Comparison of Microcosm Tests and a Field Demonstration of Cometabolic Air Sparging With Propane for the Bioremediation of Trichloroethylene and cis-Dichloroethylene

Cometabolic air sparging (CAS) is an innovative form of conventional air sparging, and is designed to degrade or remove chlorinated aliphatic hydrocarbon compounds (CAHs) in groundwater and to potentially treat these contaminants in the vadose zone. A CAS demonstration was conducted at McClellan AFB, California, for removal of chloroethenes (TCE, cis-DCE) from groundwater using propane as the cometabolic substrate. In support of this field demonstration both groundwater and vadose zone microcosm studies were performed. The microcosms were created with groundwater and aquifer materials from the demonstration site. Concentrations of compounds in the microcosms were created to mimic conditions where the demonstration was performed. 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. Results from the first season of field-testing showed propane-utilizers could be effectively stimulated in the saturated zone with repeated intermediate sparging of propane and air. The lag time for effective propane utilization to be observed in the field was about 30 to 40 days, while in laboratory microcosms the lag period was about 12 days. Consistent with the field tests the groundwater microcosms showed cis-DCE was more rapidly transformed than TCE. Microcosm tests also indicated that propane inhibited the transformation of cis-DCE and TCE, and as observed in the field, most of the transformation of these compounds occurred after propane was reduced to low concentrations. In the field demonstration propane utilization rates and rates of CAH removal slowed after three to four months of repeated propane additions, which coincided with the depletion of nitrogen (as nitrate) in the treatment zone. Similar results were obtained with repeated additions of propane to the microcosms. In the field test ammonia was added to the propane/air mixture to provide a bioavailable nitrogen source. This resulted in enhanced rates of propane utilization and CAH transformation in the saturated zone. Microcosm results also showed ammonia was effective in enhancing propane utilization and CAH transformation rates, and the transformations caused a toxic effect that significantly increased the propane-utilizers requirements for nitrogen. A 2 to 4-fold increase in nitrogen consumption was observed when CAHs were transformed. Propane utilization was observed to be much slower in the vadose zone of the field demonstration, compared to the saturated zone, and very limited CAH transformation was observed. Propane utilization rates in the vadose zone microcosms were an order of magnitude lower than what was observed in the saturated microcosms. Bioavailable nitrogen was required to maintain propane utilization rates. Higher CAH concentrations were found to inhibit the stimulation of the propane-utilizers under these conditions. Thus the vadose microcosms yielded results that were consistent with the field demonstration.