Imaging and characterisation of subsurface solute transport using electrical resistivity tomography (ERT) and equivalent transport models

We assess the usefulness of electrical resistivity tomography (ERT) in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers. A field tracer experiment was conducted at the Krauthausen test site, Germany. The spatial and temporal evolution of the injected NaBr solute plume was monitored in a 2D ERT image plane located downstream of the injection well for 90 days. Since ERT maps changes in bulk electrical conductivity, the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation. The concentration maps are then analysed using an equivalent convection–dispersion model (CDM), which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity. As demonstrated by associated synthetic model studies, ERT resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm. Since for heterogeneous media, local concentrations within the plume deviate from those predicted by the equivalent CDM, we also interpret the ERT-derived pixel breakthrough curves in terms of an equivalent stream-tube model (STM). The STM represents transport in the aquifer by a set of 1D convection–dispersion processes, allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified. We believe that the observed tracer plume is satisfactorily described by the equivalent CDM, probably because the tracer plume was small relative to the heterogeneity scale of the aquifer. Even though application of the STM revealed some deviation from the ideal homogeneous case, the equivalent dispersivity in the STM matches the longitudinal dispersivity of the CDM closely, consistent with predominantly homogeneous mixing. However, the STM analysis illustrates how ERT results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers.