International audienceThe characterization of flow and transport in fractured media is particularly challenging because hydraulic conductivityand transport properties are often strongly dependent on the geometric structure of fractures at differentscales. In addition to advection and dispersion, heat transfer is also affected by thermal retardation and damping,which results from fracture-matrix diffusion. Here, we derive analytical expressions for thermal retardation anddamping for different fracture geometries and we show, from modeling and field experiments, that estimation ofthermal retardation and damping may provide new constraints on fracture geometry. We use the developed expressionsto interpret the results of single well thermal tracer tests performed in a crystalline rock aquifer at theexperimental site of Ploemeur (H+ observatory network). Thermal breakthrough is monitored with Fiber-OpticDistributed Temperature Sensing (FO-DTS), which allows the temperature monitoring with high spatial and temporalresolution. We demonstrate that the observed thermal response indicates that heat transfer is controlled by achannel fracture of large diameter rather than by a parallel plate fracture. These results point to a strong reductionof fracture-matrix exchange by flow channeling. These findings, which bring new insights on the effect of flowchanneling on heat transfer in fractured rocks, show how heat recovery in geothermal systems may be controlledby fracture geometry. This highlights the interest of thermal tracer tests as a complement to solute tracer tests toinfer fracture aperture and geometry
