifer to buffer water quality. Most of the wells with evidence of AMD incursion are older wells that were not cased over their entire depth. It is possible that the open-hole method of completion has allowed some AMD-related water to infiltrate by gravity to the lower Madison aquifer. In this study, stable isotopes of SO 4 2 - proved to be a more powerful and sensitive tool to track AMD contamination compared to stable isotopes of water or conventional water-quality sampling.