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

ASPECT CONSULTING 96 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 6.5.2.2.5 Mercury As with copper, mercury concentrations that exceed the PCL of 0.1 mg/kg 34 are widespread within Unit B soil (Figure 6-B8). A statistical compliance evaluation for all mercury soil data from Unit B indicates a 95 percent UCL of 0.11 mg/kg, with more than 10 percent of the reported concentrations exceeding the PCL, and 14 samples that contain mercury at more than two times the PCL. The compliance statistics requirements that take into account the natural background level for mercury indicate that the frequency of exceedance can be 13 percent, but that the magnitude of exceedance is still two times the PCL. Because the 95 percent UCL is above the PCL, 21 percent of samples exceed the PCL, and 14 samples have mercury concentrations that are more than two times the PCL, Unit B mercury soil concentrations are not in compliance with the PCLs. Groundwater dissolved mercury concentrations above the stringent 0.025 µg/L PCL were detected in several wells within the central and western portions of Unit B, but there are no dissolved mercury exceedances detected at shoreline wells or the intertidal PW-03 and PW-04 porewater sample locations (Figure 6-B8; Tables 6-10 and 6-22). Inland from the shoreline, exceedances of mercury are sporadic, and the magnitude of exceedances are relatively small in most samples (Table 6-10). The highest concentration of mercury reported in Unit B groundwater is 0.286 µg/L at well PM-MW-04 when its pH had increased to 11.6 (Table 6-10; Figure 6-B8). 6.5.2.2.6 Nickel Dissolved nickel concentrations exceeding the 8.2 µg/L groundwater PCL were detected in seven Unit B wells, including four shoreline well locations (Figure 6-B9). However, soil nickel concentrations greater than the 48 mg/kg PCL (based on natural background) were detected in only two inland locations, Boiler-B-04 and REC5-HA-02 (Table 6-9). A statistical compliance evaluation for all nickel soil data in Unit B indicates a 95 percent UCL of 27 mg/kg, with less than 10 percent of samples exceeding the PCL, and no concentrations that exceed two times the PCL (96 mg/kg). Based on the results of this evaluation, Unit B nickel concentrations are in statistical compliance with the PCL. The highest concentrations of nickel detected in Upland Area groundwater are at Unit B shoreline wells RCD-MW-101 (6.9 to 159 µg/L, average 59 µg/L), UST70-MW-02 (2 to 308 µg/L, average 44 µg/L), and REC3-MW-01/01R (5 to 83 µg/L, average 37 µg/L) (Table 6-10). The wide range of detected concentrations at each of these wells may be attributable to geochemical influences related to nearshore seawater mixing. The most reliable data regarding dissolved nickel transport to the marine environment are the empirical dissolved nickel results from intertidal porewater samples PW-03 and PW- 04, collected in the intertidal zone immediately west of Unit B, which were consistently less than the PCL (Table 6-22; Figure 6-B9). As described for Unit A (Section 6.5.1.2.4), the lack of nickel sources in Unit B soil, and higher groundwater nickel occurring in shoreline wells than in inland wells, suggests that the elevated groundwater nickel concentrations in shoreline groundwater may be due to release of naturally occurring nickel in the presence of more oxidizing groundwater along 34 Based on natural background and adjusted for the practical quantitation limit (PQL).