HYDROCARBON BIODEGRADATION AT THE MIXING INTERFACE BETWEEN CONTAMINATED GROUNDWATER AND WETLAND SEDIMENTS UNDER NITRATE REDUCING AND METHANOGENIC CONDITIONS

In subsurface systems, biodegradation often is controlled by shifts in the terminal electron accepting processes (TEAPs) that occur at mixing interfaces between different water masses. This study seeks to understand the controls on rates of hydrocarbon biodegradation at a mixing interface between hydrocarbon-contaminated groundwater and a shallow wetland, in Bemidji, MN. Hydrocarbon degradation under nitrate-reducing and methanogenic conditions was investigated using innovative in-situ microcosms (ISMs) to measure rates of change over 8 weeks. The ISM samplers were designed to investigate linked microbiological and geochemical controls on rates by allowing measurements of changes in the chemistry of water in direct contact with a known in-situ microbial population. The ISM samplers contained an inner chamber filled with native wetland sediments that were allowed to incubate for about 2 weeks. The inner chamber was then closed to the surrounding environment and amended with a test solution designed to investigate biodegradation under either nitrate-reducing (N-ISMs) or methanogenic conditions (M-ISMs). Both test solutions were composed of native, contaminated groundwater extracted from the aquifer and augmented with a tracer (bromide) and BTEX (benzene, toluene, ethylbenzene and m,p-xylenes) compounds; the N-ISM solutions also contained nitrate. Analysis of ISM sediments shows that nitrate reduction and organic carbon degradation rates are related to the microbial communities present in the sediment. After a 24-hour equilibration period, nitrate loss was observed in duplicate N-ISMs at rates of 0.19 and 0.11 mg/L per hour. Hydrocarbon degradation rates (in mg/L per hour) for the duplicate N-ISMs during the same time period were 2.08 and 2.83 for benzene, 0.017 and 0.81 for toluene , 0.12 and 0.18 for ethylbenzene, and 0.12 and 0.17 for xylenes. The duplicate M-ISMs produced up to 3 mg/L methane and greater than 20 mg/L acetate, whereas undetectable methane and acetate in the N-ISMs indicated that microbial reactions in the N-ISMs were dominated by nitrate reduction. Rate data from the M-ISMs show that biodegradation rates of BTEX compounds under methanogenic conditions was roughly 10 times slower than those associated with nitrate reduction.