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0 Groundwater Monitoring ARCADIS Report for the Semiannual Period from January 1 through June 30,2012 Lincoln Center,Stockton, California Thirty-seven of the 58 monitoring wells are completed in the A-zone aquifer,which is encountered between approximately 30 feet and 80 feet below ground surface(bgs). Eighteen of the 58 monitoring wells are completed in the B-zone aquifer,which is encountered between approximately 90 feet and 140 feet bgs; and three of the 58 monitoring wells are completed In the C-zone aquifer,which is encountered between approximately 180 feet and 250 feet bgs. In addition, 19 piezometers completed in the A-zone and seven piezometers completed in the B-zone were monitored for water levels during the reporting period. 2.1 Groundwater Level Measurements On June 11 and 13,2012,depth to groundwater was measured at 58 monitoring wells and 26 piezometers. Groundwater elevations measured at the Site during the reporting period are summarized in Table 1. Figure 3 shows groundwater elevation contours for the A-zone aquifer based on water levels measured at the Site on June 11 and 13, 2012.These data indicate that shallow groundwater beneath the Site generally flows to the northeast,and that the magnitude of the horizontal hydraulic gradient in the A-zone aquifer was approximately 0.005 foot per foot(fVft).The flow direction is consistent with previous data collected at the Site.The groundwater elevation measured at well MW-102A was not representative of A-zone static groundwater level conditions at the Site during the June 2012 monitoring event, and thus was not used in contouring for this reporting period.Groundwater elevation data collected from MW-102A have historically been inconsistent with the data collected from nearby A-zone monitoring wells. One possible explanation far this discrepancy is that MWA 02A is constructed with a longer screen interval—approximately 30 feet in contrast to the 20-foot(MW-100A and Mw-011 A)and 25-foot(MW-101A)screen intervals used for neighboring wells. Figure 4 presents groundwater elevation contours for the B-zone aquifer based on water levels measured at the Site on June 11 and 13,2012.These data indicate that B-zone groundwater beneath the Site generally flows to the northeast,and that the magnitude of the horizontal hydraulic gradient in the B-zone aquifer was approximately 0.0065 ftlft. The flow direction is consistent with data collected during previous monitoring events. The groundwater elevation measured in well MW-2038-1 was not representative of B-zone static groundwater level conditions at the Site during the June 2012 monitoring event,and thus was not used in contouring for this reporting period. MW-203B-1, MW-203B-2,and MW-2036-3 are co-located wells screened within different depth intervals. Historically,the groundwater elevation measured at this location has varied m,Wwln_GW gI2 EMN5750 cc 4