EVALUATING THE EFFECT OF ETHANOL AND NITRATE AS CO-CONTAMINANTS ON THE BIODEGRADATION OF BTEX AT AN AQUIFER-WETLAND INTERFACE USING PUSH-PULL TESTS

The increased use of ethanol as fuel makes it prudent to understand the physical and biogeochemical behavior of mixed petroleum-ethanol contamination. We hypothesize that when ethanol is released in the presence of petroleum hydrocarbons, the ethanol will act initially as a solvent as well as an alternate electron donor. This may affect transport, including total plume length, maximum concentrations, as well as biodegradation rates. With careful interpretation, push-pull tests have been effectively used to conduct controlled in-situ experiments of reaction rate and transport properties. This study shows the results of two sets of single-well push-pull tests used to quantify the physical (ex. sorption and enhanced solubility) and biogeochemical processes (ex. biodegradation) that occur with a mix of monoaromatic hydrocarbons, principally benzene, toluene, ethylbenzene and xylenes (BTEX) and ethanol. Effects were evaluated under nitrate-reducing and methanogenic conditions in a wetland contaminated with crude oil in 1979 near Bemidji, MN, USA. In June 2011, two single-well push-pull tests were conducted over two weeks and in June 2012, four single-well push-pull tests were conducted over nine weeks. Test solutions made of water from the contaminated aquifer (amended with tracer and BTEX with and without nitrate and/or ethanol) were injected (pushed) into the wetland to generate an aquifer-wetland interface and slowly removed over time. Although tracer data show that the flow paths within the wetland are quite heterogeneous, notable differences in the biogeochemistry between the experiments were observed. For example, in 2011 and 2012, under methanogenic conditions, acetate was produced in far greater concentrations in experiments with ethanol. In addition, in the 2011, two-week experiments, the data show that the rate of benzene loss under methanogenic conditions was greater in the presence of ethanol. Similarly, tests showed increased rates of nitrate loss in experiments with a combination of electron donors (ethanol and BTEX) as compared to tests with BTEX alone. Few studies have looked at the impact of multiple electron donors (ex BTEX and ethanol) on overall contaminant fate. Our results suggest the presence of ethanol affects total concentrations of BTEX as well as rates of biodegradation.