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Abstract

Groundwater flow in coastal aquifers is influenced by density differences between freshwater and saline water, forming a complex density-dependent flow system. The relatively narrow zone where freshwater and saline water mix is characterized by strong geochemical gradients that can drive reactions. Understanding the dynamics of this mixing zone is crucial for understanding the biogeochemical reactions that occur during saline intrusion. At the Kidd II site adjacent to the Fraser River Estuary in Vancouver, BC, saltwater intrudes into a near-surface confined aquifer. Fresh groundwater containing dissolved organic matter sourced from young, Quaternary silts flows horizontally in the aquifer until it encounters the intruding sulfate-rich saline water. Dissolved iron and manganese concentrations peak at 370 mg/L and 12 mg/L along an ~2 – 4 m thick mixing zone between the saline water and the overlying freshwater. In this study, we first characterize flow and dispersive mixing in a confined sand aquifer subjected to saline intrusion, using both field observations and numerical modeling. Then we use 2D reactive transport modeling to interpret the geochemical processes in the aquifer, focusing on the elevated concentrations of iron and manganese in the mixing zone. We show that the elevated dissolved Fe and Mn concentrations are best explained by reductive dissolution of iron and manganese oxides coupled with sulfate reduction and concomitant oxidation of bisulfide. Secondary mineral precipitation and re-oxidation reactions affect pH, alkalinity, and metal mobility to different extents. Hydrodynamic mixing of fresh and saline groundwater controls the location and intensity of reactions. Our results highlight the importance of accurately estimating dispersivity values to model the width of the mixing zone, which strongly controls geochemical reactions at the site. These findings provide insights into the processes driving freshwater–saline water mixing in coastal aquifers and offer guidance for future modeling studies on geochemical dynamics during saline intrusion. The Kidd II site is a good example of the complex physical and geochemical processes that control iron and manganese dynamics in organic-rich, deltaic saline water intrusion systems.