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REPORT ON ADDITIONAL SITE ASSESSMENT AND RECOMMENDATIONS 5 7-Eleven Store No 14117 2725 Country Club Blvd Stockton CA December 12,2001 40 RESULTS ' 41 Geology and Hydrogeology The site is located in the northern part of the San Joaquin Valley at an elevation of approximately one foot above sea level Shallow stratigraphy In the area consists of Quaternary alluvial/deltaic ' deposits The sand units are generally fresh-water-bearing east and southeast of Stockton, but brackish or saline water is present In these strata In the delta area west of Stockton (California ' Department of Water Resources, 1967, Bulletin No 146) The saline water/fresh-water boundary trends north to south across the western part of Stockton The brackish/saline water west of this boundary is often overlain by a layer of fresh groundwater to about 50 feet below grade, therefore, ' shallower groundwater monitoring wells may encounter fresh groundwater on both sides of the boundary ' Sod types encountered during drilling generally consist of Interbedded clayey fine sand, silty clay and clayey silt, with a laterally-continuous silty/very fine sand layer at about 18 feet to total depth (about 20 feet below grade) In the previously drilled deeper boring MW-2 silty sand was encountered from 18 to 28 feet and fine to medium sand from 38 feet to total depths of 51 feet below grade During drilling, groundwater was encountered at about 10 feet below grade, equilibrating at similar depths (Table 1) The groundwater flow on November 5, 2001, was southerly at a hydraulic gradient of about 0 006 foot per foot (Table 2, Figure 2) Historically(based on data collected over the last two years), the ' gradient fluctuates between south to northwest The shallow groundwater and proximity of some of the monitoring wells to the former and current underground storage tank pits are probably ' responsible for local perturbations and fluctuations in groundwater flow direction which may not reflect regional flow direction rA formal 2,000-foot sensitive receptor investigation (to identify all municipal and/or domestic wells in the area) has not been conducted for this site ' 42 Results of Soil Sample Analyses Recent sod analytical data (Table 2) indicate the lateral extent of TPH-G, BTEX, MTBE, and other oxygenates has been delineated by non-detectable levels (except for 0 006 mg/kg total xylenes In well MW-7)to the west, northwest, and south of the known plume area Historical data are included in Table 2A Copies of the analytical results and chain-of-custody documents are included In Appendix D