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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 95 6.5.2.2.3 Copper Copper concentrations that exceed the PCL of 36 mg/kg, which is based on natural background, are widespread within Unit B soil (Figure 6-B6). Historically, the highest concentrations of metals, including copper, were detected in soil in and around the BBH area that was removed as part of the second IA (Aspect, 2021). A statistical evaluation for all remaining copper soil data from Unit B indicates a 95 percent UCL of 22 mg/kg, which is below the PCL of 36 mg/kg, with 10.4 percent of the samples exceeding the PCL (34/328), and three samples that contain copper at more than two times the PCL. The MTCA compliance statistics requirements that take into account the natural background level for copper indicate that the frequency of exceedance can be 13 percent, and the magnitude of exceedance can be 3.6 times the PCL (130 mg/kg). The highest detected concentration of copper in Unit B is 99.2 mg/kg. Therefore, the results of the statistical analysis indicate that the copper data for Unit B soils are in compliance with the PCL since the 95 percent UCL is below the PCL, less than 13 percent of the samples exceed the PCL, and no samples contain copper at a concentration greater than 3.6 times the PCL. Groundwater copper concentrations exceeding the stringent 3.1 µg/L PCL are widespread throughout Unit B, as depicted on Figure 6-B6. The highest concentrations of copper in groundwater in Unit B are reported in well UST71-MW-103 (up to 66.5 µg/L when pH was 12.1) and PM-MW-04 (68.6 µg/L when pH was 11.6) (Table 6-10). As shown on Figure 6-B6, groundwater copper concentrations are below the PCL in shoreline well PM-MW-07 located downgradient of PM-MW-04. Groundwater copper concentrations at the REC3-MW-01/01R shoreline location, downgradient of well UST71-MW-103, were low, ranging from 0.3 to 4.95 µg/L in the original and replacement well (Table 6-10). No dissolved copper exceedances were identified in intertidal porewater samples from the PW-03 location downgradient of REC3-MW-01/01R (Figure 6-B6; Table 6-10) even though the maximum total concentration of copper at PW-03 exceeded its PCL by five times (15.8 ug/L). Likewise, no dissolved copper exceedances were detected in the PW-04 porewater sample location downgradient of shoreline wells in the UST 70 IA area where groundwater copper exceedances were detected (Figure 6-B6). 6.5.2.2.4 Lead Lead was detected above the PCL in one soil sample from the GF-B-13 location near the eastern (upgradient) side of Unit B (Table 6-9; Figure 6-B7) and inconsistently in groundwater at four wells (Table 6-10; Figure 6-B7). Lead concentrations are below the PCL in all groundwater samples collected from the shoreline wells and in the PW-03 and PW-04 intertidal porewater samples (Table 6-22). A statistical evaluation of all lead soil data from Unit B indicates a 95 percent UCL of 51 mg/kg for unsaturated soil and 17 mg/kg for saturated soil, which are below the soil PCLs of 1,000 mg/kg and 81 mg/kg, respectively, with less than 1 percent of samples (1/247) exceeding the PCL. One concentration of lead in saturated soil in Unit B, reported at 115 mg/kg, exceeds two times the PCL (112 mg/kg). Unit B lead soil concentrations are in compliance with PCLs for unsaturated soil but not for saturated soil. However, groundwater data from wells positioned downgradient of the GF-B-13 location demonstrates empirically that lead is not leaching from saturated soil exceeding the PCL at those locations (Figure 6-B7).