Push-Pull Test for Determining In-Situ Biodegradation Rates of Ethanol

Ethanol significantly alters the biogeochemistry of ethanol/petroleum contaminant plumes through the effects of cosolvency and preferential biodegradation. The persistence of biofuel plumes depends on the in situ biodegradation of ethanol, including microbial acclimation period, spatial variation, if any, in the rate of degradation, and dependence of rate on the initial ethanol concentration. In this study, the Push-Pull Test (PPT) was used within a former fuel spill site at Vandenberg Air Force Base to derive ethanol biodegradation rates. Sequential PPTs can be performed within a single well to determine the response of the native microbial communities to repeated exposure to the injected reactant or to variations in reactant concentration. In addition, PPTs done at different wells within a site can reveal the spatial variability of reaction rates. Initial PPTs of 8 hour duration and 500 mg/L ethanol suggested insufficient exposure time for acclimation. Subsequent PPTs of 10 hours with an ethanol concentration of 100 mg/L suggested an acclimation period of approximately six hours prior to significant degradation of ethanol and consumption of sulfate, the major electron acceptor at the site. In the second of two sequential PPTs, the zero order rates increased three fold (4.6 to 12.8 ug/L/min), illustrating effects of additional acclimation. Longer exposure would apparently be required for complete acclimation, as suggested by prior results of a continuous release of an ethanol/BTEX mix during which the zero order ethanol degradation rate was 340 ug/L/min. To gain more insight on acclimation times and steady state ethanol degradation rates, we used two approaches: longer exposure times during the PPT, and bio-stimulation exercises prior to the PPT. Results of that new work will be included in the talk.