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. Soil and Groundwater Investigation Report ARCO Facility No 2168 Stockton, Califonua Page 8 coarser (medium-grained sand and gravelly sand) with depth in the southwest portion of the site (MW-8C) though remains fine-grained in the eastern portion of the site (MW-1C), with the exception of a 10-foot thick sand and gravelly sand layer located at approximately 75 feet bgs, • Sand and gravelly sand present between 75 and 85 feet bgs appears to be laterally continuous beneath the site as does the 12- to 18-foot thick clayey silt to silt layer that extends to approximately 110 feet bgs, • First encountered groundwater beneath the site at the time of sampling was approximately 19 to 22 feet bgs • Permeability rates on the two soil samples (classified as silt) collected from MW- 1 C at depths of 71 and 96 feet bgs are low (3 1 x 10-6 cm/sec and 9 3 x 10-7 cm/sec), • TPHG and BTEX compounds are primarily located 1n soil between 45 and 75 feet bgs, upgradient of the former UST's, and between 25 and 55 feet bgs, downgradtent of the former USTs, therefore the vertical extent of residual impact appears to be characterized, • MTBE was detected at relatively low concentrations in only two soil samples collected at 35 and 45 feet bgs to boring/well MW-8C, • Concentrations of TPHG and BTEX compounds significantly decrease in groundwater samples collected from depth-discreet monitoring wells MW-1 (screened from 35 to 55 feet bgs) through MW-1C (screened from 105 to 120 feet bgs), [e g , 18,300 ug/l TPHG to MW-1, whereas MW-1C contains 53 7 ug/l, 8,600 ug/l benzene to MW-1, whereas MW-1C contains 6 3 ug/1], • Relatively high concentrations of TPHG and BTEX compounds were detected in upgradient well MW-8B, which is screened across what appears to be a laterally continuous gravelly sand layer, confirming lateral migration from an upgradient source, • Concentrations of TPHG and BTEX compounds significantly decrease to groundwater samples collected from depth-discreet monitoring wells MW-8B (screened from 75 to 90 feet bgs) to MW-8C (screened from 105 to 120 feet bgs), [e g , 1,130 ug/l TPHG in MW-8B, whereas MW-8C contains 68 5 ug/l, 425 ug/1 benzene in MW-8B, whereas MW-8C contains 15 6 ug/1), • Dissolved concentrations of TPHG, BTEX compounds, and MTBE attenuate with depth and the vertical extent of impact appears to be adequately characterized, • MTBE was not detected in groundwater sampled collected from groundwater monitoring wells MW-1B, MW-1C, MW-8B, and MW-8C