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tMr. Nuel Henderson <br /> Page 3 of 4 <br /> ' EDB was not detected in any of the wells sampled. 1,2-DCA was detected in wells MW 10, <br /> • MW11, MW15B and MW15C. Detected concentrations of 1,2-DCA ranged from 2.3 ppb <br /> (MW15C) to 12 ppb (MW10). Analytical results for gasoline oxygenates, EDB and 1,2-DCA <br /> are summarized in Table 3. Attachment B contains laboratory reports and chain of custody <br /> ' documentation. <br /> Vapor Extraction Wells <br /> ' TPHg and BTEX concentrations in water samples collected from VW4 and VW6 were within <br /> historical ranges. MTBE was not detected in either well. The water sample collected from <br /> ' vapor well VW4 contained 1,2-DCA at a concentration of 49 ppb. This additive was not <br /> detected in the sample collected from VW6. TPHg and BTEX concentrations are summarized <br /> in Table 2. 1,2-DCA, EDB and gasoline oxygenate additive concentrations are summarized in <br /> tTable 3. Attachment B contains laboratory reports and chain of custody documentation. <br /> ' GROUNDWATER SAMPLING SUMMARY AND DISCUSSION <br /> • Water levels fell during this quarter. Groundwater elevation data continue to show an <br /> easterly-northeasterly groundwater flow direction. Well cluster MWI5AIBIC continues to <br /> exhibit an apparent upward vertical gradient component, while at the MW 16A/B well <br /> cluster a downward vertical gradient component was observed during this event. <br /> • 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 the concentrations are stable to decreasing. <br /> ' 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 MW 12 and MW15B where concentrations have risen <br /> ' over time. The highest level of MTBE to date was observed in MW15B during this <br /> sampling event (940 ppb). <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 to an annual schedule. Given the massive quantity of <br /> historical groundwater data at this site, continued quarterly sampling is not warranted. <br /> Transfer ownership or sampling responsibility for the wells that are affected by the offsite <br /> release(s) to the appropriate responsible parties. <br /> ' 11GZA_FSWOL_IIDATA\GROUNDZLIROEKIQMRS200414gm2004.doc <br /> I <br />