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

ASPECT CONSULTING 104 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 Downgradient of this location, dissolved mercury concentrations in the sample collected from the Seep-03 location were below the PCL (Figure 6-C8; Table 6-22). 6.5.3.3 Un-ionized Ammonia Detected concentrations of un-ionized ammonia were below the PCL in both groundwater samples collected from shoreline well SHB-MW-2 and the intertidal Seep-03 sample (Table 6-12). 6.5.3.4 Sulfide Total dissolved sulfide was not detected in either groundwater sample collected from shoreline well SHB-MW-2 (Table 6-12). 6.5.3.5 Other Constituent Exceedances Vinyl chloride was detected above the PCL in groundwater collected from Unit C well SHB-MW-02 both times it was sampled for VOCs (Figure 6-C12). Vinyl chloride has otherwise not been reported above the PCL in any of the shoreline groundwater or intertidal porewater/seep samples (Table 6-22). The vinyl chloride groundwater PCL is based on protection of human health for exposure to groundwater discharging as surface water. Vinyl chloride in groundwater at well SHB-MW-02 is further discussed in the shoreline water quality section (Section 6.5.7). 6.5.3.6 Summary for Unit C Outside of Former Log Pond Within Unit C outside of the Former Log Pond, the IAs successfully removed TPH- contaminated soil from the SHB-MW-01 area to meet the soil PCLs for TPH and PAHs, and PCB-contaminated soil from the Hydraulic Barker area to meet the soil PCL for PCBs. The Hydraulic Barker IA also removed TPH contamination, but the residual soil TPH concentration at the base of that excavation exceeds the 7,700 mg/kg area-specific PCL based on residual saturation. In addition, residual soil metals concentrations (copper, mercury, and zinc) at the limits of both excavation areas are greater than soil PCLs. Naphthalene in the area of well GF9-MW-03 does not pose a risk to the East Waterway and, based on the most recent sample data, no longer poses a risk with respect to VI. The sample of intertidal seepage along the Unit C shoreline contained concentrations of metals, PAHs, and NH3 less than respective PCLs based on surface water protection. The existing analytical data for Unit C outside of the Log Pond provide sufficient information for the development and evaluation of cleanup action alternatives in accordance with MTCA. 6.5.4 Log Pond Area of Unit C Investigation of the Log Pond Area of Unit C was performed in phases beginning in 2012 and ending as one of the last phases of work conducted for the RI in March through July 2017, as described in Section 4.2.7. Appendix K includes the geophysical survey report, test pit exploration logs and select photos of soil cores depicting subsurface conditions in the Log Pond Area. Explorations completed in 2017 (including the LP-B- explorations and wells LP-MW-03 through LP-MW-07; Figure 6-C1) were advanced through the entire thickness of fill in the former log pond to native estuarine/alluvial soil, which was