Geochemical evolution of acidic ground water at a reclaimed surface coal mine in western Pennsylvania

Concentrations of dissolved sulfate and acidity in ground water increase downflow in mine spoil and underlying bedrock at a reclaimed surface coal mine in the bituminous field of western Pennsylvania. Elevated dissolved sulfate and negligible oxygen in ground water from bedrock about 100 feet below the water table suggest that pyritic sulfur is oxidized below the water table, in a system closed to oxygen. Geochemical models for the oxidation of pyrite (FeS₂) and production of sulfate (SO₄^2-) and acid (H⁺) are presented to explain the potential role of oxygen (O₂) and ferric iron (Fe³⁺) as oxidants. Oxidation of pyrite by O₂ and Fe³⁺ can occur under oxic conditions above the water table, whereas oxidation by Fe³⁺ also can occur under anoxic conditions below the water table. The hydrated ferric-sulfate minerals roemerite [Fe²⁺Fe₄³⁺(SO₄)₄·14H₂O], copiapite [Fe²⁺Fe₄³⁺(SO₄)₆(OH)₂·20H₂0], and coquimbite [Fe₂(SO₄)₃·9H₂O] were identified with FeS₂ in coal samples, and form on the oxidizing surface of pyrite in an oxic system above the water table. These soluble ferric-sulfate 11 salts11 can dissolve with recharge waters or a rising water table releasing Fe³⁺, SO₄^2-. and H⁺, which can be transported along closed-system ground-water flow paths to pyrite reaction sites where O₂ may be absent. The Fe³⁺ transported to these sites can oxidize pyritic sulfur. The computer programs WATEQ4F and NEWBAL were used to compute chemical speciation and mass transfer, respectively, considering mineral dissolution and precipitation reactions plus mixing of waters from different upflow zones. Alternative mass-balance models indicate that (a) extremely large quantities of O₂, over 100 times its aqueous solubility, can generate the observed concentrations of dissolved SO₄^2- from FeS₂, or (b) under anoxic conditions, Fe³⁺ from dissolved ferric-sulfate minerals can oxidize FeS₂ along closed-system ground-water flow paths. In a system open to O₂, such as in the unsaturated zone, the aqueous solubility of O₂ is not limiting, and oxidation of pyrite by O₂ and Fe³⁺ accounts for most SO₄^2- and Fe²⁺ observed in acidic ground water. However, in a system closed to O₂, such as in the saturated zone, O₂ solubility is limiting; hence, ferric oxidation of pyrite is a reasonable explanation for the observed elevated SO₄^2- with increasing depth below the water table.