Assessing permafrost structures in headwater aquifers: an example from the Ojos del Salado massif, Andes mountains

In arid regions, the hydrological evolution of high mountains is a matter of concern under current climate forcing and increasing freshwater demand. Mountain surface hydrology is key for water storage and release and determines the amount and quality of freshwater supply for downstream ecosystems, so predicting their evolution under climate change scenarios requires detailed spatial data on subsurface hydrodynamic properties.In the present contribution, a semi-direct characterization of periglacial areas and permafrost zonation was carried out along an altitudinal transect at the Ojos del Salado massif (27°06’ S; 68°32’ W) between 4,550 and 5,830 m a.s.l. by integrating geophysics (electrical resistivity tomography; ERT) and decade-long surface temperature datasets. ERT data evidence a permafrost altitudinal gradient from a negative control at 4,550 m a.s.l. up to consistent (>100 kΩm) permafrost-related resistivities above 5,260 m a.s.l. These resistivity structures are assumed to act as confining layers, accounting for thicknesses of 8 and 25 m at the Atacama (5,260 m a.s.l.) and Tejos (5,830 m a.s.l.) sites, respectively. The geophysically determined permafrost distribution is coherent with temperature-based Frost number estimates at all sites surveyed.The results presented here are required for aquifer parameterization under short- and mid-term hydrological connectivity changes, being therefore relevant for a better understanding of groundwater storage dynamics upon permafrost degradation in arid regions.