ARCHIVED REPORTS_SUMMARY OF GROUNDWATER MONITORING

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amec'0 Mr. Howard Hold California Regional Water Quality Control Board Central Valley Region April 27, 2010 Page 3 Status: The three additional groundwater monitoring events were completed at the Lathrop Facility. As described in the Semi-Annual Groundwater Monitoring Report (AMEC Geomatrix, 2010), the groundwater monitoring events were conducted in December 2009, January 2010, and February 2010. The results of the three monitoring events are included in this letter. • Action Item #3: Provide the RWQCB with a detailed summary of the work completed to date that was outlined in the Order(including dates and tasks completed by MonierLifetile) and the rationale for rescinding the Order. As indicated during the October 22, 2009, meeting, rescinding the Order may only require the approval of the Executive Officer rather than a formal RWQCB hearing. The approval process will be determined by the RWQCB. Status: We are currently preparing a letter for submittal to the RWQCB to request site closure and to document the work completed under the Order for the Lathrop and French Camp Facilities. SUMMARY OF RECENT GROUNDWATER MONITORING EVENTS AT LATHROP FACILITY The following sections present the results of the three monthly groundwater monitoring events conducted at the Lathrop Facility in December 2009, January 2010, and February 2010. As described below, groundwater analytical data are compared to historical data, and as prescribed in the Order, the water quality goal (WQG) for TPH-diesel of 100 micrograms per liter (pg/L) (Marshack, 2001)5, and the California Department of Public Health (CDPH) Maximum Contaminant Level (MCL) for Cr and Cr VI of 50 pg/L. GROUNDWATER GRADIENT AND FLOW DIRECTION Depths to groundwater and groundwater elevations for the Lathrop Facility are listed in Table 1. Groundwater elevation contours for the three monthly monitoring events at the Lathrop Facility are shown on Figures 2 through 4. During this monitoring period, groundwater depths, elevations and flow directions were consistent with previous monitoring events as discussed below. December 2009 During the December 2009 monitoring event, groundwater was encountered at elevations of 1.82 feet below mean sea level (bmsl) in well LPW-1 (cross-gradient well), 1.91 feet bmsl in well LPW-2 (downgradient well), and 0.56 feet bmsl in well LPW-3 (upgradient well). Groundwater elevations at each well are approximately 0.87 to 1.56 feet higher than the groundwater 5 Marshack, J.B., 2001,A Compilation of Water Quality Goals, California Environmental Protection Agency Regional Water Quality Control Board, Central Valley Region, November. PA10626.000.0\Docs\Semi-Annual Groundwater Monitoring&Sampling\Lathrop Monthly Groundwater Monitoring\Ltr_042710.doc