The growing demand for renewable energy leads to an increase in the development of 1 geothermal energy projects and heat has become a common tracer in hydrology and 2 hydrogeology. Designing geothermal systems requires a multidisciplinary approach including 3 geological and hydrogeological aspects. In this context, electrical resistivity tomography 4 (ERT) can bring relevant, qualitative and quantitative information on the temperature 5 distribution in operating shallow geothermal systems or during heat tracing experiments. We 6 followed a heat tracing experiment in an alluvial aquifer using cross-borehole time-lapse 7 ERT. Heated water was injected in a well while water of the aquifer was extracted at another 8well. An ERT section was set up across the main flow direction. The results of ERT were 9 transformed into temperature using calibrated petrophysical relationships. These ERT-derived 10 temperatures were then compared to direct temperature measurements in control piezometers 11 collected with distributed temperature sensing (DTS) and groundwater temperature loggers.

[en] Geothermal energy systems, closed or open, are increasingly considered for heating and/or cooling buildings. The efficiency of such systems depends on the thermal properties of the subsurface. Therefore, feasibility and impact studies performed prior to their installation should include a field characterization of thermal properties and a heat transfer model using parameter values measured in situ. However, there is a lack of in situ experiments and methodology for performing such a field characterization, especially for open systems. This study presents an in situ experiment designed for estimating heat transfer parameters in shallow alluvial aquifers with focus on the specific heat capacity. This experiment consists in simultaneously injecting hot water and a chemical tracer into the aquifer and monitoring the evolution of groundwater temperature and concentration in the recovery well (and possibly in other piezometers located down gradient). Temperature and concentrations are then used for estimating the specific heat capacity. The first method for estimating this parameter is based on a modeling in series of the chemical tracer and temperature breakthrough curves at the recovery well. The second method is based on an energy balance. The values of specific heat capacity estimated for both methods (2.30 and 2.54 MJ/m3/K) for the experimental site in the alluvial aquifer of the Meuse River (Belgium) are almost identical and consistent with values found in the literature. Temperature breakthrough curves in other piezometers are not required for estimating the specific heat capacity. However, they highlight that heat transfer in the alluvial aquifer of the Meuse River is complex and contrasted with different dominant process depending on the depth leading to significant vertical heat exchange between upper and lower part of the aquifer. Furthermore, these temperature breakthrough curves could be included in the calibration of a complex heat transfer model for estimating the entire set of heat transfer parameters and their spatial distribution by inverse modeling

Using heat as an active tracer for aquifer characterization is a topic of increasing interest. In this study, we investigate the potential of using heat tracer tests for characterization of a shallow alluvial aquifer. A thermal tracer test was conducted in the alluvial aquifer of the Meuse River, Belgium. The tracing experiment consisted in simultaneously injecting heated water and a dye tracer in a piezometer and monitoring the evolution of groundwater temperature and tracer concentration in the recovery well and in monitoring wells. To get insights in the 3D characteristics of the heat transport mechanisms, temperature data from a large number of observation wells distributed throughout the field site (space-filling arrangement) were used.Temperature breakthrough curves in observation wells are contrasted with what would be expected in an ideal layered aquifer. They reveal strongly unequal lateral and vertical components of the transport mechanisms. The observed complex behavior of the heat plume was explained by the groundwater flow gradient on the site and heterogeneity of hydraulic conductivity field. Moreover, due to high injection temperatures during the field experiment a temperature-induced fluid density effect on heat transport occurred. By using a flow and heat transport numerical model with variable density coupled with the pilot point inverse approach, main preferential flow paths were delineated.

International audienceMeasurement of groundwater fluxes is the basis of all hydrogeological study, from hydraulics characterization to the most advanced reactive transport modelling. Usual groundwater fluxes estimation with Darcy's law may lead to cumulated errors on spatial variability, especially in fractured aquifers where local direct measurement of groundwater fluxes becomes necessary.In the present study, both classical Point Dilution Method (PDM) and Finite Volume Point Dilution Method (FVPDM) are compared on the fractured crystalline aquifer of Ploemeur, France. The manipulation includes the first use of the FVPDM in a fractured aquifer using a double packer. This configuration limits the vertical extend of the tested zone to target a precise fracture zone of the aquifer. The result of this experiment is a continuous monitoring of groundwater fluxes that lasted for more than 4 days.Measurements of groundwater flow rate in the fracture (Qt) by PDM provide good estimates only if the mixing volume (Vw) (volume of water in which the tracer is mixed) is precisely known. Conversely, the FVPDM allows for an independent estimation of Vw and Qt, leading to better precision in case of complex experimental setup such as the one used. The precision of a PDM does not rely on the duration of the experiment while a FVPDM may require long experimental duration to guarantees a good precision.Classical PDM should then be used for rapid estimation of groundwater flux using simple experimental setup. On the other hand, the FVPDM is a more precise method that has a great potential for development but may require longer duration experiment to achieve a good precision if the groundwater fluxes investigated are low and/or the mixing volume is large

16Compound-specific isotope analysis (CSIA) is a powerful tool to track contaminant fate in 17 groundwater. However, the application of CSIA to chlorinated ethanes has received little 18 attention so far. These compounds are toxic and prevalent groundwater contaminants of 19 environmental concern. The high susceptibility of chlorinated ethanes like 20 1,1,1-trichloroethane (1,1,1-TCA) to be transformed via different competing pathways (biotic 21 and abiotic) complicates the assessment of their fate in the subsurface. In this study, the use of 22 a dual C-Cl isotope approach to identify the active degradation pathways of 1,1,1-TCA is 23 This study demonstrates that a dual C-Cl isotope approach can strongly improve the 36 qualitative and quantitative assessment of 1,1,1-TCA degradation processes in the field. 37 38