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

Approximately one-cubic kilometer of sand- and gravel-rich Pleistocene glacial outburst megaflood sediment accumulated in a series of five subparallel, SW-NE oriented, up to ~3.5 km wide and ~19 km long paleochannels that are an important component of a basalt-dominated aquifer system on the semi-arid, ~450 km² West Plains area, near Spokane, Washington. Paleochannel deposits on the West Plains have been used for artificial recharge and recovery (ASR) wells and for storm-water disposal sites in this rapidly growing suburban and rural area. Recent recognition of perfluorooctanoic acid (PFOA) contamination, however, led to the closure of a municipal ASR well, highlighting the challenges of developing a sustainable ground-water supply in this complex aquifer system-where the interconnections between basalt flow and paleochannel elements, as well as the genesis and heterogeneities of the paleochannel deposits, are just beginning to emerge. Geologic and hydrogeologic data from more than 1,400 water-well reports, surface exposures, and modeling reveal the interconnections between the paleochannel deposits and surrounding basalt flows in three-dimensions. Initially incised into the Miocene Columbia River Basalt and further excavated by Pleistocene glacial-outburst megaflooding, the West Plains paleochannels and their associated deposits are in direct subsurface contact with fractured and often hydraulically conductive basalt flows. Many of the paleochannel deposits are obscured beneath meters-thick, latest Pleistocene and Holocene, sand-rich eolian and fluvial sedimentary deposits. Sediment-filled closed depressions at the bases of the paleochannels likely formed by pervasive turbulent bursts (‘kolks’) during peak megaflood flows and have promising potential to store groundwater. However, because of the relatively higher permeability of the unconfined paleochannel deposits, they also are potential conduits for infiltration of contaminants to deeper parts of the aquifer system. Ultimately, identifying potential infiltration and recharge pathways as well as contaminant sources on the West Plains will require additional borehole data and a multi-scale understanding of this geologically complex sedimentary and basaltic aquifer system.