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Geosyntec 3. RATIONALE, OBJECTIVES AND APPROACH This section summarizes the rationale, objectives, assumptions, and considerations for the proposed former source area ISCO injections. 3.1 Rationale The recent increases in COC concentrations observed in the A-zone wells in the former source area as described in Section 2.5 coincide with an increase in groundwater elevations following substantial precipitation events in the 4th Quarter 2016 and 1st Quarter 2017 (Geosyntec, 2018). The elevated COC concentrations have since remained high, with the higher groundwater elevations likely due to mobilization of residual COC mass in the historical capillary fringe (Geosyntec, 2018). The activities proposed in this Work Plan will target the lower vadose zone, the capillary fringe and groundwater in the A-zone beneath the former source area. Results from the 2014 ISCO Pilot Test (Geosyntec, 2014) indicated "appreciable COC degradation was achievable in the on-Site, shallow A-zone groundwater treatment zone". Furthermore, the continuous injection of KMnO4-dosed water as part of the ISCO recirculation system operation has resulted in two to four orders of magnitude reduction in COC concentrations in downgradient groundwater.Therefore,KMn04 injection has been demonstrated to be successful in addressing COC concentrations at the Site. Based on initial injection rates at the Pacific Avenue injection wells and experience at other remediation sites in the California Central Valley, flow rates of 10 gallons per minute (gpm) or more are anticipated to be achieved using direct-push injection in the A-zone and overlying vadose zone which has moderate to high permeability.A 10-foot ROI using the injection layout shown in Figure 4 is expected to be logistically efficient and provide adequate coverage of COC impacts during injection.Therefore,we propose ISCO injections at 24 locations as shown on Figure 4. 3.2 Objectives and Approach The primary objective of the proposed ISCO injections is to destroy residual COC mass underlying the former source area(near the capillary fi•inge)and reduce flux of mass from the capillary fringe into groundwater, thereby expediting the timeframe of Site cleanup. Based on the lateral and vertical extent of elevated PCE impacts described in Section 2.5, the injections are proposed to be implemented below the former source area along Benjamin Holt Drive extending from monitoring wells MW-006A and PT-I I and terminating upgradient ofMW- 130AU and CHMW-005A(Figure 4).The injections will target depths from approximately 15 feet bgs(in the capillary fringe)to 45 feet bgs(within the saturated upper A-zone aquifer). Because of their installation within the upper A-zone aquifer, the existing monitoring wells will be used for performance monitoring of the proposed injection program. Former source area injections are proposed to be implemented using direct-push technology. Direct-push technology allows injections in specific target intervals without the need for injection Fanner Source Area Injection Work Plan 6 August 08,2019