Characterization of groundwater flux and direction using Active-Distributed Temperature Sensing

In heterogeneous aquifers, accurately characterizing groundwater flux and direction is crucial for predicting contaminant transport. Among emerging methods, Active-Distributed Temperature Sensing (Active-DTS) has proven to be highly effective for estimating groundwater fluxes at high spatial resolution in porous media. Active-DTS measurements involve heating a Fiber Optic (FO) cable and monitoring the associated temperature response. The temperature elevation measured along the heated section directly depends on the groundwater flux in the aquifer, with higher flux resulting in lower temperature elevation and faster temperature stabilization.

While this method is particularly effective in unconsolidated porous media, its application in consolidated aquifers is limited. In such cases, the heated fiber optic cable must be installed outside the piezometer within the gravel filter. As it has been already studied, the presence of any piezometer induces the distortion of the natural groundwater flow field in its vicinity. In this configuration, the temperature increase measured during Active-DTS measurements is highly dependent on the position of the FO cable relative to the flow direction. This means that the FO cable must be aligned with the natural flow streamlines for the measurements to accurately represent the actual groundwater flux. Unfortunately, the effective position of the FO cable is often unknown, introducing significant uncertainties in groundwater flux estimates.

To address these limitations, we propose an innovative approach for estimating groundwater flow direction and flow within consolidated aquifers. This novel setup involves the vertical deployment of multiple heatable FO cables in the gravel filter surrounding the piezometer.

First numerical modelling indicates that this approach is promising for estimating groundwater flow direction. This configuration allows for the sequential heating of individual FO cables while tracking the displacement of the resulting heat plume using the other cables. By repeating this process, the groundwater flow direction can be determined. The presence of multiple heated FO cables facilitates the estimation of flux at various locations within the gravel filter, providing insights into the groundwater flow distortion and flux within the aquifer.