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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 17 As an illustration of the rise in wet season water levels following mill demolition, the top plot on Figure 2-15 depicts wet season water levels for those two wells from February 2012, 2015, and 2016, and March 2017. Between February 2012 and March 2017, the wet season groundwater level in the area of those wells rose on average about 1.4 feet. There are multiple wells with wet season water level data over time following demolition that, on average, also illustrate the long-term water level rise. The bottom plot on Figure 2-15 depicts changes in wet season water levels relative to the first wet season measurements following demolition (February 2014) for ten wells having data from February 2014, February 2016, and March 2017; there are no wells with data for those events plus February 2015. The magnitude of change varies between wells, but, on average, groundwater levels across the Upland Area are approximately 0.7 feet higher in March 2017 (44 months after removal of Site impervious surfaces) than in February 2014 (8 months after impervious removal). Note that the March 2017 data were collected toward the end of the wettest wet season on record for the Puget Sound region, which is reflected by the increased rates of rise between February 2016 and March 2017. WATER LEVELS WITHIN LOG POND FILL Within the footprint of the former Log Pond, the water level data from wells LP-MW-1 and LP-MW-2 demonstrate significantly higher water table elevations than in surrounding wells during wet-season measurement events (Figures 2-10 through 2-13). The apparent cause for the water table mound is that the Log Pond fill is considerably siltier, and thus considerably lower permeability, than the dredge fill throughout much of the rest of the Upland Area, as documented by the numerous subsurface explorations drilled in, and by hydraulic conductivity testing of, the Log Pond fill soils (discussed in Section 2.4.4.2.3). The low permeability of the fill is also consistent with lack of tidal response 8 observed in shoreline wells within the Log Pond footprint. The water levels in wells LP-MW-1 and LP-MW-2 exhibit considerable seasonality over the complete period of monitoring: for example, maximum fluctuations of 5.5 and 7.7 feet in wells LP-MW-1 and LP-MW-2 respectively, compared to 2.1 feet of maximum fluctuation in well GF-MW-2 located just south of the Log Pond (data in Table A-1). It appears that the low permeability of the Log Pond fill reduces the rate of recharge infiltration, reflected as high (mounded) groundwater elevations in the wet season, which then decline greatly by the peak dry season. Vertical hydraulic gradients between the uppermost part of the Log Pond fill and the underlying sawdust unit can be calculated using 2017 water level data from two pairs of wells: LP-MW-1 (fill) and LP-MW-3 (sawdust) near the center of the Log Pond, and MW- 6 (fill) and LP-MW-7 (sawdust) located at the shoreline. The vertical gradients are calculated as the differences in groundwater elevations divided by the difference in midpoint elevations of the well screens for the paired shallow/deep wells at each location. Following installation of the deep wells in June 2017, water level data are available from 8 The magnitude of tidally induced groundwater level fluctuation (“tidal efficiency”) in a well is proportional to the hydraulic conductivity of the aquifer between the well and the tidal interface (Ferris, 1963).