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September 25 2001 <br /> ERI 205505 R02 Tosco Service Station 5886,Stoekton Callforwa <br /> 2.3 Site Geolo and H dro eolo <br /> Sediments encountered beneath and in the vicinity of the site typically consist of heterogenous mixtures <br /> 1 of unconsolidated slit, clay, and fine- to medium-grained sand Geologic cross-sections A-A' (Plate 3) <br /> 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 Historical groundwater monitoring and <br /> sampling data indicate that groundwater underlies the site at depths ranging from approximately 5 1 <br /> to 11 3 feet below ground surface (bgs) The groundwater flow direction has ranged from southwest to <br /> northwest with the predominant flow direction towards the southwest The calculated hydraulic <br /> gradient has ranged from 0 002 to 0 006 Historical groundwater flow direction data are provided on <br /> the Groundwater Flow Direction Rose 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 /> ithrough MW10S/1/ D (Appendix C), and a review of the quarterly groundwater monitoring data with <br /> respect to the groundwater elevations in the dual-casing wells MW4SID through MW7SID and nested <br /> wells MW8SII/D through MW IOSAID (Appendix C), the groundwater beneath and downgradient of <br /> 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 18 and December 28, 1999) <br /> The purpose of the feasibility test was to evaluate the effectiveness of AS/SVE as a remedial alternative <br /> for hydrocarbon-impacted groundwater beneath the site and to evaluate vadose-zone characteristics <br /> with respect to movement, extraction, and concentrations of vapor-phase hydrocarbons <br /> The results of the feasibility testing indicated the following <br /> 1 . Approximately 0 007 pounds of total petroleum hydrocarbons as gasoline (TPHg) and 0 061 <br /> pounds of MTBE were removed by SVE <br /> The estimated effective vacuum radius of influence (ROI) for SVE is approximately 30 feet <br /> I <br /> 5 <br />