Hydrogeologic and Geochemical Distinctions in Freshwater‐Brine Systems of an Andean Salar

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 work combines geochemical and hydrogeologic data with remote sensing analysis to address differences in water zones in the marginal environments of the salar. Water samples from across the inflow to brine transition were collected over the period 2012–2016 and analyzed for ẟD, 3H, 87Sr/86Sr and major and minor elements. The ẟD values range from about −64‰ to +20‰, 3H as Rmod from 0.01 to 0.36 and the 87Sr/86Sr from 0.70750 to 0.70804 with highest ẟD and 3H occurring in the regions of open water. Geochemical modeling results indicate inflow and shallow transition zone waters are saturated with respect to calcite, whereas all others are saturated with respect to calcite, gypsum, and halite. Long‐term remote‐sensing of surface water body extents indicate that extreme precipitation events are the primary driver of surface area changes as exemplified by an increase in size of the lagoons by a factor of 2.7 after a storm. A new conceptual model of the freshwater to brine transition zone that incorporates variability in aquifer geology, hydrology and geochemistry to explain important marginal water bodies is presented. The subsurface brines in the transition zone and the halite nucleus are geochemically distinct compared to the groundwater discharge features (e.g., lagoons) over modern time scales which aids in conceptualizing the transition zone system. Plain Language Summary Salar (salt flat or playa) systems are of intense global focus because of associated water and resource issues. The freshwater‐brine systems that are unique to these areas support important ecosystems as well as both resource development and community needs. This study includes a novel integrated analysis to demonstrate how the water bodies in the marginal zones of salars are controlled by the amount of fresh water flowing from higher elevation below the ground surface, the properties of the complex subsurface geology, and climate variability. The focus is on how freshwater recharge influences physical and chemical properties of marginal water bodies including springs, wetlands, lagoons, and transitional pools. In addition, the two types of brines found in the marginal zone and from the center of the Salar de Atacama are distinct and evidently disconnected from the sh