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Vapor Intrusion Assessment Work Plan Project number:60653972 milligrams per cubic meter(mg/m3)for 1,1-DCE in four samples, 0.92 mg/m3 TCE in one sample,and less than 0.03 and 0.38 mg/m3 for PCE and toluene,respectively.After achieving low and asymptotic concentrations of VOCs in the four vapor extraction wells and the confirmation soil vapor sampling, soil closure was achieved in 2001 for no further soil action(Parsons 2001), and the SVE treatment system was disassembled in December 2001. Because GWETS VOC mass removal resulted in decreasing the mass from over 20 pounds per year in 2000 to 0.8 pounds per year in 2015 (Parsons 2016a),Parsons submitted a Proposal for MNA Study at Applied Aerospace Structures Corporation Stockton, California recommending that a MNA/rebound study be conducted(Parsons 2013). On January 8,2014, CVRWQCB responded and indicated that TCE remained above WQOs in the source area and requested to conduct pumping rate optimization to determine whether the VOC recovery could be enhanced by decreasing the pump rate. If enhanced TCE removal could not be achieved by decreasing the pump rate, CVRWQCB would consider the MNA study proposal.In 2016,Parsons submitted the Optimization for Groundwater Pumping,which concluded that VOC recovery could not be enhanced by decreasing pumping rates(Parson 2016b). In May 2016, an In Situ Chemical Oxidation(ISCO)Work Plan(AECOM Technical Services,Inc. [AECOM] 2016a)using sodium persulfate catalyzed by iron-ethylenediaminetetraacetic acid(Fe-EDTA) was submitted for CVRWQCB's review to treat the residual groundwater VOCs in the source or remaining areas with relatively high concentrations. The purpose of ISCO treatment was to accelerate the reduction of the residual groundwater VOC mass prior to transit to naturally occurring processes to attenuate any remaining groundwater VOC contamination.AECOM received CVRWQCB's review comments on the ISCO Work Plan in February 2017 and submitted responses on November 3,2017. With CVRWQCB's approval,the GWETS operation was suspended on December 20, 2016,to allow the groundwater level to recover in preparation for ISCO injection. The scope of work for the groundwater monitoring event in July 2017 was modified to address several of CVRWQCB's review comments on the ISCO Work Plan.This additional sampling included the collection of baseline metals and total dissolved solids samples to ensure compliance with the requirements of the Waste Discharge Requirements Permit under the General Order for In-Situ Groundwater Remediation And Discharge of Treated Groundwater to Land(CVRWQCB Order No.R5-2015-0012). The baseline evaluation results,and proposed path forward were submitted as part ofAECOM's response to CVRWQCB's ISCO Work Plan comments on November 3,2017. Due to the observation of consistent VOC concentrations detected and the lack of plume migration based on the July 2017 sampling results(AECOM 2017), CVRWQCB agreed to keep the GWETS offline while collecting more post-GWETS data in order to evaluate another remedial alternative. For example, a round of MNA parameter sampling was performed in August 2018 to assess whether MNA was ongoing, while multiple rounds of Site-wide VOC sampling were performed to confirm the stabilized concentration trends.Based on the review of the post-GWETS data, CVRWQCB recommended during a January 10, 2019 conference call with AECOM that a Proposed Remedial Alternative Work Plan,which may include MNA and a contingency plan,be submitted to replace the current ineffective GWETS in order to continue Site remediation. Subsequently,AECOM submitted the Proposed Remedial Alternative Work Plan(dated March 15, 2019)for the continuation of Site remediation. CVRWQCB provided review comments on the Proposed Remedial Alternative Work Plan on July 1,2019, and AECOM submitted the Revised Proposed Remedial Alternative Work Plan(AECOM 2019a)to incorporate CVRWQCB's review comments. Subsequently, CVRWQCB issued MRP No.R5-2019-0906(dated November 6,2019)to request MNA to start during the 3rd Quarter 2019. In the Annual Monitored Natural Attenuation Report 4th Quarter 2021 —3rd Quarter 2022 prepared by AECOM groundwater sampling results indicated generally stable VOC concentrations in sampled wells, with only small concentration increases in MW-13,MW-17,and GW-2R(AECOM, 2022).Upon review of the report, CVRWQCB indicated further soil gas investigation up to the current regulatory standard Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM