HYDROGEOLOGIC FRAMEWORK OF EASTERN JEFFERSON COUNTY: IMPLICATIONS FOR SURFACE WATER-GROUND WATER INTERACTIONS

The hydrogeologic framework and preliminary interactions between surface water and ground water were determined as part of a study of the water resources in the Chimacum Creek Watershed and other significant drainages within eastern Jefferson County. The study will assist local watershed planners in assessing the status of water resources and the potential impacts of groundwater development on surface water systems.

The surficial geology of the Chimacum Basin was compiled from existing sources, modified using LIDAR imagery, and used along with drillers’ logs from more than 110 wells to define the hydrogeology. Quaternary glacial deposits, which are as much as 900 feet thick and overlie bedrock, form four hydrogeologic units: a recessional outwash aquifer, a till confining unit, an advance outwash aquifer, and an older glacial sequence. The advance outwash aquifer is the most productive source of groundwater in the area, although ground water is found throughout the glacial deposits in discontinuous lenses of sand and gravel.

Time-synchronous stream discharge measurements made in June and October 2002 were used to identify reaches of Tarboo Creek, Chimacum Creek, and its tributaries, and the lower reaches of the Big and Little Quilcene Rivers that were gaining or losing water to the groundwater system. Vertical hydraulic and thermal gradients measured with in-stream minipiezometers and piezometers with nested temperature sensors provided additional data to refine the boundaries between gaining and losing reaches and define seasonal variations in surface water–ground water exchanges. Each of the creeks examined had a unique pattern of gaining and losing reaches that reflect changes in the geology or transmissivity of the material underlying the streambed. Significant surface water losses were found at transitions between Quaternary, valley-filling peat deposits and recessional outwash on Chimacum Creek and on the alluvial plain near the mouths of the Big and Little Quilcene Rivers. Seasonal variations in the magnitudes of gains and losses were likely the result of changes in the altitude of the surrounding water table relative to river stage.