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Geosyntec° consultants • The extent of PFAS and vertical extent of VOCs below 90 feet bgs are Navy-retained responsibilities per the ESCA, and therefore those data gaps are not being addressed by the Port. • The RRI Terminal rail line construction project has commenced in the second quarter of 2025 and is estimated to continue over approximately two years, so the new wells in the Landfill Area will not be installed this year. As reported in the 2024 GMR, Geosyntec recommends preparing an RI/FS for Site 4 and the Site 49 Central Area following determination if barium is naturally occurring. Groundwater data collected per this Work Plan for Site 4 and Site 49 Central Area will be utilized to support the preparation of the RDFS. Specifically,biogeochemical data collected and continued concentration trend monitoring of COCs will help determine if MNA is a suitable treatment method for Site 4 and Site 49 Central Area OUs. Continued concentration trend monitoring of COCs may be reduced or modified at Site 4 and Site 49 Central Area OUs for future monitoring events depending on the recommendations of the RI/FS. 1.5.1 Radionuclide Monitoring in the Landfill Area As reported in the 2024 GMR, radionuclides are no longer considered a groundwater COC in the Landfill Area, and therefore additional sampling is not proposed. Per the Consent Agreement, radium is only considered a COC in the Landfill Area at IAS-05;however,review of data collected by the Navy in the late 1990s demonstrated that concentrations of radium were not detected in groundwater above screening criteria and therefore radium was recommended to be removed as a COC at IAS-05 (Geosyntec 2018b). In 2019, the CVRWQCB and DTSC requested that radionuclides be included as a COPC for IAS-03/-04 as the Navy identified the site as a potential disposal area for material containing radium-226 (PRC 1994). In response, Geosyntec initiated monitoring for radionuclides, including radium-226, radium-228, and gross radioactivity (alpha and beta) at IAS-03/-04 as well as IAS-16, which is hydraulically downgradient of IAS-03/04, in 2019. Since this date, four years' worth of radionuclide monitoring data has been collected. In 2022 through 2024, potassium-40 was also analyzed to assess radioactivity. Radionuclides, being radium-226, radium-228, and gross radioactivity, were not detected above the California maximum contaminant level (MCL) from monitoring data collected between 2019 through 2024 with one exception. Gross beta was reported over the MCL in one monitoring well (W-047) during one sampling event (2019); the sample collected from the well the subsequent year was below the MCL. The concentration of gross beta reported from the well was minimally over the MCL of 50 picocuries per liter (pCi/L) at a concentration of 52.9 pCi/L (56.3 pCi/L detected in the sample duplicate) (Geosyntec 2025). Review of available groundwater data in the greater Stockton area from the Groundwater information System(GAMA)(California State Water Resources Control Board and United States Geological Survey) demonstrates the concentrations of gross alpha,gross beta,radium-226, and radium-228 detected in groundwater at RRI are similar and/or below the greater Stockton area concentrations. Radionuclides have also been evaluated in surface water to better understand the magnitude and extent of radionuclides within the Landfill Area. Radium-228 was detected in two samples collected above the initial ecological screening level (2.7 pCi/L), one Southern Landfill Area RRI 2025 Groundwater Monitoring Work Plan 8 May 2025