Estimation of groundwater flux with active distributed temperature sensing and the finite volume point dilution method: a field comparison

Considering the importance of characterizing groundwater flow for assessing recharge and contaminant transport, this study investigates the potential of two field methods to estimate groundwater fluxes in consolidated aquifers. To accomplish this, both the finite volume point dilution method (FVPDM) test and active distributed temperature sensing (Active-DTS) measurements were conducted in a single piezometer in a chalk aquifer. The FVPDM is a single-well tracer experiment, that provides a measurement of the groundwater flow rate across the tested piezometer. Whereas the Active-DTS method was performed by deploying a fiber-optic (FO) cable outside the piezometer within the gravel filter. The Active-DTS method provided high spatial resolution and local groundwater flux estimates along the heated section. Numerical simulations were used to assess the distortion of the groundwater flow field induced by the presence of the well, demonstrating that the groundwater flux is maximum within the well screen, where the FVPDM test was conducted. In the immediate vicinity of the well, where the heated FO cable was installed, the groundwater flux is lower, and the flow pattern consisted of converging and diverging flow lines. Therefore, the position of the heated FO cable relative to the flow direction is critical and can have a significant impact on the estimation of the groundwater flux. Thus, even if the deployment of the FO cable within the gravel pack minimizes convective effects and opens up interesting perspectives to estimate vertical heterogeneities, this approach may be limited if the position of the FO cable relative to the flow direction is not well known.