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Interim Corrective Action Plan—Tosco(76)Service Station No. 11.195, Lathrop,California August 28,2002 3.0 CORRECTIVE ACTION PLAN COMPONENTS 3.1 Impact Assessment 1. The physical and chemical characteristics of the hazardous substance or its constituents, including their toxicity, persistence, and potential for migration in water, soil, and air: Based on the analytical results for both soil and groundwater samples collected to date, the primary contaminant appears to be gasoline (BTEX constituents and MtBE). Material Safety Data Sheets (MSDS) for currently distributed unleaded gasolines containing ethanol are attached to this CAP in Appendix A. The MSDS sheets describe the physical and chemical characteristics of gasoline and its components. The analytical results of the groundwater samples collected from the monitoring wells at and in the vicinity of the site indicate that dissolved concentrations of petroleum hydrocarbons are present in the wells. Free product has never been detected at the site. It is GR's understanding that as of January 1, 2001,Tosco no longer delivers fuel containing MtBE to service stations in northern California. Ethanol is the current oxygenate utilized in Tosco's gasoline, and has replaced MtBE. The shallow MtBE plume beneath the site is predominantly defined, except to the south/southwest where additional monitoring wells will be proposed in GR's forthcoming Work Plan. Previous investigations at the site indicate that the plume is largely confined to a narrow band along the transect of monitoring wells MW-10,MW-14 and MW-16, although detections of petroleum hydrocarbons in other offsite wells indicate that additional lateral delineation is necessary in the downgradient direction. An iso-concentration map for MtBE in shallow groundwater is presented on Figure 4. 2. The hydrogeologic characteristics of the site and the surrounding area where the unauthorized release has migrated or might migrate: The most recent monitoring and sampling event at the site was conducted on June 18, 2002. On this date, the measured depth to groundwater in the monitoring wells ranged from 7.69 to 1.1.94 feet btoc. The groundwater flow direction at the site was toward the south, with a gradient of 0.03. Previous subsurface exploration events indicate that the site is predominantly underlain by silty sand to approximately 8 to 16 feet bgs. Underlying the silty sand is poorly-graded to well-graded sand to a depth of approximately 25 feet bgs,the total depth explored. The site geology was presented previously in Section 2.3 of this report. 140193.13 5