ABSTRACT:

Preferential flow between rivers and aquifers in alluvial floodplains may be a core component of shallow groundwater transport and, consequently, its understand- ing 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 demon- strates 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.

ABSTRACT:

Geophysical and hydrogeological data can be used to estimate aquifer hydraulic parameters and test alternative conceptual models of subsurface hydrology. Here we confirm the existence of a relic sand-dominated channel-belt in an alluvial floodplain using electrical resistivity tomography and time-domain electromagnetics. After converting the bulk resistivity structure to porosity, we use hydrological modeling to gain new insights into the hydraulics and compartmentalization of a heterogeneous alluvial floodplain aquifer system. We discovered that rainwater infiltration fills the initially dry channel-belt laterally rather than from direct infiltration from above. This new understanding of preferential flow paths around and into sand-dominated channel-belts outlines how the architecture of an alluvial floodplain determines its response to natural hydrologic disturbances, thereby providing an improved basis for making informed decisions about water management strategies.