In groundwater hydrology, the characterization of the distribution of groundwater flow within the critical zone received considerable attention in the last decades (Freeze & Cherry, 1979). Our ability to quantify groundwater flow greatly controls our ability to characterize aquifers, predict contaminant transport, and understand biogeochemical reactions and processes occurring in the subsurface (Kalbus et al., 2009; Poeter & Gaylord, 1990). Groundwater flow at interfaces such as recharge and discharge areas also plays a key role in the preservation of groundwater-dependent ecosystems (Kalbus et al., 2006; Sophocleous, 2002). The quantification of groundwater fluxes is also particularly relevant for geothermal energy since they control heat exchange and storage capacities (Diao et al., 2004). Similarly, the characterization of seepage through dams, dikes, and reservoirs is also critical for geotechnical engineering (Foster et al., 2000). The spatial distribution of groundwater fluxes is largely driven by subsurface heterogeneities. Thus, in past decades, the characterization of the distribution of groundwater fluxes and their quantification relied on the capacity of characterizing and modeling the spatial variability of hydraulic conductivities (de Marsily, 1976). Considering the challenge in characterizing the field variability of hydraulic properties, the use of heat as a tracer has been widely developed and applied to characterize flow in aquifers or at interfaces such as the hyporheic zone (

Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using heating cable was verified in the field as an option to conduct this heat injection experiment with a low power source and a compact equipment. Two thermal response tests using heating cable sections and a continuous heating cable were performed in two experimental heat exchangers on different sites in Canada and France. The temperature evolution during the tests was monitored using submersible sensors and fiber optic distributed temperature sensing. Free convection that can occur in the pipe of the heat exchanger was evaluated using the Rayleigh number stability criterion. The finite and infinite line source equations were used to reproduce temperature variations along the heating cable sections and continuous heating cable, respectively. The thermal conductivity profile of each site was inferred and the uncertainly of the test was evaluated. A mean thermal conductivity 15% higher than that revealed with the conventional test was estimated with heating cable sections. The thermal conductivity evaluated using the continuous heating cable corresponds to the value estimated during the conventional test. The average uncertainly associated with the heating cable section test was 15.18%, while an uncertainty of 2.14% was estimated for the test with the continuous heating cable. According to the Rayleigh number stability criterion, significant free convection can occur during the heat injection period when heating cable sections are used. The continuous heating cable with a low power source is a promising method to perform thermal response tests and further tests could be carried out in deep boreholes to verify its applicability.