SEDIMENTARY ARCHITECTURE OF THE SPOKANE AQUIFER AND ITS HYDROGEOLOGIC IMPLICATIONS

The Spokane Valley-Rathdrum Prairie (SVRP) aquifer system is contained within a thick sequence of mostly coarse-grained unconsolidated sediments deposited by repeated catastrophic floods escaping Glacial Lake Missoula during the late Pleistocene. Recent seismic reflection profiling work has resulted in increased understanding of the geometry of the base of the aquifer, which lays on either pre-Tertiary crystalline rock, Miocene flood basalts (Columbia River Basalts) or related interbeds (Latah Formation). The high-energy scour and fill episodes in the valley were, on some occasions, modified by the presence of a high stand of Glacial Lake Columbia that promoted the accumulation of silts and clays. Viewing the unconfined aquifer body as a homogeneous and isotropic unit is not appropriate. Glaciolacustrine units can locally act as aquitards or leaky confining layers, for example in the Hillyard Trough and around Coeur d'Alene, and the down-valley fining of the glaciofluvial deposits lowers the hydraulic conductivity from east to west in the SVRP aquifer, from >10,000 ft/day to <500 ft/day. Glaciolacustrine deposits also serve to perch groundwater along the Little Spokane River where numerous springs have developed as discharge points from the aquifer. Little is known of the sedimentologic variation with depth in the SVRP aquifer as few wells penetrate the full saturated thickness of the body, though the overall architecture is envisioned to be similar to that of a coarse-grained braided fluvial environment of deposition. Sand and gravel mining in a deep pit has exposed a strongly cemented layer of sediments beneath the groundwater surface, about 5 ft thick and of unknown lateral extent, that perhaps represents a paleo-water table surface. The present day alluvial bed of the Spokane River is inset within the glaciofluvial flood deposits, and significant exchange of water takes place between the SVRP aquifer and the river along multiple influent and effluent reaches, where head differences, river bed area and hydraulic conductivity are controlling factors. A growing population and large municipal and industrial wells have had little affect on regional water levels in the SVRP aquifer given the generally high hydraulic conductivity and prolific flow rates, however, river flows may be impacted by increasing groundwater withdrawals.