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Ernie's General Store - 3 - 22 November 2011 <br /> 4407 E. Waterloo Rd., Stocfln <br /> San Joaquin County <br /> (Attached Figure 9) and at a depth between 15.5' and 50' bgs, which is deeper than normal <br /> construction worker exposure depths. Leaching from soil to groundwater is not sufficient to <br /> cause groundwater concentrations to increase significantly, and dissolved concentrations <br /> mass removal is evident by declining minimum concentrations (currently one order of <br /> magnitude lower than 2006-2007) and stable maximum concentrations in 2006 and 2008, <br /> indicating that natural attenuation is occurring. Residual petroleum hydrocarbons in site soils <br /> and groundwater do not pose a threat to human health or the environment from vapor <br /> intrusion, soil and groundwater to Site workers or the public. The RP indicated in a meeting <br /> with Regional Board staff on 13 May 2010 that land use (gasoline service station and <br /> convenience store) is not expected to change in the foreseeable future. <br /> Additional soil or groundwater remediation is not cost effective or necessary. The residual <br /> groundwater plume will continue to decline, no supply or domestic wells are threatened, site <br /> use (gasoline service station and convenience store) is not proposed to change in the <br /> foreseeable future, residual soil contamination is located at depths greater than typical worker <br /> exposure, and soil vapor testing has shown no vapor intrusion risk to the Site store or the <br /> nearby restaurant from a groundwater plume at the depths to water greater than 50' bgs or <br /> from residual soil contamination at depths greater than 15.5' bgs. <br /> Sensitive Receptor Survey <br /> There are three public supply wells located within 2,000 feet of the site. The closest public <br /> supply well (active) is 1 ,050 feet northwest of the Site. One domestic well is located 900 feet <br /> to the south of the Site. The consultant states the groundwater plume has not traveled more <br /> than 100' offsite to the east (attached Figure 10). Based on the limited lateral extent of the <br /> groundwater plume, the public supply and domestic wells are not threatened by the <br /> groundwater plume. <br /> Site Geology and Hydrogeology <br /> Site soils are described as sands, silts and clays to 150' bgs, the maximum extent of <br /> investigations. Boring logs and geologic cross sections show discontinuous coarse sand <br /> layers: one located between 14' and 16' bgs to the northwest of the USTs, a larger coarse <br /> sand unit from 20' to 30' bgs underneath the USTs and a laterally continuous coarse sand unit <br /> from 70' bgs to 100' bgs, and from 120' to 145' bgs beneath the Site. Several fine units less <br /> than 10' thick, consisting of silty sands, silt and clays, are logged between the coarse sand <br /> units. Between 2003 and 2010, depth to water varied from 57' bgs to 76' bgs. The <br /> groundwater gradient varied from 0.0002 ft/ft/ to 0.04 ft/ft. Measured groundwater flow <br /> direction, predominantly towards the northeast, varied from northeast to northwest. <br /> Residual Mass Calculation <br /> The mass of TPHg removed from soil by excavation and SVE was not reported in the <br /> available documents. The consultant estimated TPHg remaining in soil as approximately <br /> 91 lbs. and TPHg remaining in groundwater as 0.11 lbs. Attached Graph 2 of the MW-4 TPHg <br /> data provides a conservative estimate for the date to reach and remain below Water Quality <br />