On the use of Distributed Temperature Sensing for characterizing groundwater dynamics and intermittent fluxes in heterogeneous aquifers

Heat is known to be an excellent tracer for characterizing groundwater flows in the sub-surface. Steady-state hydraulic conditions are often assumed to derive groundwater flows components from temperature measurements. Here, we investigate the interest of Fiber-Optic Distributed Temperature Sensing (FO-DTS) to monitor groundwater dynamics and intermittent fluxes in heterogeneous aquifers. The main advantage of FO-DTS is to provide a continuous temperature monitoring in boreholes at an unprecedented spatial resolution. Through few field examples, we show how such a spatial and temporal monitoring make possible the detection of small groundwater changes and intermittent fluxes. Field experiments consist in temperature monitoring for a few days on two experimental sites where fiber-optic cables has been deployed in few 100 m deep boreholes. The two experimental sites are belonging to the Network of Hydrogeological Sites H+ (http://hplus.ore.fr). Groundwater flows are mainly associated with sub-horizontal karstic structures and sub-vertical fractures at the Poitiers Experimental Hydrogeological Site (SEH), while the site of Ploemeur (Brittany, France) is as highly fractured aquifer in granitic terrains. In both sites, temperature monitoring is either achieved in natural conditions or during hydraulic tests in nearby boreholes. Changes in borehole temperature are commonly observed in most boreholes during hydraulic tests in nearby boreholes, showing that temperature is a good proxy of flow is such conditions. Such temperature data may be easily used to deduce the connectivity between the different karstic levels or fractures and to estimate the hydraulic properties of the aquifer. More surprisingly, intermittent fluxes and daily changes were also observed during temperature monitoring in ambient conditions or recovery tests. These changes, which reveals the sensitivity of the method for detecting flow changes, are interpreted as the consequence of small variations in hydraulic head within the different fractures or karstic levels intersected by the boreholes. FO-DTS appear thus as a promising tool for imaging heterogeneities and monitoring changes in flow dynamic in heterogeneous aquifers during the hydrological cycle.