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Geosyntec consultants consistently increasing trend), another round of SVE will be implemented. Alternatively, if the first two performance monitoring events show low or decreasing concentrations near or below the CHHSL, suggesting a low rebound rate, the third monitoring event will be deferred. If the soil vapor concentrations continue to be low and stable(below or near the CHHSL)after the first three performance monitoring events, additional confirmation samples may be collected on an extended schedule to confirm the trend. With continued mass removal, it is expected that the durations between subsequent SVE efforts may be increasingly longer and limited by the rate of back- diffusion. Because the duration between individual performance monitoring events and subsequent SVE events is subject to change and may need to be adapted based on the results of the preceding sampling event(s). The Trust will routinely confer with the CVRWQCB to discuss any proposed changes in the approach, sample collection or further SVE events. Geosyntec believes that this approach of intermittent SVE operation is the most effective way to remediate soil vapor, achieve back-diffusion limited mass removal, and potentially determine the point at which SVE may be technically infeasible for further cleanup of VOCs at the Site. ISCO Approach 3.4 ISCO APPROACH The conceptual site model and groundwater monitoring data suggests the potential presence of residual PCE mass sorbed to the historic capillary fringe near MW-007A that may be presenting an ongoing source to groundwater as demonstrated by stable PCE concentrations at MW-007A and MW-009A (Geosyntec, 2021a; Geosyntec, 2021c). Therefore, in addition to the implementation of SVE for shallow soil vapor,Geosyntec proposes to conduct ISCO in the vicinity of MW-007A to address residual PCE impacts in groundwater. It is anticipated that three or four direct-push injection points will be installed near MW-007A and injected with 5 to 10 percent of KMn04 solution. The injections will target depths ranging from approximately 40 feet bgs (capillary fringe) to 70 feet bgs (within the saturated zone). Each location will receive between 2,500 and 7,500 gallons of the KMn04 solution. With groundwater flow to the east, residual KMn04 and an influx of cleaner water from upgradient is expected to address the residual concentrations at MW-009A. The scope of the ISCO activities is not certain at this point and will need to be refined based on the results of SVE performance monitoring and groundwater monitoring data. Implementation of SVE in the deeper well (10-20 feet bgs) may contribute to a consistent reduction in groundwater concentrations at MW-007A. In this case,the ISCO scope of work may be modified or eliminated. Alternatively, if SVE and groundwater data suggests no effect of SVE on the capillary fringe or groundwater concentrations, ISCO will be conducted to target the capillary fringe and groundwater to reduce PCE concentrations to the extent practicable. Northern Parking Lot Remedial Plan 5 August 6,2021