ACIDIC-MINE-DRAINAGE PROJECTS IN PENNSYLVANIA
Submitted by Chuck Cravotta

PA226: Effects of Nutrients on the Formation of Acidic Mine Drainage

Problem: Recent studies of acidic ground-water quality at several reclaimed surface coal mines in the bituminous field of western Pennsylvania (Ohio and Susquehanna River Basins) indicate that (1) limestone, applied as a surficial additive to mine spoil, may not produce sufficient alkalinity to neutralize acid and minimize the transport of iron, aluminum, and other metals, and (2) sewage sludge additions, as a topsoil substitute, on mine spoil may enhance the formation of acidic ground water by nourishing iron-oxidizing bacteria, which catalyze the principal acid-forming reaction, the oxidation of pyrite. The effects of mine-spoil hydrology and chemistry on lime requirement and of sewage sludge on mine-drainage quality need to be understood by the mining industry and mining and waste-disposal regulators to develop more effective methods to prevent or mitigate AMD. This project evaluates the effects of water saturation and of different additives, including calcareous compounds, inorganic nutrients, and sewage sludge, on the production of acid and the transport of metals from pyritic, coaly shale. The shale, which was a source of acidic drainage at an active coal mine, was subjected to leaching in a series of laboratory experiments to investigate biogeochemical interactions and water-quality variations associated with the use of calcareous additives and sewage sludge for coal-mine reclamation.

Collaborators: USGS-WRD is working with the Pennsylvania Department of Environmental Protection, Bureau of Mining and Reclamation, with matching funds from the Federal-State Cooperative Program.

Activities: During January 1994-February 1995, laboratory leaching tests were conducted using crushed shale packed in columns and suspended in shake flasks (slurries). Tests were conducted at ambient temperatures of 20-28oC and over elapsed times of 6 to 39 weeks under three different hydrologic scenarios: variably saturated, aerobic; continuously saturated, stagnant; and continuously saturated, aerobic. Biologically active and sterilized conditions were evaluated to test if specific chemical additives promoted, reduced, or prevented microbial catalysis of acid-forming oxidation reactions. Deionized water was used as the influent when solids, including sewage sludge, N-P-K fertilizer, or CaCO3 were added on top of the shale at the beginning of the experiment. Solutions containing dissolved NH3, NO3, PO4, or K from chloride or sodium salts were added to the shale as the influent in other experiments. Most-probable numbers of iron- and ammonium-oxidizing bacteria and concentrations of chemicals in influent and effluent and chemicals and minerals in solids were analyzed to determine causal relations between microbial activity, acid formation, nutrient additives, and leachate quality. Estimates of cumulative transport of sulfate and metals were compared to evaluate relative effects of different additives and water saturation on pyrite oxidation and metals transport.

In general, rates of pyrite oxidation, indicated by transport of SO4 and Fe, were greater under biologically active conditions than under sterilized conditions and were greatest under variably saturated, aerobic conditions. Under variably saturated, aerobic (moist) conditions, effluent pH was 2; under continuously saturated, stagnant (wet) conditions, without and with CaCO3, pH was 3 and greater than 5, respectively. Under wet conditions, oxidation of pyrite is minimized and Fe-Mn oxide and carbonate minerals decompose releasing Ca, Mg, Fe, Mn, Ni, and Zn. However, under moist conditions, active oxidation of pyrite releases acid, SO4, Fe, and Cu, and the acid aggressively decomposes kaolinite, Fe-Mn oxides, and carbonate minerals, releasing Al and other metals. Rates of pyrite oxidation and transport of SO4 and metals were reduced by the addition of CaCO3; production of alkalinity was significant only if CaCO3 was maintained under wet conditions. Positive linear correlations between densities of iron-oxidizing bacteria and concentrations of NH3, NO3, PO4, SO4, acidity, and metals in leachate suggest that nitrogen- and phosphorus-rich materials added to pyritic shale can encourage the growth of iron-oxidizing bacteria and the formation of AMD. However, sewage sludge, N-P-K fertilizer, and other nutrient additives produced only subtle increases in transport of SO4 and metals. Coupled reduction of NO3 to NH3 and oxidation of pyrite were not significant, even under continuously saturated, biologically active conditions. However, the combined effects of direct chemical oxidation of FeS2 by NO3 and indirect effects of NH3 on microbial catalysis of FeS2 oxidation could be significant.

Project Chief: Chuck Cravotta (tel.: 717-730-6963).