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

ASPECT CONSULTING 108 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 6.5.4.2.4 Mercury Prior to the second IA, concentrations of mercury up to 3.8 mg/kg were present in soil on the east side of the chip reclaim conveyor foundation (Aspect, 2019b). As discussed above, the second IA removed more than 400 tons of contaminated soil from the Log Pond Chip Conveyor Area. Four of 13 compliance soil samples collected at the limits of the second IA Log Pond Chip Conveyor area contained mercury above but less than two times the soil PCL of 0.1, with concentrations ranging from 0.11 mg/kg to 0.17 mg/kg (Table 6-9). Similar to copper, of the 17 locations where mercury is reported in soil at concentrations above the PCL, five contain exceedances only in the sawdust layer. However, mercury is not present at concentrations above the groundwater PCL in groundwater samples collected from deep wells LP-MW-03, LP-MW-05, LP-MW-06, or LP-MW -07, empirically demonstrating that mercury is not leaching to groundwater from the sawdust layer at concentrations of concern (Table 6-10). Dissolved mercury exceeds the groundwater PCL of 0.025 µg/L in Log Pond wells LP-MW-01 and LP-MW-02, with concentrations ranging from 0.033 µg/L to 0.248 µg/L and positive correlations between elevated mercury concentrations and elevated groundwater pH (Table 6-10). Dissolved mercury concentrations in intertidal porewater samples from the PW-05 location were less than the PCL (Figure 6-C8; Table 6-22). 6.5.4.2.5 Nickel The only soil exceedance for nickel in Unit C is the 48.5 mg/kg detection at the HW-MW-01 location, relative to the 48 mg/kg PCL based on natural background concentrations. A statistical evaluation of all nickel soil data from Unit C indicates a 95 percent UCL of 27 mg/kg, which is below the soil PCL of 48 mg/kg, with 1.1 percent of samples (1/93) exceeding the PCL, and no samples with concentrations greater than two times the PCL. Nickel soil concentrations within Unit C are in compliance with the PCL. Groundwater nickel exceedances occur in the same wells as the other groundwater metals exceedances occur including shoreline wells MW-06 and LP-MW-02 (Figure 6-C9). Nickel exceedances were inconsistently detected at all Log Pond Area wells and were only detected above the PCL during a single sampling event in the two shoreline wells (Table 6-10). Dissolved nickel concentrations in intertidal porewater samples from the PW-5 location were less than the PCL (Figure 6-C9; Table 6-22). Shoreline groundwater in Unit C is generally not as aerobic (less than 1 mg/L on average) as measured at multiple shoreline wells in Units A and B (Table 6-11). The low D.O. is a further indication of the lack of tidal exchange with shoreline groundwater zone within the Log Pond (discussed in Section 2.4.4). These data are consistent with the hypothesis for higher nickel concentrations present in aerobic shoreline groundwater in Units A and B: that phenomenon is not occurring in the largely anaerobic shoreline groundwater within the Log Pond Area of Unit C. 6.5.4.3 PCBs Concentrations of total PCB Aroclors exceed the saturated soil PCL at six locations within the Log Pond (Figure 6-C12). At five of these locations, the PCB exceedances are reported