An Active-Distributed Temperature Sensing method for measuringgroundwater flow velocities into streambed sediments at high spatialresolution.

International audienceUnderstanding and quantifying groundwater and surface water interactions are key elements for the managementof water quality and quantity but also for the preservation of groundwater dependent ecosystems and riparianhabitat. Localizing and quantifying these exchanges remains challenging since exchange processes vary both intime and space leading to an extreme heterogeneity in the distribution of fluid exchanges. Given the interest incharacterizing hyporheic flows and quantifying interactions between groundwater and surface water, we developeda new method for measuring groundwater flow velocities in streambed sediments with an unprecedented highspatial resolution.The experimental setup consists in heating an armored fiber-optic cable, previously deployed along thestreambed within the sediments of a first-order and intermittent stream. According to heat transport principle,the temperature increase along the cable depends mainly on the sediments thermal properties and ongroundwater flow velocities that may dissipate heat through heat advection. Thus, the temperature reachedafter a given time and at a given location should be a function of local groundwater flow velocities. Thermalresponse measured along heated the fiber-optic cable are interpreted by assuming that the heated cable canbe modelled as a Moving Infinite Line Source (MILS), i.e. a thermal source, using the analytical solutiondeveloped by Metzger et al in 2004. To validate the use of the analytical solution and to estimate the uncertaintyand limits associated to the method, a 2D numerical model has been developed. This model simulates steadystate fluid flow and transient heat transfer using the Conjugate Heat Transfer module of COMSOL Multiphysics ®.After few hours of heating, the measured temperature at steady state is particularly variable along the sectionwith temperature increases from 16 to 36C. By applying the analytical model, we show that it is possibleto reproduce field measurements by varying flow velocity only. Therefore, this suggests that the thermal responsevariability can simply be associated with local variations of groundwater fluxes. These field tests allowedcharacterizing the groundwater flow velocities distribution into the streambed as a lognormal distribution rangingbetween 8e-7 and 6e-5 m/s