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SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 3 <br /> 16 - <br /> 7-Eleven Store No 14117, 2725 Country Club Blvd Stockton,CA June 20,2002 <br /> 20 GEOLOGY/HYDROGEOLOGY <br /> The site is located in the northern part of the San Joaquin Valley at an elevation of approximately one foot <br /> above sea level Shallow stratigraphy In the area consists of Quaternary alluvial/deltaic deposits The <br /> sand units are generally fresh-water-bearing east and southeast of Stockton, but brackish or saline water <br /> ' is present in these strata In the delta area west of Stockton (California Department of Water Resources, <br /> 1967, Bulletin No 146) The saline water/fresh-water boundary trends north to south across the western <br /> ' part of Stockton The brackish/saline water west of this boundary is often overlain by a layer of fresh <br /> groundwater to about 50 feet bg, therefore, shallower groundwater monitoring wells may encounter fresh <br /> groundwater on both sides of the boundary <br /> Soils encountered to 18 or 20 feet bg generally consist of interbedded clayey, silty, fine sand, silty clay <br /> and clayey silt (Figure 3) The clayey, silty, fine sand occurs at the base of this shallow section and <br /> ' appears to be lenticular in some wells <br /> In the previously-drilled deeper boring MW-2, silty sand was encountered from 18 to 28 feet and fine to <br /> medium sand from 38 feet to total depths of 51 feet below bg Cross-section lines are shown on Figure 2, <br /> and cross-sections are shown on Figure 3 n {, ; _�, ,,,,t <br /> As shown in Figure 3, it appears as if the primary migration pathway for groundwater and dissolved <br /> constituents is the shallower sand lens Aquitard thickness between the shallowest sand and the next- <br /> deepest sand bed may limit vertical migration of gasoline constituents assuming a vertical gradient is not <br /> imposed by artificial or natural means <br /> During drilling, groundwater was encountered at about 10 feet bg, equilibrating at similar depths <br /> Groundwater flow for this site shifts direction due to seasonal and/or tidal influences, and the gradient is <br /> ' generally gentle <br /> 30 MTBEITBA FATE AND TRANSPORT STUDY <br /> ' The potential distribution of MTBE (and by inference, TBA as a co-occurring oxygenate with similar <br /> adsorption/retardation characteristics) at a future date, given current site conditions unmodified by <br /> ' remediation activities, was run for the site using WinTran software (Appendix A) <br /> 14117Scm2 doc <br />