USING STABLE ISOTOPES TO TRACE CONTAMINATION OF THE MADISON LIMESTONE AQUIFER FROM ACIDIC COAL MINE DRAINAGE, CENTRAL MONTANA

The Stockett-Centerville area, near Great Falls, Montana, was extensively mined for coal by underground room-and-pillar methods in the late 1800’s through the early 1950’s. The coal seams are rich in pyrite and are located at the top of the Jurassic Morrison Fm. After closure, the mine portals were sealed and the workings became flooded. Many of the mines have drains that are point sources of acid rock drainage (ARD) which flows into dry streambeds and soaks into the ground. People living in the area rely on the Mississippian Madison Group limestone, which is located several hundred feet below surface and stratigraphically below the coal, for their drinking water. Twelve domestic water wells completed in the Madison near former coal centers were sampled for water chemistry and stable isotope analysis of water and dissolved sulfate. The groundwater samples plot close to the meteoric water line and reveal no clear evidence of mixing with water from the ARD drains, the latter being slightly enriched in ¹⁸O by evaporation. In contrast, the sulfates have isotopic compositions that plot on a mixing line between isotopically-heavy “background” sulfate in the Madison Aquifer (represented by Giant Springs and Big Spring, two very large natural springs with combined discharge > 10 m³/sec; avg. d³⁴S = 13.6‰ and d¹⁸O = 10.8‰) and isotopically-light sulfate from ARD waters associated with the coal mines (avg. d³⁴S = -14.4‰ and d¹⁸O = -10.0‰). The only domestic well in our study that shows no evidence of mixing with ARD sulfate is located upgradient of the coal mines. For the majority of the other wells, the percentage of sulfate derived from ARD is estimated to be between 5 and 40%. However, two wells show a much greater extent of mine water influence, with > 80% of their dissolved sulfate apparently coming from ARD sources. These two wells have elevated sulfate concentrations, but otherwise show no ARD-associated water quality problems, such as low pH or high concentrations of heavy metals. This underscores how the Madison limestone can effectively buffer groundwater chemistry. In this study, stable isotopes of sulfate have proven to be a much more powerful and sensitive tool to track ARD contamination compared to stable isotopes of water or conventional water quality sampling.