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Groundwater elevation measurements from the momtonng wells indicate the magutude of the horizontal hydraulic gradient at the site to be very low (i e , less than approximately 0 0001 feet per feet),with an apparent direction toward the south and southwest Petroleum analytes or constituents generally are not detected in groundwater samples collected from ' monitoring wells located less than 100 feet down gradient of the affected soil (MW-5 and MW-6) Therefore, the detections of TPHd in monitoring well MW-2, which is screened across petroleum affected soil, likely represents either petroleum entrained on soil particulates in groundwater(turbidity) i ' or globules of petroleum collected with the groundwater sample and are not representative of dissolved phase petroleum The TPH concentrations in MW-3 and MW-4 represent conditions at the I ' soiVgroundwater interface i Shallow groundwater within and downgradient of the petroleum affected soil is not currently used and I ' is not anticipated to be used in the near future A well survey was conducted by ATC in 2000 that searched Department of Water Resources (DWR) files, performed a site and vicinity reconnaissance, and contacted the City of Stockton and California Water Service for wells within 2000 feet of the site ' vicinity(ATC 2000) According to ATC, the nearest well is located approximately 400 feet southeast of the site and is a monitoring well The nearest well potentially used for drinking water is a domestic well located approximately 1,300 feet to the south of the site In addition, San Joaquin County ' Department of Public Health Services ordinances require a minimum of 50 foot sanitary seal at the top of any public water supply well These sanitary seal requirements would preclude shallow affected water at the site from future use as a source of municipal water The nearest surface water is the • Stockton deepwater channel, located approximately 650 feet north of the site The following is a summary of findings from the October 2004 CAP ' 1 The lateral extent of reported petroleum-related detections in groundwater is limited to the vicinity of the petroleum-impacted area and the volume is small ' 2 No dissolved "plume" of petroleum affected groundwater associated with this petroleum- impacted area is present ' 3 It is highly unlikely that this localized area of petroleum affected shallow groundwater will migrate beyond its current limited spatial extent given that the release occurred at least 16 years ago ' 4 Shallow groundwater is not and likely will not be used directly as a source of drinking water 5 Environmental impacts (e g, to groundwater supply wells or surface water bodies) outside the immediate area of the former UST location are not likely to occur under current conditions 35 Human Health Risks The October 2004 CAP identified the environmental screenilig levels(Eels)applicable to the petroleum constituents detected at the Stockton Elevator Table 5 compares the maximum concentrations of petroleum constituents detected in vadose zone soil to environmental screening levels (ESLs) The ' comparison below is of soil site conditions to IMs for deep soils(>3m bgs) where groundwater is not • a current or potential source of drinking water as applied to the Residential Land use screening levels 1 14 ' R1CONTGRAN15tockton\CAP\CAPADDENDUM