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I Fuller Mobile Nome Park ' 36S E. Louise Ave., Lathrop, California May 28, 1998 • Phase II Site Characterization - Continued Page: 6 ' and the 310,000 ug/kg found at B-11 at 12 5' in depth in the 1998 sample These comparisons are used based on the proximity to the former UST location ' 6.0 Groundwater Conditions The water table was approximately 10 feet below ground level as the borings were being ' drilled During the quarterly event on April 8, 1998, subsequent water levels before sampling recorded water levels in the vicinity of 8 5 feet below ground level (Table 3) ' A groundwater gradient 0 0160 ft/ft was calculated (Figure 4) The direction is S63°W across the site I 7.0 Conclusions and Recommendations IThe soil contamination "plume" has been defined at this site A groundwater contamination plume exists at varing depths from approximately 10' bgl to approximately lie 32 5' bgl at the eastern boundary of the former underground tanks The lateral and vertical extent is completely defined Only one (1) monitoring well showed some traces of contamination The groundwater gradient direction is (at the time of sampling - April 8, 1998) to the SW With the shallow water table and the exceptionally flat groundwater gradient, contaminants have been moving laterally with the variable gradients caused by pumping of wells, irrigation of nearby fields and precipitation The Delhi sand that covers I the area is a very permeable sand that overlies a sandy silt (in some areas) that is in contact with the water table This is evidenced by contamination over 50 feet down gradient and over 60 feet laterally from the past gradients, and both soil and water samples have higher Icontamination levels to the south and west of the former tank site The following facts support the vertical and lateral definition of both soil and ' groundwater ' The most recent groundwater data collected indicates only Diesel contamination present at monitoring well #7 This 60 ug/L of Diesel is within the State of Californias allowable level of 100 ug/L All other wells tested non-detected for all analytes The water sample ' collected from boring B-12 at 12 5' bgl indicates 3000 ug/L of Gasoline, 4 4 ug/L of Ethyl-Benzene and 33 ug/L of total Xylenes These levels of Ethyl-Benzene and total Xylenes are well below the established allowable levels (Xylenes - 1,750 ug/L, E-Benzene ' 700 ug/L) The Gasoline level does exceed the allowable level of 5 ug/L The defined mass of the secondary source of petroleum hydrocarbon is calculated to be approximately 1,150cubic yards in quantity underlying the former UST location The