Geoelectrical Imaging of Hyporheic Exchange and Mixing of River Water and Groundwater in a Large Regulated River

Hyporheic mixing and surface water-groundwater interactions are critical processes in aquatic environments. Yet, there is a lack of methods for assessing the spatial extent and distribution of these mixing zones. This study applied time-lapse electrical resistivity (ER) imaging in a 60-m wide and 0.7-m deep alluvial river whose stage periodically varied by 0.7 m due to dam operations to assess dynamic hyporheic mixing and surface water-groundwater interactions. Sixteen channel-spanning repeat ER tomograms (2D sections) over one flood cycle captured the dynamic ER distribution. We mapped a laterally discontinuous hyporheic zone, which had mainly river water circulating through it, several meters into the bed. Underneath the hyporheic zone was a transitional mixing zone intermittently flushed by mixing river water and deep groundwater. Minimally mixed groundwater dominated the deepest areas. ER imaging allows for unraveling hyporheic and deep mixing zone dynamics in large regulated rivers.