Events

A groundwater contaminant plume in California comprised of a mixture of 1,4-dioxane and chlorinated hydrocarbons was selected as the site to demonstrate aerobic cometabolism stimulated by multiple primary substrates. In preparation for implementation in the field, microcosm and packed column studies with groundwater and aquifer solids from the site were conducted in the laboratory. Microcosm studies demonstrated that neither isobutane- nor methane-utilizing bacteria could not be stimulated under site conditions, likely due to the high concentration of 1,1-dichloroethene (1,1-DCE) which forms a toxic epoxide when cometabolically oxidized. However, an isobutane-utilizing bacterium, Rhodococcus sp. ENV493, and a mixed methanotrophic culture, ENV494M, were enriched from the site. When bioaugmented in microcosms, isobutane-grown ENV493 cometabolized 1,1-DCE, 1,2-cis-dichloroethene (cis-DCE), chloroform, and 1,4-dioxane. Subsequent bioaugmentation with methane-grown ENV494M resulted in the transformation of trichloroethene (TCE), thereby suggesting the multiple primary substrate approach and bioaugmentation with site bacteria was a viable approach for treatment of the site’s complex contaminant mixture. However, Michaelis-Menten/Monod kinetic modeling analysis showed primary substrate utilization and cometabolic transformations proceeded more slowly than expected. A high first-order decay coefficient of >1/day was required to simulate the experimental data, highlighting that biological activity ceased after just a few days without the presence of primary substrate. Experiments in a packed column were used to simulate the groundwater recirculation strategy planned for the field. Effluent was recirculated through the column four times father than influent groundwater was added. Isobutane was introduced to the column through dissolution in site groundwater. Isobutane transport was significantly retarded relative to a bromide tracer. No uptake of isobutane or contaminant transformation was observed for 40 days prior to bioaugmentation with ENV493, further demonstrating bioaugmentation will be required in the field. The aquifer sediment packed into the column showed high abiotic oxygen demand. Continuous addition of 100 mg/L hydrogen peroxide was required to maintain oxygen concentrations greater than 3 mg/L throughout the column. After bioaugmentation, simultaneous transformation of 1,1-DCE, cis-DCE, chloroform, and 1,4-dioxane as well as isobutane utilization was observed. Cis-DCE epoxide, the transformation product of the cometabolic oxidation of cis-DCE, was observed as cis-DCE was transformed, offering further evidence that cometabolic transformation was occurring in the column. However, an increase in hydrogen peroxide to 300 mg/L was required to sustain transformation, indicating substantial oxygen addition will be required in the field. Analysis of concentrations at intermediate points along the column showed the majority of transformation occurred in the third of the column closest to the port where ENV493 was introduced. Methane and ENV494M are currently being introduced into the column to facilitate TCE transformation. In addition, the numerical model used to assess transformations in the microcosms will be expanded to include the influence of flow, transport, and retardation in the column.

This project presenter is available for live video chat on Sept. 1, 2020 from 10:15 a.m. - 12:00 p.m. PDT.