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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 73 The pH effect of groundwater seasonally coming into and out of contact with CM is illustrated clearly by data collected from Log Pond well LP-MW-1. Well LP-MW-1 historically had the highest pH readings measured on the Site (up to 13.0 in November 2013) due to the facts that (1) its well screen was shallow enough that it intercepted the thick layer of CM in that area, and (2) the Log Pond fill has low enough permeability that precipitation does not infiltrate there nearly as readily as the dredge fill outside of the Log Pond (refer to Section 2.4.4.2.3). Consequently, when the seasonal rains began, the groundwater level within the Log Pond footprint “mounded up” because infiltration is so slow. The mounded groundwater submerged CM within the well’s screened interval, which created very high pH groundwater in the shallow depth interval screened by LP-MW-1. Figure 6-P10 illustrates the relationship of groundwater level and pH measured at well LP-MW-1 over time. The bottom of the CM at that well is depicted for comparison with groundwater elevations on the figure. Between February and August 2016, the groundwater level at well LP-MW-1 dropped approximately 3.5 feet, to below the bottom of the CM at that location, with a corresponding groundwater pH drop from 11.2 to 8.6 in that well. By March 2017, the groundwater level at LP-MW-1 rose nearly 4 feet relative to August 2016 and was 0.4 feet higher than that measured in February 2016. In the peak dry-season conditions of September 2017, the water level was again below the bottom of the CM and the groundwater pH was 8.15—the same phenomenon observed in the dry season of 2016. During groundwater pH monitoring conducted for the CM removal project, the pH readings in LP-MW-1 ranged from 7.0 to 7.5 between August 31 and September 22, 2020. During this period, on September 3, the water level was measured to be 7.0 feet below the well’s top of casing and thus below the base of the CM. Between September 24 and 26, 2020, approximately 1.6 inches of rain fell in Everett. By September 28, the pH in LP-MW-1 had risen to 9.7; the water level was not measured that day. On September 29, when a pH of 10.5 was measured, the water level had risen to 3.3 feet below top of casing. Comparing the September 3 and September 29 readings, the water level rose about 3.7 feet, submerging the base of the CM, and the pH increased about 3.3 standard units in response (Figure 6-P10). These data, in combination with low pH measured in the deeper wells installed in 2017 within the Log Pond (LP-MW-3 through LP-MW-7), indicate that the elevated groundwater pH was limited to a shallow depth interval beneath the CM layer. In areas where the CM did not contact groundwater, relatively higher groundwater pH occurred where the thickness of the CM layer was greatest and where the water table was shallowest. The depth of the water table generally increases from east to west across the Upland Area. Apart from the localized CM-filled subsurface structures, the thickest occurrences of CM—greater than 5 feet—were generally present within the center of the Log Pond and within the Tissue Mill areas. 6.3.1 Time Trends for Groundwater pH While there is seasonality in the groundwater pH, the data collected between 2013 and 2021 also demonstrate that, prior to CM removal, the elevated pH was gradually declining