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ARCADIS Ms. Lori Duncan <br /> October 8, 2008 <br /> • The calculated groundwater flow was toward the northeast at an <br /> approximate horizontal hydraulic gradient of 0.001 ft/ft <br /> • TPH-DRO was detected in groundwater samples collected from all five <br /> monitoring wells. Given the distribution of similar concentrations across <br /> the site, TPH-DRO may be a regional issue in this area of Stockton <br /> • TPH-GRO, toluene, ethylbenzene, xylene MTBE, TBA, and 1,2-DCA, <br /> were detected in groundwater samples collected from monitoring well <br /> MW-1 at concentrations generally consistent with or lower than results of <br /> past quarterly monitoring and sampling events <br /> • 1,2-DCA was detected at a concentration above the California Primary <br /> �! MCL in the groundwater sample collected from MW-1 <br /> • Concentrations of TPH-MO, benzene, DIPE, ETBE, TAME, methanol, <br /> ethanol, and EDB were not detected above their respective MDLs in <br /> groundwater samples collected during the second quarter 2008 sampling <br /> event <br /> Based on the results presented above, the petroleum hydrocarbon impacts to soil <br /> and groundwater appear to be isolated to.the former dispenser islands at the site. <br /> Recommendation <br /> ARCADIS recommends the inclusion of wells MW-1, MW-2, MW-3, MWA and MW-5 <br /> into the groundwater monitoring and sampling program for the site. <br /> In addition, the petroleum impacts to soil and groundwater appear to be isolated to <br /> the former dispenser island, therefore ARCADIS recommends advancing the deep <br /> soil boring proposed within the Site Assessment Work Plan Addendum(ARCADIS, <br /> December 18, 2007) near existing monitoring well MW-1 as shown on Figure 1. As <br /> proposed in the work plan addendum, the soil boring will be advanced to a depth of <br /> approximately 150 feet bgs and a deep temporary well installed based on field <br /> observations. Up to 15 soil samples will be collected at approximately 10-feet <br /> intervals and one groundwater sample to be submitted for laboratory analysis. <br /> 4 <br /> Page: <br /> \\arcaCsusbffcedata\Swmmento-CA\EMACHEVRON130907516 Delivmbles\Repals%wessment\309075 Assessment Report_181810950.doc 9/10 <br />