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

ASPECT CONSULTING 110 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 Within the deep sawdust layer beneath the Former Log Pond, the 2017 groundwater NH3 concentrations at inland wells LP-MW-03, LP-MW-05, and LP-MW-06 ranged from 0.036 mg/L to 0.11 mg/L, and 0.36 mg/L NH3 was detected at shoreline well LP-MW-07. The elevated NH3 concentrations in these wells are attributable to decomposition of the sawdust. One of two 2014 samples of intertidal porewater collected from the PW-5 location contained a concentration of un-ionized ammonia (0.19 mg/L) exceeding the PCL (Table 6-12). However, NH3 concentrations were below the PCL in the three surface water samples LP-SW-01, LP-SW-02, and LP-SW-03 collected at the base of the rip rap along the Log Pond Area shoreline, which is consistent with the high dissolved oxygen content of the surface water. 6.5.4.6 Sulfide Groundwater H2S concentrations in the Log Pond Area shoreline wells and in most inland wells exceeded the 0.002 mg/L PCL (0.007 to 0.087 mg/L; Table 6-12). Shoreline well REC6-MW-02 has the highest detected H2S concentration (0.087 mg/L), consistent with the most strongly reducing groundwater conditions observed in this well. In contrast to NH3, H2S is not stable at pH values greater than approximately 8 (refer to Appendix H), and, therefore, the lowest reported H2S concentration occurs in high-pH well LP-MW-01 in the Log Pond interior (7 x 10-8 mg/L; Table 6-12). Concentrations of total dissolved sulfide, which can include mineral sulfides in colloidal form, were typically greater than H2S concentrations in the groundwater samples. This was particularly notable in the July 2017 sample from well REC6-MW-02 (16.2 mg/L total dissolved sulfide vs. 0.087 mg/L H2S; Table 6-12). One of two 2014 samples of intertidal porewater collected from the PW-05 location contained a H2S concentration (0.005 mg/L) slightly exceeding the PCL. Concentrations of H2S were below the PCL in surface samples LP-SW-01 and LP-SW-0336, which is consistent with the high dissolved oxygen content of the surface water (Table 6-12). 6.5.4.7 Summary for Log Pond Area of Unit C Within the Log Pond Area, hydrogeologic data indicate that groundwater has limited hydraulic connection with the East Waterway due to the low permeability of the log pond fill. Groundwater exceedances for the metals arsenic, copper, lead, mercury, and nickel are inconsistently detected at shoreline wells LP-MW-02, MW-06, and REC6-MW-02 (Figure 6-S2). Unlike elevated nickel concentrations observed in Units A and B shoreline wells, nickel concentrations in the Log Pond Area shoreline wells are low, consistent with low groundwater D.O. attributable to the groundwater’s limited tidal exchange. Total cPAHs (TEQ) in groundwater exceed the PCL at inland wells, but not at the shoreline wells or in the sample of intertidal seep discharge. It is noted that the reporting 36 Free sulfide data for the LP-SW-02 location could not be collected because the DGT cartridge was damaged during deployment (see Section 4.1.7).