SALT PRECIPITATION ALONG A BRINE-TO-FRESHWATER INTERFACE AS A REACTIVE HYDRAULIC BARRIER IN ARID AQUIFERS

Abstract:

Adjacent to a dense (1.2 g⸳cm^-1) brine underlying a salt flat, we analyzed freshwater equivalent (FWE) head for nested wells with depths above and below a shallow (0.001 m⸳m^-1) brine-to-freshwater interface. As this analysis combined with observations of secondary precipitation of salts indicates confined hydraulic conditions, we further investigated the role of secondary porosity by analyzing groundwater samples for major cations and anions, alkalinity, and temperature. Samples were collected along a profile parallel to projected groundwater flow that transects the interface and zoned evaporite facies, from carbonate to gypsum, and halite at the edge of the salt flat. The hydraulic impact of salt precipitation-driven closure of pores was investigated by combining geochemical modeling of saturation indices and machine learning based on thermodynamic databases with reactive transport modeling in PHREEQC, and coupling results with density-driven flow simulations in SEAWAT. Results indicate that the probability of halite precipitation laterally increases by 0.6-1.5% and gypsum precipitation increases by up to 1.4% for every 100 meters moving towards the salt flat within the upper 10 meters of the transition zone, which corresponds with halite and gypsum saturation indices progressively nearing saturation adjacent to the salt flat margin. Sensitivity analyses indicate that observed temperatures lead to halite and gypsum precipitation rates that are respectively 4.4-10% and up to 3.3% lower between the transition zone and the margin when compared to equal temperatures, suggesting that occlusion rates are influenced but not defined by temperature. CO_2(aq) distribution in equilibrium is similarly sensitive to temperature but remains highest within the transition zone, indicating that hydraulics along the interface control CO_2(aq) fluxes in these systems. SEAWAT simulations support that the resulting porosity reduction calculated from reactive transport results can create the hydraulic conditions observed in the field that lead to the above-mentioned FWE values. This work supports previous findings of the sensitivity of CO₂-brine-rock fluxes with permeability and highlights the importance of the brine-to-freshwater interface as a reactive barrier exerting a control on subsurface porosity.