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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 85 Naphthalene was not detected above the groundwater PCL in groundwater in Unit A with one exception:  Along the north edge of the Bunker C ASTs Area IA excavation, the first groundwater sample collected from well BCT-MW-108 following the IA contained a naphthalene concentration (130 µg/L) exceeding the 89 µg/L VI-based PCL. Detected naphthalene concentrations in the subsequent seven samples collected from that well were below the PCL. This groundwater data suggests that the 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 beneath (as long as the warehouse remains as an effective cap) and west of the warehouse 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. 6.5.1.2.2 Metals Copper, mercury, and nickel were each detected in Unit A groundwater samples at concentrations greater than their respective PCLs (Figures 6-A7, 6-A9, and 6-A10, respectively; Table 6-9). During the Phase 2 ESA, prior to the first IA, groundwater lead exceedances were also detected in well MW-4 located within the Bunker C ASTs IA area. However, given the large quantities of groundwater pumped from that area during the first IA, those pre-IA groundwater data are no longer representative of current conditions. Lead was detected above the soil PCL based on groundwater protection in two saturated soil samples collected from boring GF-B-15A (Table 6-9; Figure 6-A8). Zinc was also detected in one or more Unit A soil samples at concentrations exceeding soil PCLs based on groundwater protection (Figure 6-A11; Table 6-9). However, lead and zinc exceedances are not detected in Unit A groundwater (Figure 6-A11; Table 6-10), indicating that lead and zinc concentrations in Unit A soil are protective of groundwater in accordance with MTCA. At each of the 10 wells with copper, mercury, and/or nickel exceedances in groundwater, the exceedances were not consistently replicated in repeated sampling. In fact, the exceedances were rarely detected in more than half the samples collected over time at a well (Table 6-10). The copper, mercury, and nickel data are described below. 6.5.1.2.2.1 COPPER Copper is present in soil above the PCL, which is based on natural background concentrations, at five locations within Unit A (Figure 6-A7; Table 6-9). A statistical evaluation of all copper soil data from Unit A32 indicates a 95 percent UCL of 18 mg/kg, which is below the PCL of 36 mg/kg, with 9 percent of the samples (7/78) exceeding the PCL. The compliance statistics requirements that take into account the natural background level for copper (Appendix I) indicate that the magnitude of exceedance can be three times 32 This includes the Warehouse Subarea