Imaging and quantifying salt-tracer transport in a riparian groundwater system by means of 3D ERT monitoring

Abstract

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‰, the ERT time-lapse images demonstrate that the plume's center of mass and its front propagate with velocities of 2 × 10 −4 m∕s and 5 × 10 −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 timelapse 3D surface ERT to monitor flow pathways in a challenging perialpine environment over larger scales than is practically possible with crosshole 3D ERT.

Assessing the effect of different river water level interpolation schemes on modeled groundwater residence times

“…Hydraulic conductivities were estimated to range from 4 Â 10 À3 to 4 Â 10 À2 m/s by slug tests, a pumping test and a salt tracer test (Diem et al, 2010;Doetsch et al, 2012). The aquifer is underlain by a lacustrine clay layer, which forms the lower hydraulic boundary.…”

Covariance-constrained difference inversion of time-lapse electrical resistivity tomography data

“…Among the available geophysical techniques, time-lapse electrical resistivity tomography (ERT) is one of the most popular methods . Given its sensitivity to numerous soil/rock properties, ERT has been applied in various contexts, such as salt-tracer experiments (Doetsch et al, 2012b;Robert et al, 2012), dynamics of infiltration and saturation in the vadose zone (Binley et al, 2002;Koestel et al, 2008), monitoring of permafrost (Krautblatter et al, 2010;Supper et al, 2014), interaction between surface and groundwater (Coscia et al, 2011), and more recently to CO 2 sequestration (Carrigan et al, 2013) and heat-tracing experiments Arato et al, 2015).…”

Morphological, hydrological, biogeochemical and ecological changes and challenges in river restoration – the Thur River case study

“…It was then possible to postulate a likely limit between old fluvial deposits and those associated with deposition at the time when the river was channelized (Doetsch et al, 2012a). Geophysical monitoring allowed three-dimensional imaging of groundwater flow patterns that originate from infiltrating river water using cross-hole geophysics (Coscia et al, 2012b) or by using injections of artificial saline tracers in combination with surface-based geophysics (Doetsch et al, 2012b). We could also estimate groundwater velocities and image the hydraulic conductivity field by combining natural-tracer and hydraulictomography data with geophysical data in a joint inversion framework (Lochbühler et al, 2013).…”