IMPACTS OF THE GEOMETRY & CLIMATIC SENSITIVITY OF BRINE-TO-FRESHWATER INTERFACES ON GROUNDWATER-FED WETLANDS IN ARID BASINS

Arid basins often contain brine-to-freshwater interfaces as evaporation outpaces recharge and results in evaporite accumulation and saline groundwater. Salar de Atacama (SdA) in northern Chile is an ideal analog for such basins with a groundwater-dependent ecosystem that coincides with the confluence of the brine-to-freshwater interface. Observations suggest that the interface influences the location of groundwater discharge features at SdA. We therefore attempt to constrain the controls on an interface’s geometry and climatic sensitivity in order to assess the potential sensitivity of groundwater-fed springs and wetlands in arid basins. We compare density-dependent groundwater models that reflect the hydrostratigraphic framework (HSF) of SdA as a mature salt flat (salar) to geostatistical variations of the HSF’s distribution of hydraulic conductivity. The salar’s model framework is based on an integration of core data, surficial geology, and permeability data from pumping tests. Three sets of geostatistical realizations have different ratios of continuous stratigraphic features that represent aquitards. Each realization of the model experienced a series of hydrologic perturbations from a steady-state. The perturbations consist of changes in modeled recharge and evapotranspiration. Metrics for evaluating interface sensitivity include time to reach a new equilibrium state from the beginning of the time period of the hydrologic perturbation as well as the length of vertical and horizontal migration that the interface experiences over this time. Results show that climatic perturbations in a basin are a first order control on a brine-to-freshwater interface’s geometry and sensitivity. However, we observe that stratigraphic features significantly impact both an interface’s surface expression and behavior at depth by limiting its sensitivity and increasing its reaction time to surficial hydrologic changes. Our analysis suggests that stratigraphic features can impact the sensitivity of brine-to-freshwater interfaces in arid regions, and thus can impact the sustainability of groundwater-fed ecosystems in arid climates.