Paper No. 5
Presentation Time: 2:50 PM
MICROBIALLY-MEDIATED ARSENIC CYCLING AT THE LAVA CAP MINE SUPERFUND SITE, NEVADA COUNTY, CA
FOSTER, Andrea L., U.S. Geological Survey, 345 Middlefield Rd., MS 901, Menlo Park, CA 94025, ASHLEY, Roger P., US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, ONA-NGUEMA, Georges, Institut de Minéralogie et de Physique des milieux condensés, University of Paris, Case 115, 4 Place Jussieu, Paris, 75252, France, RYTUBA, James J., US Geological Survey, 345 Middlefield Road, MS901, Menlo Park, CA 94025 and BROWN Jr, Gordon E., Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, afoster@usgs.gov
The Lava Cap Mine site (LCMS) is one of many historically-mined, low-sulfide, quartz-hosted gold deposits in the western Sierra Nevada foothills that do not produce acid mine drainage but nevertheless pose an environmental risk due elevated arsenic (As) in mine adit water and tailings. We have been studying geochemical/microbiological processes at the LCMS for several years and report here a summary of our findings. Oxidation of arsenian pyrite and arsenopyrite in mine workings is the ultimate source of dissolved As in mine adit water and downstream surface drainages. As(III) in adit water quickly oxidizes to As(V) and a fraction adsorbs on iron (Fe) (hydr)oxide phases in surface drainages. Dissolved inorganic As(V) dominates the aqueous speciation in Lost Lake (originally a tailings retention pond), although there is evidence for limited arsenic cycling in planktonic lake biota. X-ray absorption spectroscopy (XAS) indicates approximately half as much oxidation of reduced-As phases in tailings submerged for approximately 50 years (< 35%, n =5) than in tailings estimated to have been air-exposed for < 3 years at the time of analysis (n =2). Arseniosiderite (Ca
2Fe(AsO
4)
3 3H
2O) was identified by XAS as a major weathering product of intermittently submerged and subareal tailings (average of 22% of total As; n =4), but As(V) sorbed on Fe (hydr)oxides was identified only in subareal tailings samples.
Suboxic waters at the LCMS contain macroscopic, rust-colored microbial flocs that sequester more As than the original tailings material (dry weight average = 0.9 % As, n = 21). Within the floc, As(III) and As(V) adsorb to biogenic Fe (hydr)oxide produced by enzymes embedded in the sheaths of Leptothrix species. Several lines of evidence (most-probable number estimates, quantitative PCR, and DNA sequence variation) suggest that the Fe-floc community harbors microorganisms that can remobilize sorbed As by reductive dissolution of Fe (hydr)oxide, respiration of As(V), or indirect reduction of Fe (III) and/or As(V) by fermenters. Insufficient sulfate and/or lack of strongly reducing conditions precludes the formation of arsenic sulfides or arsenic-bearing iron sulfides that might sequester arsenic for longer periods.
