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2905 West Benjamin Holt Drive, Stockton, California March 16, 1994 Chevron U.S.A. Products Company Page 4 Data collected during the above stages of soil assessment indicate the following: • Unauthorized releases from the three gasoline tanks in the former UST pit located on the southern portion of the service station property were the main source of hydrocarbons found in soil samples collected on the Chevron property. Potential contributions from operation of the new USTs and product lines have not been investigated. ■ Hydrocarbon-impacted soils are concentrated in the southern half of the service station property in and around the former UST pit. Hydrocarbon-impacted soils have been detected as far as 90-100 feet east (MW-2), 40-50 feet north (MW-5), and 20-30 feet south (MW-1) of the former UST pit. Lateral boundaries of hydrocarbon-impacted soils are complicated to the south by the presence of impacted material related to ARCO unauthorized releases. ■ Hydrocarbon-impacted soils are concentrated vertically at 15-16 feet BGS throughout the impacted area. The highest TPH-G concentrations in hydrocarbon-impacted soils occur from 14-16 feet BGS in and around the former UST pit. TPH-G concentrations are low at 5 ft. BGS except for an isolated area near the northeast corner of the former UST pit. TPH-G concentrations are an order of magnitude lower at 19-20 feet BGS than those measured at 15-16 feet BGS within the same sampling area. Lateral and vertical distribution of soil-vapor hydrocarbon concentrations are similar to the distribution of hydrocarbon-impacted soil determined from laboratory analytical results of soil samples collected during drilling and excavation. High concentrations of volatile hydrocarbons in the southernmost vapor sample indicate that the southern boundary of the impacted soil may be influenced by unauthorized releases at the neighboring ARCO station. Based on the above observations, hydrocarbon-impacted soils related to the Chevron former UST pit are concentrated within the southern half of the site in a zone between 10 and 20 ft. SGS and highly concentrated at 15 ft. BGS (Figure 3). 2.3 Results of Groundwater Assessment Groundwater beneath the Chevron service station flows to the east-northeast at a hydraulic gradient of approximately 0.005 as indicated by potentiometric surface maps constructed from data collected during high and low groundwater elevation periods (Figures 4 and 5). High stands of the water table occur between March and July and low stands occur between September and December (Figures 6, 7, and 8). The ARCO groundwater recovery system appears to have only local capture. It is unknown what effect, if any, the municipal well located east of the Chevron service station has on shallow groundwater flow. Historical analytical data for groundwater are summarized in Table 2. Groundwater samples collected during the initial investigation in 1989 were above State maximum contaminant levels (MCL) for benzene �P 5073PAR.APT GROUNDWATER TECHNOLOGY