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Mr. Nuel Henderson <br /> Page 3 of 4 <br /> and MW15B (1.6 ppb). These wells are affected by MTBE releases originating at 101 S. <br /> Wilson Way and/or 130 S. Wilson Way. <br /> EDB was not detccted in any of the wells sampled. 1,2-DCA was detected in each of the <br /> sampled wells, except MW7 and MW1513. Concentrations of 1,2--DCA ranged from 0.8 ppb <br /> in MW8 to 400 ppb in MW1313. Analytical results for gasoline oxygenates, EDB and 1,2- <br /> DCA are summarized in Table 3. Attachment C contains laboratory reports and chain of <br /> custody documentation. <br /> Vapor Extraction Wells <br /> TPHg and BTEX concentrations in the water sample collected from VW1 were within <br /> historical ranges. MTBE was not detected in well VWL The water sample collected from <br /> vapor well VW1 contained 1,2-DCA at a concentration of 1,200 ppb. TPHg and BTEX <br /> concentrations are summarized in Table 2. 1,2-DCA, EDB and gasoline oxygenate additive <br /> h concentrations are summarized in Table 3. Attachment C contains laboratory reports and <br /> chain of custody documentation. <br /> GROUNDWATER SAMPLING SUMMARY AND DISCUSSION <br /> Water levels fell during this quarter. Groundwater elevation data continue to show a <br /> predominantly easterly-northeasterly groundwater flow direction. An upward vertical <br /> gradient component was observed across the site. <br /> o Petroleum hydrocarbon concentrations in the wells affected by the onsite release were <br /> within historical ranges. The extent of groundwater contamination associated with the <br /> onsite release is stable and contaminant concentrations are stable to decreasing. <br /> o The plume of contamination originating from offsite source(s) is continuing to spread. <br /> High levels of MTBE persist in wells along the southern portion of the property. MTBE <br /> has also affected downgradient wells MW11, MW 12, and MW 15B. The concentration of <br /> MTBE detected in MW 12 during this event was the highest ever recorded in that well. <br /> RECOMMENDATIONS <br /> Groundwater Monitoring and Sampling <br /> Reduce the sampling frequency for wells that are monitoring the plume of contamination <br /> originating from the onsite release. Given the massive quantity of historical groundwater data <br /> at this site, continued quarterly sampling is not warranted. We also recommend that sampling <br /> responsibility for the wells that are affected by the offsite release(s) be transferred to the <br /> appropriate responsible parties. As requested in your letter of August 31 we propose the <br /> G:\GROLNDZE\ROEKIQMRQ005\4qm2OO5.doc <br />