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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 109 in samples collected from the sawdust layer at depths of 31 to 49 feet bgs (Table 6-18). The highest detected concentration of PCB Aroclors in soil in the Former Log Pond is 5.4 mg/kg, in a sample collected at a depth of 23 feet bgs from boring LP-B-06; the sample collected at a depth of 31 feet from this boring had no detectable PCBs (Table 6-18). All other soil PCL exceedances are at or below 0.6 mg/kg. There is no PCB Aroclor groundwater data for the Former Log Pond; however, concentrations of total PCB Congeners are above the PCL in groundwater samples collected from four of eight wells (Table 6-20). The total PCB congener exceedances are in both shallow (LP-MW-01 and LP-MW-04) and deep wells (LP-MW-03 and LP-MW-06). Concentrations of total PCB congeners were not reported above the PCL in the groundwater samples collected from the shoreline wells (Figure 6-C12). 6.5.4.4 Other Constituents PCP was detected at a concentration of 1.7 µg/L in a groundwater sample collected from well LP-MW-01, which is just slightly above the PCL of 0.5 µg/L, (Table 6-17). PCP has not been detected above the reporting limit in any of the 46 soil samples collected from the Log Pond Area (Table 6-16). PCP has not been detected above reporting limits in groundwater samples collected from shallow well MW-06, located downgradient of well LP-MW-01 and along the shoreline (Figure 6-C1). Vinyl chloride has been detected in groundwater at two Log Pond Area wells (LP-MW-04 and LP-MW-06; Figure 6-C12) at concentrations exceeding the 0.2 µg/L PCL, ranging from 0.7 to 0.96 µg/L (Table 6-17). Monitoring wells LP-MW-04 and LP-MW-06 are located more than 500 feet from the shoreline and vinyl chloride has not been detected in any of the groundwater samples collected from downgradient wells (Table 6-17; Figure 6-C1). 6.5.4.5 Un-Ionized Ammonia Un-ionized ammonia (NH3) was detected at concentrations greater than the 0.035 mg/L PCL in one or more groundwater samples collected from each of the nine Log Pond Area monitoring wells (Table 6-12; Figure 6-S3). The elevated NH3 concentrations are likely attributable to a combined decomposition of the large quantities of wood chips stored at the surface for decades during mill operations and the buried woody debris. Within the inland portion of the Former Log Pond, the highest groundwater concentrations of NH3 (up to 13.0 mg/L) have been detected at inland well LP-MW-01. The high NH3 concentrations in the LP-MW-01 well are a result of high groundwater pH, which favors the un-ionized form of ammonia (refer to Appendix H), at the time of the measurements. Notably, the NH3 concentration detected in LP-MW-01 in July 2017 (0.89 mg/L; Table 6-12 was ten times lower than the average concentration measured in 2013–2014 (9.0 mg/L; Table 6-12). This suggests water quality improvement in the center of the Former Log Pond area during the five years following removal of the wood chips from there. Substantially lower concentrations of NH3 were detected at the shoreline wells within the Log Pond Area, but they all exceeded the PCL (0.36 mg/L or less at wells LP-MW-02, MW-06, and REC6-MW-02; Table 6-12 and Figure 6-S3).