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Mr. Brattain October 19, 2020 Page 2 concentrations and the WDR monitoring program is intended to evaluate performance and the effectiveness of the in-situ chemical oxidation (ISCO) recirculation system (ISCO recirculation system) Final Remedy as described in the Remedial Design/Remedial Action (RD/RA) Work Plana. Considering that the ISCO injections are complete and the WDR-MRP program was not specific to the monitoring requirements for the current phase of remediation, the most efficient approach for modifying the monitoring program will likely include rescinding the current WDR- MRP and issuing a new WDR-MRP more applicable to the current phase and that allows modifications to the monitoring program as the remediation status at the Site evolves in the future. This request presents relevant background information, the decision-making process and discussion of data assessed in support of the proposed modifications to the groundwater monitoring programs and to destroy wells that are no longer in use. Background In accordance with the Federal Consent Decree issued by the Court, the Trust is responsible for implementing remedial action at the Site in accordance with the Site Response Plan and under the oversight of the CVRWQCB. The Trust maintains an aggressive schedule for implementing this work with a requirement to reach closure for the Site under the insurance coverage timeframe. As described in the RD/RA Work Plan, the Final Remedy for the Site consists of several phases of remediation to address Site constituents of concern(COCs)remaining in groundwater.The RD/RA Work Plan called for Phase 1 to operate for 18 months and consist of ISCO injection along Pacific Avenue with ongoing extraction along Inglewood Avenue. Phase 2 was also planned for 18 months and included ISCO injection transects across the downgradient plume. Phase 3 was planned for five years and consisted of clean water recirculation with injection along Pacific Avenue and extraction along Inglewood Avenue. Based on positive results in Phase 1, ISCO injections continued longer than originally planned in Phase 1. The Phase 1 ISCO injections into injection wells along Pacific Avenue continued for approximately two years between October 2016 and September 2018. Transition to Phase 2 commenced in September 2018 with an update of the groundwater model for the Site. The model showed that better mass removal would be accomplished by enhancing groundwater extraction at the downgradient plume margin and at mid-plume instead of the planned ISCO transect injections across the downgradient plume. In response to the model results, 3 Geosyntec.2015.Remedial Design/Remedial Action Work Plan,Lincoln Center,Stockton,California. 9 October engineers I scientists I innovators