Combining Different Sensing Approaches for Characterising Groundwater Fluxes in Sandbox Experiments

While permeability can be routinely estimated from conventional groundwater field methods, it remains extremely difficult to measure groundwater flux variability in heterogeneous aquifers. Recently, groundwater flux quantification has been demonstrated by Simon et al. (2021) using actively heated fibre optic Distributed Temperature Sensing (Active-DTS). The method is based on the spatial and temporal monitoring of the thermal response to a controlled heat source. However, one limitation in assessing the variability of groundwater fluxes in the field, comes from the DTS spatial resolution which is limited to 0.5 to a meter, depending on the DTS unit used. To improve our ability to measure groundwater flux at high spatial resolution, we tested and compared different sensing approaches in laboratory experiments.

Experiments have been conducted in perfectly controlled conditions at two different scales, a laboratory-scale sand tank and a large-scale experimental basin. The sand tank consists of a 3×0.3×0.3 m tank installed in an experimental hall in Rennes, France. It allows to measure groundwater flux in different layers of sand having different permeability. The large-scale experiment was conducted at the Site Contrôlé Expérimental de Recherche pour la réhabilitation des Eaux et des Sols (SCERES) platform in Strasbourg, France. This 25×12×3 m experimental basin equipped with pumps controlling the upstream and downstream head allows to reproduce flow in an artificial porous aquifer. An advantage of this platform is its large scale, which compared to conventionally used experimental sand tanks, potentially minimise boundary effects. To investigate our ability to measure groundwater flux at high spatial resolution, a fibre optic cable was installed parallel to a comb comprised of alternating layers of high-density polyethylene (HDPE) barrier and sand layers with variable thickness. Here, we present preliminary results obtained in both experimental setups, for different heating period and for different hydraulic conditions.