Resolving Flow-Dependent Indicators of Groundwater Exchange in the Columbia River, Washington

Authors

Michael Gooseff¹* ([email protected]), Margaret Digiorno¹, Neil Terry², Martin Briggs², Xingyuan Chen³, Evan Arntzen³

Institutions

¹University of Colorado–Boulder, CO; ²U.S. Geological Survey; ³Pacific Northwest National Laboratory, Richland, WA

Abstract

Hydrologic exchange flows (HEFs), the movement of water between a river channel and the adjacent subsurface, are important for water quality and river corridor ecosystem function. The spatial distributions of HEFs in rivers are influenced by hydrologic conditions in the surrounding aquifer, hydraulic conditions in the channel, and the spatial distribution of alluvium with variable sediment properties under and around the channel. Exchange with adjacent aquifers is less understood in large rivers because tracer injections and other techniques one might use in streams are much more difficult due to water depth and higher flows. In this study researchers ask whether large-scale geologic units in the area drive HEF locations more than the finer-scale sediment types and the riverbed morphology along a 75 km reach of the Columbia River near the Hanford Site in eastern Washington. To determine the locations of HEFs, the team measured temperature, specific conductance, and dissolved Radon-222 along the riverbed during three sampling events in 2021/2022. The team used a FloaTEM system to identify the locations of changes in the large-scale geology and compared these to the locations of HEFs.

Together these methods provided a more complete picture of what drives HEF locations. Researchers observed water quality anomalies in similar locations to 3D numerical modeling experiments and past field research in the study area but did not find a single factor that completely explained the locations of HEFs. Though sensitive to surface water inflows, this method is useful for quickly surveying long reaches of river and determining locations for more in-depth investigations of HEF dynamics.