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1 ' Fuller Mobile Home Park 36S 1« Louise Ave., Lathrop, California December 20, 1993 Phase HSite Characterization Page: S 4.0 Soil Conditions rThe subsurface lithology of the Fuller Mobile Home Paris site in Lathrop, California, is characterized by silty, clayey sands with thin interbedded nits and clays This is typical of the valley fill deposits in the region Analyses of soil samples show BTEX, Gasoline & 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 cuttmig (chainsaw) activities and shop/garage activities immediately adjacent to the well No BTEX or Diesel contamination was detected (Table 2) The actual sampling interval of the split spoon sampler is 18 inches with three 6-inch brass tubes The laboratory sampie is taken from the bottom tube to minimize any contammation from previous intervals whose soil may have sloughed to the bottom of the hole Therefore. soil contamination may actually be limited to an area around the former tank Lite, and the bulk of the fuel that leaked from the tanks is in the groundwater- S. roundwaterS.0 Groundwater Conditions The water table was approximately 10 feet below ground level as the wells were being drilled (Nov 17 & 18th, 1993) Subsequent water levels after development and before sampling recorded water levels in the vicinity of 11 feet below ground level MW-3 has a protective steel casing installed approximately 2 5' above ground level and gives misleading "depth to water" readings The maximum groundwater elevation variation across the site is 0 05 feet (Table 1) After the wells were surveyed (Appendix E), a groundwater gradient of 0 0012 ft/ft was calculated at N 52 degrees E(Figure 4) 6.0 Conclusions and Recommendations With the exception of an isolated interval of gasoline at 5 feet in MW-2, a contamination plume exists from near 10 feet below ground level down to the water table in the area of the former underground tanks The lateral extent is not defined, because all borings and wells showed contamination To delineate the contamination plume, additional bonngs/momtonng ' wells will be required at locations proposed on Figure 2 The groundwater gradient direction (at the time of drilling, November 17/18, 19931 is offsite to the NE, into a residential area But with the shallow water table, the exceptionally flat groundwater gradient, and the sand lithology, the contaminants have probably been moving laterally with the variable seasonal water table Additionally, the vertical limits of contamination need to be defined E