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ERI 205505 R01 Tosco Service Station 5886,Stockton,Cahforma December 28, 1999 4.4 Cost COmAarLson Of Remedial Alternatives r� ERI performed a comparison of costs associated with the installation and operation of the remedial alternatives discussed above This comparison is presented as Table 3 I � 5.0 RECOMMENDED CORRECTIVE ACTION Dissolved hydrocarbons in groundwater are delineated vertically and laterally downgradient (southwest) of the site ERI recommends installation of an extraction well in the UST cavity and periodic purging ' of groundwater from the UST cavity for groundwater remediation ERI recommends that periodic purging continue until dissolved hydrocarbon and MTBE concentrations decrease to asymptotic levels ' At that time, ERI recommends performance of a risk-based corrective action (RBCA) analysis, in accordance with methods outlined by the American Society of Testing Materials (ASTM) Designation ' E 1739-95 i I ' 5.1 Groundwater Treatment The proposed groundwater treatment system will consist of a 4-inch diameter extraction well constructed in the UST cavity The extraction well will extend to a depth of approximately 12 feet into ' the UST cavity backfill The proposed location of the extraction well is shown on Plate 2 Groundwater will be periodically purged from.the UST cavity utilizing a VAC truck The groundwater ' will be transported to the Tosco refinery in Rodeo, California for disposal ' Following completion of the extraction well, a VAC truck will be mobilized to the site to purge up to 5000 gallons of groundwater from the UST cavity The groundwater will initially be purged once per ' week for a period of four weeks, after which purging will continue on a monthly schedule Quarterly groundwater monitoring and sampling data will be used to evaluate the success of purging for ' remediation of dissolved hydrocarbons in groundwater and plume stability 8