Use of heat as a groundwater tracer in fractured rock hydrology

International audienceCrystalline rocks aquifers are often difficult to characterize since flows are mainly localized in few fractures. Inparticular, the geometry and the connections of the main flow paths are often only partly constrained with classicalhydraulic tests. Here, we show through few examples how heat can be used to characterize groundwaterflows in fractured rocks at the borehole, inter-borehole and watershed scale. Estimating flows from temperaturemeasurements requires heat advection to be the dominant process of heat transport, but this condition is generallymet in fractured rock at least within the few structures where flow is highly channelized. At the borehole scale,groundwater temperature variations with depth can be used to locate permeable fractures and to estimates boreholeflows. Measurements can be done with classical multi-parameters probes, but also with recent technologies such asFiber Optic Distributed Temperature Sensing (FO-DTS) which allows to measure temperature over long distanceswith an excellent spatial and temporal resolution. In addition, we show how a distributed borehole flowmeter canbe achieved using an armored fiber-optic cable and measuring the difference in temperature between a heatedand unheated cable that is a function of the fluid velocity. At the inter-borehole scale, temperature changes duringcross-borehole hydraulic tests allow to identify the connections and the hydraulic properties of the main flowpaths between boreholes. At the aquifer scale, groundwater temperature may be monitored to record temperaturechanges and estimate groundwater origin. In the example chosen, the main water supply comes from a depth of atleast 300 meters through relatively deep groundwater circulation within a major permeable fault zone. The influenceof groundwater extraction is clearly identified through groundwater temperature monitoring. These examplesillustrate the advantages and limitations of using heat and groundwater temperature measurements for fracturedrock hydrology