INTRAFORMATIONAL DISCONTINUITY CONTROLS ON VASHON ADVANCE OUTWASH WATER LEVELS, BEAR CREEK PLATEAU, KING COUNTY, WASHINGTON

Water levels in Vashon advance (Qva) glaciofluvial sediments are controlled by intraformational discontinuities at a combination borrow pit/soils management site located on the Bear Creek Plateau in King County, Washington. Exploration drilling and aquifer testing identified two distinct ground water zones within the footprint of the 300- by 900-foot borrow pit. Ground water elevations in the southeast portion of the pit are about 40 to 50 feet lower than ground water elevations to the northwest. A series of thin northwest-dipping silt beds extend across the site and form a hydraulic barrier to regional flow. The hydraulic barrier forces ground water to flow along the interface between an overlying scouring surface and the underlying silt subcrop. South of the subcrop limit, ground water flows vertically downward before encountering the regional water table.

A dewatering system was designed to dewater the northern cell of the borrow pit to maximize aggregate extraction. Multiple dewatering wells extended through the Qva into underlying Olympia-age fluvial sands and gravels. Boundary conditions between the Olympia sediments and Qva, and intraformational boundaries within the Qva resulted in rapid dewatering.