Groundwater Salinity Dynamics in Coastal Aquifers

Coastal aquifers host a complex interplay between freshwater and saline waters driven by natural processes and human activities. Sea-level fluctuations, tidal forcing and groundwater abstraction alter hydraulic gradients, prompting saltwater to migrate inland and fresh groundwater to discharge offshore. Density contrasts between fresh and saline water lead to convective mixing, while geological heterogeneity governs the pathways and rates of salt intrusion. Submarine groundwater discharge returns both meteoric and recirculated seawater to the coastal ocean, influencing coastal biogeochemistry. Offshore freshened groundwater systems, emplaced during past low-sea-level stands, represent substantial but often hidden water reserves. Understanding the spatial distribution, driving mechanisms and long-term evolution of salinity patterns in coastal aquifers is essential for sustainable water management, ecosystem health and adaptation to climate-driven sea-level rise.

Research from Nature Portfolio

Recent studies have revealed vast offshore freshwater bodies previously undetected by conventional surveys. In one regionally focused investigation, combined marine geophysics, pore-water geochemistry and palaeo-hydrological modelling uncovered a large freshened groundwater reservoir extending more than fifty kilometres seaward of a major estuary, suggesting significant potable water potential and novel insights into sedimentary controls on recharge. A separate investigation using controlled-source electromagnetic, seismic reflection and borehole data characterised an offshore freshened aquifer system on an active margin, demonstrating how fault-driven conduits and past sea-level lowstands shaped its architecture and volumetric distribution. Foundational work along the U.S. Atlantic margin employed shallow-water electromagnetic mapping to image continuous low-salinity aquifers up to ninety kilometres offshore, attributing their emplacement to glacial-eustatic processes and modern onshore–offshore hydrologic connectivity. These combined findings highlight the global prevalence of offshore freshened groundwater and the importance of integrated geophysical and hydrogeological approaches to quantify its extent and controls.

Groundwater Salinity Dynamics in Coastal Aquifers publication trend

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

Technical terms

Aquifer: A permeable geological formation capable of storing and transmitting groundwater.

Saltwater intrusion: Landward movement of saline water into freshwater aquifers driven by hydraulic gradients or density differences.

Submarine groundwater discharge (SGD): Flow of groundwater and recirculated seawater from coastal aquifers into the marine environment.

Offshore freshened groundwater (OFG): Sub-seafloor low-salinity water bodies emplaced during past sea-level lowstands within continental margins.

Density-dependent flow: Groundwater movement controlled by variations in fluid density, typically between fresh and saline water.

References

1. Offshore freshened groundwater in the Pearl River estuary and shelf as a significant water resource. Nature Communications (2023).
2. 3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight. Nature Communications (2020).
3. Aquifer systems extending far offshore on the U.S. Atlantic margin. Scientific Reports (2019).
4. Autonomous Large-Scale Radon Mapping and Buoyant Plume Modeling Quantify Deep Submarine Groundwater Discharge: A Novel Approach Based on a Self-Sufficient Open Ocean Vehicle. Environmental Science and Technology (2023).
5. Global Impact of Sea Level Rise on Coastal Fresh Groundwater Resources. Earth's Future (2024).
6. A global coastal permeability dataset (CoPerm 1.0). Scientific Data (2024).