Time-lapse inversion of resistivity data reveals infiltration response of crystalline substratum following rainfall in an Alpine catchment

Predicting groundwater recharge, storage, and transport in mountain environments is challenging due to high spatiotemporal variability and limited data. In this context, we report on a time-lapse Electrical Resistivity Tomography (ERT) survey conducted in an Alpine catchment in October 2024 involving measurements before and after a major rainfall event. Hydraulic head (yearly variations of ~40 m) and water temperature (decreasing long-term trend) have been monitored since 2010 in the Val d’Ursé sub-catchment (Poschiavo, Switzerland) through an instrumented borehole located at an elevation of ~2300 m. A 470-meter long ERT profile was centred on this borehole, where the groundwater level was located at ~25 m depth at the time of the measurements. The profile crosses a geological interface between orthogneiss, within which the borehole is located, and schist. The soil and/or alluvial cover of the crystalline substratum is very thin or absent. Using the PyGIMLi framework, we inverted apparent resistivity data collected before and after a major rainfall event with cumulative precipitation of ~50 mm within ~10 hours. The timelapse results obtained by difference inversion of the Wenner-Schlumberger data indicate a clear signature of infiltration in terms of a well-resolved decrease in resistivity on the order of 10 % in the upper 20 m of the unsaturated orthogneiss.  Following these initial results, intensive geophysical field campaigns are being planned for the next field season, which will primarily target the period shortly after snowmelt, when groundwater storage is at its maximum, and the period from late summer to early fall, when groundwater storage is much lower. In addition to ERT surveys, we plan to carry out self-potential monitoring as well as time-lapse gravity and drone-based ground-penetrating radar (GPR) measurements along the current profile and in its vicinity, which notably include a prominent rock glacier.