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To be presented at 1999 Petrweum Hydrocarbons Conference, Houston, Texa- MTBE, as appropriate, then resealed and returned to dark incubation. Results — UCD tests Figure 2 shows the average headspace MTBE concentration in nonsterile and sterilized control systems over a 320 -hour experiment. In the uninoculated samples, there is evidence of slow MTBE degradation over the first 150 hours, after which the degradation rate increased dramatically until MTBE was not detectable at 240 hours. The systems were re -spiked with 10 ug MTBE per mL at 310 hours. Approximately 80% of the second spike degraded rapidly within the next 14 hours at which time the experiment was terminated (data not shown). These data suggest that the indigenous microbial consortia present in the VAFB soils may rapidly degrade MTBE after an acclimation period, if supplied with sufficient oxygen and MTBE to allow growth to a critical biomass. The slow degradation rate observed over the first 150 hours suggests that MTBE oxidation can occur when cell populations are low. This is important for low concentration plumes where there may not be enough MTBE to allow cells to grow to a critical biomass. Materials and Methods — UW tests Given the encouraging results from the UCD studies, further work was initiated at UW to explore MTBE utilization by indigenous microbes in systems with sediment/water relationships more closely resembling subsurface conditions. The previously collected contaminated location groundwater and soil were both homogenized and the water was purged with nitrogen to drive off volatile organics prior to microcosm assembly. Approximately 200 g of soil was introduced to each of two 1 L bottles specially designed with double positive seals and a side port fitted with a mininert" port to facilitate sample extraction, re -spiking, and headspace equilibration. An MTBE stock solution was prepared by adding sufficient neat MTBE to 4 L of site water to result in a concentration of approximately 2 mg/L. 500 mL of this stock was added to each microcosm bottle and the bottle was then sealed. At the start, during, and near the end of stock addition, samples of the stock were collected for later analysis. The volume of water added ensured that the sediments were fully saturated and isolated from the headspace, which more closely represents in situ conditions. It was assumed that the water would equilibrate with the headspace to provide a constant dissolved oxygen concentration of 120 100 Q 80 E a. a. 0 w 60 0 00 .E E 40 0 o: 0 20 50 100 150 200 250 300 350 elapsed time (hours) Figure 2. MTBE concentration history in the UCD microcosms. roughly 8 mg/L to the microbes in the sediments. The constructed systems had roughly 420 mL of headspace, which contained sufficient oxygen to theoretically degrade all the MTBE introduced, assuming the reaction imposed a stoichiometric ratio of oxygen demand of 3:1. The systems were incubated in the dark at room Page 4 Sterile (2) Uninoculated (3) PM I Inoculated (3) 9 50 100 150 200 250 300 350 elapsed time (hours) Figure 2. MTBE concentration history in the UCD microcosms. roughly 8 mg/L to the microbes in the sediments. The constructed systems had roughly 420 mL of headspace, which contained sufficient oxygen to theoretically degrade all the MTBE introduced, assuming the reaction imposed a stoichiometric ratio of oxygen demand of 3:1. The systems were incubated in the dark at room Page 4