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9110 Thornton Page 2 <br /> sou, CONTAMINATION <br /> The report included an estimate of 100 feet for the radius of influence based on the January <br /> 13, 1994 vapor extraction test. Evaluating Figure 10 of the report, PHS/EHD questions the <br /> determination that the effective radius of influence was 100 feet. There appears to be a <br /> significant reduction of the observed vacuum within 20 feet of the test well. Based on the <br /> vapor sample results observed during the testing period, it appears that the effective radius <br /> of influence during operation of the vapor extraction system was Iikely less than 100 feet. <br /> The results of vapor samples were included in Table 5 and some confusion occurred with <br /> respect to the designation of MW1A, MW4A, PW3A and MW1B, MW4B, and PW3B. <br /> Some notation should be included that indicates that these samples were collected from the <br /> same respective well although at different times. <br /> SUMMARY <br /> The report did not indicate what happened to the tanks between 1986 when the <br /> unauthorized release was detected and 1991 when the tanks were removed. Please provide <br /> this information in the next background section of the quarterly report due August 30, 1994. <br /> The vapor extraction testing which was performed indicated that residual petroleum <br /> hydrocarbons continued to be evidenced in the subsurface. Please provide an estimate of <br /> the volume remaining and its distribution in the next quarterly report. <br /> The report stated that the potential hydrocarbon extraction rate was 0.55 lbs/day; however, <br /> there was no indication of the length of time it might take to extract the residual soil <br /> contamination evidenced. <br /> Additional soil confirmation sampling will be required to estimate the volume of residual soil <br /> contamination. Please submit by August 30, 1994 a work plan to perform the requested <br /> investigation along with a drilling permit application and $89 fee. <br /> If you have any questions, comments, or wish to schedule a sampling inspection, please <br /> contact Mary Meays at (209) 468-0337. <br /> Donna Heran, REHS, Director <br /> Environmental Health Division <br /> Mary Meays, Senior REHS Diane M. Hinson, REHS <br /> Site Mitigation Unit Supervisor <br /> MM19110THOR <br /> c: Elizabeth Thayer, CVRWQCB <br /> c: Brian Garber, GTI West Sacramento Office <br />