Despite current and projected future reliance on lithium as a resource, deficiencies remain in genesis models of closed-basin Li brines. Subsurface geochemical interactions between water and bulk solid phases from lacustrine sediments, are shown here to be the most important process for brine genesis and sustainability of the Clayton Valley, NV brine deposit. A new subsurface basin model was developed and used to select Li-bearing solids to test the release mechanisms for Li. Ash (20-350 ppm Li) and bulk sediments (1000-1700 ppm Li) samples across depths in the basin represent the majority of the subsurface Li-bearing materials. Temperature dependent (25-95 ^oC) batch reaction experiments using low-salinity groundwater from the basin indicate a positive relationship between the amount of Li released and temperature. Four-step sequential extractions on a subset of bulk sediments indicate most Li is released from water and weak acid-soluble portions with approximately 30% of the total Li contained in the sediments released overall. We conceptualize that lithium is released from these samples via three mechanisms: 1) release of adsorbed Li; 2) cation exchange of Li and Mg and; 3) possible minor release from silicate structure at elevated temperatures. Based on these results and the abundance of Li-bearing sediments in the subsurface we estimate the mean Li mass in the basin materials to be between 24.4 to 58.0 Mt. This Li provides a continuous supply from water-rock interactions. This is now the largest known accumulation of Li in a basin-fill continental setting on a global scale.

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.