Resumen de Hydrogeology of salt flats: the salar de atacama example
This thesis aims to advance the knowledge of the hydrogeological functioning of salt flats in order to achieve a sustainable management of their mineral resources and ecosystems. Salt flats are a major source of Li, B, I, K, Mg and NaCl, and they host some of the most outstanding ecosystems on earth. Around 25 % of Li world reserves are located in the Salar de Atacama (NE of Chile). The exploitation of this raw material, highly valued in the technological and pharmaceutical industries, is carried out by brine pumping. Brine pumping can affect the natural hydrogeological regime of salt flats and, in many cases, the possible impact on their ecosystems is unknown.
This thesis contributes to (1) explain the geometry and groundwater flow of the mixing zone (saline interface) located around salt flats, (2) define the water balance and hydrodynamics of the Salar de Atacama under the natural regime, (3) analyse and quantify the impacts caused by brine pumping, (4) improve the design of the brine exploitations in salt flats and (5) question the traditional ideas on the origin of the Li-enrichment in the Salar de Atacama.
The groundwater recharged in the mountains is forced to flow upward when it reaches the mixing zone around the salt flat due to its lower density, feeding the lake and wetland ecosystems. The first 3D mapping of a salt flat mixing zone has been carried out in the Salar de Atacama and it has shown a slope of the mixing zone much lower than previous assumptions. The 2D modelling of the mixing zone demonstrated as the permeability of the upper aquifer increases, the slope of the mixing zone decreases, resulting in a shallower mixing zone. Thus, the permeability of the upper aquifer, mostly constituted of very permeable karstified evaporites and alluvial deposits, is critical to the geometry of the mixing zone.
The hydrogeological conceptual model of the Salar de Atacama, prior to brine pumping (natural regime), has been defined and validated using a 3D steady-state groundwater flow model. The asymmetric distribution of the recharge by infiltration of rainwater and, above all, the evaporation discharge results in a water table whose minimum hydraulic head is located in the easternmost nucleus, close to the eastern mixing zone.
The 3D transient-state modelling of the Salar de Atacama basin from 1986 to 2018 has allowed to explain the hydrodynamics and water balance impacts caused by brine exploitation. From 1994 to 2015, under the mining regime, the brine pumping caused an additional drawdown of the water table that triggered a decrease of the phreatic evaporation. This effect has been defined as "damping capacity" and it allows to counteract the disturbances occasioned by natural or anthropogenic events on the water table and balance of salt flat basins. The damping capacity compensated, at least in part, the decrease of the evaporation discharge, but nowadays it is already amortized in the nucleus and the mixing zone begins to be impacted.
The 3D modelling of a hypothetical salt flat under brine exploitation has contributed to optimize the spatial distribution of the pumping wells in terms of impact on the water balance. Brine exploitations should consider to distribute the pumping outflow in the greatest extent possible to take full advantage of the damping capacity.
The results of the hydro-thermo-haline modelling of the Salar de Atacama basin has suggested the Salar Fault System as the main contributor for the extreme Li-enrichment, either through the rise of Li-rich brines from a deep hydrothermal reservoir or remobilization of ancient layers of Li-enriched salts and/or clays by non-evaporated recharge waters coming from the W. The persistence of the mixing zones in depth also dismissed previous ideas of (1) leaking of brines from the present salt flats or (2) leaching of hypothetical salt flats buried by volcanic eruptions in the Altiplano-Puna as sources for the extreme Li-enrichment of the Salar de Atacama brines.
