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USING δ13C TO PREDICT THE RATE OF BIODEGRADATION OF MTBE
Citation:
WILSON, J. T., P. KAISER, J. WEAVER, T. KUDER, P. PHILP, AND S. DAUGHERTY. USING δ13C TO PREDICT THE RATE OF BIODEGRADATION OF MTBE. Presented at Battelle Conference 8th International In-Situ and On-Site Bioremediation Symposium, Baltimore, MD, June 06 - 09, 2005.
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Description:
It is difficult to estimate the rate of natural biodegradation of MTBE at field scale. Dispersion in the aquifer or dilution in the well can give a false impression of attenuation along a flow path. The first product of MTBE biodegradation is TBA. Many gasoline spills contain TBA as a component of the gasoline, which makes it difficult to determine the extent of biodegradation of MTBE from relative concentrations of TAB and MTBE in the ground water. The conventional unit for the relative abundance of l3C in a compound expresses the relative abundance as the ratio of l3C to l2C in the compound normalized to the ratio in an isotopic standard (δl3C = (Rsample/Rstandard - 1) * 1000). During MTBE biodegradation, δl3C increases in the residual MTBE. A regression of δl3C on the natural logarithm of the fraction of MTBE remaining is a straight line. The slope of the regression is termed the isotopic enrichment factor (ε). The value of ε depends on the mechanism of biodegradation. During biodegradation of MTBE by aerobic bacteria, values of ε vary between -1.4 and -2.4. The value of ε for anaerobic biodegradation of MTBE is near -12. The δl3C of MTBE was determined in ground water from four wells at a gasoline spill site in Orange County California. The natural logarithm of the fraction of MTBE remaining after biodegradation was estimated by subtracting the δl3C of MTBE in gasoline from the δl3C of MTBE in the ground water sample, then dividing the difference by the isotopic enrichment factor. The heaviest reported value of δl3C in gasoline (¬27.4°/(00) and a conservative value of ε(-12) were used to estimate the faction of MTBE remaining. This approach puts a conservative boundary on the extent of biodegradation of the MTBE. Biodegradation was at least as extensive as estimated. This estimate of MTBE biodegradation is not influenced by dispersion in the aquifer or by dilution in the monitoring well. The MTBE concentration in the most contaminated well was 11,000 µg/L. The concentration in three down gradient wells was 869,47, and 19 flg/L. The δl3C of MTBE in the most contaminated well was -22.88°/00. The extent of biodegradation was minimal, 75% of the MTBE remained after biodegradation. The δl3C of MTBE in the down gradient wells was 6.84°/00, 9.83°/00, and 18.11°/00 respectively, corresponding to a fraction remaining that was no more than 5.8%, 4.5% and 2.3% of the MTBE originally released, or 7.7%, 6.0% and 3.0% of the MTBE in the most contaminated well. The average hydraulic conductivity based on slug tests of monitoring wells was 36 feet per day. The average hydraulic gradient was 0.0023 feet/feet based on thirteen rounds of water table elevations. Assuming an effective porosity of 0.25, the travel time from the most contaminated well to the down gradient wells was 0.26, 0.37 and 0.31 years. The corresponding first order rate of natural biodegradation of MTBE between the most contaminated well and the three down gradient wells was 9.8, 7.4 and 11 per year. These rates consistent with rates of anaerobic biodegradation in microcosm studies at other sites.