Geophysical insights into the shallow groundwater system of the Salar de Huasco, northern Chile

Altiplano salt flats in northern Chile are areas of high hydrological and ecological importance, due to the availability of water in extremely arid environments. Groundwater dynamics in these areas are complex, largely because of density driven flow. The Salar de Huasco (20.2°S; 68.8°W; 4,164 m a.s.l.) is a salt flat located in an endorheic basin in the Chilean altiplano, which has little anthropogenic disturbance. The main surface water expressions in the basin are a shallow saline lagoon with an area of ∼2 km², and the Collacagua river that infiltrates ∼8 km north of the lagoon. The salt flat counts with two monitoring stations, with meteorological, soil and groundwater sensors. Amongst other geophysical methods, electrical resistivity tomography (ERT) data have been acquired on the salt flat during austral spring. Six 160m ERT profiles were measured using a Schlumberger array, in the study area. Two profiles were measured outside the salt flat, where the Collacagua river completely reinfiltrates. Two other profiles were measured inside the salt flat, near one of the monitoring sites; and the last two were measured along its edge, at the second monitoring site. The profiles were inverted using pyGIMLi, an open-source python library. The inversion models revealed three main geo-electrical units: a resistive unit (∼600 Ωm), interpreted as dry sediment or ignimbrite, an intermediate unit (∼100 Ωm), corresponding to freshwater-saturated sediment, and a rather conductive unit (∼40 Ωm), associated with sediment saturated in water with higher salinity, or sediments with a smaller grain size saturated with freshwater. The preliminary interpretation of the models indicates a groundwater depth between 0 and 10 m, and a brine-freshwater mixing zone reaching a minimum depth of 20 m in the northernmost monitoring station. The spatial distribution of these structures supports a conceptual model of an upwelling of freshwater pushed from beneath by a denser saline wedge, never considered at the study site. The ERT profiles allow to distinguish vertical and horizontal variations in electrical resistivity, aiding to further understand and characterize the groundwater systems in this salt flat. If confirmed by complementary measurements and analyses, this upwelling groundwater flow could be significant in maintaining superficial ecosystems.