Modeling flow and reactive transport to explain mineral zoning in the Atacama salt flat aquifer, Chile

Abstract

Geochemical processes are typically neglected when modeling groundwater flow in highly saline systems. In the Atacama salt flat, northern Chile, field data indicate that the main aquifer presents a mineralogical zoning consisting of carbonates, sulfates and halite, which is a result of precipitation and/or dissolution reactions. When precipitation and/or dissolution of minerals are active processes in the aquifer, physical properties that control groundwater flow, such as porosity and permeability, would also change. Until now, groundwater flow in the Atacama salt flat has been simulated with conventional constant- or variable-density flow models. This study, for the first time, investigated the importance of incorporating geochemical processes in modeling the Atacama salt flat aquifer. In particular, the impact of mineral precipitation/dissolution on aquifer’s physical properties and flow patterns was studied. To this end, the SHEMAT code was utilized to develop a two-dimensional numerical groundwater flow model in the eastern border of the Atacama salt flat. The SHEMAT code solves the classic groundwater flow equation coupled to the reactive transport of ions and mineral precipitation, with variable density, viscosity, porosity and permeability. When geochemical processes were included in the simulations, the model reproduced reasonably well the mineral zoning evidenced by field observations. Mineral precipitation was responsible of a strong permeability reduction in areas of the aquifer associated with halite and calcite formation. Permeability alterations were responsible of changes in the equivalent freshwater head that resulted in significant flow pattern variations compared to variable-density flow models. These results highlight the importance of considering geochemical processes that modify the aquifer’s physical properties when modeling groundwater flow in highly saline aquifers such as the Atacama salt flat aquifer.