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

ASPECT CONSULTING 18 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 July and September 2017 and the following vertical gradients calculated within the Log Pond (Table 2-1):  Inland well pair (LP-MW-1/LP-MW-3). Downward vertical gradients are calculated during both monitoring events (0.17 ft/ft and 0.06 ft/ft downward in July and September, respectively), with an average of 0.12 ft/ft downward; and  Shoreline well pair (MW-6/LP-MW-7). Vertical gradients change from upward (0.014 ft/ft) in July 2017 to downward (0.0026 ft/ft) in September 2017; the average of the two values is 0.006 ft/ft in an upward direction. The lower-magnitude vertical gradients relative to the inland location, and the change from slight upward to very slight downward gradients, measured at the shoreline is consistent with groundwater approaching the surface water interface. 2.4.4.2.3 Hydraulic Conductivity Estimates TIDAL STUDY-BASED ESTIMATES FOR FILL OUTSIDE OF LOG POND Hydraulic conductivity (K) of the fill unit outside of the Log Pond was estimated from the tidal study data, using the stage ratio and time lag methods of Ferris (1963). The Ferris (1963) methods are intended to provide a K estimate representing the entire fill unit between the well and the tide (area of groundwater discharge to the East Waterway). The Ferris methods were developed for confined aquifer conditions, but can be applied for unconfined aquifers if the observed tidal fluctuation in the aquifer is relatively small compared to the aquifer thickness, and the observation well is far enough from the submarine outcrop so that vertical flow is not a significant component of the flow path (Millham and Howes, 1995). Because the fill unit is on the order of 40 feet thick along the shoreline, these conditions are considered to be met sufficiently to use the methods to estimate K. Groundwater levels within the Log Pond show negligible tidal influence, so this method could not be applied to the Log Pond fill. The stage ratio method uses the tidal efficiency measured at a well and the distance between the well and the tide (point of discharge) as key input data. The time lag method uses the tidal lag measured at a well and the distance between the well and the tide as key input data. In each calculation, the tidal period for the semidiurnal Puget Sound tides is 12.4 hours, and the aquifer thickness at each well was estimated as an assumed 40-foot depth for the bottom of fill (near the shoreline, based on the geologic cross sections) minus the average measured depth to water below grade for the various water level measurements at the well. The aquifer storage coefficient (specific yield for unconfined aquifer) for the reworked alluvial materials was estimated as 0.1 based on the following literature estimates and best professional judgement:  United States Geological Survey (USGS, 1966) estimates for various facies of unconsolidated alluvium: 0.04 for silt; 0.16 for very fine sand; 0.23 for fine sand.  A model-calibrated value (0.02) for dredge fill at the Port of Seattle’s Terminal 30 (S.S. Papadopoulos and Associates, 2006).  A USGS (2003) estimate (0.13) for an alluvial aquifer in California.