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Geosynte& consultants monitoring report will include a description of the groundwater monitoring network at RRI following the completion of the 2025 well maintenance activities. If data gaps in the monitoring network outside of the Landfill Area are identified, then additional well installations will be proposed in the 2026 groundwater monitoring work plan. 3.3 Sampling and Analysis Wells sampled for VOCs will be analyzed by USEPA Method 8260B (Table 4). Wells sampled for TPH gasoline will be analyzed by USEPA Method 8260B and TPH diesel and motor oil will be analyzed by USEPA Method 8015B with silica gel cleanup (Table 4). As noted in Section 1.5, preparation of an RDFS was recommended for Site 4 and the Site 49 Central Area following determination of whether concentrations of barium in groundwater are representative of background. Wells W-020 (Site 4), W-304 (Site 49 Central Area), and W-336 (Site 49 Central Area) will additionally be analyzed for the below biogeochemical parameters to support preparation of the RDFS. Biogeochemical data collected will be used to assess the feasibility of MNA and in situ treatment technologies for each operable unit: • Field parameters: oxygen reduction potential (ORP), pH, dissolved oxygen (DO), and specific conductance • Nitrate, nitrite, sulfate, and chloride by USEPA Method 300.0 • Ferrous iron and alkalinity by Standard Method(SM) 2320B • Total sulfide by SM 4500 • Manganese by USEPA Method 6010B • Total organic carbon by SM 5310B • Methane, ethane, ethene, and carbon dioxide by USEPA Method RSK-175 • Functional gene (i.e., vcrA,bvcA, and tceA) enumeration by SiREM Lab Gene-Trac® 3.3.1 Field Methods and Procedures Field work will be performed under the supervision of a California licensed professional geologist or engineer in accordance with the SOP for low-flow sampling provided in Appendix C and the Health and Safety Plan(HASP)provided in Appendix D. Confluence Environmental of Rio Linda, California, will perform groundwater sample collection under the direction of Geosyntec using bladder pumps. The groundwater samples will be collected at a mid-screen depth in each well. Within each OU, groundwater sampling will be conducted starting with the wells that have had the lowest VOC concentrations, with progressive sampling of the wells with higher VOC concentrations. Prior to and after each use, reusable sampling equipment will be properly decontaminated as described in the SOP in Appendix C. Quality assurance/quality control samples will be collected in accordance with the Quality Assurance Project Plan provided in Appendix E. The QR codes installed in 2024 will be used during the sampling event. The QR codes allow for the field technician to access well identification and construction details as well as well inspection, RRI 2025 Groundwater Monitoring Work Plan 14 May 2025