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GeosyntecO <br /> consultants <br /> sample and one Northern Landfill Area sample (2.83 and 3.39 pCi/L at SW-07/07B and SW-13, <br /> respectively). The initial ecological screening level equated to the lowest no observable effect <br /> concentration (NOEL) for aquatic receptors in the ECORISK Database v.4.1 (Los Alamos <br /> National Laboratory [LANL] 2017). However, the measured concentrations were well below the <br /> corresponding lowest low observable effect concentration (LOEC) for radium-228 of 27 pCi/L, <br /> also for aquatic receptors (LANL 2017). In addition, radium-228 concentrations are below the <br /> drinking water standard for combined radium-226/228 of 5 pCi/L (United States Environmental <br /> Protection Agency [USEPA] 2000). <br /> Gross beta and gross alpha emitters were detected in some samples collected from the Landfill <br /> Area (SW-06, SW-07/07B, SW-13, SW-13B; Appendix B). While there are no ecological <br /> screening levels for gross alpha or gross beta, the gross alpha maximum contaminant level for <br /> drinking water is 15 pCi/L and the gross beta USEPA trigger value is 50 pCi/L, both of which are <br /> well above measured concentrations in the Southern and Northern Landfill Areas. <br /> Monitoring data collected demonstrates that there is minimal frequency of exceedance for <br /> radionuclides in groundwater and surface water, a low magnitude of exceedance, as well as a low <br /> spatial distribution. Radionuclide concentrations are similar or below the range of concentrations <br /> detected in groundwater in the greater Stockton area, demonstrating a negligible risk as <br /> concentrations are consistent with background. Therefore,radionuclides are not considered a COC <br /> in groundwater within the Landfill area and are not recommended for groundwater sampling <br /> moving forward. <br /> RRI 2025 Groundwater Monitoring Work Plan 9 May 2025 <br />