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GEOLOGY AND HYDROGEOLOGY OF THE SITE The site is located on surface soils belonging to the Stockton Adobe Clay and Honeycut Clay series They are derived from recent alluvial fan and floodplain deposits The 40 feet of explored section consisted of clay and silt overlying interbedded lenses of silty and clayey sands below 20 feet The groundwater in the vicinity of the site is assumed to be semiconfined Groundwater elevation at the site varies from a wet year high of 7 feet bgs to about 35-40 feet bgs during the previous drought period Groundwater flow direction at the site has generally been to the west since December 1997, see Figure 3 (June 8, 2000) OVERVIEW OF WORKPLAN This workplan is designed to provide information to delineate the downgradient lateral extent of the groundwater plume and for the eventual cost/feasibility analysis of at least two remedial options for the site including the overexcavation of the contaminated soils and air sparging / vapor extraction ® The pump test portion of this workplan will be conducted after the installation of additional monitoring and recovery wells This will help provide an estimate of the volume of water that will be generated during the excavation of a trench and installation " of air sparging / vapor extraction piping through the contaminated soil plume The water volume and flow rates encountered will in turn factor into the cost and feasibility of overexcavating the entire soil contamination plume The groundwater elevation at the site has increased steadily since the removal of the UST's The bulk of the contaminated soil plume now lies below the top of groundwater throughout the yearly hydrologic cycle This situation does not allow for efficient removal of hydrocarbons by vapor extraction alone This workplan proposes to test the feasibility of using vapor extraction coupled with air sparging through a portion of the submerged soil contamination plume Air sparging tubing will be installed at the base of a narrow trench that will be excavated through the center of the soil contamination plume The trench will be located close enough to two existing monitor wells and the proposed recovery well (RI) to allow monitoring of the effects of the system beyond the immediate environment of the trench The trench will be backfilled with pea gravel Horizontal vapor extraction piping will be installed within the trench above the top of groundwater Surface air will be sparged through the submerged tubing at a rate capable of enhancing biodegradation of the soil and groundwater contaminants for one to two months Groundwater samples will be collected periodically from R1, MW1 and MW3 and another temporary well located 2