Density-dependent groundwater flow and hydrogeological response to brine extraction and reinjection in salt flat

The increasing demand of Li, required in the electrification of motoring industry, has intensified the exploitation of brines in salt flats, as one of the main known sources of this element. Continental salt falt are hydrogeological systems characterized by extreme salinity gradients that generate significant spatial variations in groundwater density and give rise to complex flow patterns dominated by thermohaline circulation. Despite their relevance, density-dependent processes are often simplified or neglected in hydrogeological models used to assess brine exploitation, introducing substantial uncertainty in the interpretation of system behavior and associated impacts.

In this contribution, a coupled numerical modelling approach is presented to analyze the hydrogeological response of a salar subject to brine exploitation under variable-density flow conditions. First, a three-dimensional regional-scale groundwater model was developed and calibrated against observed hydraulic heads and salinity distribution. The model represents basin-scale flow patterns and incorporates existing brine pumping associated with exploitation, providing a reference framework for evaluating anthropogenic perturbations.

Building on the calibrated regional model, the system response to a controlled brine injection test associated with Direct Lithium Extraction (DLE) schemes was investigated. The simulations allow assessment of the spatial and temporal evolution of the injected brine plume, as well as the interaction between pumping- or injection-induced hydraulic gradients and buoyancy forces related to density contrasts

Results indicate that both brine extraction and reinjection induce non-linear and spatially propagated responses that are strongly controlled by density contrasts and hydrostratigraphic architecture, and that differ markedly from predictions obtained using constant-density approaches. The study highlights the necessity of explicitly accounting for density-dependent flow when evaluating the impacts of brine exploitation and reinjection strategies, and provides a physically consistent modelling framework to support the assessment and management of salars under conventional and DLE schemes. Those models would help to predict not only the evolution of phreatic level in a strategy based on DLE, but also would predict groundwater flow paths and the stability of the freshwater–brine mixing zone in marginal areas.