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ameLAO <br /> Mr. Howard Hold <br /> California Regional Water Quality Control Board <br /> Central Valley Region <br /> April 27, 2010 <br /> Page 4 <br /> elevations calculated from the November 2009 semi-annual monitoring event (AMEC <br /> Geomatrix, 2010). Overall, the groundwater elevations were within the range of previously <br /> calculated elevations for the Lathrop Facility (Table 1). <br /> Based on the water level measurements collected in December 2009, the average groundwater <br /> gradient in the vicinity of the former stockpile is 0.004 feet per foot (ft/ft)with a northeasterly flow <br /> direction. Interpreted groundwater elevation contours for December 2009 are shown on Figure 2. <br /> The calculated groundwater gradient and flow direction are consistent with the historical data for <br /> the Lathrop Facility. <br /> January 2010 <br /> During the January 2010 monitoring event, groundwater was encountered at elevations of <br /> 0.66 feet bmsl in well LPW-1 (cross-gradient well), 0.61 feet bmsl in well LPW-2 (downgradient <br /> well), and 3.25 feet above mean sea level (amsl) in well LPW-3 (upgradient well). Groundwater <br /> elevations at each well are approximately 1.16 to 3.81 feet higher than the groundwater <br /> elevations calculated from the December 2009 monitoring event. Overall, the groundwater <br /> elevations were within the range of previously calculated elevations for the Lathrop Facility <br /> (Table 1). <br /> Based on the water level measurements collected in January 2010, the average groundwater <br /> gradient in the vicinity of the former stockpile is 0.009 ft/ft with a northeasterly flow direction. <br /> Interpreted groundwater elevation contours for January 2010 are shown on Figure 3. <br /> The calculated groundwater gradient and flow direction are consistent with the historical data for <br /> the Lathrop Facility. <br /> February 2010 <br /> During the February 2010 monitoring event, groundwater was encountered at elevations of <br /> 0.62 feet amsl in well LPW-1 (cross-gradient well), 0.44 feet amsl in well LPW-2 (downgradient <br /> well), and 3.12 feet amsl in well LPW-3 (upgradient well). Groundwater elevations at wells <br /> LPW-1 and LPW-2 are approximately 1.05 to 1.28 feet higher than the groundwater elevations <br /> calculated from the January 2010 monitoring event; the groundwater elevation at well LPW-3 is <br /> approximately 0.13 feet lower. Overall, the groundwater elevations were within the range of <br /> previously calculated elevations for the Lathrop Facility (Table 1). <br /> Based on the water level measurements collected in February 2010, the average groundwater <br /> gradient in the vicinity of the former stockpile is 0.007 ft/ft with a northeasterly flow direction. <br /> Interpreted groundwater elevation contours for February 2010 are shown on Figure 4. <br /> The calculated groundwater gradient and flow direction are consistent with the historical data for <br /> the Lathrop Facility. <br /> P110626.000.0\Docs\Semi-Annua1 Groundwater Monitoring&Sampling\Lathrop Monthly Groundwater Monitoring\Ltr_042710.doc <br />