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10 November 2003 <br /> S AGE-NC Project No 97-0249 <br /> Page 7 of 8 <br /> • The highest off-site concentrations of dissolved MTBE were detected in the Flag City <br />' Chevron samples collected from Hydrogeologic Unit 1 extraction wells EW-5 and EW-8 at <br /> 22 µg/1 (Figure 11) and from Hydrologic Unit 2 well MW-8A at 420 �Ig/l (Figure 12) <br /> • Previously,low concentrations of TBA and 1,2-DCA have been detected from wells MW-1, <br /> MW-2, MW-3, MW-5, MW-6, MW-8A, MW-l0A and MW-11A suggesting that only a <br /> minor TBA-impact and 1,2-DCA-impact to ground water in Hydrogeologic Unit 1 is present <br /> at the site <br /> • The lateral and vertical extent of the dissolved petroleum hydrocarbon plume is not known <br /> toward the northeast of Three B's Truck Plaza well MW-3, toward the south of Three B's <br />' Truck Plaza well MW-6 and toward the south of Flag City Chevron wells MW-16/17118 <br /> (Figures 9, 10, 11 and 12) <br /> 1 <br /> 5.0. RECOMMENDATIONS <br /> Based upon the results of the third quarter 2003 investigation., AGE recommends <br /> • Continued quarterly ground water monitoring of wells MW-1 through MW-12, the next <br /> quarterly ground water monitoring event is tentatively scheduled for December 2003 <br /> • Continue to conduct future ground water monitoring events concurrently with the Flag City <br /> Chevron site (6421 Capitol Avenue), <br /> • Transferring responsibility of well MW-13 (located approximately 20 feet west of the Three <br /> B's Truck Plaza above ground tank area) from the Flag City Chevron investigation to the <br /> Three B's Truck Plaza investigation <br /> • Additional ground water monitoring wells(screened in Hydrogeologic Unit 1)are warranted <br /> in the area northeast of Three B's Truck Plaza well MW-3, south-southeast of Three B's <br /> Truck Plaza well MW-6 and southwest of MW-16/17/18, installation of the additional <br /> ground water monitoring wells should aid in the preparation of a site conceptual model <br /> Based on site data collected, the presence of three municipal water wells (CSA-1, CSA-2 and <br /> CSA-3)within a 2,000 foot radius of the site,the presence of a local domestic water well on the site, <br /> and due to 1) the high concentrations of contaminants (i e MTBE, BTEX) encountered at the site <br /> that are significantly above the DHS's MCLS, 2) the high mobility of MTBE dissolved in ground <br /> water, AGE strongly recommends that an active ground water interim remediation system be <br /> implemented at the site as soon as possible to establish hydraulic control and inhibit potential off-site <br /> migration of the contaminated ground water plume towards any of the above mentioned sensitive <br /> treceptors <br /> i <br /> Advanced GeoEnvironmental,Inc <br /> 1 <br />