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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 113 Outside of the IA excavation areas, the results of soil and groundwater sampling within Unit D indicated no concentrations of TPH in soil or groundwater above the PCLs, with one exception. A single soil sample collected from a depth of 15 feet bgs at well TM-MW-05 contained TPH-O at a concentration greater than the PCL (4,325 mg/kg); soil samples collected from depths of 5 and 18 feet bgs in this same location did not contain detectable TPH (Table 6-13). TPH was not detected in either of the groundwater samples collected from well TM-MW-5, which is screened across the depth interval of the impacted soil (Table 6-14). Those groundwater data indicate empirically that the detected soil TPH concentration is protective of groundwater in accordance with MTCA. Besides the UST 29/Latex Spill area, low-level total cPAHs (TEQ) exceedances were detected above the PCL in groundwater collected from two monitoring wells within Unit D: up to 0.033 µg/L at TM-MW-01 and up to 0.019 µg/L at TM-MW-02 (Figure 6-D3; Table 6-14). Total cPAHs (TMEQ) were detected at concentrations exceeding the PCL in soil samples collected across Unit D as shown on Figure 6-D3. Total cPAHs (TEQ) have not been detected above the PCL in any of the groundwater samples collected from the four wells located at the CN-B-2 excavation area, or downgradient of wells TM-MW-01 and TM-MW-02 where groundwater cPAH exceedances are detected, or in any of the 10 shoreline wells located within Unit D (Figure 6-D3). These data indicate that cPAH concentrations in groundwater above its PCL have not migrated to the shoreline in Unit D. 6.5.5.2 Metals Soil concentrations in exceedance of PCLs for arsenic, copper, lead, mercury, nickel, and zinc were detected at sampling locations scattered across Unit D. Lead and zinc were not detected in Unit D groundwater at concentrations exceeding the PCL. The metals detected in groundwater at concentrations above PCLs—arsenic, copper, lead, mercury, and nickel—are described below. 6.5.5.2.1 Arsenic Arsenic was detected in groundwater at concentrations greater than the PCL at three monitoring wells but was not detected above the PCL in groundwater at the Unit D shoreline wells or in intertidal porewater/seep samples (Figure 6-D5). Soil arsenic exceedances (up to 43 mg/kg) were detected at four locations within Unit D (Figure 6-D5). A statistical evaluation of all arsenic soil data from Unit D indicates a 95 percent UCL of 7.1 mg/kg, which is below the soil PCL of 20 mg/kg, with 3 percent of samples (4/147) exceeding the PCL, and no concentrations of arsenic that are greater than two times the PCL. Based on these results, Unit D arsenic soil concentrations are in compliance with the PCL. 6.5.5.2.2 Copper There are numerous locations with detected copper exceedances in Unit D soil and groundwater, but there is little if any correlation in the locations of the exceedances for soil versus groundwater (Figure 6-D6).