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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 89 leaching from the OMS area soil at levels that result in exceedances of the groundwater PCLs, subject to long-term groundwater monitoring. 6.5.1.2.4 Un-Ionized Ammonia One sample of groundwater collected from Unit A contained a concentration of un-ionized ammonia slightly greater than the 0.035 mg/L PCL (0.068 mg/L in September 2012 sample from well REC1-MW-09); however, the concentrations of un-ionized ammonia detected in this well during the two subsequent sampling events were less than the PCL (Table 6-22). The highest detected concentration was less than the biological no observable effects concentration (NOEC) of 0.46 mg/L for un-ionized ammonia used in the Dredged Material Management Program (DMMP) (Kendall and Barton, 2004). The detected concentration is within the range of background concentrations for Puget Sound sediments (up to 0.2 mg/L; USGS, 2010). Detected concentrations of un-ionized ammonia were below the PCL in all samples from the other Unit A shoreline wells MW-01, MW-02, REC7-MW-03, and REC7-MW-04, and in samples collected from SEEP-01 (Table 6-12; Figure 6S-3). 6.5.1.3 Summary for Unit A Unit A Groundwater PCL exceedances are detected for copper, mercury, and nickel at scattered locations across Unit A. None of the exceeding metals are consistently detected in any well, and the magnitudes of exceedance are generally low. Notably, a low-level mercury exceedance was detected in only one groundwater sample across the entire unit, and not at a shoreline well. A marginal copper exceedance was detected inconsistently in groundwater samples from one of six shoreline monitoring wells, whereas nickel exceedances are detected consistently in groundwater samples from five of six shoreline monitoring wells. There are not elevated nickel concentrations detected in Unit A soil, and the detected groundwater nickel concentrations are higher in shoreline wells than in inland wells. The collective data suggest that the elevated shoreline groundwater nickel concentrations may be attributable to release of naturally occurring nickel from sulfide minerals interacting with oxygenated seawater within the aquifer’s tidal mixing zone. There are no source concentrations of these metals identified in Unit A soil. The highest concentration of total PCBs in soil in the Upland Area is located in a small volume of soil at the southeast corner of the OMS area IA excavation that could not practicably be removed during the second IA. Groundwater data collected from nearby historical OMS area well OMS-MW-02 did not identify detectable PCB concentrations indicating that, despite the small residual area with relatively high concentration in soil, the leaching to groundwater pathway may not be a concern for PCBs in Unit A, subject to long-term groundwater monitoring. A low-level exceedance of un-ionized ammonia is detected in one of 23 shoreline groundwater samples collected from Unit A, but the detected concentration was not reproduced in subsequent sampling. The existing analytical data for Unit A soil and groundwater provide sufficient information for the development and evaluation of cleanup action alternatives in accordance with MTCA.