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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 107 at MIE-SB-3) (Figure 6-C5; Table 6-9). As discussed in Section 6.5.3.2.1, arsenic soil concentrations within Unit C as a whole are in compliance with the PCL. In groundwater, arsenic is detected above the PCL in well HW-MW-01 at the Hazardous Waste Cage, and in shoreline well LP-MW-02 (Figure 6-C5). Arsenic was not detected exceeding the PCL in any of the other shoreline wells or in either of the porewater samples collected at PW-5 located offshore from well LP-MW-02. 6.5.4.2.2 Copper As observed in Unit C outside of the Log Pond and in other Site Units, dissolved copper concentrations exceeding the 3.1 µg/L groundwater PCL are widespread in shallow groundwater across the Log Pond area of Unit C (Figure 6-C6). The highest reported concentrations of copper in groundwater (up to 31.6 µg/L) are in well LP-MW-01 and correlate to high groundwater pH (up to pH 13; Table 6-10). The elevated groundwater pH at the well was likely a result of the locally higher water table saturating the lower portion of the CM that was in place in this area at the time of sampling as described in Section 6.3. At two of three shoreline wells, REC6-MW-02 and MW-06, concentrations of copper in groundwater exceeding the PCL were reported in samples collected in 2012 but four subsequent sampling events at each well did not identify copper above the PCL (Table 6-10). Dissolved copper was detected above the PCL at well LP-MW-02 during two of three sampling events but concentrations in intertidal porewater samples from the PW-5 location, located downgradient of well LP-MW-02, were less than the PCL (Figure 6-C6; Table 6-22). The highest concentrations of copper in soil are reported in the sawdust layer, which is located at depths greater than 30 feet bgs. Except for a single soil sample collected from fill soil at boring LP-B-06, the concentrations of copper reported above the soil PCL in the sawdust layer are all above 100 mg/kg (ranging from 105 mg/kg to 141 mg/kg), while the concentrations of copper reported in overlying fill soil are all below 100 mg/kg (ranging from 36.8 mg/kg to 81.9 mg/kg) (Table 6-9). However, copper was not reported above the groundwater PCL in samples collected from deep wells in the Log Pond (Table 6-10), which demonstrates empirically that copper is not leaching from the sawdust layer at concentrations that pose a risk to the East Waterway. 6.5.4.2.3 Lead Lead was detected in Unit C groundwater above the PCL in the Log Pond at shoreline well LP-MW-02 (Figure 6-C7), however, only one of three dissolved groundwater lead concentrations at well LP-MW-02 are above the groundwater PCL and that sample also had a high groundwater pH (Table 6-10). Lead was not detected in soil at well LP-MW-02 above the soil PCL of 81 mg/kg established for the protection of groundwater. The maximum soil detection of lead within the Log Pond Area (303 mg/kg) occurs at the HW-MW-01 location, where dissolved lead concentrations were less than the PCL in groundwater (Table 6-10); the groundwater data appear to indicate empirically that soil lead concentrations are protective of groundwater in accordance with MTCA. The intertidal porewater lead concentration detected at the PW-5 location was less than the PCL (Table 6-22).