Electrical Resistivity Imaging (ERI) of solute transport under transient
conditions within a model hillslope transect.
Abstract
The direct observation of water movement via Electrical Resistivity
Imaging (ERI) can leverage the understanding of the processes that lead
to the occurrence of variable residence times (RT) within the Critical
Zone (CZ). While hydrological processes at natural landscapes are often
space and time-variable, quantitatively estimating solute transport with
ERI under transient conditions is challenging due to necessary
considerations of moisture states and electrical properties of the
medium. Here, we introduce the use of Periodic Steady State (PSS) theory
applied to electrical resistivity of soils to provide a simple solution
to the problems and report a laboratory experiment to test the proposed
method. We used a 1 m3 sloping lysimeter to represent the hydrological
functioning of natural hillslopes, equipped with electrodes to provide
cross-borehole images of bulk soil electrical conductivity and performed
a 28-days experiment in which a periodic irrigation was applied. A
saline tracer was added to the lysimeter in two irrigation pulses and
subsequent pulses were applied until the tracer was flushed out.
ERT-surveys and estimates of background soil-water conductivity were
used to quantitatively estimate solute breakthrough throughout the
different cross-sections. Integrated lysimeter-scale images were
superimposed with the water table progression throughout the experiment
to leverage the understanding of flow and transport processes
responsible for the tracer mobilization. Our study introduces a novel
method for laboratory experimentation at mesocosm scales using ERT and
provides valuable insight into the role of water table dynamics in
mediating the occurrence of variable flow pathways within hillslopes.