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To be presented at 1999 Petr�.,cum Hydrocarbons Conference, Houston, Text, colonies. Our investigations revealed there were relatively small differences in the numbers of alkane degrading microorganisms in each sample. The "background" and "contaminated" n -hexane -stimulated microcosms contained 6.3 x 106 and 3.4 x 106 cfu/mL respectively. The "background" and "contaminated" isopentane-stimulated microcosms contained 2.2 x 106 and 3.9 x 106 cfu/mL respectively. Our group at Waterloo has been working on a passive release system well suited to oxygen and other gases, and thus wished to identify a gaseous alkane candidate for release in the field. In a separate experiment we examined whether populations of alkane -degrading microorganisms could be stimulated in enrichment cultures seeded with samples of MTBE -impacted sediment obtained from near UW9ML. Samples of sediment (-5g) were placed in sterile plastic tubes with sterile mineral slats medium (45 mL). The tubes were briefly sonicated (5 x 30 s) in an ultra sonic water bath. The mixture was then allowed to separate and samples (50-250 µL) of the liquid phase were used to seed enrichment cultures using either propane, n -butane or isobutane as a sole source of carbon and energy. The enrichments were conducted in stoppered glass serum vials (120 mL) which contained mineral salts medium (30 mL) and a gas phase composed of 50% alkane in air. The enrichments were incubated at 30° C for 3 weeks. Samples (50-250 µL) were then removed and used to inoculate a second series of enrichment cultures. These second series of enrichments are ongoing but we have observed growth in both the enrichments using propane and n -butane while the isobutane enrichment has so far provided no evidence for microbial growth. To determine whether any of these alkane substrates promoted the growth on MTBE -degrading microorganisms the contents of the original enrichment were harvested by centrifugation and the resulting cell and sediment pellet was washed and finally resuspended in sodium phosphate buffer (50 mM, pH 7.0). Samples (100 µL) of this material were incubated in stoppered glass serum vials which contained sodium phosphate buffer (900 µL) and MTBE (1 µmol). The reactions were initiated by the addition of cells and the reaction vials were incubated in a shaking water bath (30° C 150 rpm). Samples (2 µL) were removed at the start of the incubation and after 1 hr and were analyzed by gas chromatography for the depletion of MTBE and the production of recognized MTBE oxidation products (tertiary butyl formate (TBF) and tertiary butyl alcohol (TBA)). In all cases we observed the consumption of MTBE although neither TBA nor TBF accumulation was observed. Cells grown on propane, n -butane and isobutane consumed 92, 166 and 210 nmoles MTBE respectively. Our current efforts are directed at reproducing these observations using samples obtained from the secondary enrichment cultures and the use of selective oxygenase inhibitors which will allow us to account for abiotic losses of MTBE. Summary The lab experiments conducted to date indicate that supplying sufficient oxygen to stimulate indigenous MTBE degraders may be a promising treatment option, provided the natural oxygen demand observed in the VAFB soils can be eventually satisfied. The PM 1 bacterial isolate is also capable of rapid MTBE degradation in VAFB soils, raising the possibility of amendment of a portion of the aquifer with the organisms and releasing oxygen upgradient. Oxygen can be efficiently and effectively released into groundwater using passive diffusive devices such as those described by Wilson and Mackay (1995) and used in ongoing pilot tests at VAFB. These devices are also capable of passive delivery of gaseous alkanes to promote MTBE cometabolism. As indicated in batch microcosm studies, indigenous VAFB organisms are capable of isopentane, isobutane, hexane propane, and n -butane degradation. Isopentane and hexane degraders also seem to express the enzyme necessary for MTBE oxidation as well. If this is also true of some or all of the light gaseous alkanes, it may be possible to stimulate in situ cometabolism of MTBE by the pulsed release of oxygen and alkane in some strategic fashion. The promise of all three methods to enhance MTBE degradation shown in these lab studies has resulted in 3 separate pilot tests being initiated at VAFB. Results of these tests are anticipated within the next 2 years. References Borden, R.C., R.A. Daniel, L.E. LeBrun IV, and C.W. Davis. 1997. Intrinsic Biodegradation of MTBE and BTEX in a Gasoline - Contaminated Aquifer. Water Resour. Res., 33, 1105-1115. Cowan, R.M., and K. Park. 1996. Biodegradation of the Gasoline Oxygenates MTBE, ETBE, TAME, TBA and TAA by Aerobic Mixed Cultures. Proceedings of the 29x' Mid -Atlantic Industrial and Hazardous Waste Conference, Buffalo, NY., 523-530. Page 8