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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 81 leaching value. However, the soil PCL for total cPAH (TMEQ) is based on protection of groundwater (with groundwater protective of discharge to the East Waterway), and following completion of the interim action, there are no groundwater cPAH exceedances in wells downgradient of these soil exceedance locations, nor anywhere in Unit A (Figure 6-A4). The groundwater data demonstrate empirically that the cPAH concentrations in Unit A soil, as long as the warehouse remains as an effective cap, appear to be protective of groundwater in accordance with MTCA, subject to future long-term groundwater monitoring. Naphthalene was detected in four Unit A soil samples at concentrations exceeding the soil PCL (Figure 6-A6; Table 6-7). Two of these soil samples were located on the IA excavation sidewall along the warehouse north wall, where inaccessible TPH-impacted soil remains in place. The soil PCL for naphthalene is based on groundwater protection. However, there are no naphthalene exceedances detected in groundwater from monitoring wells located within or downgradient of the Warehouse Subarea during four or eight rounds of monitoring following the IA soil excavation (Table 6-8). The groundwater data demonstrate empirically that the naphthalene concentrations in soil, as long as the warehouse remains as an effective cap, beneath the warehouse appear to be protective of groundwater in accordance with MTCA, subject to long-term groundwater monitoring. Although soil TPH concentrations exceed soil PCLs, groundwater TPH concentrations in monitoring wells beneath, adjacent, and downgradient of the warehouse do not have corresponding groundwater TPH exceedances with one exception:  Along the south edge of the Bunker C ASTs Area IA excavation, the first groundwater sample collected from well BCT-MW-103 following the IA contained TPH-G at a concentration of 1,100 µg/l, which slightly exceeds the PCL of 1,000 µg/l. Detected TPH-G concentrations in the subsequent seven samples collected from that well were well below the PCL. This groundwater data suggests that the initial exceedance is likely attributable to hydrocarbons mobilized during the large-scale excavation that preceded the first round of confirmational groundwater sampling. The current groundwater TPH data appear to indicate empirically that the soil TPH concentrations in the Warehouse Subarea, as long as the warehouse remains as an effective cap, are protective of groundwater in accordance with MTCA (WAC 173-340-747(9)). Any additional groundwater monitoring requirements for this area will be incorporated into the future long-term groundwater monitoring plan for the Site. 6.5.1.1.2 Warehouse Subarea Vapor Intrusion Assessment The data collected as part of the VI assessment in the warehouse is provided on Tables 6-15a–6-15d. Tables 6-15a, 6-15b, and 6-15c present the sub-slab soil gas and indoor air data compared to industrial, commercial, and unrestricted screening levels, respectively. Hydrocarbon concentrations detected during both rounds of indoor air sampling within the ground floor of the Warehouse meet MTCA Method C indoor air PCLs for industrial use (Table 6-15a). Hydrocarbon concentrations detected in samples of sub-slab air collected below the floor slab also meet screening levels for industrial use.