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Training exercises at Texas Air Force Base resulted in an impact to soil and groundwater by trichloroethylene (TCE) and carbon tetrachloride. A monitored natural attenuation study conducted indicated that chlorinated solvents at the plume would naturally decline to acceptable levels after 30 years of monitoring.
After over 14 years of monitoring, contaminant reductions were not proceeding as quickly as predicted and additional remediation was advised. It was determined that limited anaerobic activity was present and biostimulation was a viable option. A pilot test was conducted on-site.
The objective of the test was to determine the feasibility of injection into the shallow, discontinuous groundwater unit. The pilot test was also designed to determine if the residual TCE and carbon tetrachloride could be biodegraded through enhanced bioremediation and reduced to below 5 ppb (the Texas Risk Reduction Program limit). Follow-up injections were applied downgradient of the source area and also in a barrier configuration several hundred feet dowgradient of the source were implemented about 6 months after the pilot test.
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The combination of the pilot test and full-scale application resulted in a 97.8% reduction in carbon tetrachloride 8 months post-injection, with chloroform, methylene chloride, and chloromethane remediation daughter products on a downward trend. High TCE concentrations were effectively treated as well with TCE peaking after injections to 126 ppb within a 3-month period to 0.2 ppb.