In arid and semiarid regions, persistent hydrological imbalances illuminate the considerable gaps in our spatiotemporal understanding of fundamental catchment‐scale governing mechanisms. The Salar de Atacama basin is the most extreme example of groundwater‐dominated continental basins and therefore is an ideal place to probe these unresolved questions. Geochemical and hydrophysical observations indicate that groundwaters discharging into the basin reflect a large regional system integrated over 10²–10⁴ year timescales. The groundwater here, as in other arid regions, is a critical freshwater resource subject to substantial demand from competing interests, particularly as development of its world‐class lithium brine deposit expands. Utilizing a uniquely large and comprehensive set of H and O isotopes in water, we demonstrate that much of the presumed recharge area on the Altiplano‐Puna plateau exhibits isotopic signatures quite distinct from waters presently discharging within the endorheic Salar de Atacama watershed. δ¹⁸O values of predicted inflow source waters are 3.6‰ to 5.6‰ higher than modern plateau waters, and ³H data from 87 discrete samples indicate that nearly all of this inflow is composed of premodern recharge (i.e., fossil water). Under plausible conditions, these distinctions cannot be explained solely by natural variability in modern meteoric inputs or by steady state groundwater flow. We present a conceptual model revealing the extensive influence of transient draining of fossil groundwater storage augmented by regional interbasin flow from the Andes. Our analysis provides robust constraints on fundamental mechanisms governing this arid continental groundwater system and a framework within which to address persistent uncertainties in similar systems worldwide.

Demand for lithium for batteries is growing rapidly with the global push to decarbonize energy systems. The Salar de Atacama, Chile holds ∼42% of the planet's reserves in the form of brine hosted in massive evaporite aquifers. The mining of these brines and associated freshwater use has raised concerns over the environmental responsibility of lithium extraction, yet large uncertainties remain regarding fundamental aspects of governing hydrological processes in these environments. This incomplete understanding has led to the perpetuation of misconceptions about what constitutes sustainable or renewable water use and therefore what justifies responsible allocation. We present an integrated hydrological assessment using tritium and stable oxygen, and hydrogen isotopes paired with remotely sensed and terrestrial hydroclimate data to define unique sources of water distinguished by residence time, physical characteristics, and connectivity to modern climate. Our results describe the impacts of prolonged drought on surface and groundwaters and demonstrate that nearly all inflow to the basin is composed of water recharged >65 years ago. Still, modern precipitation is critical to sustaining important wetlands around the salar. Recent large rain events have increased surface water and vegetation extents and terrestrial water storage while mining‐related water withdrawals have continued. As we show, poor conceptualizations of these complex hydrological systems have perpetuated the misallocation of water and the misattribution of impacts. These fundamental issues apply to arid regions globally. Our new framework for hydrological assessment in these basins moves beyond calculating gross inputs‐outputs at a steady state to include all compartmentalized stores that constitute “modern” budgets.

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 R_mod 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.

High-elevation wetlands in South America are not well described despite their high sensitivity to human impact and unique biodiversity. We describe the hydroclimatological and limnological characteristics of 21 wetlands on the High Andean Plateau of Argentina, synthesizing information gathered over ten years (2010–2020). We collected physical-chemical, phytoplankton, and zooplankton data and counted flamingos in each wetland. We also conducted an extensive analysis of climatic patterns and hydrological responses since 1985. These wetlands are shallow, with a wide range of salinity (from fresh to brine), mostly alkaline, and are dominated by carbonate and gypsum deposits and sodium-chloride waters. They tend to have high nutrient concentrations. Plankton shows a low species richness and moderate to high dominance of taxa. Flamingos are highly dependent on the presence of Bacillariophyta, which appears to be positively linked to silica and soluble reactive phosphorus availability. Climatic conditions show a strong region-wide increase in average air temperature since the mid-1980s and a decrease in precipitation between 1985–1999 and 2000–2020. These high-elevation wetlands are fundamentally sensitive systems; therefore, having baseline information becomes imperative to understanding the impact of climatic changes and other human perturbations. This work attempts to advance the body of scientific knowledge of these unique wetland systems.