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

ASPECT CONSULTING 114 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 A statistical evaluation of Unit D soil copper concentrations indicates that the 95 percent UCL of 24 mg/kg is below the PCL of 36 mg/kg, 11.8 percent of samples (17/144) exceed the PCL, and two samples have copper concentrations that are more than two times the PCL. With revised compliance statistics that take into account the natural background level for copper, the frequency of exceedance can be 14 percent and the magnitude of exceedance can be 3.3 times the PCL (118 mg/kg). One soil result from Unit D contains copper at a concentration that is more than 3.3 times the PCL (166 mg/kg). However, a sample hypothesis test with a confidence of 95 percent, using the Wilcoxon- Mann-Whitney test, indicates no statistical difference between Ecology’s natural background data set for copper and the data set for Unit D. Considering these results, copper soil concentrations within Unit D achieve statistical compliance. On the north end of Unit D, shoreline wells MW-05 and NRP-MW-03 each had one or more copper exceedances in groundwater during the 2012 sampling events (up to 3.6 and 15.9 µg/L, respectively), but no exceedances were detected in either well during the 2013 and 2014 sampling events (Table 6-10). The concentrations of dissolved copper detected in intertidal porewater sample PW-07 and in seep samples Seep-10, -11, and -12, all located downgradient of the two shoreline wells with groundwater exceedances (MW-05 and NRP-MW-03), were less than the PCL (Table 6-22). 6.5.5.2.3 Mercury Mercury was detected exceeding the groundwater PCL of 0.025 µg/L in two Unit D locations (Table 6-10; Figure 6-D8). The highest reported concentration of mercury in groundwater in Unit D is at well UST-29-MW-101 where dissolved mercury was detected at a concentration of 0.199 µg/L. Mercury was detected in Unit D soil at concentrations exceeding the PCL at six locations (Figure 6-D8). A statistical evaluation of Unit D mercury soil concentrations indicates a 95 percent UCL of 0.06 mg/kg, which is below the PCL of 0.1 mg/kg, 4.1 percent of the samples (6/147) exceed the PCL, and there are two samples that contain mercury at more than two times the PCL. Based on this data, Unit D mercury concentrations do not comply statistically with the PCL. There is no apparent spatial correlation between mercury exceedances in soil versus in groundwater (Figure 6-D8). The highest concentration of mercury in soil in Unit D is located at well TM-MW-04, where groundwater data collected during three separate sampling events does not contain mercury above the groundwater PCLs (Table 6-10), demonstrating empirically that mercury is not leaching to groundwater at levels that result in exceedances of the PCLs. Mercury is below the PCLs in groundwater collected from shoreline monitoring wells, including well REC6-MW-02, which is in Unit C, but downgradient from well UST29-MW-101, or in intertidal porewater or seep samples (Figure 6-D8). 6.5.5.2.4 Nickel Three groundwater nickel exceedances were detected in Unit D groundwater, at inland wells TM-MW-02, TM-MW-04, and UST29-MW-101 (Figure 6-D9; Table 6-10). There is only one location within Unit D where soil nickel concentrations exceed the PCL, at boring TM-B-02 (Figure 6-D9). However, Unit D nickel concentrations do not statistically