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Geosyntec° consultants ■ The vertical extent of the VOC plume should be evaluated by the Navy. Analysis of a grab-groundwater sample GGW-25 from 85-90 feet bgs reported VOCs at concentrations above the drinking water screening levels. Per the ESCA, the Navy retains responsibility at RRI below 90 feet bgs. ■ The extent of PFAS in groundwater is being evaluated by the Navy;per the ESCA, this is a Navy-retained responsibility. 1.5 Objectives The objectives of the 2025 groundwater monitoring program are to continue to monitor concentration trends of COCs associated with each OU to support preparation of an RDFS. In addition, well maintenance items that were not able to be implemented in 2024 or require follow- on work, as described in the 2024 Annual Groundwater Monitoring Report (2024 GMR) (Geosyntec 2025), will be addressed. Those activities include the following: • Conduct well maintenance activities at Site 3, Site 4, Site 49 Central Area, and UST 817B Central Area, including resurveying the top of casing (TOC) elevation for monitoring wells with outdated survey data, well replacement, well redevelopment, well location activities, and other maintenance activities as needed (well repairs and well protection, etc.). • Collect data to assess biogeochemical conditions at W-020, W-304, and W-336 to continue to evaluate the feasibility of monitored natural attenuation(NINA) and in situ treatment technologies at Site 4 and Site 49 Central Area OUs. • Sample for VOCs at select wells at Site 4, Site 49 Central Area, UST 817B Central Area, Landfill Area, and Site 3 to confirm the downgradient and vertical extent of the VOC groundwater plumes. • Sample for TPH at select wells at Site 49 Central Area, UST 817B Central Area, and Landfill Area to evaluate concentration trends. The data gaps described in Section 1.4 will not be addressed in this 2025 GMWP as they are being assessed separately or are a Navy retained responsibility, as described below: • An evaluation of existing barium groundwater data is currently underway to determine if barium may be considered naturally occurring. This analysis will address the data gap at Site 4 and the Site 49 Central Area to determine if barium is a COC and therefore additional sampling is not proposed. A site-wide metals background analysis is also planned to be performed and therefore additional metals data will not be collected in the Landfill Area. • An investigation to address soil,groundwater,and soil vapor data gaps at the UST 817B Central Area was performed under a separate work plan, as noted above, and investigation data is being utilized to determine if the Site meets the criteria presented in the State Water Resources Control Board's Low-Threat Underground Storage Tank Case Closure Policy (LTCP; State Water Resources Control Board 2012) or if further investigation or remediation is warranted. RRI 2025 Groundwater Monitoring Work Plan 7 May 2025