Designing a multi-scale sampling system of stream–aquifer interfaces in a sedimentary basin

• Methodological Framework to study stream–aquifer interaction.
• Imaging of structural heterogeneities at the regional and local scales.
• Geostatistical interpolation of piezometric head snapshot campaigns.
• Hyporheic zone temperature profiles used as a tracer of the flow.
• Quantification of water fluxes using thermo–hydro modeling.

A methodological framework is proposed for designing a multi-scale sampling system to quantify the stream–aquifer interactions in a multi-layer aquifer system. First, geophysical and drilling investigations are performed to assess the regional structure of the aquifer system and the local connectivity between streams and aquifer units. At the catchment scale, the investigations permit to define the composition of the upper aquifer unit. At the local scale, the connectivity status between streams and aquifer units is evaluated using various settings for electrical resistivity tomographies. These geophysical investigations are then used to select local monitoring stations (LMSs) along the stream network. Moreover, piezometric head maps representative of low and high flow regimes are interpolated using geostatistics, which provides distributions of both piezometric heads and standard deviations of the estimation error (STEE). The map of STEE is used to define the location of new piezometers. Altogether, the sampling system allows for monitoring water exchanges on a 40 km2 watershed along 6 km of the stream network, with a finer hydro-geophysical sampling at each LMS. Finally, temperature profiles in the HZ are interpreted with a coupled thermo–hydro finite element code at the upstream station of the domain. Multiple simulations indicate first proof of evidence for a gaining stream in the upstream part of the sampling domain.