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Sources of Water and Solutes to the Salar de Atacama, Chile: A Coupled Hydrologic, Geochemical, and Groundwater Modeling Study

Abstract
Focused groundwater discharge in endorheic basins provides opportunities to investigate mechanisms for closing hydrologic budgets in arid regions. The Salar de Atacama (SdA), a closed basin in northern Chile, has accumulated over 1800 km3 of halite and a lithium-rich brine since the late Miocene primarily through evapotranspiration of groundwater. The hydrologic balance of SdA and sources of water and solutes required to explain this deposit are not well constrained. An adapted chloride mass balance method drawing on a database of over 200 water sample sites is applied to a remotely-sensed precipitation dataset to estimate spatially-distributed modern groundwater recharge. Comparing groundwater recharge to evapotranspiration in a steady-state water budget constrains potential regional-scale watersheds on the Altiplano-Puna Plateau. The sodium mass balance of the deposit is used to predict long-term water discharge from the basin and place modern fluxes in a paleo-hydrologic context. A 2D groundwater model informed by published paleoclimate reconstructions evaluates whether draining groundwater storage contributes to the modern hydrologic system. Modern recharge from precipitation in the topographic watershed is extremely small compared to evapotranspiration. The missing water is sourced from precipitation in an area over 4 times larger than the topographic watershed, and groundwater recharged during wetter periods in the late Pleistocene is still actively draining and discharging from storage without a corresponding input into the system. These results have implications for lake-level based paleoclimate reconstructions, conceptualizations of watershed boundaries and water resource management.
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