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ENt accumulation of rainfall and snowfall in the Sierra Nevada (United States Department of Agriculture, Soil Survey of San Joaquin County, 1992) The nearest body of water is the Mokelumne River located approximately 2,200 feet east of the subject property (U S G S Thornton Quadrangle, 1978) A water well located along the eastern boundary of the subject property is used for irrigation purposes only The residence receives its domestic water from the San Joaquin Municipal Water District 40 SITE CHARACTERIZATION In November 1994, three soil borings (SBA, SB3, and S134) were advanced and sampled at the subject site by RAH Environmental, Inc (Figure 2) Soil samples collected from the borings were analyzed for the presence of TPH-g and BTEX Laboratory analysis of the soil sample collected from soil boring SBA at a depth of 20 feet bgs detected concentrations of TPH-g at 1,000 mg/Kg and benzene at 6 8 mg/Kg Groundwater samples were collected through hollow-stem augers at the location of the sod borings and analyzed for the presence of TPH-g and BTEX The analytical results indicated the ' greatest concentrations were detected at the location of SBA On the basis of these results, RAH Environmental, Inc concluded that the source of the petroleum hydrocarbons identified at the location of drill hole SBA most probably resulted from the former UST on the Lopez property Historically, groundwater has fluctuated from approximately 4 5 feet to/12 5 feet Ings (Figure 3) � The inferred groundwater flow direction was to the southeast, at an-average hydrogeologic gradient of 0 006 feet per foot (Figure 4) ' 1• � In February 1996, Fugro West, Inc (Fugro) witnessed the advancement of five drill holes (MW-1, MW-2, MW-3, SB-A1, and SB-A2) at the locations illustrated on Figure 2 The drill boles were F ' advanced to approximate depths ranging from 20 5 to 35 5 feet Drill holes MWA, MW-2, and MW-3 were completed as 2-inch diameter groundwater monitoring wells TPH-g was detected at 5,500 mg/Kg and benzene at 22 mg/Kg in drill hole SB-A2 at a depth of 16 feet bgs These soil ' samples were collected from the saturated zone From February 13, 1996 to November 26, 1996, Fugro conducted groundwater monitoring ' activities ENSR conducted groundwater monitoring activities during the first and second quarters of 1997 Low levels of TPH-g (between 51 micrograms per liter [µg/L] and 110 µglL) have been detected intermittently in groundwater monitoring wells MW-1, MW-2 and MW-3 Concentrations ' of BTEX have not been detected at or above the reporting limit Methyl tertiary-butyl ether (MTBE) was not detected at or above the reporting limit The presence of petroleum hydrocarbon impacted soils in the identified saturated soil zone, ' most likely resulted from lowering of the water table by groundwater pumping activities conducted at the Tri-Valley Growers (TVG) facility, beginning in 1991 The groundwater pumping activities most likelya rested a smear zone as the water table was drawn down -- 1 ' L IENSR18716100218716002 c1s2 appb doc 2 11/13/98