ARCHIVED REPORTS_SUMMARY OF GROUNDWATER MONITORING

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ameO Mr. Howard Hold California Regional Water Quality Control Board Central Valley Region April 27, 2010 Page 5 GROUNDWATER MONITORING RESULTS Sample analytical results from the three monthly groundwater monitoring events and previous monitoring events at the Lathrop Facility are summarized in Table 2. Laboratory analytical results are presented on Figure 5. Groundwater sample concentration trends for TPH-diesel, Cr, and Cr VI for the wells at the Lathrop Facility are shown on Figures 6 through 11. Field sampling records are presented in Appendix A. Laboratory analytical reports and chain-of- custody documentation are presented in Appendix B. During the monitoring period, the groundwater sample results for TPH-diesel, total and dissolved Cr, and dissolved Cr VI were consistent with previous monitoring data as described in the following section. In summary, TPH-diesel was not detected above the laboratory reporting limit in all three wells. Results for total Cr and dissolved Cr and dissolved Cr VI were below the CDPH MCL of 50 pg/L for Cr. TPH-Diesel TPH-diesel was not detected above the laboratory reporting limit of 50 pg/L in the groundwater samples from all three monitoring wells (LPW-1, LPW-2, and LPW-3) during the sampling period. Since May 2007, TPH-diesel has not been detected above the diesel WQG (100 pg/L) in the groundwater samples collected at the Lathrop Facility. For the most part, historical detections of petroleum hydrocarbons in the groundwater samples were classified by the laboratory as weathered diesel or higher boiling point petroleum hydrocarbons. Furthermore, TPH-diesel has been mainly reported in groundwater samples from the well (LPW-1) located cross-gradient to the former stockpile (adjacent to the eastern boundary of the Lathrop Facility), twice in the groundwater samples from the upgradient well (LPW-3), and only once in the groundwater samples from the downgradient well (LPW-2). Since the removal of the stockpile in October 2007, over 2 years of groundwater monitoring has been conducted at the Lathrop Facility (eight groundwater sampling events). During this period, the concentrations of TPH- diesel in groundwater were reported as non-detect. Therefore, with respect to TPH-diesel, groundwater concentrations are stable (Figures 6 through 11). Cr and Cr VI December 2009 Total Cr was detected in the groundwater samples from monitoring wells LPW-1 and LPW-2 at concentrations of 6.3J6 and 14 pg/L, respectively. Dissolved Cr was detected in the groundwater samples from wells LPW-1 and LPW-2 at concentrations of 5AJ and 12 pg/L, respectively. Total and dissolved Cr were not detected above the laboratory reporting limit 6 J = Estimated concentration between the laboratory reporting limit and method detection limit. P:\10626.000.0\Docs\Semi-Annual Groundwater Monitoring&Sampling\Lathrop Monthly Groundwater Monitoring\Ltr_042710.doc