ARCHIVED REPORTS_SECOND 2008 SEMI-ANNUAL GROUNDWATER MONITORING AND SAMPLING REPORT

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' M. Scott Mansholt—CEMC Second Semi-Annual 2008 Groundwater Monitoring and Sampling Report September 26,2008 Vernalis Pump Station Page 6 of 7 I0 Well MW-5 has had only one TPHd exceedance(150µg/L in the first quarter of 2007) since quarterly sampling began. Subsequent sampling events have not reported TPHd exceedances from this well. I • Off-site well MW-6 has never had a detection of TPHd above the WQO/ESL. Well MW-6 was the only well that indicated the presence of one or more PAHs at concentrations above their ' respective WQOs or ESLs since monitoring began; no PAH exceedances have been reported since the first quarter of 2007. • Off-site well MW-7 was the only well where TPHd was consistently detected at elevated concentrations with the exception of the second and third quarter 2007 sampling events, which reported non-detectable concentrations. 0 PAH analysis for all on-site wells has been discontinued and oleophilic sock inspection has been reduced from quarterly to semi-annual frequency in accordance with the RWQCB letter to Chevron dated May 8, 2008. ' • Off-site monitoring wells MW-6 and MW-7 were properly abandoned on August 24 and 25, 2008. MW-4 will be abandoned at a future date in accordance with the RWQCB letter to ' Chevron dated May 8, 2008. 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: • Continue semi-annual inspection/replacement of the oleophilic sock in MW-1. • Discontinue semi-annual monitoring and sampling of on-site wells MW-2,MW-3, and MW-5. Wells MW-3 and MW-5 have each had a singular TPHd exceedance since quarterly sampling began, and well MW-2 has only had sporadic TPHd exceedances. The history of monitoring data shows evidence of a stable plume. Prepare a human health screening evaluation (HHSE)which covers the on-site parcels(Arcos and ' Bellini) and the off-site parcels owned by Bogetti. o No additional on-site soil or groundwater investigation was requested by the RWQCB in ' its response to the May 2005 site investigation results. The lateral and vertical extent of affected soil and groundwater on-site is delineated and sufficient data exist to prepare an HHSE. o The RWQCB concurred'that the off-site extent of affected soil and groundwater were delineated during the February 2008 Vernalis Offsite investigation and that an HHSE 1 4. RWQCB,2006. Review—Soil and Groundwater Investigation Report, Vernalis Pump Station,35500 Welty males, Road Vernalis, San Joaquin and Stanislaus Counties. September 20. ' 5. RWQCB,2008. Review—Soil and Groundwater Investigation Report, Vernalis offsite, Vernalis,San Joaquin and Stanislaus Counties. July 28. 1