HR: 0800h
AN: B31A-0960    [Abstracts]
TI: Altered Groundwater Inflow Affects Biogeochemical Patterns in Iron Rich and Acidic Sediments
AU: * Knorr, K
EM: kh.knorr@uni-bayreuth.de
AF: Limnological Research Station University of Bayreuth, Universitaetsstr. 30, Bayreuth, 95440 Germany
AU: Blodau, C
EM: christian.blodau@uni-bayreuth.de
AF: Limnological Research Station University of Bayreuth, Universitaetsstr. 30, Bayreuth, 95440 Germany
AB: Sediments of acidic mining lakes are characterized by steep geochemical gradients. Typically, anoxic and moderately acidic (pH 5) groundwater high in ferrous iron and sulfate passes the sediment (10-20 cm) and the pH drops to values as low as 2.8 in the lake water. In the sediment, bacterial iron reduction predominates; sulfate reduction is viable only at a pH of >5. The uppermost sediment is oxic, ferrous iron is oxidized and precipitates. We tested the hypothesis, that the inflow of moderately acidic (pH 5), anoxic groundwater increases the retention of ferrous iron as sulfide or carbonate in the sediment of an acidic mining lake by altering pore water chemistry and elevating the sediment pH. In a laboratory column experiment, lake sediment was percolated with groundwater of different iron and sulfate concentrations (15 and 10 mmol/l vs. 1 and 1 mmol/l) and flow rates (0, 5 and 20 mm/d). Effects on sediment and pore water chemistry were estimated by sampling the percolate, solid phase and pore water. The effect of different treatments in the first 6 weeks of incubation at 10 °C was low. The outflow remained low in pH and acidity production predominated (4.7-31.4 mol m^-2~a^-1). High Fe and sulfate concentrations in the outflow indicated reductive dissolution of iron hydroxosulfate minerals. The addition of DOC (25 mg/l) to the percolate and a temperature of 20 °C initiated sulfate reduction (0.3-6.6 mol H+ eq. m^-2~a^-1) after 60-80 days and increased the pH in the outflow of columns with fast flow rates (20 mm/d) to ~5. Higher pH accelerated schwertmannite transformation into goethite by a factor of >2 but also iron release from the sediment, especially in treatments receiving low concentrations of iron and sulfate. In those treatments the induced groundwater flow remobilised substantial quantities of iron (1.2-20.5 mol m^-2~a^-1) and sulfate (8.7-20 mol m^-2~a^-1), making up >50 % of the total load in the outflow. Contrarily, in the treatments receiving dump impacted groundwater, a remarkable amount of ferrous iron was retained (up to 40.5 mol m^-2~a^-1). The study demonstrates that changes in groundwater inflow will affect geochemical gradients and biogeochemical processes in iron rich and acidic sediments.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0448 Geomicrobiology
DE: 0463 Microbe/mineral interactions
DE: 0471 Oxidation/reduction reactions (4851)
DE: 0488 Sulfur cycling
SC: Biogeosciences [B]
MN: Fall Meeting 2005