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

ASPECT CONSULTING 88 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 Again, well REC7-MW-4 does not fit the pattern (average nickel in REC7-MW-4 is driven by one high detection; Table 6-10); however, there is not another reason apparent for the shoreline groundwater nickel exceedances based on the other Unit A soil and groundwater data. This phenomenon of higher dissolved nickel with higher D.O. is also observed in Unit B shoreline wells, as described below in Section 6.4.2.2.6. No dissolved nickel exceedances were detected in the SEEP-1 samples (Table 6-22). Contaminant transport modeling indicates that a groundwater nickel concentration of approximately 380 µg/L in Unit A shoreline monitoring wells is estimated to be protective of intertidal sediment and surface water quality (Table 6-22). All detected dissolved nickel concentrations in Unit A shoreline wells are considerably less than that value (Table 6-10). 6.5.1.2.3 Polychlorinated Biphenyls There are two Unit A exceedances of total PCBs in soil: a single sample collected from boring GF-B-15A, where total PCBs were reported at a concentration of 0.22 mg/kg in saturated soil, and a sidewall sample collected from second IA OMS area where total PCBs were reported at a concentration of 24 mg/kg, above the MTCA and federal EPA (40 C.F.R. 761.61) industrial land use soil standard of 10 mg/kg, in unsaturated soil (Table 6-18). The soil that contains total PCBs (which is composed entirely of Aroclor 1254) at the southeast corner of the OMS area excavation is located near the top of the steep shoreline bank of the off-loading dock slip and next to a 36-inch diameter wood stave pipe that is located within the upper portion of that bank. Based on consultation with Ecology during the second IA, the OMS area excavation was completed to its maximum practicable limits and further excavation was deemed not feasible without potentially compromising the stability of the bank and risking slope failure into the East Waterway (Aspect, 2021). There is limited groundwater data for PCBs in Unit A but a single sample collected from historical OMS area well OMS-MW-02 in 2015 did not contain PCBs above the laboratory reporting limits, indicating empirically that PCBs may not be MW-1 MW-2 OMS-MW-1 OMS-MW-2REC1-MW-4 REC1-MW-5 REC1-MW-6 REC1-MW-8 REC1-MW-9 REC1-MW-14 REC7-MW-3 REC7-MW-4 0 2 4 6 8 10 12 14 16 0 1 2 3 4 5 6 7Avg. Nickel in ug/LAvg. Dissolved Oxygen in mg/L Unit A Groundwater: Nickel vs. D.O. Shoreline Wells