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Additional Site Assessment and Work Plan and Conceptual Site Model PREM DOR 433 Scotts Avenue, Stockton, California 1.0. INTRODUCTION At the request of Dr. Sonya F. Olbrantz of Masonite International/Prem Dor, Advanced GeoEnvironmental,Inc.(AGE)has prepared this work plan and conceptual site model for 433 West Scotts Avenue,Stockton,California(site).The scope of work includes the advancement of four soil borings and one-paired cone penetrometer test boring for the collection of soil and ground water samples. The location of the site is illustrated on Figure 1.A plan of the site is illustrated on Figure 2. Additional site background information is provided in Appendix A. The San Joaquin County Environmental Health Department(EHD)has directed by letter dated 21 December 2004 that a work plan be prepared to investigate the extent of contamination in the soil and ground water at the site (Appendix B). This work plan has been prepared in accordance with Central Valley Regional Water Quality Control.Board(RWQCB) guidelines for the investigation of underground storage tanks (UST) sites. 2.0. CONCEPTUAL SITE MODEL Based on the data collected to date from soil borings and monitoring well installations, AGE has developed a conceptual site model (CSM) (Figures 3 and 4) for the release; migration and distribution of the contaminants in the subsurface as described below: The soil profile beneath the site is generally composed of silt with sand and gravel (fill material) from surface grade to 6 feet below surface grade (bsg); clays, clayey silts and clayey sands from 6 feet bsg to 21 feet bsg; fine to coarse sand and silty sand from 21 feet to 29 feet bsg (35 feet at MW-2);clay with medium to high toughness and plasticity from 29 feet bsg to 36 feet bsg;silt from 36 feet bsg to 49 feet bsg; and fine sand from 49-feet bsg to 52 feet bsg or beyond. Since initiation of ground water monitoring in October 2003, depth to ground water measurements have fluctuated between approximately 18.5 feet and 21.feet below the top of the casing(btoc).The inferred ground water flow direction generally has been towards the northeast, four times, at gradients ranging between 0.005 ft/ft and 0.022 ft/ft; ground water flow was towards the east- northeast once at a gradient of 0.002 ft/ft. Releases of petroleum hydrocarbons occurred from the former on-site USTs. Once released to the subsurface,the contaminants migrated through the fine grained unit directly below the UST area to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered a more porous sand unit and the water table and began to migrate laterally as well as vertically;the direction of lateral migration should be towards the northeast, in the preferred ground water flow direction; however, maps of contaminant distribution suggest a more uniform spread away'from the former Advanced GeoEnviromnental,Inc.