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Work Plan for Additional Site Assessment and Interim Remediation/Feasibility Testing ARCO Facility No.5469 Stockton,California Page 3 Depth to groundwater at the site has ranged between 36 and 40 feet bas. Wells MW-1 through MW-8 are screened across the upper sandy unit, while wells MW-9 and MW-10 are screened across the lower sandy unit. Analytical results of the soil samples collected during the exploratory investigations conducted to date did not contain TPHG and benzene in the top 40 feet (vadose zone). Low concentrations (<10 mg/kg) of petroleum hydrocarbons and MTBE were detected in soil samples collected below the product piping and dispensers. MTBE has been consistently detected in the groundwater samples collected during the quarterly monitoring events from monitoring wells MW-4, MW-5, MW-6, MW-7, MW-8 and MW-10. However, TPHG and benzene have not been detected in wells MW-5 and MW-6. Petroleum hydrocarbons and oxygenates have not been detected in wells MW-1, MW-2, and MW-3. MTBE concentrations in groundwater during themost recent sampling event ranged from 16,000 micrograms per liter (µg/L) to 24,000 µg/L (see Appendix A for historical and 4th Quarter 2001 analytical results). The lateral extent of the dissolved hydrocarbons appears to be delineated to the northwest (well MW-1), southeast (well MW-2), and southwest (well MW-3). Presence of dissolved petroleum hydrocarbons in well MW-10 indicates that the vertical extent of impact appears to extend deeper than 75 feet bgs. Review of documents prepared by Ground Zero Analysis, Inc. (GZA), and discussions with PHS/EHD indicate that there are two other petroleum hydrocarbon plumes originating from 102 Wilson Way (Knowles property) and 101 Wilson Way (Top Filling property) in the vicinity of the ARCO facility. The Knowles property is located immediately north 'of the ARCO facility, while the Top Filling property is located northwest of the ARCO facility. Petroleum hydrocarbons and oxygenates have been detected at both the Knowles and Top Filling properties. Based on information gathered by GZA and Advanced GeoEnvironmental (AGE), high concentrations of MTBE appear to be migrating from the Top Filling site. Additional Assessment of H drocarbon Im act The groundwater flow direction and extent of impact indicate that additional assessment is warranted towards the east and northeast of the ARCO facility to .complete the delineation of the lateral extent of impact to groundwater. It is our understanding that GZA is going to be characterizing the impact to groundwater along Market Street and Eugenia Street. PAARM54691Work Plan15469,ATkpin 03-2002.doc