Determining BTEX Biodegradation Rates Using In Situ Microcosms at the Bemidji Site, Minnesota: Trials and Tribulations

By E. Michael Godsy, Ean Warren, Isabelle M. Cozzarelli, Barbara A. Bekins, and Robert P. Eganhouse

This report is available in pdf format: MikeGodsy.pdf 68KB

ABSTRACT

In situ microcosms (ISMs) were installed in an aquifer contaminated by crude oil to study the in situ biodegradation of monoaromatic hydrocarbons. One was placed in an area where iron reduction predominates (ISM-Fe) and the other in an area where methanogenesis predominates (ISM-CH₄). Numerous problems were encountered during installation and operation of the ISM-CH₄ microcosm and therefore monitoring of it has been temporarily discontinued. Biodegradation rates determined from field concentrations suggested that degradation for the ISM-Fe should proceed in the following order: (1) toluene and o-xylene - ~12 days; (2) m-, p-xylene - ~400 days; (3) benzene - 700 days; (4) ethylbenzene - ~1600 days. Samples taken from the ISM-Fe at days 36 and 68 indicated that toluene biodegradation was occurring. Iron, manganese, CO₂, alkalinity, and low molecular weight volatile fatty acids increased while toluene concentration decreased by approximately 20 percent. All other organic and inorganic compounds remained at initial levels including the tracer with the exception of sodium, chloride, and sulfate. These species increased sharply starting at approximately day 8 and have remained elevated since then for an unknown reason.