| Field and laboratory studies evaluated biogeochemical interactions and transport processes affecting pyrite oxidation and the formation of acidic mine drainage (AMD) at surface coal mines. Field studies were conducted during the first 2-3 years after sewage sludge was surficially applied to promote revegetation at two reclaimed mines in western Pennsylvania. Laboratory leaching tests where sludge and inorganic nutrients were added to pyritic, coaly shale also were conducted to address the following questions: (1) Does the sludge decrease pyrite oxidation by functioning as an {dollar}rm Osb2{dollar}-consumptive barrier at the mine surface? (2) Is the sludge a significant source of metals and nutrients to ground water at a mine? (3) Do nutrients leached from the sludge promote the activity of iron-oxidizing bacteria and increased pyrite oxidation? (4) Is nitrate reduction coupled with pyrite oxidation a significant process in acidic mine spoil?; Field data for rock, pore-gas, and ground-water chemistry indicated that surficially applied sewage sludge was not an effective barrier to {dollar}rm Osb2{dollar} influx and did not decrease pyrite oxidation in underlying zones; contributed nutrients, but not trace metals, to ground water; and enhanced the acidification of ground water. Median concentrations of {dollar}rm NOsb3sp-, SOsb4sp{lcub}2-{rcub},{dollar} Fe, Al, Mn, Cd, Cr, Ni, and Zn and abundances of iron-oxidizing bacteria were greater in ground water from sludge-treated spoil than from untreated spoil with similar mineral and chemical composition. Positive correlations between abundances of iron-oxidizing bacteria and concentrations of {dollar}rm NHsb4sp+, NOsb3sp-, SOsb4sp{lcub}2-{rcub},{dollar} acidity, and metals in the ground water indicated the bacteria were associated with AMD and the nitrogen leached from sludge could have promoted bacterial oxidation of pyrite. Concentrations of {dollar}rm NOsb3sp-{dollar} frequently exceeded 10 mg/L as N in vadose-water samples from sludge-treated spoil but were less than 3 mg/L in underlying ground water. The decrease in {dollar}rm NOsb3sp-{dollar} concentrations resulted from dilution and {dollar}rm NOsb3sp-{dollar} reduction coupled with oxidation of pyrite or Fe(II)-Fe(III) hydroxysulfates.; Results of column leaching experiments generally supported the interpretations of field data and indicated that effects from sewage sludge and {dollar}rm NOsb3sp-{dollar} were small relative to hydrological and mineralogical factors affecting AMD formation. Pyrite oxidation rates were greatest under variably saturated, aerobic conditions and least under continuously saturated, stagnant conditions. The addition of {dollar}rm CaCOsb3{dollar} decreased pyrite oxidation rates. |
