SITE INFORMATION AND CORRESPONDENCE

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INTRODUCTION UNOCAL Corporation petitioned the Underground Storage Tank Cleanup Fund manager seeking review of the decision of the San Joaquin County Public Environmental Health Division (County) not to close their UST release site located at 1665 Pacific Avenue, Stockton, California. This report presents a summary of the site's history, its hydrogeologic setting and associated groundwater impacts. This report also addresses the 7 potential for current site conditions to adversely affect beneficial uses of groundwater. BACKGROUND The site is located in a commercial/residential area of Stockton and was an ! operating service station from about 1947 to 1988; gasoline was dispensed from two ! 10,000 gallon capacity USTs. Since demolition of the station and removal of the USTs in 1988, the site has remained vacant. The site is underlain by flood plain sediments (clay, silt and sand) of the Pliestocene Victor Formation. In the vicinity of the site, groundwater in the more permeable strata of Victor Formation is generally of inferior quality and is not viewed by local water purveyors as a source of drinking water. Underlying the Victor Formation are the alluvial sediments (clay, silt, sand, and gravel) of the Plio-Pliestocene Laguna Formation. Groundwater in the Laguna Formation is used extensively throughout the area for municipal, industrial, and agricultural supply. There are no water supply wells within 800 feet of the site and those municipal wells located in the general area are routinely constructed with annular seals which extend to depths of 150 to 200 feet so as to exclude the shallower poor quality groundwater in the Victor Formation. The nearest surface water, a canal which extends easterly from the Port of Stockton, is about 2,000 feet to the west. Shallow groundwater at the site flows in a general easterly direction with a gradient of about 0.002. In January 1988, analyses of soil samples from five soil borings drilled to depths of 20 to 30 feet indicated that a release had occurred at the site. In February 1988, three groundwater monitor wells were installed to assess site groundwater conditions. The initial groundwater samples from the three wells revealed an absence of gasoline constituents in groundwater near the location of the two gasoline USTs, high concentrations (99,000 ppb TPH-g and 5,700 ppb benzene) in the vicinity of the easterly pump island, and low concentrations (170 ppb TPH-g and 1 ppb benzene) in the vicinity of the waste oil UST. Data gained from the well installations also indicated that the groundwater was confined and that it flowed in an easterly direction. In March 1988, the two 10,000-gallon capacity gasoline USTs, the waste oil UST, ! and the dispensers and associated piping were removed from the site. Visual inspection of the bottom and side-walls of the gasoline UST excavation revealed that soil was affected by a release prompting UNOCAL to over-excavate an estimated 850 cubic yards of soil to a final depth of 28 to 30 feet'. Soil samples collected from the bottom and side wall of the ' The volume of soil excavated is a point of contention with the County. UNOCAL indicates in its Site Closure Summary report that an estimated 1,100 cubic yards of soil were excavated while the County,in I response to the report, contends that only 350 cubic yards are documented to have been excavated. However, in a March 12, 1991 letter to UNOCAL, the County states that;"...approximately 1300-1400 1