Importance of Exposure History When Using Single Well Push‐Pull Tests to Quantify In Situ Ethanol Biodegradation Rates

Abstract

Single well push‐pull tests (PPTs) were used to characterize in situ biodegradation rates of ethanol in groundwater at a leaking underground fuel tank (LUFT) site at Site 60, Vandenberg Air Force Base (VAFB), CA. For the tests, local groundwater was spiked with bromide and ethanol and injected at different times into three different wells throughout the experimental area. The spiked water was allowed to remain in the aquifer for 1 to 15.9 h prior to extraction. Biodegradation of ethanol was not observed within 15 h of the aquifer’s first exposure to ethanol near any test well; the ethanol/Br ratio was nearly constant in the extraction samples. Biostimulation treatments (ethanol injections) over the course of 1 to 2 weeks resulted in a linear decrease in ln(ethanol/Br) with time in the extraction samples indicating that ethanol was biodegrading with a first order rate constant of about 0.3/h. After exposing an area to ethanol for 3 months, the biodegradation rate increased further by about a factor of 2. Ethanol degradation rates in the aquifer at this site were temporally variable based on the ethanol exposure history. Our results suggest that PPTs were an effective tool for examining such variability. PPT investigations should be valuable at other locations because ethanol degradation rates in groundwater should vary spatially and temporally depending on the type and timing of fuel releases as well as other factors that control the history of ethanol exposure to an aquifer.