Hydrogeologic and Geochemical Distinctions in Salar Freshwater Brine Systems

Lee Ann Munk, David F Boutt, Brendan J Moran , Sarah Virginia McKnight, Jordan Jenckes

The Salar de Atacama contains one of the world’s most important lithium resources and hosts unique and fragile desert ecosystems. Water use issues of the hyper-arid region have placed it at the center of global attention. This investigation is the first robust assessment of a salar system to incorporate geology, hydrogeology, and geochemistry of the aquifer system in the inflow, transition zone and the nucleus. Multiple physico-chemical parameters including conductivity, temperature, Li and Na, and multiple isotopic indicators (3H, ẟD, and 87Sr/86Sr) all conclude that the transition zone water zones are distinct and separated from the brine in the halite nucleus. Geochemical modeling indicates that the inflow and transition waters are saturated with respect to calcite whereas lagoons, transition zone margin, halite nucleus margin and nucleus waters are saturated with respect to calcite, gypsum, and halite, and the transition zone brines at depth display a broader range of saturation states as compared to the nucleus brines. Long-term remote-sensing of surface water body extents suggest that extreme precipitation events are the primary driver of surface area changes (by a factor of 2.7 after storm). A major finding from this work is that the subsurface brines in the transition zone and the halite nucleus are geochemically and hydraulically disconnected from the groundwater discharge features (lagoons) over modern time scales which has far reaching implications for understanding the link between brine and freshwater.