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McDowell&Davis Towing Geoprobe Soil Boring&Revised Risk Assessment and Conceptual Site Model Report Page: 4 ar All soil samples were delivered, under chain of custody, to an analytical laboratory licensed by the State of California for hazardous materials work. The samples were analyzed for Total Petroleum Hydrocarbons (Gasoline), BTEX & MTBE by Method 8020 and 8260. Analyses were conducted by GeoAnalytical Laboratories, Inc., in Modesto, California. �. All rinse water was removed by the driller, V&W Drilling, Inc. Drill cuttings are not produced by this method. 3.0 CONCEPTUAL SITE MODEL r. 3.1 Hydrogeology The McDowell and Davis site is characterized fine to medium sands with sporadic discontinuous silty and clay rich units. Fine-grained units are present at depths of 7 to 17 feet and at depths of 42 to 55 feet in the western portion of the site. A continuous unit that could provide a barrier to the vertical movement of the contaminants was not recognized in the subsurface investigation. A revised hydrogeologic cross section of the dispenser area (Section I — P) is presented in Figure 3. The trace of the cross section is shown on the site map Figure 2. The surface soils are mapped by the Soil Conservation Service as "Delhi Loamy Sand", a wind modified alluvium derived from the underlying Modesto Formation. The sands and silts / clays are part of the Pleistocene Modesto Formation, a distal facies of the Ancestral Stanislaus River alluvial fan/delta system. Silt and clay rich units are common in the Modesto Formation resulting from fluctuations in the level of ice age lakes, which occupied the central portions of the Central Valley during the Pliestocene. Global warming episodes, not related to the activities of man, caused .. glaciers in the Sierra Nevada Range to alternately advance and retreat in the Ancestral Stanislaus River Valley resulting in fluctuating lake levels in the Central Valley. 3.2 Contaminant Distribution •• Soils contamination associated with the former underground storage tanks was recognized at two locations during the removal and closure of the underground storage tanks, at the western end of the center tank, TK2, and at the western dispenser island (grab sample No. 7). Drilling in the vicinity of the underground tank failed to recognized impacted soils limiting the lateral extent of the release. The release at the underground storage tank is characterized by very low concentrations of MTBE and -- an absence of benzene. The dispenser island release was confirmed by soil samples collected from depths of 15 and 25 feet in boring B2. The lateral extent of the release is limited as shown by the absence of contamination in boring B-5. The release is characterized by extremely