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A field-scale fixed bed bioreactor was used to successfully treat an MTBE-contaminated aquifer in North Hollywood, CA without requiring inoculation with introduced bacteria. Native bacteria from the MTBE-impacted aquifer rapidly colonized the bioreactor, entering the bioreactor in the contaminated groundwater pumped from the site, and biodegraded MTBE with greater than 99 % removal efficiency. DNA sequencing of the 16S rRNA gene identified MTBE-degrading bacteria Methylibium petroleiphilum in the bioreactor. Quantitative PCR showed M. petroleiphilum enriched by three orders of magnitude in the bioreactor above densities pre-existing in the groundwater. Because treatment was carried out by indigenous rather than introduced organisms, regulatory approval was obtained for implementation of a full-scale bioreactor to continue treatment of the aquifer. In addition, after confirmation of MTBE removal in the bioreactor to below maximum contaminant limit levels (MCL; MTBE = 5 μg L(-1)), treated water was approved for reinjection back into the aquifer rather than requiring discharge to a water treatment system. This is the first treatment system in California to be approved for reinjection of biologically treated effluent into a drinking water aquifer. This study demonstrated the potential for using native microbial communities already present in the aquifer as an inoculum for ex-situ bioreactors, circumventing the need to establish non-native, non-acclimated and potentially costly inoculants. Understanding and harnessing the metabolic potential of native organisms circumvents some of the issues associated with introducing non-native organisms into drinking water aquifers, and can provide a low-cost and efficient remediation technology that can streamline future bioremediation approval processes.

The chemical kinetics of the free-radical-induced degradation of the gasoline oxygenate methyl tert-butyl ether (MTBE) in water have been investigated. Rate constants for the reaction of MTBE with the hydroxyl radical, hydrated electron, and hydrogen atom were determined in aqueous solution at room temperature, using electron pulse radiolysis and absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. The rate constant for hydroxyl radical reaction of (1.71 +/- 0.02) x 10(9) M(-1) s(-1) showed that the oxidative process was the dominant pathway, relative to MTBE reaction with hydrogen atoms, (3.49 +/- 0.06) x 10(6) M(-1) s(-1), or hydrated electrons, <8.0 x 10(6) M(-1) s(-1). The hydroxyl radical reaction gives a transient carbon-centered radical which subsequently reacts with dissolved oxygen to form peroxyl radicals, the rate constant for this reaction was (2.17 +/- 0.06) x 10(9) M(-1) s(-1). The second-order decay of the MTBE peroxyl radical was 2k = (6.0 +/- 0.3) x 10(8) M(-1) s(-1). These rate constants, along with preliminary MTBE degradation product distribution measurements, were incorporated into a kinetic model that compared the predicted MTBE removal from water against experimental measurements performed under large-scale electron beam treatment conditions.