Saltwater Intrusion Dynamics in Coastal Aquifers

Saltwater intrusion occurs when denser marine waters encroach upon freshwater in coastal aquifers, driven by variables such as sea-level rise, excessive groundwater pumping and reduced recharge. This process forms a characteristic saltwater wedge beneath a freshwater lens, with a mixing or transition zone in between. The intrusion dynamics are governed by density-driven flow, aquifer heterogeneity, hydraulic conductivity and boundary conditions including tidal fluctuations and inland water-table variations. Geological factors such as stratigraphy, anisotropy and permeability contrasts determine the shape and landward extent of the saltwater interface, while human activities and climate-induced changes in recharge and sea levels expedite or retard intrusion. Understanding these interactions is essential for sustainable management of coastal freshwater resources, for designing monitoring networks and for implementing measures such as managed aquifer recharge, pumping restrictions and hydraulic barriers.

Research from Nature Portfolio

Analyses of a quarter-million coastal groundwater-level observations across the contiguous United States reveal that over 15 % of coastlines exhibit water tables below sea level, creating landward hydraulic gradients conducive to seawater intrusion. The study highlights regional hotspots along the East, Gulf and parts of the West Coast where pumping and subsidence amplify vulnerability, and underscores the compounding effects of future sea-level rise and greater water demand. In a separate multidisciplinary investigation of a karstic aquifer in Western Australia, long-term field data, numerical simulation and electrical resistivity imaging were combined to map the geometry of the seawater interface. Findings demonstrate how hydraulic anisotropy and spatially correlated heterogeneity control intrusion length and mixing-zone thickness, and reveal limitations of conventional monitoring wells for constraining rapid pressure and salinity changes near the freshwater–saltwater interface.

Saltwater Intrusion Dynamics in Coastal Aquifers publication trend

The graph below shows the total number of articles in saltwater intrusion dynamics in coastal aquifers across all publications each year (not limited to Nature Index journals).

Technical terms

Saltwater Intrusion: The landward movement of saline water into freshwater aquifers, often forming a wedge-shaped interface beneath the freshwater lens.

Saltwater Wedge: The triangular intrusion of denser saline water under a freshwater lens in a coastal aquifer.

Density-Driven Flow: Groundwater movement controlled by spatial variations in fluid density, typically due to salinity contrasts.

Hydraulic Conductivity: A measure of an aquifer’s ability to transmit water, often varying with direction (anisotropy).

Freshwater–Saltwater Interface: The transitional boundary zone in an aquifer where freshwater and saltwater mix, defined by a gradient in salinity.

References

1. Controls on coastal saline groundwater across North America. Environmental Research Letters (2025).
2. Climate‐Induced Saltwater Intrusion in 2100: Recharge‐Driven Severity, Sea Level‐Driven Prevalence. Geophysical Research Letters (2024).
3. Comparing Darcy’s Law and the Brinkman Equation for Numerical Simulations of Saltwater Intrusion. Sustainability (2023).
4. Groundwater level observations in 250,000 coastal US wells reveal scope of potential seawater intrusion. Nature Communications (2020).