EVALUATING THE EFFECTS CO-CONTAMINANTS ETHANOL AND NITRATE ON BTEX BIODEGRADATION USING A MULTI-SPECIES NUMERICAL MODEL

Four, single well, push-pull tests were conducted in a wetland near Bemidji, MN to study the impact of ethanol and nitrate as co-contaminants on the biodegradation of benzene, toluene, ethylbenzene, and xylenes (BTEX). Injection solutions for the four tests included BTEX, BTEX and ethanol, BTEX and nitrate, and BTEX, ethanol, and nitrate, respectively. Each injection solution included a conservative tracer. The four wells were sampled over 63 days to monitor how ethanol and nitrate affected BTEX biodegradation. To quantify biodegradation rates, we simulated data from the present push-pull tests using a multi-species reactive transport model (PPTEST). PPTEST allows the user to investigate physical transport processes, sorption, and microbial reaction kinetics from single well push-pull test data; it is an improvement over estimating reaction rates with analytical approximations as it allows arbitrary-order and user-defined reactions to be modeled. For example, an analytical evaluation of the field data from these tests could be interpreted to suggest that BTEX was produced in the aquifer over the course of the experiments because BTEX concentrations remained high relative to the conservative tracer. However, PPTEST shows that this apparent production is an artifact of differential transport of BTEX and the tracer. Additional analysis of the data indicate that the presence of ethanol plays a complex role in the physical and biogeochemical processes that affect overall BTEX concentrations. For example, since BTEX is more soluble in ethanol than in water, we hypothesize that ethanol acts as a solvent for BTEX, increasing aqueous concentrations. In addition, since ethanol is microbially preferred over BTEX as an electron donor, ethanol is likely consumed prior to BTEX biodegradation, and as ethanol is consumed, the solubility, and therefore aqueous concentrations of BTEX, decreases. Ethanol concentrations decrease more rapidly in the presence of nitrate due to thermodynamic controls, which likely decreases the solubility of BTEX and inhibits biodegradation. The results of this study will be useful for understanding the role of co-contaminants in the biodegradation of BTEX in a variety of anaerobic electron accepting conditions. Findings from this study are important for understanding the water quality impacts of fuel spills as the addition of ethanol to gasoline becomes more common.