ROLE OF COAL MINE DRAINAGE IN CARBON CYCLING: EVIDENCE FROM A MULTIPROXY APPROACH

In streams impacted by coal mine drainage the rates of chemical weathering can be significantly accelerated due to production of sulfuric acid via pyrite oxidation. This sulfuric acid enhanced weathering could potentially release old carbon sequestered in the bedrocks in form of bicarbonate. The added carbon could potentially escape to the atmosphere via CO₂ evasion, or exported as dissolved inorganic carbon. The goal of this study was to test the applicability of stable isotopes to understand the sources and fate of carbon in a coal mine discharge site located at the base of the Pennsylvanian (≈290 Ma) Pittsburgh Formation of the Monongahela Group, in the eastern limb of the Irwin Syncline. The study site is Iron Falls (40°15'22.40"N, 79°47'41.67"W), located near Blythdale, PA in Allegheny County, approximately 32 km south east of Pittsburgh. Water at this site has natural high dissolved CO₂ concentrations. Samples were collected for analysis of major cations, anions, alkalinity, carbon isotope composition of dissolved inorganic carbon (DIC), and S and O isotope composition of dissolved sulfate. Geochemical measurements taken in field include pH, DO, temperature, conductivity and dissolved CO₂ concentrations. Preliminary data indicates that stable isotopes can be used in conjunction with routine geochemistry to understand sources of carbon at this site. In addition, the carbon isotope signatures of dissolved inorganic carbon (δ¹³C_DIC) seem to be very sensitive to small shifts in dissolved CO₂ concentrations in the range of 0.01-0.20 g/L demonstrating the power of this technique to monitor minor CO₂ variations.