Hydrogeologic and Geochemical Distinctions in Salar Freshwater Brine Systems

Abstract

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.

Impact of Hydrostratigraphic Continuity on Brine‐to‐Freshwater Interface Dynamics: Implications From a Two‐Dimensional Parametric Study in an Arid and Endorheic Basin

“…The hydrostratigraphic correlation is based on over 50 hydraulic tests that have occurred in the area. Hydrostratigraphic interpretations were separated into five lithologic facies with assigned hydraulic conductivity based on the conceptualization in Munk et al (2020): medium-grain clastic from alluvial fan deposits (10 m⋅d −1 ), fine-grain carbonate (1 m⋅d −1 ), vuggy carbonate (100 m⋅d −1 ), un-fractured ignimbrite (0.01 m⋅d −1 ), and interbedded gypsum and carbonate (0.1 m⋅d −1 ). Hydraulic conductivity values were determined within one standard deviation from the average for each lithostratigraphic facies.…”

“…Figure 2 illustrates how salars can develop a range of stratigraphically complex aquifers, and how density-driven freshwater upwelling occurs above the brine-to-freshwater interface in brine-bearing aquifers. Their climate-sensitive change in areal extent can create a series of interbedded lithologies (Houston, 2009), and this interbedding at SdA is specifically documented in Munk et al (2020). Since brine-bearing aquifers commonly exist in tectonically active endorheic basins (Yager et al, 2017), faulting among lithologies further complicate the lateral continuity of subsurface heterogeneity and produce interface geometries that defy theory when intersecting fault systems (Yechieli, 2000).…”