Scientific Investigations Report 2007–5041
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5041
The Spokane Valley-Rathdrum Prairie (SVRP) aquifer is the sole source of drinking water for more than 500,000 residents in Spokane County, Washington, and Bonner and Kootenai Counties, Idaho. Recent and projected urban, suburban, and industrial/commercial growth has raised concerns about potential impacts on water availability and water quality in the SVRP aquifer and the Spokane and Little Spokane Rivers. This report presents the hydrogeologic framework and water-budget components of the study area compiled and interpreted by the U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources and Washington State Department of Ecology for the SVRP aquifer. Descriptions of the geologic history, hydrogeologic framework, water-budget components, and further data needs are provided in this document.
The SVRP aquifer consists primarily of thick layers of coarse-grained sediments—gravels, cobbles, and boulders—deposited during a series of outburst floods resulting from repeated collapse of the ice dam that impounded ancient Glacial Lake Missoula. Sources of recharge to the aquifer include infiltration from precipitation, return flow from water applied at land surface, seepage from the Spokane and Little Spokane Rivers and adjacent lakes, and surface-water and ground-water inflow from tributary basins. The aquifer primarily discharges into the Spokane and Little Spokane Rivers and through pumping wells.
A simplified geologic model of the Rathdrum Prairie and Spokane Valley includes filling of the ancient Rathdrum-Spokane River valley with generally unknown amounts of Miocene basalts and interbedded sediments followed by a period of downcutting, repeated cycles of glacial and interglacial sedimentation, and finally the repeated and catastrophic cycles of outburst flooding from Glacial Lake Missoula. In most places, the SVRP aquifer is bounded by bedrock of pre-Tertiary granite or metasedimentary rocks, or Miocene basalt and associated sedimentary deposits. The base or bottom boundary of the aquifer is uncertain except along the margins or in shallower parts of the aquifer where wells have penetrated the entire thickness of the aquifer and reached bedrock or silt and clay deposits.
Fine-grained layers are scattered throughout the SVRP aquifer at considerably different altitudes and with considerably different thicknesses. In the Hillyard Trough, a massive fine-grained layer with an altitude of about 1,670 feet and an average thickness of 215 feet separates the aquifer into upper and lower units. The southern extent of the layer is uncertain, but is believed to be within about 2 miles of downtown Spokane. The fine-grained layer that occurs in the Hillyard Trough also is present in the Little Spokane River Arm of the aquifer with a more variable altitude that ranges from about 1,500 to 1,700 feet and an average thickness of 130 feet. Most of the Spokane Valley part of the aquifer is void of fine-grained layers except near the margins of the valley and near the mouths of lakes. In the Rathdrum Prairie, multiple fine-grained layers are scattered throughout the aquifer with altitudes ranging from 1,653 to 2,392 feet with thicknesses ranging from 1 to more than 135 feet.
The altitude of the base of the aquifer ranges from less than 1,800 feet near Lake Pend Oreille to less than 1,200 feet near the aquifer’s outlet near Long Lake. The thickness of the aquifer is more than 800 feet in the northwestern part of the northern Rathdrum Prairie, through the West Channel area, and through the west-central part of the Rathdrum Prairie. In Washington, the areas of greatest thickness, more than 600 feet, are mapped in the central parts of the Spokane Valley, the City of Spokane, and the Hillyard Trough.
Based on this study’s data set, the median well yield for the SVRP aquifer, the Basalt and fine-grained interbeds unit, and the Bedrock unit are 100, 10, and 8 gallons per minute, respectively. The median specific capacity for wells completed in the SVRP aquifer, the Basalt and fine-grained interbeds unit, and the Bedrock unit are 200, 8.3, and 0.66 gallons per minute per foot, respectively.
Recharge or inflow to the SVRP aquifer occurs from six main sources: the Spokane River, lakes, precipitation over the aquifer, tributaries, infiltration from landscape irrigation and septic systems, and subsurface inflow. Total estimated mean annual inflow to the aquifer is 1,471 cubic feet per second. Discharge or outflow from the SVRP aquifer occurs from five main sources: the Spokane River, the Little Spokane River, pumpage, underflow to Long Lake, and infiltration of ground water to sewers. Total estimated mean annual outflow from the SVRP aquifer is 1,468 cubic feet per second.
Several data needs were identified during this investigation that would provide for a more refined characterization of the hydrogeologic framework and water-budget components for the SVRP aquifer study area. Deep drilling along the axis of the SVRP aquifer would identify the presence or absence of fine-grained layers, and their thickness in the deepest parts of the aquifer and determine the depth to the bottom of the aquifer where data are currently unavailable. A more detailed analysis of the geologic and hydrologic setting near the southern ends of Spirit and Hoodoo Valleys would help determine the location of the ground-water divide between the two valleys and the Rathdrum Prairie. Better estimates of seepage into the aquifer from Coeur d’Alene Lake and Lake Pend Oreille and underflow from the aquifer to Long Lake would strengthen the recharge and discharge estimates currently available for the aquifer. Well-designed hydrochemical studies using environmental tracers and ground-water age dating could reduce uncertainty in some ground-water budget estimates and improve definition of ground-water flow paths.
U.S.
Department of the Interior | U.S. Geological
Survey
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