Effects of Nutrients on the Formation of Acidic Mine Drainage
ProblemRecent 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.
CollaboratorsUSGS-WRD is working with the Pennsylvania Department of Environmental Protection, Bureau of Mining and Reclamation, with matching funds from the Federal-State Cooperative Program.
ActivitiesDuring 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 increased or decreased 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 nutrients (NH3, NO3-, PO43-, 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 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 dissolved SO4 and Fe, were greater under biologically active conditions than under bacteriostatic conditions and were greatest under variably saturated, aerobic conditions. Under discontinuously 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 was minimized and Fe-Mn oxide and carbonate minerals decomposed. However, under moist conditions, active oxidation of pyrite released acid, SO42- and Fe3+, and the acid aggressively decomposed kaolinite, Fe-Mn oxides, and carbonate minerals, releasing Al3+, Mn2+, and other metals. Rates of pyrite oxidation and transport of SO42- and metals were reduced by the addition of CaCO3; production of alkalinity was significant only if CaCO3 was maintained under wet conditions. The measured pyrite oxidation rates were comparable with those reported for other laboratory tests, but were 10 to 100 times greater than rates estimated for field tests, owing to the high surface areas of small particles (<1-cm diameter) used in laboratory tests.
Positive linear correlations between number of iron-oxidizing bacteria and concentrations of total NH3, PO4, SO4, metals, and acidity in leachate suggest that (1) nitrogen- and/or phosphorus-rich materials added to pyritic shale can encourage the growth of iron-oxidizing bacteria and the oxidation of pyrite, or (2) nitrogen and phosphorus solubility and transport are enhanced under acidic conditions. Because of extreme acidification under variably saturated conditions (pH 1.5-2), mineral sources of nutrients rapidly decomposed producing soluble species, NH4+ and H3PO4o. Nevertheless, additions of sewage sludge and small concentrations of NO3- (0.1 and 1 mmol/L) increased the rate of pyrite oxidation by 30 percent or more, as indicated by significantly higher transport of acidity, SO4, and Fe during 12 to 18 weeks of leaching relative to untreated controls. However, the addition of 10 mmol/L of NO3-, 1 mmol/L of NH4+, H2PO4-, or K+, or equivalent amounts of solid N-P-K fertilizer did not increase pyrite oxidation rates. Results for 10 mmol/L of NO3- are consistent with reports that indicate high NO3- concentrations can be toxic to iron-oxidizing bacteria. For the tests with addition of 1 mmol/L of NO3-, decreases in NO3- and correlative increases in NH4+ and SO42- in the leachate indicated that pyrite oxidation coupled with reduction of NO3- to NH4+ could have occurred in variably saturated columns, but was not significant in continuously saturated columns relative to controls. Microbial catalysis of the NO3-reduction and pyrite-oxidation reactions was suggested because high concentrations of NO3- (=10mmol/L) were inhibitory, producing decreased transport of SO42-, whereas low concentrations of NO3- (<1 mmol/L) produced greater rates of SO42- production.
Preliminary results of this project were published in an abstract, "Effects of water saturation and microbial activity on acid production and metals transport from pyritic shale" (Cravotta, C.A. III, and Bird, P.H., 1995, EOS, v. 76, no. 17, p. S149). The complete results, including experimental data and detailed discussion, were reported in "Municipal sludge use in coal-mine reclamation and potential effects on the formation of acidic mine drainage" (Charles A. Cravotta III, 1996, Unpublished Ph.D. thesis, Department of Geosciences, Pennsylvania State University, University Park, Pa 16802).
Project ChiefChuck Cravotta