Resolving flow-dependent indicators of groundwater exchange in the Columbia River, WA

Hydrologic exchange flows (HEFs), which are the movement of water between a river channel and the adjacent subsurface, are important for water quantity, quality, and ecosystem function. The spatial distributions of HEFs in streams are influenced by hydrologic conditions in the aquifer, hydraulic conditions in the channel, and the spatial distribution of permeability under and around the channel. Exchange with adjacent aquifers is less well understood in large rivers because tracer injections are much more difficult due to water depth and higher flows than in small streams where they have been used extensively. In this study we 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 we measured temperature, specific conductance, and dissolved Radon-222 along the riverbed during three sampling events in 2021/2022. We used a FloaTEM system to identify the locations of changes in the large-scale geology and compared these to the locations of HEFs. Though each method we used had some shortcomings together they provided a more complete picture of what drives HEF locations. We observed water quality anomalies in similar locations to 3D numerical modeling experiments and past field research in the study area, but we did not find a single factor that completely explained the locations of HEFs. Though sensitive to surface water inflows, our method is useful for quickly surveying long reaches of river and determining locations for more in-depth investigations of HEF dynamics.