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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 103 Groundwater arsenic concentrations exceeding the 9 µg/L PCL were detected in two of the GF-9 area wells and in well SHB-MW-101 (Figure 6-C5), at concentrations ranging from 10.6 µg/L to 15.1 µg/L (Table 6-10). Well GF9-MW-03 is located downgradient of GF9-MW-04, which is the only location within Unit C (outside of the Log Pond Area) with arsenic in soil at concentrations above the soil PCL (Figure 6-C5). Dissolved arsenic concentrations were less than the PCL in the sample from intertidal Seep-3 collected west of Unit C (Figure 6-C5) (Table 6-22). 6.5.3.2.2 Copper Similar to adjacent Units B and D, copper is present in soil and groundwater throughout Unit C, outside of the Log Pond Area, at concentrations exceeding the PCLs (Figure 6-C6). Historically, the highest copper concentrations in Unit C groundwater were reported at well HB-MW-1R (up to 152 µg/L). Although there was no soil metals data for HB-MW-1R, copper was reported up to concentrations of 143 mg/kg in soil samples collected from original well HB-MW-01 (Aspect, 2013a). That soil was removed as part of the second IA at the Hydraulic Barker area (Aspect, 2021). Seven soil samples analyzed for metals from the final excavation limits of the Hydraulic Barker area contained copper above the PCL of 36 mg/kg, based on natural background, with concentrations ranging from 39.6 mg/kg to 64.4 mg/kg, indicating that the relatively high concentration of copper that was likely acting as a source to groundwater was removed (Table 6-9). A statistical evaluation of all soil copper data from the Hydraulic Barker area indicated a 95 percent UCL of 41 mg/kg, which is just above the natural background-based PCL of 36 mg/kg, and no individual concentration is more than two times the PCL (Aspect, 2021). Outside of the Hydraulic Barker area, copper concentrations in groundwater exceed the PCL at four locations: two GF9- wells, SHB-MW-101 and SHB-MW-02 (Figure 6-C6). Of these locations, there are only single exceedances of the PCL except at well SHB-MW-101 where copper was detected above the PCLs in 3 of 9 groundwater samples (Table 6-10). Downgradient of this location, the dissolved copper concentration collected from the intertidal Seep-03 location were below the PCL (Figure 6-C6; Table 6-22). 6.5.3.2.3 Mercury As with arsenic and copper, the highest historical Unit C groundwater concentrations of mercury (up to 0.41 µg/L) were detected consistently in well HB-MW-1R prior to its removal in the second IA area. Five soil samples collected at the final limits of the second IA at the Hydraulic Barker area contained mercury exceeding the PCL with concentrations that range from 0.15 mg/kg to 0.34 mg/kg (Table 6-9). A statistical evaluation of all mercury soil data from the Hydraulic Barker area indicated a 95 percent UCL of 0.14 mg/kg, which is only slightly above the soil PCL of 0.1 mg/kg (Aspect, 2021). Wells GF9-MW-02 and SHB-MW-101 also had groundwater mercury exceedances (Table 6-10). Of these locations, there is only a single exceedance of the PCL at well GF9-MW-02 (Table 6-10). Mercury was detected above the PCL in 3 of 9 groundwater samples collected from well SHB-MW-101 (at concentrations less than 1 in 10 billion), but was not reported above the PCL in the last four sampling events (Table 6-10).