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Technical Memorandum April 7, 2017 Page 2 The area geology and hydrogeology have been delineated into four groundwater zones — shallow zone (to 75 feet below ground surface (ft-bgs)), upper intermediate (75 to 100 ft-bgs) and lower intermediate zones (100 to 125 ft-bgs), deep zone (125 to 175 ft-bgs), and the deeper zone (>175 ft-bgs). Groundwater is encountered at approximately 50 feet ft-bgs and fluctuates throughout the year due to changes in precipitation, recharge, leakage from the Mokelumne River, and pumping by municipal and agricultural irrigation wells within the City and the region. Groundwater flow directions are typically to the south-southeast in the shallow, intermediate, and deep groundwater zones, and to the southeast in the deeper groundwater zone. The LCP source area extraction well EW-06 was shut down on August 19, 2014. Monthly monitoring indicates that contaminant concentrations have remained stable in the source area. Based on recent groundwater monitoring, the southern extent of the LCP extends to Mission Street and Stockton Street, where monitoring well MW-23B is located. PCE concentration at MW-25B, which is an upper intermediate zone well screened from 86 to 96 ft-bgs, is similar to the historical PCE concentrations observed at the shallow zone monitoring well G-13, which is screened from 63 to 68 ft-bgs. This would suggest that PCE has migrated from the shallower to the deeper zones. Additionally, PCE concentrations below the California Primary MCL have also been historically detected in City well 06R. Figure 1 shows PCE concentrations at the upper intermediate zone monitoring wells (except for wells G-13 and G-17A which are shallow zone monitoring wells) and the extent of the LCP in the intermediate zone for November 2017. As shown on Figure 1, the higher PCE concentrations in the plume, as represented by the 500 micrograms per liter (1,1,g/L) contour, extend south of Tokay Street between Church Street and School Street. The City, as part of its ongoing remediation efforts for the LCP, evaluated the placement of one extraction well to remediate the higher PCE concentrations (represented by the 500 ilg/L contour) of the LCP in the upper intermediate zone. Based on analysis of the data and groundwater flow and transport modeling, one extraction well, EW-7, located on the Needham Elementary School property near the intersection of Church Street and West Tokay Street, pumping at a rate of 50 gallons per minute (gpm) and screened between 75 and 125 ft-bgs, was found to adequately capture and contain the migration of the LCP. The proposed location of EW-7 was approved by the Water Board in a letter dated March 9, 2015. EW-7 was installed in October 2016 along with construction of the groundwater extraction and treatment (GWET) system. However, during system startup activities, it was observed that EW-7 did not meet the required extraction rate to hydraulically contain and capture groundwater contamination in the LCP mid-plume area. Hence, it was determined that the City would identify potential locations for EW-7R to capture and mitigate the migration of the LCP PCE plume in this area. The selection of the optimum location for EW-7R is described below. Location Selection, Utility Clearance and Permitting To evaluate the suitability of potential EW-7R locations, piezocone penetration testing (CPT) and grab groundwater sampling was conducted to characterize subsurface stratigraphy and vertical contaminant concentrations at select locations. On January 30, 2017, Langan personnel mobilized to the site to meet with Lyman Chang and Andrew Richle of the City. Based on these discussions, two options for the preferred location of EW-7R were identified: one on W Tokay Street, between Church Street and S School Street, and the other on the corner of Spruce Street and S School Street. The two locations were marked for utilities by a private utility locator and Underground Services WEST YOST ASSOCIATES \ A213 \ 17-17-37 \ wp LPEDW-7RLocation \ 040517_2M