MULTIDECADAL GEOCHEMICAL EVOLUTION OF ACID MINE DRAINAGE IN AN APPALACHIAN COAL BASIN

Discharges from coal mines release leachates high in heavy metals, sulfates, and total dissolved solids into waterways, requiring costly, long-term remediation strategies. In some cases, net-acidic mine drainage transitions to net-alkaline over time. Understanding this evolution will improve predictive models critical for environmental remediation and policy making. The Irwin Coal Basin (ICB) in Pennsylvania contains a structurally confined series of mine pools from abandoned Pittsburgh coal seam mines [1] that transition from net-acidic to net-alkaline over ~25 km, with geochemistry spanning nearly the range of Appalachian coal mine drainage [2]. In the eight major discharges from the ICB, alkalinity and pH increase with mine pool depth and residence time.

Historical water-quality data and recent (2021-22) bimonthly sampling of ICB discharges were used to evaluate temporal and spatial trends over five decades. Since the 1970s, all discharges increased in pH and decreased in acidity, sulfate, and iron concentrations. In deep minepools (69-94 m depth), alkalinity increased between 123-228 mg/L (as CaCO₃) to values as high as 363 mg/L. Sodium concentrations increased by up to 456 mg/L with high [Na]/[Cl] ratios that cannot be explained by halite dissolution or deep brines. The correlation of Na and alkalinity are consistent with cation exchange on overburden clays driving carbonate dissolution. Directly measured dissolved CO₂ and dissolved inorganic carbon (DIC) concentrations are consistent with other Appalachian mine discharges [3]. Elevated PCO2 and DIC values are likely due to carbonate mineral weathering from sulfuric acid. Decay curves applied to discharges with known mine closure dates indicate that average acidity concentration decreased by 2-5% per year. The rapid Fe decay (8% per year) and low Fe concentrations of unflooded minepools could be due to Fe(III) mineral precipitation while in flooded minepools, high Fe(II), high pH, and low O₂ could reflect reductive dissolution of Fe(III) hydroxides or siderite dissolution contributions under equilibrium conditions due to elevated PCO₂.

[1] Winters W.R., Capo R.C., 2004, Ground Water 42: 700-710; [2] Cravotta III, C.A., 2008, Appl. Geochem. 23: 166-202; [3] Vesper, D.J., Moore, J.E., Adams, J.P., 2016, Env. Earth Sci. 75: 340.