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-Ms. Laurie Cotulla <br /> February 14, 1991 <br /> Page 2 <br /> The attached graphs show the relationship of depth to water versus time in <br /> each of the five monitoring wells onsite. Above each depth to water <br /> point, on the graphs, is the benzene concentration presented in parts per <br /> billion (ppb) . From these graphs you can notice that: 1) benzene <br /> concentrations showed no appreciable decreases between the June 1988 and <br /> September 1989 sampling events; 2) benzene concentrations were reduced to <br /> nondetectable between the September 1989 and December 1989 sampling <br /> events, the period during which the vapor extraction system was operated; <br /> 3) benzene concentrations remained nondetectable even after ground-water <br /> levels returned to July 1989 levels (where benzene concentrations were as <br /> high as 74 parts per billion) . <br /> Your letter also states that "the groundwater plume of contamination has <br /> not been adequately defined. Additional wells are necessary in <br /> downgradient locations of the former tank excavation and piping system". <br /> To address this issue, please refer to the attached Potentiometric Map <br /> from Applied GeoSystems' most recent monitoring report. <br /> We have drawn in ground water flow lines in accordance with the <br /> potentiometric surface contour lines. The flow lines indicate that water <br /> flows from wells MW-1, MW-3, and MW-4 toward MW-2. Water beneath the area <br /> of the underground distribution lines and dispensers flows toward MW-2 and <br /> then MW-5 (wells which have historically had very low to nondetectable <br /> concentrations of petroleum hydrocarbons) . <br /> We would appreciate it if you would please review this new information <br /> carefully and reevaluate our request for site closure. We would <br /> appreciate a technical review of our request by Mr. Jim Maughan, or other <br /> appropriate personnel of the Central Valley Regional Water Quality Control <br /> Board. Please do not hesitate to call if you have any questions regarding <br /> this information. <br /> Sincerely, <br /> ULTRAMAR INC. <br /> Dembroff <br /> Manager <br /> Marketing Environmental Department <br /> Enclosures: EVAX Technologies' letter report <br /> Depth-to-water graphs with benzene concentrations (2 pages) <br /> Potentiometric Map (modified from Applied GeoSystems) <br /> cc w/encl : Mr. Jim Maughan - CVRWQCB, Sacramento, California <br /> 02141cot <br />