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McDowell&Davis Towing Risk Assessment and Conceptual Site Model 3.0 CONCEPTUAL SITE MODEL ti.. 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. Hydrogeologic cross sections are presented in Figures 3 and 4. The trace of the cross sections 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 1 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. Groundwater was measured at depths of 61 to 64 feet below ground surface. Groundwater flow fluctuated in direction from Southeast to Northwest. There does not appear to be a correlation between flow direction and seasonal climatic changes. Groundwater gradients calculated for the monitoring events ranged from flat to extremely flat (0.018 to 0.002 ft/ft). A summery of ground water flow information is presented below. Groundwater Flow Date Flow Direction Gradient(ft/ft) 11/29/2000 South 73 East 0.002 .� 03/05/2001 South 35 East 0.004 06/27/2001 North 35 West 0.014 11/20/2001 North 52 West 0.017 02/01/2002 South 52 West 0.018 05/06/2002 North 23 East 0.016 3.2 Contaminant Distribution Soils contamination associated 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 ,r 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 z.. an absence of benzene. 3