UST-8 (Building 1138) was a former gas station at Marine Corps Air Ground Combat Center (MCAGCC) that was in operation from mid 1950’s to 1993. The six former gasoline tanks leaked an unknown quantity of fuel hydrocarbons into the vadose zone to depths of over 200 feet bgs. Dissolved fuel hydrocarbon in groundwater beneath the site was determined to be the result of diffusion of soil gas vapors to groundwater.
PEM staff prepared a Corrective Action Plan (CAP) for the former UST site, recommending soil vapor extraction (SVE) for the vadoze zone and monitored natural attenuation (MNA) for the groundwater. Groundwater at the site ranged from 250 to 350 feet bgs. Following CAP approval from the California Regional Water Quality Control Board (RWQCB), PEM staff installed and operated a SVE system from 2007 to 2015, and performed annual groundwater sampling. During this period, total cumulative TPH mass removed by the SVE system was approximately 89,000 pounds (as hexane equivalents). The system was effective in reducing contaminant concentrations in soil/soil gas to levels protective of human health and significantly reduced source area contaminant concentrations. Diffusion of TPH-impacted soil gas to groundwater was not occurring at a higher rate than the rate of natural attenuation, as indicated by the decrease of contaminant concentrations in groundwater over time as a result of natural attenuation. Therefore, it was concluded that further active remediation to soil / soil gas was not warranted given that soil / soil gas objectives had been achieved.
In 2015, a second CAP was prepared for former UST-8. The CAP provided all site background information, results of environmental activities at the site, site cleanup requirements, and a corrective action recommendation. Based on the evaluation of alternatives, MNA was determined to represent the most cost-effective remedial alternative for groundwater at the site, and was the recommended remedial alternative for groundwater. When groundwater monitoring results indicate a stable plume, the site will be recommended for closure with a restriction in the Base Master Plan against using impacted site groundwater for potable or irrigation purposes until MCLs are met.