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how destroyed approximately 59 pounds of TPHg. Oxygen injections were performed in the A, B, and B' Zones. 3. A Zone Historical Data Review GHD reviewed historical and current groundwater analytical data to determine if continued groundwater monitoring and sampling of A Zone wells is still appropriate. Based on analytical data from the most recent semi-annual groundwater monitoring and sampling event conducted during the first quarter 2020, two of the five A Zone wells contained 1,2-DCA concentrations above the water quality objective (WQO). Based on current 1,2-DCA concentrations, monitoring of A Zone wells will continue to further evaluate 1,2-DCA concentration trends and to define the extent of 1,2- DCA in the A Zone. Destruction of A Zone wells is not proposed at this time. Figure 4 shows current dissolved 1,2-DCA concentrations in the A Zone. 4. 1 ,2-DCA Groundwater Analytical Data During the May 13, 2020 meeting between GHD and CVRWQCB, CVRWQCB staff notified GHD of 1,2-DCA groundwater analytical data that is present in GeoTracker but not included in GHD's historical groundwater analytical data table. GHD conducted a file review of all groundwater monitoring and sampling reports available in GeoTracker and incorporated the 1,2-DCA analytical data into the historical data table. The updated historical groundwater analytical data are presented in Table 1 and will be included in future semi-annual groundwater reports. 5. Proposed Scope of Work As discussed during the May 13, 2020 meeting, GHD proposes to install three offsite groundwater monitoring wells to provide additional definition of dissolved petroleum hydrocarbon plumes in the B Zone and C Zone. Total petroleum hydrocarbons as gasoline (TPHg) and 1,2-DCA concentrations exceeding WQOs extend offsite to the north and northeast. Proposed well MW-23 will be installed to the northeast of the site in the City of Stockton's right-of-way, Sperry Avenue, to define the extent of 1,2-DCA in the B Zone. Proposed wells MW-25 and MW-26 will be co-located to the north of the site and installed in the C Zone and B Zone, respectively, to define the offsite extent of TPHg and 1,2-DCA. The locations of the proposed monitoring wells are shown on Figures 2 and 3. Groundwater elevation and hydrocarbon isoconcentration contours in the B Zone and C Zone from the most recent groundwater monitoring and sampling event conducting during the first quarter 2020 are presented on Figures 5 through 12. Given the proximity of the site to potential offsite residential receptors, the extent of hydrocarbon concentrations exceeding WQOs, and the known presence of water supply well"Well 10", GHD proposes to conduct additional work to identify other known water supply wells in the vicinity of the site and to confirm that nearby residences are on a water service. Further details of the proposed assessment and well survey work are discussed below. GHD I Well Installation Work Plan Addendum 111193145(5)I Page 3