The infiltration of river water into aquifers is of high relevance to
drinking-water production and is a key driver of biogeochemical
processes in the hyporheic and riparian zone, but the distribution and
quantification of the infiltrating water are difficult to determine
using conventional hydrological methods (e.g., borehole logging and
tracer tests). By time-lapse inverting crosshole ERT (electrical
resistivity tomography) monitoring data, we imaged groundwater flow
patterns driven by river water infiltrating a perialpine gravel aquifer
in northeastern Switzerland. This was possible because the electrical
resistivity of the infiltrating water changed during rainfall-runoff
events. Our time-lapse resistivity models indicated rather complex flow
patterns as a result of spatially heterogeneous bank filtration and
aquifer heterogeneity. The upper part of the aquifer was most affected
by the river infiltrate, and the highest groundwater velocities and
possible preferential flow occurred at shallow to intermediate depths.
Time series of the reconstructed resistivity models matched groundwater
electrical resistivity data recorded on borehole loggers in the upper
and middle parts of the aquifer, whereas the resistivity models
displayed smaller variations and delayed responses with respect to the
logging data. in the lower part. This study demonstrated that crosshole
ERT monitoring of natural electrical resistivity variations of river
infiltrate could be used to image and quantify 3D bank filtration and
aquifer dynamics at a high spatial resolution.