Determining groundwater flow paths of infiltrated river water is
necessary for studying biochemical processes in the riparian zone, but
their characterization is complicated by strong temporal and spatial
heterogeneity. We investigated to what extent repeat 3D surface
electrical resistance tomography (ERT) can be used to monitor transport
of a salt-tracer plume under close to natural gradient conditions. The
aim is to estimate groundwater flow velocities and pathways at a site
located within a riparian groundwater system adjacent to the perialpine
Thur River in northeastern Switzerland. Our ERT time-lapse images
provide constraints on the plume's shape, flow direction, and velocity.
These images allow the movement of the plume to be followed for 35 m.
Although the hydraulic gradient is only 1.43 parts per thousand, the ERT
time-lapse images demonstrate that the plume's center of mass and its
front propagate with velocities of 2x10(-4) m/s and 5x10(-4) m/s,
respectively. These velocities are compatible with groundwater
resistivity monitoring data in two observation wells 5 m from the
injection well. Five additional sensors in the 5-30 m distance range did
not detect the plume. Comparison of the ERT time-lapse images with a
groundwater transport model and time-lapse inversions of synthetic ERT
data indicate that the movement of the plume can be described for the
first 6 h after injection by a uniform transport model. Subsurface
heterogeneity causes a change of the plume's direction and velocity at
later times. Our results demonstrate the effectiveness of using
time-lapse 3D surface ERT to monitor flow pathways in a challenging
perialpine environment over larger scales than is practically possible
with crosshole 3D ERT.