ROLE OF QUATERNARY GLACIAL OUTBURST MEGAFLOOD PALEOCHANNEL DEPOSITS IN A BASALT-DOMINATED AQUIFER SYSTEM, WEST PLAINS AREA OF EASTERN WASHINGTON

Approximately 2 km2 of sand- and gravel-rich Pleistocene glacial-outburst megaflood sediment has accumulated in a series of five subparallel paleochannels that are an important component of a basalt-dominated aquifer system on the semiarid, ~450-km2 West Plains area. Located near Spokane, Washington (USA), the West Plains paleochannel deposits have been used for artificial storage and recovery wells and for planned and passive stormwater disposal in this rapidly growing area. Recent recognition of perfluorooctanoic acid contamination and excessive drawdowns have led to the closure of multiple municipal wells on the West Plains, highlighting the challenges of developing a sustainable groundwater supply in this complex aquifer system. Geologic and hydrogeologic data, from more than 1,400 water-well reports, surface exposures, existing groundwater geochemistry reports, and modeling, reveal the interconnections between the paleochannel deposits and surrounding basalt flows in three dimensions (3D). Initially incised into the Miocene Columbia River Basalt and excavated by Pleistocene glacial-outburst megaflooding, the West Plains paleochannels are projected in unprecedented 3D. This study’s modeled interactions between the more highly hydraulically conductive (100×) paleochannel sedimentary deposits and surrounding basalt beds provide insight into 14C ages and tritium values that had been measured previously. Sediment-filled closed depressions at the bases of the paleochannels, which are features likely formed by turbulent bursts (‘kolks’) during peak megaflood flows, are potentially promising groundwater storage sites. However, because of the relatively higher permeability of the unconfined paleochannel deposits, the closed depressions also are potential conduits for infiltration of contamination to deeper parts of the aquifer system.