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Geosyntec consultants 3. RATIONALE, OBJECTIVES, AND APPROACH The rationale, objectives,assumptions, and considerations for the activities proposed in this Work Plan are detailed below. 3.1 Rationale In response to the CVRWQCB request (CVRWQCB, 2021), a combination of SVE, ISCO, and excavation were evaluated as potentially applicable technologies to reduce the PCE concentrations in shallow soil vapor near SG-100 to the commercial CHHSLs (603 gg/m3) cleanup goal. Considering the historical effectiveness of SVE in the area, the limitations of each of the technologies considered as criteria for use, implementability with minimal disruption to Site activities, and the presence of only low-level residual impacts. Based on the analysis of alternatives, shallow SVE was determined to be the most effective technology in addressing soil vapor impacts and was selected as the preferred alternative. ISCO will not address current shallow soil vapor impacts and is generally ineffective for addressing residual mass sorbed onto low permeability soils unless high pressure injections are conducted, which may be impracticable due to nearby stormwater and sanitary sewer infrastructure. Excavation will not address soil vapor impacts and would be impracticable for the slightly deeper soil vapor impacts. As described previously (Geosyntec, 2021b), slow back-diffusion of residual PCE in low permeability soils volatilizing into higher-permeability zones over time is the likely cause of the elevated PCE concentrations observed near SG-100 after cleanup goals were achieved in 2013. It is expected that the SVE will rapidly reduce current soil vapor concentrations and result in asymptotic conditions beyond which follow-up soil vapor recovery efforts may not be cost- effective, at least in the short-term. As is the case with typical SVE operations assessing rebound conditions, follow-up SVE efforts are implemented after allowing time for soil vapor concentrations to rebound through back-diffusion from lower permeability soils. This cycle is repeated as reasonable and practicable to facilitate mass removal. It has been well demonstrated that the rate of back-diffusion of soil vapor is dependent on volatile organic compound (VOC) mass sorbed onto lower permeability soils. With multiple cycles of SVE,the rate of back-diffusion is expected to decrease, therefore resulting in longer durations for soil vapor concentrations to accumulate to levels that may warrant subsequent SVE efforts. Because only a small residual mass of PCE is expected to remain sorbed to the lower permeability soils near SG-100, the proposed SVE application for this area may be limited by the rate of back-diffusion after a few applications and may be technically impracticable thereafter. Data collected during and after the SVE activities proposed in this Work Plan will allow for long-term performance monitoring and to determine whether follow-up SVE will be practicable. To address the capillary fringe and residual groundwater impacts at MW-007A,ISCO was selected as the preferred alternative considering its effectiveness at the Former Source Area near monitoring wells MW-006A and PT-11,south of Benjamin Holt Drive(Geosyntec,2020). While the proposed SVE activities are intended to address shallower soil vapor concentrations, the interconnected hydrostratigraphy of subsurface soils near SG-100 may allow SVE to extract residual mass sorbed to the capillary fringe and potentially decrease groundwater concentrations. While ISCO may eventually be necessary to address residual groundwater impacts, the extent of ISCO injections may be limited or focused on the remaining impacts if SVE is successful in addressing capillary Northern Parking Lot Remedial Plan 3 August 6,2021