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Project No S8100-06-IIA .lune 29, 1995 Soil Remediation Alternatives ' Without hydrocarbon source removal or reduction, petroleum hydrocarbon mass in vadose zone soil will continue to have a negative effect on future groundwater quality beneath the site The continued redistribution of hydrocarbon mass from the vadose zone to groundwater will likely result in the continued migration of dissolved TPHg away from the source area A "no action" alternative for the hydrocarbon impacted soil would result in continued future groundwater impacts and would preclude Caltrans' ability ' to secure regulatory closure for the site For these reasons, the "no action" alternative will not be given further consideration The direct removal of soil impacted with petroleum hydrocarbons would require extensive excavation ' beneath the site to depths ranging up to 19 8 meters (65 feet) bgs Specific economic and technical issues associated with excavation include the design and construction of a shoring system that would be required to provide slope stability around the entire excavation perimeter and the necessity to transport the t excavated soil to a treatment facility or a landfill for disposal For obvious economic reasons, this option will not be given further consideration Vapor extraction has evolved into one of the most successful in situ soil treatment technologies for the removal of volatile compounds from vadose zone soil and has been demonstrated to be an effective and economically practical alternative to other remediation alternatives In general, the effectiveness of a vapor extraction system(VES) is only limited by the permeability of vadose zone sediments,the volatility of the compounds to be extracted and the ability to install vapor extraction wells at locations that will provide adequate air flow through the impacted soil zones The performance of a vapor extraction field ' pilot test will be required to obtain design criteria for a soil vapor extraction system and to test the effectiveness of source removal from vapor extraction well VW-1 ' Groundwater Remediation AIternatives Groundwater quality data from May 1995 indicates that dissolved TPHg and benzene exist in onsite groundwater at maximum reported concentrations of 2,900 and 400 ug/L, respectively The benzene concentrations are in excess of the California Maximum Contaminant Level (MCL) of 1 0 ug/1L The extent of dissolved benzene in onsite groundwater will likely prevent the regulatory approval of a "no 1 remediation" option for groundwater Currently, the most common remedial technologies for groundwater impacted with gasoline are groundwater extraction with aboveground treatment (pump & treat), air sparging and in-situ 1 bioremediation Pump and treat technology was the most commonly employed groundwater remediation alternative when UST corrective actions were first mandated in the early 1980's in California In-situ bioremediation, which employs groundwater extraction and re-injection combined with the addition of microorganisms and nutrients to the infected groundwater, has also been proved to be an effective remediation alternative Air sparging combined with vapor extraction (to capture organics stripped from the groundwater) is a relatively new groundwater restoration method and, with respect to cost, is currently viewed as an attractive alternative to groundwater remediation technologies requiring the extraction of groundwater 14