HYDROGEOLOGY OF THE ORTING LAKE PLATEAU, PIERCE COUNTY, WASHINGTON

The Orting Lake Plateau (Plateau) located in Pierce County, Washington is a glaciated upland in the southern Puget Sound Lowland delineated by South Prairie Creek to the east, Carbon River to the south and southwest, and the Puyallup River Valley to the west. The upland is partially dissected by Fennel Creek to the north. Regional studies established the Pleistocene-age glacial and non-glacial stratigraphy of the plateau including the Orting Drift, Alderton Formation, Stuck Drift, Puyallup Formation and Vashon Drift. Hydrogeologic conditions of the 5000+ acre plateau, have been evaluated from over 50 borings/wells, more than 400 exploration pits, petrographic sand analysis, long-term groundwater level and stream flow monitoring, aquifer testing, and field mapping.

Orting Lake and multiple wetlands are present on the Plateau, however, no surface water streams connect upland surface water to the adjacent valleys. The only major stream system on the Plateau is from groundwater forming a major spring system at the headwaters of Canyonfalls Creek. Groundwater flow beneath the Plateau is generally to the west and northwest toward Canyonfalls Creek where flows range from about 10 to 25 cfs. Groundwater also discharges at additional spring systems along Fennel Creek and at various smaller spring locations around the margins of the Plateau. The spring systems at Canyonfalls Creek and Fennel Creek all discharge at a similar elevation and are controlled by low permeability mudflow deposits contained in the Puyallup Formation. Groundwater flow is concentrated in highly permeable non-glacial and glacial fluvial deposits overlying the Puyallup Formation mudflows.

Petrographic analysis of sand and diamicton suggest the Plateau is underlain by previously unrecognized glacial and nonglacial units. The nonglacial unit has an easterly-derived, Cascade provenance with interbedded volcanic fluvial deposits (60-80% volcanic lithics/total lithics) and lahar/lahar run-out deposits (85-90% volcanic lithics/total lithics). These volcanic sediments have substantial Mount Rainier hypersthene-phyric andesite grains. Given the provenance, weathering, and stratigraphic position these nonglacial sediments are tentatively correlated with the Whidbey Formation.