Combining experimental and modelling approaches to monitor the transport of an artificial tracer through the hyporheic zone

In order to advance methodologies used in the investigation of Hyporheic Zone (HZ) mixing processes, this article combines experimental and modelling tools to follow a tracer injected into the river and infiltrating into the HZ. A highly concentrated sodium chloride solution was injected into the river; (i) the river conductivity, (ii) the riverbed resistivity by Electrical Resistivity Tomography (ERT) and (iii) vertically distributed chloride concentrations within the HZ were monitored. Both ERT and concentration measurements showed an infiltration depth of the tracer of 35 cm, and a partial recovery after injection, which was faster within the superficial layer that was found to be more resistive according to the ERT initial image. The modelling approach used the HydroGeoSphere code to model the coupling between river surface flows and HZ groundwater flows and transport processes. The model set‐up involved a 50 cm high existing riverbed step, a vertical contrast in HZ saturated hydraulic conductivity and the aquifer discharge flux. Fitting the vertical chloride profile, the adjusted values were 5 × 10−2 m s−1 for the saturated hydraulic conductivity of the first highly permeable layer below the riverbed, and 4 × 10−6 m s−1 for the aquifer discharge flux. The bottom layer saturated hydraulic conductivity was found to be at least 10 times lower than the value within the first layer. Numerical simulations showed that the two main parameters controlling the mixing within the HZ were the groundwater discharge and the saturated hydraulic conductivity first sediment layer of the riverbed. The riverbed step was found to be less significant here compared to these two parameters. The combination of experimental and modelling tools allowed us to quantify the aquifer discharge flux, which is complicated to investigate in the field without any model. Results of this study showed that combining modelling with ERT and vertically distributed chloride sampling allows the quantification of the main factors controlling the hyporheic exchange. The mixing processes within the hyporheic zone were monitored during a tracer test experiment with the help of vertical chloride sampling and ERT measurements. The observed results were successfully reproduced by a model built with the HydroGeoSphere code. Coupling various experimental measurements with a modelling approach helped in understanding the processes that drive the river fluxes through the hyporheic zone and allowed quantif