MEASUREMENT OF HYDROCARBON BIODEGRADATION RATES USING IN SITU TECHNIQUES

Due to the prevalence of hydrocarbon-contaminated aquifers, there has been considerable interest in identifying the processes that control their fate. Investigations of the in situ distribution of alkylbenzenes in a crude-oil contaminated aquifer near Bemidji, MN, revealed that their fate is related to their dissolution into ground water and their potential to biodegrade under different redox conditions. Degradation of aromatic hydrocarbons in the anoxic plume is limited by the availability of iron oxides which are used as electron acceptors by iron-reducing microorganisms. The anoxic plume evolves from iron-reducing to methanogenic conditions over time and, thus, an understanding of the potential transport of hydrocarbons in this system requires that the biodegradation rates of hydrocarbons be quantified under these conditions. In order to quantify the biodegradation rates we used an in situ microcosm approach, which allowed for the isolation of a small region of the aquifer with minimal disturbance to the microbial populations. The microcosm was installed in the iron-reducing zone of the plume and the loss rates of benzene, toluene, xylenes and ethylbenzene were compared to the loss rates of these compounds observed at the plume scale. Results from the microcosm showed that toluene and o-xylene degraded first under iron-reducing conditions with first order biodegradation rates of 1.82E-02 per day and 1.35E-02 per day respectively. Benzene and ethylbenzene degradation was not observed in the microcosms until > 90% of the toluene and xylenes were gone (at 198 days). Benzene and ethylbenzene biodegradation rates were 3.94E-03 per day and 1.46E-03 per day, respectively. These results compare well with rate estimates from inverse modeling based on hydrocarbon concentration data in the plume. Despite their similar physical and chemical properties, isomeric alkylbenzenes, isopropylbenzene and 1-methyl-3-ethylbenzene, had different removal rates (4.06E-03 per day and 1.49E-02 per day respectively) in the microcosm, consistent with the plume-scale observations, indicating the differences in their removal rates can be attributed to biologically mediated processes.