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Site Closure Summary Fuller Mobile Home Park August 8, 1997 365 E. Louise Avenue, Lathrop, California Page: 4 ' The continental deposits of quaternary age crop out primarily along the major rivers and streams of valley and also other low lying areas These include river flood basin deposits ' and sand dunes The river deposits, which include channel and flood plain deposits are considered to be the most permeable deposits in the valley, but they are not tapped by wells Flood basin deposits are largely clay and silt deposited in the flood basins during ' floods on the major streams They grade laterally into the river deposits and into the younger alluvial fan deposits The basin deposits are generally of low permeability, with only a few irrigation wells producing appreciable quantities of water (Olmstead and Davis, 1961) ' 3.0 Soil Conditions ' The subsurface lithology of the Fuller Mobile Home Park site in Lathrop, California, is characterized by silty, clayey sands with thin interbedded silts and clays This is typical of the valley fill deposits in the region (Appendix D) Analyses of soil samples show BTEX, ' Gasoline and Diesel contamination in the samples below 10 feet, except for one isolated gasoline analysis at five feet in MW-2 This sample was probably from wood cutting (chainsaw) activities and shop/garage activities immediately adjacent to the well No BTEX or Diesel contamination was detected (Table 2) Lithologic cross-sections were constructed from data from all borings (Figures 5 & 6) tContamination cross-sections were constructed from analytical data from all borings (Figures 7 & 8) Soil samples from B-5 & 6 and MW-4, 5, 6 & 7 were all ND (Table 1) The lateral extent of soil contamination has been defined (Figure 9) 4.0 Groundwater Conditions The water table was approximately 12 feet below ground level as the wells were being drilled Subsequent water levels before sampling recorded water levels in the vicinity of 11 feet below ground level (Table 3) The maximum groundwater elevation variation 1 across the site was 0 11 feet The extremely shallow water table and flat gradient is very sensitive to rain and errors in measuring depth to water as small as 0 01 feet ' After the wells were surveyed, a groundwater gradient ranging between 0 0004 and 0 0011 ft/ft was calculated (Figure 3) The direction is variable across the site It appears to be influenced by the new stormwater retention ponds whose bottom elevations are approximately five feet above the water table