SITE INFORMATION AND CORRESPONDENCE FILE 1

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Chevron Chevron U.S.A. Products Company 2410 Camino Ramon,San Ramon,California•Phone (510)842-9500 Mail Address: P.O.Box 5004,San Ramon,CA 94583-0804 November 23, 1992 EIVED Mr. Michael Infuma 0 E C 0 2 1992 San Joaquin County Public Health Services ENVIi7Qi�iV9 1 A HEALTH P.O. Box 2009 l 1 1601 East Hazelton Avenue PERMIT/SERVICES Stockton, CA 95201 Re: Former Chevron Service Station #94054 2103 Country Club Blvd., Stockton Dear Mr. Infurna: Enclosed we are forwarding the Environmental Assessment Report dated November 13, 1992, prepared by our consultant Groundwater Technology, Inc. (GTI) and Environmental Risk Information and Imaging Services (ERIIS) Report dated October 19, 1992 for the above referenced site. As indicated in the GTI report, twelve (12) borings were advanced with four (4) being completed into ground water monitor wells designated MW-4 through MW-7. These wells were installed to delineate the extent of the hydrocarbon plume. The additional borings were installed to further assess the extent of the hydrocarbon contamination in the on-site soils. Soil samples collected from the drill cuttings were analyzed for total petroleum hydrocarbons as gasoline(TPH- G) and BTEX. A sample collected from MW-4 located in the vicinity of the former waste oil tank was also analyzed for total oil & grease (TOG). TPH-G concentrations ranged from non- detectable (ND) to 5,100 ppm; Benzene concentrations ranged from ND to 48 ppm. The highest concentrations were from samples collected at the 15 and 20-foot intervals above the ground water interface. TOG was reported as ND. The materials encountered during drilling consisted of clays, silty sands and sands. Ground water samples were collected and analyzed for TPH-G and BTEX. Analytical results reported benzene concentrations ranging from ND to 2,100 ppb. Groundwater samples were also collected from existing wells MW-1 through MW-3 at this time. Benzene concentrations ranged from 2,000 to 6,100 ppb. Depth to groundwater was measured at approximately 22-feet below grade, and the direction of flow is to the north-northwest with a gradient of approximately 0.006 ft/ft. The results of the ground water samples suggest that there may be an off-site source east of the site as wells MW-6 and MW-7, based on the current ground water flow direction, are cross-gradient of the referenced site. The ERIIS report presents the findings of a one-mile radius database record review. ERIIS reported that twenty (20) sites had existing underground storage tanks(UST's). Of the 20 sites, eight (8) have reported leaking underground storage tanks (LUST). As indicated in the report, two (2) of the eight (8) sites on the LUST list are located immediately to the east and west of the former Chevron site. Additionally, all available ground water elevation and flow direction data was reviewed. Historically the ground water flow direction beneath the site varied from the northeast to the southeast. Ground water elevations calculated from the current data shows a ground water flow direction to the northwest. This change in direction may possibly be attributed to the following factors: