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ERI 205505 R01 Tosco Service Station 5886,Stockton California December 28, 1999 <br /> percent of the total river discharge in California These rivers converge west of Stockton, California to <br /> create the San Joaquin-Sacramento Delta region, which drains through the Carqinez Strait into San <br /> Francisco Bay (Norris and Webb, 1990) <br /> 1_3 Site Geology-g- Hydroge-ology <br /> Sediments encountered beneath and in the vicinity of the site typically consist of heterogeneous <br /> mixtures of unconsolidated silt, clay, and foie- to medium-grained sand Geologic cross-sections A-A' <br /> (Plate 3) and B-B' (Plate 4) represent ERI's interpretation of the soil stratigraphy beneath and in the <br /> downgradient direction of groundwater flow from the site <br /> Historical groundwater monitoring and sampling data indicate that groundwater ranges in depth from <br /> approximately 5 1 to 113 feet below ground surface (bgs) The groundwater flow direction ranges <br /> ' from southwest to northwest with the predominant flow direction towards the southwest Calculated <br /> groundwater gradients range from 0 002 to 0 006 Historical groundwater flow direction and gradient <br /> data are provided on the Groundwater Flow Direction and Gradient Diagram (Plate 5) <br /> Based upon ERI's evaluation of borings B1 and CPT1 through CPT3, the continuous core samples <br /> collected from borings B8 through 1310, the boring logs and well construction details of wells MW 1 <br /> through MW 10S/I/ D (Appendix A), and a review of the quarterly groundwater monitoring data with <br /> ' respect to the groundwater elevations in the dual-casing wells MW4S & D through MW7S & D and <br /> nested wells MWBS/I/D through MWIOS/I/D (Appendix B), the groundwater beneath and <br /> downgradient of the site appears to consist of a single unconfined aquifer of variable permeability <br /> 3.0 FEASIBILITY STUDIES <br /> In May 1999, ERI performed feasibility testing at the site (ERI, October 27, 1999) The purpose of the <br /> feasibility test was to evaluate the effectiveness of AS/SVE as a remedial alternative for hydrocarbon- <br /> impacted groundwater beneath the site and to evaluate vadose-zone characteristics with respect to <br /> movement, extraction, and concentrations of vapor-phase hydrocarbons Data collected during the <br /> ' feasibility test are summarized in Table I <br /> ` 5 <br />