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

ASPECT CONSULTING 86 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 the PCL (108 mg/kg). The highest detected concentration of copper in Unit A is 87 mg/kg. The results of the statistical analysis indicate that the copper in Unit A soils are in compliance with the PCL since the 95 percent UCL of 18.mg/kg is below the PCL of 36 mg/kg, less than 10 percent of samples exceed the PCL, and no samples contain copper at a concentration greater than three times the PCL. The groundwater copper exceedances are scattered and generally low level, with all but two exceedances less than two times the stringent 3.1 µg/L PCL (Table 6-10). One groundwater sample collected from each of BCT-MW-106 and BCT-MW-107 contained dissolved copper at concentrations of 13.3 µg/l and 13.8 µg/l, respectively, which are more than four times higher than the PCL (Table 6-10). Only one out of six samples from one Unit A shoreline well marginally exceeded the copper PCL (4.35 µg/L in MW-01) (Figure 6-A7; Table 6-10). Dissolved copper concentrations were below the PCL in both samples collected from the SEEP-01 located within the intertidal shoreline (Table 6-22). Based on the spatial distribution of soil and groundwater exceedances for copper in Unit A, there are not clearly identifiable soil sources for the groundwater copper exceedances. For example, the highest soil copper concentrations (up to 265 mg/kg) were historically detected in the area of the Old Machine Shop (OMS-series explorations), but there are no groundwater copper exceedances detected historically in groundwater at either of wells OMS-MW-01 or OMS-MW-02 in that area. 6.5.1.2.2.2 MERCURY The soil mercury PCL based on leaching is the 0.1 mg/kg PQL, which is slightly above the 0.07 mg/kg natural background soil concentration (Table 5-3). Therefore, any detection of mercury in soil is an exceedance, and soil mercury exceedances are scattered across Unit A with no particular association to historical operations, as depicted on Figure 6-A9. A statistical evaluation of all mercury soil data from Unit A indicates a 95 percent UCL of 0.07 mg/kg, which is below the soil PCL of 0.1 mg/kg, with 10.3 percent of the samples exceeding the PCL (8/78), and one concentration that exceeds two times the PCL. Revised compliance statistics that take into account the natural background level for mercury indicate that the frequency of exceedance can be 15 percent, but the magnitude of exceedance remains at two times the PCL. Mercury was not detected above the laboratory reporting limit in 89.7 percent of the soil samples collected from Unit A and the 90th percentile of the population has a concentration of 0.07 mg/kg, which is below the soil PCL of 0.1 mg/kg and equal to natural background for the Puget Sound region. Consistent with the low mercury concentrations in soil, dissolved mercury was detected above the groundwater PCL in only one of 18 wells located in Unit A (well BCT-MW-105; Figure 6-A9), in only one of five samples collected from that well, and at a concentration (0.0255 µg/L) that only marginally exceeds the groundwater PCL of 0.025 µg/L (Table 6-10). No dissolved mercury exceedances were detected in wells downgradient of BCT-MW-105, or in the SEEP-01 samples (Table 6-22).