Effects of selective handling of pyritic, acid-forming materials on the chemistry of pore gas and ground water at a reclaimed surface coal mine in Clarion County, PA, USA

Abstract

A change from dragline to “selective handling” mining methods at a reclaimed surface coal mine in western Pennsylvania did not significantly affect concentrations of metals in ground water because oxidation of pyrite and dissolution of siderite were not abated. Throughout the mine, placement of pyritic material near the land surface facilitated the oxidation of pyrite, causing the consumption of oxygen (O 2 ) and release of acid, iron, and sulfate ions. Locally in the unsaturated zone, water sampled within or near pyritic zones was acidic, with concentrations of sulfate exceeding 3,000 milligrams per liter (mg/L). However, acidic conditions generally did not persist below the water table because of neutralization by carbonate minerals. Dissolution of calcite, dolomite, and siderite in unsaturated and saturated zones produced elevated concentrations of carbon dioxide (CO 2 ), alkalinity, calcium, magnesium, iron, and manganese. Alkalinity concentrations of 600 to 800 mg/L as CaCO 3 were common in water samples from the unsaturated zone in spoil, and alkalinities of 100 to 400 mg/L as CaCO 3 were common in ground-water samples from the underlying saturated zone in spoil and bedrock. Saturation indices indicated that siderite could dissolve in water throughout the spoil, but that calcite dissolution or precipitation could occur locally. Calcite dissolution could be promoted as a result of pyrite oxidation, gypsum precipitation, and calcium ion exchange for sodium. Calcite precipitation could be promoted by evapotranspiration and siderite dissolution, and corresponding increases in concentrations of alkalinity and other solutes. Partial pressures of O 2 (Po 2 ) and CO 2 (Pco 2 ) in spoil pore gas indicated that oxidation of pyrite and precipitation of ferric hydroxide, coupled with dissolution of calcite, dolomite, and siderite were the primary reactions affecting water quality. Highest vertical gradients in Po 2 , particularly in the near-surface zone (0-1 m), did not correlate with concentrations of total sulfur in spoil. This lack of correlation could indicate that total sulfur concentrations in spoil do not reflect the amount of reactive pyrite or that oxidation rates can be controlled more by rates of O 2 diffusion than the amount of pyrite. Hence, if placed in O 2 -rich zones near the land surface, even small amounts of disseminated pyritic material can be relatively significant sources of acid and mineralized water.