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

ASPECT CONSULTING 72 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 2013. As discussed in Section 4.5, all the CM was removed from the Site in 2020, in accordance with the Plan of Operations for CM Removal (K-C, 2018c) that was prepared and then implemented in consultation with the SHD. Figure 6-P1 depicts the footprint of the CM when it was in place between 2013 and 2020. The groundwater quality data collected during the RI indicate that, following placement of the CM in spring of 2013, groundwater pH increased within portions of the Upland Area where the CM was present. Figures 6-P1 through 6-P6, respectively, present groundwater pH data collected across the Upland Area in November 2013, February 2014, February 2016, August 2016, March 2017, and September 2017. Figures 6P-7 and 6-P8, respectively, present the average groundwater pH from multiple sets of measurements conducted in 2017 and then throughout the CM removal action in 2020. During the CM removal action, groundwater pH monitoring was conducted in accordance with the Work Plan for the Second Interim Action (Aspect, 2019b), and the results are presented in the Completion Report for the Second Interim Action (Aspect, 2021). Most of the Upland Area monitoring wells located outside of the Warehouse were decommissioned prior to the CM removal, so there is no groundwater pH data from wells after 2020. There is some limited pH data from exploratory test pits excavated in 2021, which is discussed herein. Figure 6-P9 presents groundwater pH data collected in June 2021, approximately 8 months after completion of the CM removal action. Table 6-11 includes the Site groundwater data set for field parameters including pH collected between 2012 and 2017. Appendix G to this report includes a tabulation of the 2020 groundwater pH data. Figure 6-P2 shows that by February 2014, approximately 8 months after placement of the CM, groundwater pH had increased throughout much of the eastern half of the Upland Area and extending farther west in the Boilers area (area of BBH-, UST71-, and UST70- series wells) and the eastern half of the Log Pond (well LP-MW-1). The pH of water infiltrating through the CM increased because calcium hydroxide [Ca(OH)2] dissolves from freshly exposed cement within the CM; the calcium hydroxide readily disassociates to produce hydroxide ions [OH-] in solution, which raises the pH of the water. Although the CM was reportedly placed above the water table during facility demolition, the removal of pavement from 32 acres of the Site resulted in increased infiltration across the surface of CM and a corresponding rise in the water table, as described in Section 2.4.4.2.2. While the water table has always been relatively shallow within the Upland Area, the water table rose enough to saturate the bottom of the CM layer during the wet season in some areas of the Site. In addition, the resulting thinner layer of unsaturated soil separating the CM and water table had limited capacity to buffer pH, thus allowing high pH infiltration to reach the water table. During the 2020 CM removal project, it was also discovered that CM had been used to fill some vaults and other subsurface structures that extended beneath the water table, particularly within the footprint of the former Tissue Mill in the northeastern portion of the Upland Area. The highest groundwater pH occurred in those areas where CM was in direct contact with groundwater. This occurred seasonally across parts of the Upland Area and occurred year- round in the northeastern portion of the site where CM filled deeper subsurface structures.