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ERI 227903 R03 Former Unocal Service Station 5098,Stockton California September 30, 1999 • Collecting soil samples from the borings and submitting the soil samples to Sequoia Analytical Laboratory, Inc (Sequoia) for analysis of gasoline hydrocarbons, oxygenated compounds, and lead scavengers The laboratory results for these soil samples are provided in Attachment F • Collecting three reconnaissance groundwater samples from boring HP1 and submitting the samples �i to Sequoia for analysis of gasoline hydrocarbons The laboratory results for the reconnaissance groundwater samples are provided in Attachment G • Coordinating groundwater monitoring and sampling of the existing wells and newly installed wells with Unocal and Gettler-Ryan, Inc (GRn of Dublm, California • Preparing and submitting a report documenting the procedures and presenting the results of the investigation (ERI, July 30, 1999) RESULTS OF INVESTIGATION/DISCUSSION Site Geology and Hydrogeolagy Sediments encountered in soil borings 131, B2, MW10A, MW14A, MW18, MW19S, MW19D, MW21S, MW21D, MW22S, MW22D, and HP1 typically consisted of layers of silt, clay and heterogeneous mixtures of silt and clay, fine-grained to medium-gramed sand, and silty or clayey sand Groundwater was encountered between approximately 33 and 40 feet bgs in the borings During second quarter 1999, groundwater flow direction from and in the vicinity of the site was south and west toward the Delta irrigation well (Plate 2) at a calculated gradient of 0 01 (GRI, July 6, 1999, Attachment B) Groundwater elevation data collected from the nested wells (MW19S & D, MW21S & D, and MW22S & D) indicates that the groundwater elevation contrast between shallow and deep well casings at each location range from approximately 0 19 to 0 5 feet Based upon this data, the upper and lower portion of the water-bearing zone are in hydraulic communication During September 1998, ERI performed a hydrogeologic investigation at the site (ERI, January 5, 1999) The hydrogeologic investigation included continuous monitoring of water levels over a 4-day period to evaluate groundwater flow direction and gradient during pumping and non-pumping periods of the Delta irrigation well operation Based upon the data collected during the hydrogeologic investigation and quarterly groundwater monitoring and sampling, the interpreted groundwater flow direction was generally towards the Delta well during both pumping and non-pumping intervals during the irrigation period This data indicates that during the seasonal pumping from the Delta well (May through October), the groundwater flow direction and gradient is controlled by groundwater extraction from the Delta well The gradient increases from approximately 0 0057 to 0 0088 (non-pumping) to approximately 0 0100 to 0 0160 while the well is pumping During the seasonal non-pumping period (reportedly November through April), the groundwater flow direction is towards the east to southeast at a gradient ranging from approximately 0 002 to 0 005 The cumulative groundwater flow direction and hydraulic gradient are shown on the Groundwater Flow Direction and Gradient Diagram (Plate 3) ERI's review of the data collected during this investigation and previous investigations, including groundwater elevation data in shallow (S) and deep (D) nested wells MW19S & D, MW21S & D, and 3