Geochemical Evolution of Acidic Ground Water at a Reclaimed Surface Coal Mine in Western Pennsylvania

Abstract

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 (FeS 2) and production of sulfate (SO~-) and acid (H+) are presented to explain the potential role of oxygen (0 2) and ferric iron (Fe 3 +) as oxidants. Oxidation of pyrite by o 2 and Fe3+ can occur under oxic conditions above the water table, whereas oxidation by Fe 3 + also can occur under anoxic conditions below the water table. The hydrated ferric-sulfate minerals roemerite [Fe 2 +Fe~+(S0 4) 4 •14H 2 0], copiapite [Fe 2 +Fel+(so 4) 6 (0H) 2 •20H 2 0], and coquimbite [Fe 2 (S0 4) 3 "9H 2 D] were identified with FeS 2 in coal samples, and form on the oxidizing surface of pyrite in an oxic system above the water table. These soluble ferric-sulfate 11 salts 11 can dissolve with recharge waters or a rising water table releasing Fe 3 +, SO~-. and H+, which can be transported along closed-system groundwater flow paths to pyrite reaction sites where o 2 may be absent. The Fe 3 + 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 o 2 , over 100 times its aqueous solubility, can generate the observed concentrations of dissolved SO~-from FeS 2 , or (b) under anoxic conditions, Fe 3 + from dissolved ferric-sulfate minerals can oxidize FeS 2 along closed-system groundwater flow paths. In a system open to o 2 , such as in the unsaturated zone, the aqueous solubility of o 2 is not limiting, and oxidation of pyrite by o 2 and Fe 3 + accounts for most SO~-and Fe 2 + observed in acidic ground water. However, in a system closed to o 2 , such as in the saturated zone, o 2 solubility is limiting; hence, ferric oxidation of pyrite is a reasonable explanation for the observed elevated SO~-with increasing depth below the water table.

Isotopic evidence of enhanced carbonate dissolution at a coal mine drainage site in Allegheny County, Pennsylvania, USA

“…In contrast, there was an increase in observed SO 4 and Fe concentrations between December and July (322-628 mg/L and 17-66 mg/L, respectively). The increased SO 4 concentration in summer could be due to rapid dissolution of secondary SO 4 -bearing minerals, such as coquimbite ðFe 3þ 2 ðSO 4 Þ 3 Á 9H 2 OÞ, pickeringite (MgAl 2 (SO 4 ) 4 Á22H 2 O), and roemerite ðFe 2þ Fe 3þ 2 ðSO 4 Þ 4 Á 14H 2 OÞ, which have been reported to form by the oxidation of pyrite in Pennsylvania coals and overburden shale during periods of low rainfall (Cravotta, 1994). During large precipitation events, these minerals may be dissolved by recharge or rising groundwater, releasing dissolved SO 4 and associated cations into the groundwater within the mine (Cravotta, 1994;Brady et al, 1998).…”

Assessment of characteristics and remedial alternatives for abandoned mine drainage : case study at Staple Bend Tunnel unit of Allegheny Portage Railroad National Historic Site, Cambria County, Pennsylvania, 2004

“…3 and 4). is a dilute acidic solution that adds some acidity, sulfate, and metals, likely due to atmospheric aerosols (acid rain) and dissolution of pyrite-oxidation products along the recharge pathway (Cravotta, 1994(Cravotta, , 2000.…”

Effects of historical coal mining and drainage from abandoned mines on streamflow and water quality in Bear Creek, Dauphin County, Pennsylvania — March 1999–December 2002

“…Manganese oxides are also sources of manganese in sedimentary rocks (Hem, 1985, p. 88-89). Other metals, including aluminum, can originate from the dissolution of aluminosilicate minerals (Cravotta, 1991). Because iron is the metal that has the largest effect on the water quality of Bear Creek, the following discussion focuses on iron production via the oxidation of pyrite.…”