HYDROCHEMISTRY OF LEGACY COAL-MINE DRAINAGE IN PENNSYLVANIA

Water-quality data for discharges from 140 legacy coal mines in the Anthracite and Bituminous Coalfields of Pennsylvania reveal important relations among the pH and dissolved solute concentrations. Observed values of pH ranged from 2.7 to 7.3. Generally, flow rate was positively correlated with pH and negatively correlated with solute concentrations. Although the concentration of Fe was not correlated with pH, the concentrations of As and Ba were positively correlated with pH, and the concentrations of SO₄, Se, Al, Mn, Zn, Cd, Ni, Pb, and Cu were negatively correlated with pH. The bituminous discharges had smaller median flow rates; greater concentrations of SO₄, Fe, Al, As, Ni, and Cu; comparable concentrations of Mn, Zn, Cd, and Se; and smaller concentrations of Ba and Pb than anthracite discharges with the same pH values. The observed relations between the pH and constituent concentrations can be attributed to (1) dilution of acidic water by near-neutral or alkaline ground water; (2) solubility control of Al, Fe, Mn, and Ba by hydroxide, sulfate, and/or carbonate minerals; and (3) aqueous SO₄-complexation and surface-complexation reactions. Sulfate complexation can account for 10 to 20 times greater concentrations of dissolved Al in SO₄-laden bituminous discharges compared to anthracite discharges at pH of 5 and also can account for 10 to 30 times greater concentrations of dissolved FeIII at equilibrium with Fe(OH)₃ at pH of 3 to 5. Lower Ba concentrations in bituminous discharges indicate that elevated SO₄ concentrations could limit Ba concentrations by the precipitation of barite (BaSO₄). Concentrations of dissolved Pb, Zn, Cd, and Cu in bituminous and anthracite discharges were orders of magnitude less than equilibrium with sulfate, carbonate, and/or hydroxide minerals. Surface complexation (adsorption) by hydrous ferric oxides (HFO) could account for the decreased concentrations of these trace cations with increased pH. Increased concentrations of As and, to a lesser extent, Se with increased pH could result from the adsorption of these oxyanions by HFO at low pH and desorption at near-neutral pH.