CLOSING WATER AND SOLUTE BUDGETS IN ARID ENDORHEIC BASINS: REFINING THE COMPLIMENTARY ROLE OF REGIONAL-SCALE GROUNDWATER FLOW AND TRANSIENT STORAGE

In the driest deserts of the world, groundwater resources are often relicts of paleohydrologic recharge events. The response time, flow paths, and timing of natural aquifer recharge place critical constraints on modern hydrologic investigations, paleoclimate reconstructions, and resource assessments, yet difficulties elucidating these processes are confounded by long residence times, deep water tables, and large uncertainties in hydrologic budgets. Focused groundwater discharge in endorheic basins, such as those in the Chilean Altiplano, provide opportunities to investigate mechanisms for closing hydrologic budgets in arid regions. The Salar de Atacama (SdA), a large endorheic basin adjacent to the Central Andes in the hyperarid Atacama Desert, has accumulated over 1800 km³ of evaporites and a lithium-rich brine since the late Miocene. We demonstrate that modern evapotranspiration is 5 to 21 times greater than modern recharge from precipitation in the topographic watershed. Multiple lines of evidence including an adapted chloride mass balance method applied to remotely sensed precipitation estimates and sodium mass balance calculations support this conclusion. We contend that the missing water needed to close the extreme hydrologic imbalance of SdA is sourced from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed, augmented by transient draining of stored groundwater. A 2D groundwater flow model of the potential hydrogeologic watershed examines whether this system is still responding to climatic forcing from past pluvial periods. Modeling predicts that during higher than modern recharge rates groundwater divides extend significantly beyond the topographic divides. After the initial release of water from transient draining the groundwater divides move further away from the topographic divide. The application of steady state assumptions to the modern hydrologic system are clearly unsupported by observations and difficult to justify in this geologic setting with extremely low recharge rates and high topographic relief.