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Geosyntec consultants 3. GROUNDWATER DATA GAPS The work conducted at RRI between 2018 and 2021 in accordance with the 2018 GMWP and addendums was designed to address the groundwater data gaps identified at RRI. This section describes outstanding data gaps that remain in the Site 3, Site 4, and Central Areas following completion of the data gaps investigation work through 2021. Data gaps related to the Landfill Area will be addressed following completion of the soil remedy and soda ash terminal construction activities; therefore, outstanding data gaps in the Landfill Area are not discussed in this 2022 GMAT. 3.1 Site 3 The monitoring wells located to the west of the former source areas at Site 3 were either unable to be located(W-102) or found to be damaged(W-025, W-211, W-096) during the well locating field activities conducted between 2018 and 2021 (Figure 4). As a result,the current cross- gradient lateral extent of VOCs in groundwater to the west of the former source areas at Site 3 remains a data gap. The grab-groundwater sample collected as part of the data gaps investigation at location GGW-25 in 2020 adjacent to the former burn pit from 85 to 90 feet bgs was reported with a VC concentration of 2.5 µg/L, which is above the MCL and DTSC Tap Water screening criteria. Per the ESCA, the Navy retains responsibility below 90 feet bgs at RRI; therefore,the vertical extent of VOCs at Site 3 is a remaining data gap that would need to be addressed by the Navy. The detection of dioxins and furans at Site 3 was inconsistent between the 2020 and 2021 monitoring events. In the 2020 monitoring event, dioxins and furans were detected in several wells where dioxins and furans were not detected in 2021. Due the inconsistency between the 2020 and 2021 groundwater sampling results,whether or not dioxins and furans are a COC in groundwater at Site 3 remains a data gap. Other groundwater data gaps previously identified for Site 3 have been addressed by the sampling conducted between 2018 and 2021. Metals were not identified as a potential COC for Site 3 in the Consent Agreement based on the historical site activities and historical Navy sampling at Site 3. Metals will be evaluated to determine if they are a groundwater COC at sites at RRI through the Work Plan to Evaluate Background Concentrations in Groundwater (Geosyntec, 2021 c). Once the three outstanding data gaps listed above have been addressed, the characterization of Site 3 groundwater will be adequate to proceed to the next step of a remedial investigation and feasibility study(RDFS). 3.2 Site 4 Grab-groundwater sampling conducted at Site 4 in 2019 defined the vertical and lateral extent of VOCs in groundwater. Following completion of the grab-groundwater sampling, a new monitoring well cluster(W-332 and W-333)was installed in 2020 to monitor the cross-gradient lateral and vertical extent of VOCs in groundwater to the west of the former source area. The concentrations of cis-1,2-DCE and VC have exceeded the MCLS or DTSC Tap Water screening criteria in the new monitoring well cluster. Therefore, monitoring of the lateral extent of VOCs in groundwater west of the former source area at Site 4 remains a data gap. RRI Groundwater Monitoring Work Plan 15 June 2022