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USING STABLE CARBON ISOTOPES TO ESTIMATE THE RATE OF NATURAL BIODEGRADATION OF MTBE AT FIELD SCALE
Citation:
WILSON, J. T., T. KUDER, P. ELLIS, AND S. J. DAUGHERTY. USING STABLE CARBON ISOTOPES TO ESTIMATE THE RATE OF NATURAL BIODEGRADATION OF MTBE AT FIELD SCALE. Presented at Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Assessment, and Remediation Conference, Costa Mesa, CA, August 17 - 19, 2005.
Impact/Purpose:
To inform the public
Description:
Natural biodegradation of fuel contaminants in ground water reduces the risk of contamination of drinking water wells. It is very difficult to estimate the natural rate of biodegradation of MTBE in ground water because its primary biodegradation product, TBA, is also a component of gasoline. As a consequence, most risk evaluations for MTBE at gasoline spill sites assume that MTBE does not biodegrade in ground water at the site. Recent studies have shown that the stable carbon isotopes in MTBE are fractionated during biodegradation. It is possible to use the ratio of stable carbon isotopes in MTBE in ground water to put a conservative boundary on the extent of biodegradation of MTBE. The ratio of stable carbon isotopes in MTBE in the ground water was measured at a gasoline spill site in Orange County, California and at a gasoline spill site in New Castle County, Delaware. The extent of biodegradation of MTBE in ground water was estimated by assuming that MTBE exhibited Rayleigh fractionation during anaerobic biodegradation, by assuming that the isotopic enrichment factor during biodegradation was -12, and by assuming that the isotopic ratio of MTBE (δ13C) in the gasoline that was originally spilled was -27.4 0. Based on these assumptions, the fraction of MTBE remaining after biodegradation in a well at the gasoline spill site in Delaware was 0.05. The well was 14 meters down gradient of the leaking underground storage tanks. Based on the hydraulic conductivity of the material sampled by the well, and the hydraulic gradient, the estimated travel time from the buried tanks to the well was 1.2 years. At a site in Orange County, California, in three wells with relatively higher concentrations of TBA and lower concentrations of MTBE, the fraction of MTBE remaining was 0.058, 0.023, and 0.045 respectively. These wells were 9.6, 11.7 and 31 meters down gradient of the plausible source of residual gasoline. The estimated travel times to the wells were 0.26, 0.31, and 0.84 years respectively. The first order rate of natural biodegradation of MTBE along a flow path to a particular monitoring well was estimated by taking the natural logarithm of the fraction of MTBE remaining in ground water, and dividing the logarithm by the estimated travel time along the flow path to the well. The estimate rate of biodegradation of MTBE during travel to the well at the site in Delaware was 3.7 per year. The rates in the three wells in the site in California were 11, 12, and 3.7 per year. These rates are rapid, equivalent to the rates of natural anaerobic biodegradation of BTEX compounds in ground water. Anaerobic biodegradation of MTBE did not occur uniformly across the site in California. At a fourth well, the predicted fraction of MTBE remaining was 0.994, even though this well was 23 meters down gradient of the source. The extent of biodegradation in the fourth well was too small to have environmental significance, and the rate was too slow to be detected.