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ERI 205505 ROI Tosco Service Station 5886,Stockton,California December 28, 1999 4.0 REMEDIAL ALTERNATIVES ERI evaluated three remedial alternatives for this CAP 1) natural attenuation, 2) AS/SVE, and 3) remediation utilizing periodic purging from the UST cavity, where dissolved hydrocarbons in groundwater are concentrated Remediation by excavation and removal of soil was not considered because it falls to address the hydrocarbon and MTBE impact to groundwater 4.1 Natural Attenuation Natural attenuation is the degradation of petroleum hydrocarbons by naturally occurring biological processes and hydrodynamic dispersion Natural attenuation is not considered an effective remedial alternative at this site based on the concentrations of petroleum hydrocarbons and MTBE in groundwater Natural attenuation was not selected since it appears that remediation goals could be ' reached in a more timely manner using a pro-active remediation approach i ' 442 AS/SVE r AS is a remedial alternative capable of volatilizing dissolved petroleum hydrocarbons in groundwater SVE is then utilized to extract the volatilized hydrocarbons from the air stream as well as extract ' hydrocarbons from the vadose zone However, while AS/SVE is feasible at the site, based upon soil vapor concentrations and removal rates, it is not economical to implement AS/SVE as a remediation alternative this site ' 443 Periodic Purging,of Groundwater Periodic purging of groundwater is a viable remedial alternative to remove dissolved hydrocarbon- impacted groundwater from beneath the site Based upon data collected during ongoingquarterly Y ' groundwater monitoring and sampling, dissolved hydrocarbons are effectively delineated both laterally and vertically in the vicinity of the site However, dissolved hydrocarbons continue to be detected in groundwater beneath the site and are concentrated in the vicinity of the UST cavity 7