This thesis focused on using microcosms to better understand the aerobic cometabolic processes of TCE and cis-DCE transformation that occurred during a Cometabolic Air Sparging (CAS) demonstration at McClellan Air Force Base. The microcosms were created with groundwater and aquifer materials from the demonstration site. Concentrations of compounds in the microcosms were maintained to mimic conditions where the demonstration was performed. Propane was used as the primary substrate to stimulate indigenous propane-utilizers present in the McClellan subsurface. The microcosms were used to test the potential of the propane-utilizers to transform the CAHs of interest, and determine their nutrient requirements while transforming these compounds. Vadose zone microcosms were also created and used to compare the cometabolic processes and nutrient requirements of the propane-utilizers under these different conditions. After the addition of propane a ten-day lag period was observed before the propane-utilizers were stimulated in all the microcosms. The presence of CAHs and excess nitrogen did not have any effect on the lag period required to stimulate these microorganisms. Microcosms that received nitrogen amendments maintained effective transformation of TCE and c-DCE with successive additions. The rate of c-DCE transformation was observed to be faster than TCE transformation. Complete removal of the CAHs occurred in these microcosms. No other nutrients, such as phosphorous, were observed to cause any nutrient limitations. However, the microcosms that only had limited amounts of nitrogen present were only able to maintain transformation ability for a short time. Propane utilization rates gradually decreased with each addition, and CAH transformation eventually ceased. This was also observed during the CAS field demonstration after successive additions of propane. Ammonia gas was added to the sparge gas in the field and propane utilization and CAH transformation resumed. Ammonia gas was added to the nitrogen-limited microcosms, and like the field demonstration, propane utilization and CAH transformation resumed. Nitrogen was found to be a critical nutrient for effective cometabolism of CAHs. Nitrogen supplied either as ammonia or nitrate was required for the propane-utilizers to maintain effective rates of propane utilization and CAH transformation ability. By comparing different sets of microcosms under different conditions, estimates were made to the amount of nitrogen required by the propane-utilizers with and without CAHs transformed. The transformation of CAHs significantly increased the propane-utilizers requirements for nitrogen. A 2.0-3.8-fold increase in was observed for nitrogen consumption when CAHs were transformed, possibly resulting from toxic effects caused by the transformations. The sparge gas used at the CAS demonstration also contained ethylene at a low concentration (1% vol/vol). The microcosm experiments with this concentration of ethylene were found not to have any negative effects on CAH transformation. The propane-utilizers were also able to maintain propane utilization and CAH transformation at high CAH concentrations. The vadose zone microcosms showed that propane utilization in the vadose zone was an order of magnitude lower than what was observed in the saturated microcosms. Also bioavailable nitrogen was required to maintain propane utilization rates. However, higher CAH concentrations were found to inhibit the stimulation of the propane-utilizers under these conditions. / Graduation date: 2002