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Mr. Mike Desso -2- 31 August 2011 two Ripon Christian School water supply wells (RC-1 and RC-2) approximately 2,520 feet northwest, City water supply well MW-10 approximately 4,270 feet northwest, and City water supply well MW-5 approximately 2,450 feet north. Nestle implemented the investigation in two phases: 1) an initial phase of cone penetrometer test (CPT) borings and push-boring groundwater sampling adjacent to monitoring well nest M-10 to identify the COC-contaminated strata and the COC concentrations within those strata; and 2) a follow-up phase of borings to characterize COG concentrations within these strata drilled along one southeast-northwest trending transect (A'-A on attached Figure 2 of the Report) and two east-west transects (B'-B and C'C, Figure 2). Transect A'-A begins at the former facility, passes through the M-1 OA location and terminates approximately 1,540 feet northwest of the former facility. Also, transect B'-B passes through the M-10 well nest. Transect C'-C passes through transect A'-A approximately 470 feet southeast of the nest and 300 feet northwest of the former facility. Attached Figures 4, 5, and 6 of the Report present geologic cross-sections of the previously-described A Zone along these transects. Central Valley Water Board Staff Comments Nestle Monitoring Well Nest M-10: The Report identifies three sandy units within the A Zone beneath the M-1 OA area: 1) the Upper A Zone between 50 and 80 feet below the ground surface (bgs), 2) the Middle A Zone between 80 and 95 feet bgs, and 3) the Lower A Zone between 100 and 115 feet bgs. The cross-sections on Figures 4, 5, and 6 show the three units as laterally continuous along transects A'-A, B'-B, and C'-C. Also, the figures show the lateral extent of COCs within the three units. However, the extent of CDCs remain unknown east, west, and northwest of each transect in the A Zone. Therefore, additional investigation work is necessary to complete the lateral characterization of COCs. Nestle Monitoring Well Nest M-13: Monitoring well nest M-13 is located approximately 600 feet west of transect C'-C, as shown on Figure 2. The geologic cross section beneath this transect, shown on Figure 6, extends from East Main Street at the Main Street overpass of U.S. Highway 99 (cone penetrometer test boring [CPT]-26]) west to an alley-way location between Stockton Avenue and South Walnut Avenue (CPT-23). Concentrations of TCE were detected in CPT-23 groundwater samples collected from all three sandy units of the A Zone. Central Valley Water Board staff note that TCE and cis-1,2-DCE were detected in April 2008 in monitoring well M-1 3A, and have been detected during all four subsequent quarterly sampling events conducted through the second quarter of 2011. Furthermore, the concentration trends for both COCs are increasing in M-1 3A. The attached graph shows the TCE and cis-1 ,2-DCE concentration trends between April 2004 and June 2011. The well is screened between 120 and 130 feet bgs. It is likely that TCE has migrated from the former facility within sandy units of the A Zone, and possibly the C-Zone, to well M-1 3A. The well screen is at the approximate boundary of the A and C Zones. Additional A and C Zone wells are needed at this location to evaluate the extent to which TCE has migrated. Also, an additional investigation soil boring drilled between CPT-23 and monitoring well nest M-13 is needed to improve the lateral characterization of TCE migration from the former facility in the A and C Zones. Nestle reduced the sampling frequency of M-1 3A from quarterly to annual in 2011. Because CDCs have recently migrated into this well and the concentrations are increasing, Nestle must