Earth's cold regions are rapidly degrading primarily driven by atmospheric warming. Globally, glaciers are losing mass at an accelerated rate leading to changes in the flux of freshwater, solutes, and sediment (

Recent investments in renewable energy infrastructure on a global scale have doubled that of new energy from nuclear power and fossil fuels (REN21, 2018), as the global energy focus is transitioning away from nonrenewable resources. This transition and the explosion of the electric vehicle industry has consequently led to an increased demand for lithium (Li) as one of the major components of batteries for electric vehicles with a projected compound annual growth rate of 18% until 2030 (Roskill, 2020). Lithium is primarily found in three main deposits: (a) pegmatites, (b) continental brines, and (c) clays (

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.

High-latitude regions are warming at rates of two to three times the global average (IPCC, 2007). Precipitation regime changes associated with the increasing temperatures will result in increased precipitation, primarily in the form of autumn and winter rain (Beamer et al., 2017). Associated with these changes, global glacier volume will decline 29-41% by 2100 compared to 2006 accompanied by a projected 20% decline in global glacier runoff (Radić et al., 2014). Ice fields and glaciers cover 18% of the 420,230 km 2 Gulf of Alaska (GoA) region and supply 47% of the freshwater water runoff (Neal et al., 2010). Looking to the future it is predicted that Alaska will experience a 30% decline in runoff by the end of the 21st century (Bliss et al., 2014) accompanied by a forecasted decrease of glacier volume between 32 ± 11 and 58 ± 14% for RCP2.6 and RCP8.5, respectively (Huss & Hock, 2015). As such, the climate change predicted for the 21st century will significantly alter the amount of freshwater discharging to the GoA along with changes in precipitation regimes. Currently, glacial fed streams have increased discharge compared to nonglacial precipitation fed streams. This paradigm will shift as coastal glacier coverage declines into the 21st century. Today, glaciers act as a control on seasonal runoff variation within a catchment. Stream discharge within a glacierized basin varies little year to year and peak runoff is generally predictable. Precipitation fed streams, conversely, have higher interannual variations in discharge due to their susceptibility to interannual climate variability. For example, Fountain and Tangborn (1985) suggest that basins with glacial coverage around 36% have the lowest year-to-year variation in discharge. Streams with less than 10% glacier cover, however, show large year to year variability in stream flow when compared to streams within watersheds with greater percentages of glacier coverage (Fountain & Tangborn, 1985).

The marginal environments of salar (e.g., salt flats) systems are unique ecological and hydrogeological regions of great importance in arid to hyper-arid climates (Pigati et al., 2014; Rosen, 1994; Warren, 2016). These distinctive places have become one of the most significant areas of concern in regions where groundwater and/or brine extraction are relied on for human use including resource development and fresh water sources used by communities (Houston et al., 2011; Tyler et al., 2006; Warren, 2010). As demand for water sources continues to increase (Gleeson et al., 2020; Wang et al., 2018; Zipper et al., 2020) it is critical to have a complete and scientifically based assessment of these transitional zone regions where freshwater discharges and evaporates above brackish and brine water in the subsurface. This process is what supports the formation of springs, wetlands or marshes (vegas), and lagoons (lagunas) that form important ecosystems on the margins of all salars on a global scale. However, the extent of development of these groundwater

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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.

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