SITE INFORMATION AND CORRESPONDENCE

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Mr.M. Scott Mansholt—CEMC First Semi-Annual 2008 Groundwater Monitoring and Sampling Report April 28,2008 Vernalis Pump Station Page 5 of 6 casing that was observed during the 2008 event. MW-7 had a TPHd detection of 390 gg/L(<50 gg/L after silica-gel cleanup)during the January 2008 monitoring, and a TPHd detection of 150 gg/L(88 gg/L after silica-gel cleanup)during October 2007 monitoring event. There were no other TPHd or PAH concentrations detected above their respective WQOs and/or ESLs. Recent and historic site groundwater monitoring data indicate that the TPHd groundwater plume appears to be localized in a small area of the site with wells MW-1 through MW-4 defining the lateral extent of the TPHd groundwater plume. Minor amounts of SPO have been observed in MW-1 since monitoring and sampling began in 1998; therefore,this well has never been sampled. Well MW-4 has not been sampled since January 2006 due to insufficient water volume, and damage to the well casing was observed during the January 2008 event. MW-4 had a TPHd detection of 290 µg/1(150 gg/L after silica- gel cleanup)when last sampled,with this detection exceeding TPHd WQO/ESL of 100 gg/L. Well MW-2 had a TPHd detection of 120 gg/L(67 gg/L after silica-gel cleanup)when sampled in January 2006, and TPHd were detected during the most recent sampling event(January 2008) at a concentration of 52 gg/L(<50 gg/L after silica-gel cleanup). Well MW-3 was last sampled in November 2006 and TPHd were non-detect below the laboratory method reporting limit of<50 gg/L,the most recent sampling event(January 2008)had a TPHd detection of 79 gg/L(<50 gg/L after silica-gel cleanup). Well MW-5 had TPHd detections of 72 gg/L in May 2005 and 150 gg/L in January 2007, and the most recent sampling event(January 2008)had a TPHd detection of 73 gg/L(<50 gg/L after silica-gel cleanup). Well MW-6 has never had a detection of TPHd above the WQO/ESL. Well MW-7 is the only well where TPHd have been consistently detected at elevated concentrations, although during the 2nd and 3rd quarter 2007,TPHd have been non-detect. MW-7 had a TPHd detection of 390 gg/L(<50 gg/L after silica-gel cleanup)during the January 2008 sampling event, and a TPHd detection of 150 gg/L(88 gg/L after silica- gel cleanup) during the October 2007 sampling event. PAHs have not been detected at concentrations above WQOs and/or ESLs in any of the site wells with the exception of well MW-6. PAHs were detected in well MW-6 above WQOs and/or ESLs from samples collected during the January 2006 and January 2007 sampling events. PAHs have not been observed in any of the wells since January 2007. SAIC recommends the following based on current and historic groundwater sampling results for this site and the apparent size and stability of the TPHd groundwater plume: • Reduce oleophilic sock inspection and replacement in well MW-1 from quarterly to semi-annual events. The semi-annual inspections should coincide with the semi-annual monitoring and sampling of the on-site wells; • Discontinue PAH analyses for all site wells; • Abandonment of MW-4 since it can no longer serve as a useful data point and wells MW-1,MW- 2,MW-3, and MW-5 would provide sufficient water-quality data for the site. • Assess the sampling frequency of off-site wells MW-6 and MW-7 based on the results of the pending off-site soil and groundwater investigation. The RWQCB requested that the off-site wells be monitored and sampled on a quarterly basis until the down-gradient extent of petroleum- affected groundwater has been characterized. An off-site soil and groundwater investigation work plan was submitted to the RWQCB on April 30, 2007, and is pending acceptance.