3815_PGSF WMVD Pkg 2_Vol 4-2_03.03.2026_BXWA_Certified

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 97 the shoreline. In particular, the groundwater at shoreline well RCD-MW-101 is oxygenated (measured D.O. of 5.4 to 11.1 mg/L; Table 6-11). This contrasts with the anaerobic (reducing) groundwater in the dredge fill throughout most of the Upland Area and is indicative of seawater intrusion into the nearshore portion of the fill. Wells UST70-MW-02 and REC3-MW-01 have lower D.O. than RCD-MW-101, but some measured values at those wells (up to 6.7 mg/L and 2.8 mg/L, respectively) are higher than typically measured farther inland (Table 6-11). Although it is a hypothesis, as discussed for the Unit A nickel data, there is not another reason apparent to explain the elevated nickel concentrations in Unit B shoreline groundwater. 6.5.2.2.7 Zinc Soil zinc concentrations greater than PCLs (PCLs equal to and slightly above natural background concentrations for saturated and unsaturated soil, respectively) are scattered throughout Unit B (Figure 6-B10; Table 6-9). A statistical evaluation of all Unit B zinc soil concentrations indicates a 95 percent UCL of 64 mg/kg for unsaturated soil and 40 mg/kg for saturated soil, which are below the soil PCLs of 100 mg/kg and 85 mg/kg, respectively, with 4.2 percent of samples (12/288) exceeding the PCL, and no saturated soil samples with zinc concentrations that exceed two times the PCL. There are five unsaturated soil samples that contain zinc at concentrations greater than two times the PCL so unsaturated soils in Unit B are not in compliance with the soil PCLs for zinc. Although zinc concentrations in unsaturated soils statistically do not comply with the PCL, the Unit B groundwater data empirically demonstrate those soil concentrations are not a significant source to groundwater. Dissolved zinc concentrations exceeding the 81 µg/L groundwater PCL were detected in a single Unit B well (UST70-MW-02) during a single sampling event and were below the PCL in eight subsequent samples collected from that well (Table 6-10). Historically, the highest Unit B groundwater zinc concentrations were reported in well BBH-MW-104 located on the south edge of the Boiler/Baghouse first IA excavation. The second IA at the BBH area removed an additional 7,500 tons of metals-contaminated soil to address the soil leaching to groundwater pathway. However, even before the second IA, no groundwater zinc exceedances were detected at historical well BBH-MW-102 located downgradient of well BBH-MW-104 or the further downgradient REC3-MW-01/01R shoreline well location, indicating limited downgradient migration of dissolved zinc (Figure 6-B10). Furthermore, dissolved zinc concentrations were less than 1 µg/L in the PW-03 and PW-04 intertidal porewater locations (Table 6-22). 6.5.2.3 Polychlorinated Biphenyls (PCBs) The second IA included excavation and removal of approximately 6,700 tons of PCB- contaminated soil from the CMS area (Aspect, 2021). Concentrations of total PCBs in soil are below PCLs in Unit B except for three saturated soil samples collected at the limits of the second IA CMS area and the first IA DAST area (Figure 6-B11). The concentrations of total PCBs exceeding the saturated soil PCL of 0.12 mg/kg, based on the protection of groundwater, range from 0.2 mg/kg to 0.3 mg/kg (Table 6-18). Concentrations of PCBs were reported above the groundwater PCL in samples collected from historical wells CMS-MW-1R and DAST-MW-101 within the CMS excavation