STABLE ISOTOPE CHARACTERISTICS OF ABANDONED COAL MINE DISCHARGES, PENNSYLVANIA

Oxygen and hydrogen isotopic compositions in waters and sulfur and oxygen isotopic compositions in dissolved sulfate were determined for samples collected in 1999 from 140 discharges from abandoned mines in the Anthracite and Bituminous Coalfields of Pennsylvania. The flow from these discharges ranged from 0.028 to 2,210 L/s. The pH ranged from 2.7 to 7.3, but had a bimodal distribution with most samples typically being either acidic (pH = 2.5 to 4) or near neutral (pH = 6 to 7). Specific conductance ranged from 0.131 to 3.980 mS/cm. Alkalinity ranged from 0 to 510 mg/L as CaCO₃. Concentrations of dissolved SO₄ (34 to 2,000 mg/L), Fe (0.046 to 512 mg/L), Mn (0.019 to 74 mg/L), and Al (0.007 to 108 mg/L) varied widely. Concentrations of dissolved O₂ were typically low (<2 mg/L). The δ¹⁸O and δD values of the waters range from -10.6 to -7.8 permil, and from -74 to -47 permil, respectively, and generally conform to the meteoric water line, which indicates that evaporation is not an important process in the geochemical evolution of these waters. The δ³⁴S values for dissolved sulfate vary widely (anthracite: δ³⁴S = 0.7 to 10.8 permil; bituminous: δ³⁴S = -10.3 to 25.2 permil), consistent with known sulfur isotope variations in pyrite from coals in the Appalachian Basin. The δ¹⁸O values for dissolved sulfate range from -6.4 to 11.1 permil; the highest values (> 4 permil) suggest that the oxidation of dissolved intermediate sulfoxyanions during sulfide oxidation may have occurred under oxygen-limited conditions. The oxygen isotope fractionation between dissolved sulfate and water varies with pH. Below pH ~ 5.5, the fractionation is generally less than 10 permil, but above pH 5.5, it reaches 20 permil. These variations are interpreted to reflect differences in the oxidant associated with pyrite oxidation. At low pH, ferric iron is the dominant oxidant and most of the oxygen in sulfate is derived from water, whereas at higher pH, molecular oxygen becomes the more important oxidant, and it contributes increased amounts of oxygen to the resulting sulfate. Thus, stable isotopes may be useful for gaining insights the conditions of sulfide oxidation in settings where the acid-neutralizing capacity of the country rocks varies significantly.