DYNAMICS AND HYDROGEOLOGY OF THE FRESHWATER/BRINE TRANSITION ZONE AT THE SALAR DE ATACAMA, CHILE

The Salar de Atacama is a hyper-arid endorheic basin in northern Chile that has accumulated approximately 1500 cubic kilometers of halite since the Miocene. Topographically driven groundwater flow off the Andean Altiplano-puna (~4500 m) plateau delivers significant amounts of water to the basin floor at ~2300 m. Characteristic of many endorheic basins, fresh groundwater entering the basin is evaporated and concentrated into high-density brine that reaches saturation with halite over a narrow transition zone. In this talk we focus on this ~8 km zone because it is this loci of robust geochemical transformations, high ground water discharge, and supports unique migratory bird habitat.

We present a 3-dimensional numerical and conceptual model of the transition zone that is parameterized using geophysical surveys, borehole information, hydraulic data, and geochemical analysis of both brine and freshwater. Due to the dynamic interaction of low-density freshwater and high-density brine we use density-driven hydrogeologic models to predict the spatial and temporal distribution of ground water discharge and salinity. Observations suggest that a deep brine extends beneath the whole of the transition zone and at least 10 km out from the salt margin. Evaporation of the shallow ground water has a strong control on the dynamics of this zone. Mineral precipitation/dissolution reactions have a strong influence on the 3-dimensional distribution of hydraulic properties of the subsurface. The simulation results have applications for assessing climatic and anthropogenic impacts of the resilience of these ground water dependent ecosystems.