An Active-Distributed Temperature Sensing method to quantifygroundwater – surface water exchanges

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. We developed a methodology to quantify groundwater and surface water interactions, by setting up anactive heat tracer experiment using fiber-optic distributed temperature sensing (FO-DTS).The experimental setup consists in heating an armoured fiber-optic cable that has been previously deployedalong the streambed within the sediments. Then, the increase in temperature along the heated cable is a functionof the thermal properties of the sediments and of the fluid flow velocity within the sediments. The cable isheated electrically through the steel armouring of the cable while the elevations in temperature are continuouslymonitored. We tested this methodology on the Kerbernez catchment, located in south-western Brittany (France)and which is part of the AgrHys hydrological observatory. We deploy the cable in a first-order stream within thissmall agricultural catchment (0.12 km2). Temperature was monitored along 60 meters of stream with a spatial andtemporal resolution respectively equal to 29 cm and 30 s. To interpret the data, we used an analytical solutiondeveloped for geothermal energy that considers advection and conduction of temperature in porous media. Tovalidate the use of the analytical solution and to define the limits of the method, a 2D numerical model has beendeveloped. This model simulates heat transport and conduction with steady state fluid flow using the ConjugateHeat Transfer module of COMSOL Multiphysics®.During heating and cooling, the measured temperature was particularly variable along the section withtemperature increases that range between 16 to 36C. This variability can directly be associated with localvariations of water fluxes by applying the appropriate analytical solution. Henceforth, it is possible to modelfield data, just by adjusting one free parameter: the flow velocity within the sediments. For the electrical powerused, these preliminary tests permits to estimate fluxes between groundwater and surface water between 8.10^-7to 6.10^-5 m.s-1. Thus, our results show that an active heat tracer experiment using fiber-optic distributed tem-perature sensing can be a promising and innovative method to quantify surface water and groundwater interaction