Thermal reactivity at the stream–aquifer interface

The evolution of water temperature at the stream–aquifer interface and the associated heat fluxes have a major influence on the ecological state. Here, a synthetic case is developed to characterize the behavior of the stream–aquifer interface submitted to meteorological and hydrological forcings as well as to various hydraulic and thermal properties, representative of a wide range of lithofacies from clay to gravel/sand. The thermal regime of the stream–aquifer system is driven by two pseudo-periodic cycles: the annual cycle and the diurnal cycle. A thermo-hydrogeological model coupled with a parameter sampling script is used. The results highlight the drivers that have the greatest influence on heat fluxes: (1) the meteorological conditions through the seasonal thermal gradients established between the stream and the aquifer during winter and summer periods, and (2) the hydraulic conductivity at the stream–aquifer interface. Depending on the hydraulic conductivity values at the stream–aquifer interface, two thermal regimes exist: for high hydraulic conductivities, advective fluxes clearly prevail, while for lower hydraulic conductivities, conductive fluxes predominate in most cases. This depth is reduced in upwelling cases and modulated by the thermal regime. The predominance of either the streambed- or aquifer-property effects depends on the equivalent hydraulic conductivity. In downwelling conditions under a given meteorological setup, stream–aquifer disconnection leads to increased advective fluxes compared with its connected counterpart. Results from this study provide better insight into heat fluxes at the stream–aquifer interface, which will ultimately result in a better stream heat-balance model.