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ERI 227903 R03 Former Unocal Service Station 5098,Stockton,Califorma September 30, 1999 MW22S & D (Attachment E), ERI's hydrogeologic investigation (ERI, January 5, 1999), and historical data Indicates the following • The strata beneath the site are in hydraulic equilibrium and comprise a single Interconnected water- bearing zone with variable permeability • The groundwater surface III the vicinity of the site has fluctuated from approximately 30 to 59 feet bgs in the wells (GRI, July 6, 1999), with a general rising trend over the past several years • The groundwater flow direction and gradient is controlled by groundwater extraction from the Delta well during May through October when the well is in episodic operation During the seasonal non-pumping period (reportedly November through April), the groundwater flow direction is towards the east to southeast • The upper aquifer beneath and in the vicinity of the site (presently defined by the vertical limits of borings and wells installed to date) appears to have a weak hydraulic connection with the lower aquifer Soil Conditions Results of laboratory analyses of soil samples are provided in Attachment F A total of 27 soil samples, collected from depths ranging from approximately 30 to 80 feet bgs were selected for laboratory analyses The laboratory analysis of the soil samples collected from borings B1, 132, MW10A, MW14A, MW18, MW19D, MW21D, and MW22D did not detect concentrations of oxygenated compounds or lead scavengers at or above laboratory method detection limits The laboratory analysis of the soil samples collected from borings MW 10A,MW 14A,MW 18, MW21D, and MW22D did not detect concentrations of total purgeable petroleum hydrocarbons as gasoline (TPPHg)or benzene at or above laboratory method detection limits TPPHg was detected in the soil samples collected from boring B 1 at 50 feet bgs, from boring B2 at 65 and 75 feet bgs, and from boring 19D at 50 to 55 feet bgs ranging from 16 parts per million (ppm) to 144 ppm The laboratory analysis results of soil samples collected during the supplemental environmental investigation(Attachment F) and previous drilling and excavation activities(Attachment C) indicate the following • The bulk of the hydrocarbon impacted soil at the site (approximately 6,000 cubic yards)was removed to a depth of approximately 53 feet bgs from the vicinity of the former pump islands (KEI, December 15, 1988) The approximate limits of the excavation area are shown on Plate 2 • Residual hydrocarbon concentrations have not been detected within the vadose zone beneath the site The vadose zone ranges from approximately 0 to 38 feet bgs 4