Saltwater Intrusion and Geogenic Metal Mobilization in Coastal Aquifers

ABSTRACT

Coastal aquifers provide critical freshwater resources to coastal regions but are increasingly threatened by natural and anthropogenic environmental changes. Among the most significant subsurface impacts is saltwater intrusion (SWI), which can make groundwater unpotable and can mobilize geogenic sediment‐bound metals, posing acute threats to groundwater quality and coastal ecosystems. This review synthesizes our current understanding of the interrelated physical and geochemical processes linking SWI to metal mobilization in coastal aquifers. We focus on vertical SWI associated with episodic events such as storm surges, coastal flooding, and extreme tides that can rapidly introduce saline water into shallow aquifers and disturb established geochemical conditions, including redox gradients. In coastal aquifers, physical processes (e.g., hydraulic and density gradients, tidal pumping) control groundwater flow and solute transport, while geochemical conditions (e.g., redox zonations, geologic formations) control chemical transformations and solubility of geogenic constituents. Associated geochemical changes induced by physical or chemical perturbations in ionic strength, pH, alkalinity, and mineral equilibria promote reactions such as reductive dissolution of iron and manganese oxides, sulfate reduction, and competitive desorption that affect the mobilization of sediment‐bound metals. We highlight key knowledge gaps, management challenges, and research opportunities to advance our understanding of the vulnerability of fresh coastal groundwater resources to salinization and concomitant metal mobilization.