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GEORGE AND MEI TERANISHI • <br /> 1600 Durham Ferry Rd. <br /> Page 4 of 4 <br /> some 70 plus years after the initial release. Of more concern is a significant sand <br /> encountered at 30 feet bsg in the three borings extending to that depth, where <br /> the ground water velocity has been determined to range from 5 feet to 134 feet <br /> per year. Ground water in sand near 50 feet bsg is monitored by MW-107, but <br /> this sand appears to be separated from the sand at 30 feet bsg by fine-grained <br /> material (clay, silt) encountered at 40 feet bsg in the three borings. With a rapid <br /> ground water velocity, a vertical barrier at 40 feet bsg, and potentially upward <br /> vertical gradients, the lack of impacted ground water in the 50-foot sand in MW- <br /> 107 does not preclude the potential for impacted ground water to be present in <br /> the 30-foot sand, which could be a significant lateral migration pathway. MW-105, <br /> which is completed near the 30-foot sand, is in the upgradient to cross-gradient <br /> direction from the suspected release area and may not exhibit an impact due to <br /> the rapid ground water velocities. EHD believes that several ground water <br /> samples should be collected from the 30-foot interval in the down-gradient <br /> directions to complete the lateral assessment of impacted ground water. <br /> The UST Fund has indicated a willingness to reimburse for continuous cores <br /> taken to identify migration pathways. EHD recommends continuous coring when <br /> performing the above investigation for just that reason. <br /> An additional vial of the ground water samples should be collected from the more <br /> impacted wells to perform focused EPA 8260 runs to improve the detection limit <br /> for MTBE. If requested, EHD will approve an annual sampling schedule for <br /> monitoring wells MW-105, MW-107, and MW-202 until site conditions are <br /> suspected to have changed to justify a return to quarterly sampling. <br /> Please submit a work plan to address the comments above by 30 August 2002. <br /> Questions or comments may be directed to the undersigned by mail (address on <br /> letter head), by e-mail at nhenderson@sicehd.com, or by telephone at <br /> (209) 468-3436. <br /> Donna Heran, REHS, Director <br /> Environmental <br /> Health Division <br /> Nuel C. Henderson, Jr., R.G. <br /> Unit IV - Site Mitigation <br /> C: Marty Hartzell — CVRWQCB <br /> Ray Kablanow, Michael Niemi — Geological Technics Inc <br />