Preferential flow between rivers and aquifers in alluvial floodplains: A key to modelling and sustainably managing shallow groundwater resources

Preferential flow between rivers and aquifers in alluvial floodplains may be a core component of shallow groundwater transport and, consequently, its understanding is key to modelling and managing groundwater resources. At a clay wedge separating present-day streamflow and bank storage from an adjacent shallow aquifer, we image a suspected sand-dominated structure. This structure cuts through the clay wedge and possesses temporally dynamic electrical resistivity as seen in time-lapse electrical resistivity tomographic (ERT) images collected over a 61-day study period. During days 11-12, following heavy rainstorms, the cross section of the electrically resistive sand fades into the background resistivity structure, reappearing the following day. This research shows that preferential flow can be imaged in time-lapse ERT in buried sand-dominated structures between a floodplain and the adjacent river. Our analysis demonstrates that sand conduits can transport infiltrated rainwater from the floodplain into the river as a bank spring and, hypothetically, at high-stage streamflow, from the river into the adjacent shallow aquifer. In both directions, these conduits for preferential flow exert an important control on the regulation and distribution of water, sediments and contaminants. This phenomenon will help hydrological models to incorporate more real-world phenomena and ultimately better prepare groundwater managers to sustainably steward shallow groundwater resources.