USING IN SITU MICROCOSMS TO SIMULATE HYDROCARBON BIODEGRADATION RATES WHEN A CONTAMINANT PLUME DISCHARGES INTO A WETLAND

The prevalence of hydrocarbon-contaminated aquifers and the high cost of engineered remediation have resulted in extensive efforts to document natural attenuation of benzene, toluene, ethylbenzene, and xylene (BTEX) under a range of redox conditions. In this study we used in-situ microcosms (ISMs) to investigate BTEX biodegradation rates in a wetland at the Bemidji, MN crude-oil spill site. Our objective was to compare rates of biodegradation of BTEX compounds in methanogenic wetland sediments using ISMs to those previously determined from monitoring well data and ISM experiments conducted in the iron-reducing zone of the aquifer. ISMs installed in the wetland contained native wetland sediments. After 2 weeks of incubation, the ISM was isolated from the surrounding environment and amended with groundwater extracted from the aquifer, a conservative tracer (Br), and BTEX. Sediments within the ISMs were analyzed for methanogens over time using quantitative PCR. Duplicate ISMs produced substantial amounts of methane (0.033 and 0.045 mg/L-day), acetate (0.46 and 0.38 mg/L-day), and propionate (0.027 and 0.121 mg/L-day), suggesting methanogenic processes. Concentrations of aqueous sulfate and nitrate (< 1 mg/L) and solid-phase iron-oxides (< 0.20 mmol/gm sed) were too low to account for the biodegradation processes. Benzene was degraded most rapidly in the wetland ISMs (0.008 mg/L-day), followed by toluene (0.0009 mg/L-day), o-xylene (0.00048 mg/L-day), ethylbenzene (0.00046 mg/L-day), and m, p-xylene (0.00024 mg/L-day). In contrast, in the aquifer ISMs the order of biodegradation under iron-reducing conditions was toluene = o-xylene > m,p-xylenes > benzene > ethylbenzene and a lag time was observed before onset of benzene biodegradation. The zero-order benzene biodegradation rates in the wetland ISMs were similar to rates determined in the iron-reducing zone of the aquifer. ISM experiments conducted in the iron-reducing aquifer also showed zero order biodegradation rates for benzene of 0.008 mg/L-day and observation well data collected in 2007 showed first order benzene biodegradation rates of 0.004 mg/L-day (in the iron-reducing zone). These results indicate BTEX-contaminated groundwater should continue to undergo natural attenuation by biodegradation if it discharged into the adjacent wetland.