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`3 c I <br /> 4 <br /> t <br /> Thrifty Oil Company#171 page 2 <br /> 1250 N. Wilson Way, Stockton. t, I. <br /> During the most recent quarterly groundwater sampling event, 1St Quarter 2002, <br /> analytical results indicated that the highest concentrations of TPHg, benzene, and MtBE <br /> were detected in monitoring well MW-5 at concentrations of1017000 µg/l, 7,260 �tgll, and <br /> 23,800 }Lgll, respectively. Other oxygenates detected ihckud,ed di-isopropyl ether {DIPE} <br /> in wells and TB-8 and tert-butanol (TBA) in wells�MW-3R, TB-1, TB-8, TB-10, and <br /> TB-12. Methanol was also detected in TB-12 at 4,600 µg11. <br /> EHD has reviewed the data for this site and concurs that SVE alone has not adequately <br /> # reduced the petroleum impact to the groundwater and�that additional remedial action is <br /> required. Additionally, EHD has the following concerns: I <br /> 1. The vertical extent of impacted groundwater has notbeen delineated and no <br /> f attempt has been made to discuss or correct this oversight. <br /> 2. The groundwater plume is not assessed laterally toward the SW-W-NW, and is <br /> not well assessed toward the southeast (TB-9, TBA 0 and TB-12 are too <br /> shallow)_ 'R <br /> 3. Groundwater flow direction at MW-5 is variable',and includes all points of the <br /> compass, with SE-SSE as the most common. Groundwater gradient is commonly <br /> high at this site, ranging from 0.008 feet per foot to Oh:15 feet per foot, often at <br /> 0.02 to 0.03 at this site, while typical gradients for the Stockton area vary 1 <br /> between 0.001 and 0.003 feet per foot, yet MtBE apparently is not moving in the j <br /> monitored direction. Why? f 1 <br /> 4. A high concentration gradient between MW-5 and the nearby monitoring welds <br /> exists but MtBE apparently is not moving far, even 6 the 65-75' sand zone. EHD <br /> is concerned that a significant migration pathway for jMtBE may have missed <br /> being identified. f <br /> 5. With 62,000} pounds of TPHg extracted by the SVE system to date, it is' <br /> surprising that the MtBE impact to groundwater is as small as currently depicted. <br /> 6. The calculation for the stagnation point used an uncommonly low gradient for this <br /> site. With gradient ranges as noted in #2 aboV'e, it would better if your consultant <br /> provides a calculation for stagnation point witha higher gradient than the 0.0065 <br /> feet/foot utilized for the calculation presented. '$ <br /> EHD feels something is missing from this site's characterization. More assessment is <br /> needed, especially to the west, possibly toward the southeast, and vertically. It may be <br /> useful to implement a CPT boring program to refine the geological/hydrogeological <br /> model, identify potential migration pathways, and to more fully characterize the site. <br /> t <br /> Although comments and evaluations for other remedial altematives were briefly <br /> discussed in the ATR-RAP, a cost and time-requirement analysis was not included. <br /> 1 Typically EHD will evaluate feasible remedial alternatives against each other and <br /> approve the most cost-effective and feasible alternative. Without a cost and time i <br /> comparison, EHD is unable to approve GWE at this time as the most cost- <br /> I <br /> effective modification to your active CAP. <br /> I <br /> itg} I <br />