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Chevron U S A Products Company April 8, 1992 02320 2241 Page 4 of Stockton The saline-water/fresh-water boundary within these sedimentary units trends north to south across the western part of Stockton The saline groundwater west of this boundary is usually overlain by a layer of fresh groundwater to a depth of approximately 50 feet BGS, therefore, shallow monitoring wells usually encounter fresh groundwater on both sides of the boundary The materials penetrated to a depth of 30 feet during drilling at the site were predominantly silty clays, clayey silts and silty fine sands with lenses of coarser sands (Appendix A) Groundwater was encountered during the recent round of drilling at 18 to 20 feet BGS 42 RESULTS OF SOIL ANALYSES Four soil samples collected from the three soil borings were submitted for laboratory analyses Soil samples submitted included the 164eet BGS samples from MW-6 and MW-7, and the 16-and 19-feet BGS samples from VW-1 Petroleum hydrocarbons were detected only in the two samples collected from VW-1 with concentrations reaching 2,700 parts per million (ppm) TPH-G and 10 ppm benzene in the 16-feet BGS sample Analytical results of soil samples are summarized in Table 2 Laboratory analyses reports and chain-of-custody manifests for the soil samples are included in Appendix C 43 RESULTS OF GROUNDWATER MONITORING AND ANALYSES Historical and recent monitoring data for the site are summarized in Table 1 Figures 2 and 3 are potentiometric surface maps constructed from data collected on March 11 and 17, 1992, respectively Figure 2 was constructed using the combined data from the cooperative monitoring event between ARCO and Chevron The potentiometric surface maps indicate that groundwater beneath the site flows to the east and northeast at a hydraulic gradient of 0 001 to 0 01 Results of the groundwater analyses indicate that dissolved petroleum hydrocarbons are present in six of the seven monitoring wells with TPH-G concentrations ranging from 150 to 13,000 parts-per-billion (ppb) and benzene from 0 7 to 3900 ppb (Figure 4 and 5) Off-site well MW-6 contained 3,900 ppb TPH-G and 63 ppb benzene Analytical results of groundwater samples are summarized in Table 1 Laboratory analyses reports and chain-of-custody manifests are included in Appendix C 2241SUM1 RPT �� GROUNDWATERELI-] TECHNOI OGY INC