ARCHIVED REPORTS_SEMI-ANNUAL GROUNDWATER MONITORING REPORT AUGUST 2010

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ameO WQG (100 fag/L) in the groundwater samples collected at this 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 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). Total Cr was detected in the groundwater samples from monitoring wells LPW-1, LPW-2, and LPW-3 at concentrations of 31 pg/L, 29 pg/L, and 3.2J pg/L ("J" is estimated) (LPW-3 duplicate at 3.OJ), respectively. Dissolved Cr was detected in the groundwater samples from wells LPW-1 and LPW-2 at concentrations of 20 pg/L and 12 pg/L, respectively. Dissolved Cr was not detected above the laboratory reporting limit (10 pg/L) in the groundwater samples from well LPW-3. The reported concentrations of total and dissolved Cr were below the CDPH MCL of 50 pg/L and within the range of concentrations detected during previous groundwater monitoring events. Dissolved Cr VI was detected in the groundwater samples from monitoring wells LPW-1 (cross-gradient), LPW-2 (downgradient), and LPW-3 (upgradient) at concentrations of 19 pg/L, 12 pg/L, and 5.6 Ng/L (LPW-3 duplicate at 4.9 pg/L), respectively. The reported concentrations of dissolved Cr VI in the groundwater samples were below the CDPH MCL of 50 pg/L and within the range of concentrations detected during previous groundwater monitoring events. Since the removal of the stockpile in October 2007, almost three years of groundwater monitoring has been conducted at the Lathrop facility (nine groundwater sampling events), and groundwater sample concentrations at wells LPW-1, LPW-2, and LPW-3 appear to be stable or decreasing (Figures 6 through 11). 4.2.2 French Camp Facility During the August 2010 sampling event, TPH-diesel was not detected above the laboratory reporting limit of 50 pg/L in the groundwater samples from the three monitoring wells (FCW-1, FCW-2, and FCW-3). At this Facility, TPH-diesel has only been detected above the WQG of 100 pg/L in the groundwater samples from well FCW-1 (cross-gradient well) between May 2004 and May 2007, in May 2008, and in May 2009. As reported during historical sampling events, the laboratory classified the petroleum hydrocarbons as higher boiling point hydrocarbons. Consistent with previous monitoring events, TPH-diesel was not detected above the laboratory reporting limit of 50 pg/L in groundwater samples from well FCW-2 (cross-to downgradient well) and well FCW-3 (upgradient well). AMSC Geomatrix, Inc. P:\10626.000.0\Docs\Semi-Annual Groundwater Monitoring&Sampling\Semi-Annual 1-10 8