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MLei]1 ]nstallation Report ARCO Facility No. 5469 Stockton, California • Page 4 (13.3 mg/kg) was detected in sample B-1-415 collected from boring BI/MW-1 at 41 feet bgs (Table 1). Soil samples collected for laboratory analysis from well Iocations MW-5 and MW-6 did not contain TPHG, BTEX, or methanol above the Iaboratory detection limits. However, the soil samples contained MTBE concentrations ranging from below the laboratory detection limit (<O.1 mg/kg) to 1.26 mg/kg (sample B-5-41.5S). The only samples containing detectable TPHG and BTEX compounds were collected from boring B4/1\4W-4. The soil analytical results are presented in Table I and summarized on the geologic cross sections (Figure 3). The average depth to groundwater measured on June 23, 2000 was approximately 36.6 feet bgs. The average groundwater gradient across the site is relatively flat (0.002 ftlft). The groundwater flow direction appears to be generally toward the southeast. The groundwater gradient and flow direction are illustrated on Figure 4. Table 2 presents the analytical results of the groundwater samples collected from the monitoring wells on June 23, 2000. TPHG and BTEX compounds were not detected in the groundwater samples collected from wells MW-1, MW-2, MW-3, MW-3 and MW-6. TPHG and benzene were detected at concentrations of 1,020 I'M and 1,470 pg/l, respectively in the sample collected from MW-4. MTBE was detected in samples collected from monitoring wells MW-3 (19.2 µ41), MW-4 (2,290 gg/1), MW-5 (8,370 µg/), and MW-6 (4,120 ga distribution of dissolved-phase TPHG, benzene, and MTBE in wells at the stet'ure 4 presents the TPHG and benzene were detected in soil and groundwater samples collected from well MW-4 located on the northern portion of the Site. These constituents were not detected in samples collected from monitoring wells MW_I, MW-5, or MW-6, which are located between well MW- 4 and the USTs. This suggests that those constituents detected in well MW-4 may be from an Offsite source. Concentrations of MTBE appear to be localized onsite in the vicinity of the USTs and northeast toward monitoring well MW-4. Limitations This report was prepared in general accordance with accepted standards of care which existed at the time this work was performed. No other warranty, expressed or implied, is made. Conclusions and recommendations are based on field observations and data obtained form this work and previous investigations. It should be recognized that definition and evaluation of geologic conditions is a diff cult and inexact an, Judgments leading to conclusions and recommendations are generally made with incomplete knowledge of the subsurface conditions • 3 1