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The Role of Magnetite in In Situ Biogeochemical Transformation
Citation:
WILSON, J. T., C. J. ADAIR, AND M. Ferrey. The Role of Magnetite in In Situ Biogeochemical Transformation . Presented at The Insitu Biogeochemical Transformation Workshop, Dallas, TX, November 05, 2009.
Impact/Purpose:
Observing the disposal of chlorinated solvents on the TCAAP contaminated groundwater in the shallow, unconsolidated sand aquifer with TCE and cis-DCE.
Description:
The former Twin Cities Army Ammunition Plant (TCAAP) is located just north of St. Paul, Minnesota. Disposal of chlorinated solvents on the TCAAP contaminated groundwater in the shallow, unconsolidated sand aquifer with TCE and cis-DCE. Concentrations of TCE and cis-DCE rapidly attenuate with distance from the source even though the ground water has high concentrations of dissolved oxygen, there is no evidence from geochemical parameters for biological reductive dechlorination, and there is no accumulation of vinyl chloride. Data from the Building 102 site on the TCAAP was used to calibrate a simple transport and fate model. The calibrated rate of degradation of TCE and cis-DCE that was necessary to achieve an acceptable correspondence to the field data was 1.9 per year and 0.8 per year, respectively. To determine whether purely abiotic reactions of TCE and cis-DCE with magnetite in the aquifer matrix could explain the attenuation of TCE and cis-DCE in ground water, sediment from the site was acquired for a microcosm study. The sediment was autoclaved to prevent biological degradation of the organic compounds. Over 879 days of incubation, vinyl chloride, ethylene, and acetylene were not detected in pore water of the microcosms. The rates of removal of TCE and cis-DCE in the microcosms corresponded within a factor of two with the rates needed to explain the removal of TCE and cis-DCE in the field scale plume at Building 102 Site. Within a reasonable margin of error, abiotic processes can explain the removal of TCE and cis-DCE in aerobic ground water at the Building 102 site. The sediment collected to construct microcosms had a mass magnetic susceptibility near 9.07 E-07 m3* kg-1. The removal was probably associated with magnetite in the aquifer matrix.