Use of Active-DTS (heat tracer experiment) and FVPDM (tracer experiment) for field quantification of groundwater fluxes

Understanding of transport processes is essential to identify the fate of contaminants in surface and subsurface water. Quantification of such transport processes requires a sound understanding and quantification of groundwater flow fields. Over the past decades, efforts have been made to develop and propose field methods that provide direct estimates of groundwater fluxes. The challenge is to propose field methods able to reflect the complexity of groundwater flow pathes in aquifer systems. In this context, we investigated the potential of two field methods to estimate groundwater fluxes in consolidated aquifers. Both FVPDM (Finite Volume Point Dilution Method) and Active-DTS (Distributed Temperature Sensing) measurements were conducted in a single piezometer in a fractured chalk aquifer. On the one hand, the FVPDM, a single-well tracer experiment, provided a measurement of the groundwater flow rate across the tested piezometer. On the other hand, the Active-DTS method was performed by deploying a Fiber-Optic (FO) cable outside the piezometer within the gravel filter. This method provided high-resolution and local groundwater flux estimates along the heated section. We relied on numerical simulations to assess the distortion of the groundwater flow field induced by the presence of the well. The groundwater flux is maximum within the well screen, where the FVPDM test was conducted. In the vicinity of the well, where the heated FO cable was installed, the groundwater flow is lower, and the flow pattern consists of converging and diverging flow lines. Thus, the position of the heated FO cable related to the flow direction is critical and can have a significant impact on the estimation of the groundwater flux. Regardless, we demonstrate that deploying the FO cable within the gravel pack is a novel and efficient approach, which opens up interesting perspectives for the use of Active-DTS measurements in consolidated aquifers to estimate vertical heterogeneities.