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Colorado Water Science Center


National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Peter B. McMahon, Rick Crowfoot 1, and Duane Wydoski 2


Methyl tert-butyl ether (MTBE) was the organic contaminant most often detected during a recent regional study of shallow groundwater in the Denver, Colorado metropolitan area. Those findings prompted a study on the effect of fuel oxidants (ethanol and MTBE) on the degradation of gasoline components (toluene and benzene) in aquifer sediments. The compounds ethanol or MTBE were added in concentrations of 0, 200, 2,000, or 20,000 ppb to sediment incubations containing 200 ppb of either toluene or benzene. Concentrations were selected to reflect the range of concentrations measured during the regional study. Experiments were carried out under oxic and under methanogenic conditions. Results indicated that ethanol increased rates of toluene degradation by about 30 percent under oxic and methanogenic conditions but decreased rates of benzene degradation by about 8 times compared to incubations having no ethanol. Rates of toluene degradation were inversely related to MTBE concentration. The highest MTBE concentration decreased toluene degradation rates by about 10 percent compared to rates in treatments having no MTBE. MTBE additions decreased rates of benzene degradation by as much as 5 times compared to incubations having no MTBE. Measurements of CO2 and CH4 production indicated that lower concentrations of ethanol may have promoted toluene degradation by cometabolic processes whereas elevated concentrations of MTBE may have initially inhibited microbial activity in the sediments. After 100 days of incubation, all of the MTBE in the 20,000 ppb MTBE/oxic treatment was consumed compared to no measurable decrease in MTBE concentrations in any other treatments. Results from this study indicate that fuel oxidants have a measurable effect on the degradation rate of gasoline components and need to be taken into account when modeling degradation processes in sediments contaminated with fuel oxidant-containing gasoline.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225
2 U.S. Geological Survey, 5293 Ward Rd., MS407, National Water-Quality Laboratory, Arvada, CO 80002

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