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4. CORRECTIVE ACTION OBJECTIVE As discussed in Section 3 4, the small pocket of affected groundwater is not migrating and will not impact surface water or water supply wells As discussed in Section 3 5, the residual petroleum does not likely pose a human health risk Therefore, corrective actions are only focused upon soil source removal Removal of this source will reduce any future potential for migration, remove free product, and will assist the natural attenuation of the groundwater concentrations 5. CORRECTIVE ACTION ALTERNATIVES EVALUATION ' A total of seven corrective action alternatives were previously evaluated in two CAPS submitted for this site The most recent CAP, dated October 5, 2004, evaluated alternatives including natural attenuation, air sparging/SVE, low temperature thermal desorption, and excavation ' Further refinement of the CAP alternatives is provided in this Addendum Three additional options are presented for excavation/off-site disposal to further determine the cost effectiveness of source removal For each alternative the costs, mass of TPH removed, effect on free product, ' and an estimate of time to achieve compliance with WQOs were compared to determine the most appropriate alternative for the Stockton Elevator ' This Addendum to the CAP presents the following three alternatives for excavation and off-site disposal of affected soil Alternative Al Excavation and off-site disposal with remedial goal addressing soils to 500 mg/kg TPH boundary (includes beth dewatering and shoring to remove soil to a depth of up to 28 feet deep or approximately 14 to 17 feet into the water table) ' . Alternative A2 Excavation and off-site disposal to approximately five feet into water table, expanding out laterally to area of MW-3 (does not include shoring, may or may not include dewatering) Alternative A3 Excavation and off-site disposal as combination of above options, to approximately 28 feet deep and laterally expanded to MW-3 (requires both shoring and ' dewatenng) 5 1 Alternative Al Excavation of soils containing 500 mg/kg TPH or more The focus of Alternative Al was to remove all soils containing more than 500 mg/kg of TPH This particular soil objective is based upon the ESL from the San Francisco Bay Region and represents the most conservative soil screening level for TPH where groundwater is a potential drinking water resource Therefore, this ESL does not apply directly to the Stockton Elevator but rather provided a baseline for comparison of source area removal The area to be excavated is defined by both historic soil borings and the soil samples collected in 2004 The lateral boundaries of this area were HA-8, HA-9, the utility pole, and MW-4 This area extends laterally approximately 67 5 ,ft by 20 ft and compares to Alternative 3 and Alternative 4 in the October 2004 CAP that measured 35 ft by 30 ft in size To remove all soils with a concentration of more than 500 mg/kg TPH requires excavation into the water table 16 R%CONTGRANIStockton\CAP\CAP ADDENDUM I