RELIC AND CONTEMPORARY WATERS OF THE DRY ANDES: GLOBAL IMPLICATIONS FOR WATER AND LITHIUM RESOURCE DEVELOPMENT

The endorheic basins of the Dry Andes (the Lithium Triangle) hold ~75% of the planet’s Lithium reserves in the form of continental salar brines. The mining of these brines and associated freshwater use has raised concerns over the environmental responsibility of its extraction. These concerns are amplified by persistent hydrological uncertainties that continue to muddle the attribution and quantification of impacts. Principally these involve the spatial and temporal connections between recharge and discharge, water budget closure, and response times to perturbations. Resolving these questions is required to develop new mines and assign water rights responsibly. We present an integrated analysis across a large swath of this region utilizing a uniquely large and comprehensive set of water tracer data (³H, ¹⁸O/²H) along with physical and geochemical measurements. These data represent a robust survey of the hydrological system including new data from shallow and deep brines within major Li-bearing basins. Our results define dominant controls between groundwater and the modern water cycle that we argue are intrinsic features of these arid endorheic systems.

We outline several novel findings about the hydrology in these environments: i) the inflows sustaining hydrological systems at the regional and watershed scale are dominated by old water (0-5% modern, 100 to >1ka) yet contemporary waters (weeks to yrs) are critical to sustaining all surface water bodies. ii) Proportions of contemporary/relic water among physical water groupings show consistent patterns and sharp divergence in relative age occurs over small spatial scales (<1km); iii) the existence of surface water bodies and their connections to modern meteoric inputs is directly regulated by persistent features such as regional water tables, permeability contrasts, and perennially flowing streams; iv) and, important distinctions can be made between modern meteoric waters by δ¹⁸O source signatures. The rapid development of lithium brines worldwide and the uncertain impacts of global Climate Change on arid regions highlight the need to resolve fundamental uncertainties about hydrological systems here. This work provides a process-based framework to resolve many of these questions and improves our ability to extract these resources responsibly.