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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 bus 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 remove soils contaminated with gasoline range hydrocarbons while in the process of installing air sparging/vapor extraction trench. The removal of the contaminated soils will expedite the remediation objectives for this site. TRENCH AND OVEREXCAVATION Figures 4A and 4B represent the plan and vertical views of the proposed air sparging/vapor extraction trench with the estimated area to be over-excavated. It is estimated that approximately 650 cubic yards of gasoline contaminated soil will be removed during the installation and the limited over-excavation of the air sparging/vapor extraction trench. The excavation will commence at the former UST area and remove contaminated soils to the 15 foot depth. Once this depth has been achieved relatively undisturbed soil samples from the base and sidewall of the excavation will be obtained for field testing using a portable gas chromatograph (Photvac 1OS50) for the presence of TPHg, Benzene and MTBE. Based on the field screening results a determination will be made to expand the over-excavation area. The excavation/trench will then proceed to the north, towards the former northern pump island area where borings THl, E and F sample results indicated high level soil contamination exists. The excavation in these areas will be dug to an estimated 20 foot depth, with potholes to approximately the 30 foot depth, dependent upon field screening results. Samples will be obtained from the sidewalls and base of the excavation to document the results as directed by San Joaquin County Health. INSTALLATION OF AIR SPARGING/VAPOR EXTRACTION PIPING After completion of the excavation, geofabric liner will be placed to line the excavation floor and sidewalls. This fabric will allow the migration of groundwater but will limit the amount of fine materials from advancing into the excavation. '/� inch clean rounded pea gravel will then be placed at the base of the excavation (approximately one foot thick) to hold the liner in place. Once the pea gravel base has been placed horizontal air sparging piping will be placed with vertical risers to connect the horizontal pipe will be positioned. '/I inch pea gravel will then be place to the six foot depth below surface (approximately one foot above the highest groundwater level). At the six foot depth a horizontal vapor extraction pipe will be placed with a vertical riser to surface. Y2 inch pea gravel will be placed on and above the vapor extraction pipe to the five foot depth, then the entire excavation will be covered with geofabric and compacted backfilled with clean road base to subgrade surface prior to be covered with asphalt to provide a vapor barrier. 2 Excavation WP 11-2000