Paleo-salt water dominates coastal aquifer salinization: A continental-scale study in China.

Seawater intrusion poses a major threat to freshwater resources in coastal aquifers worldwide. In China alone, these types of aquifers provide a vital groundwater source for nearly 400 million people. Whereas modern seawater intrusion is often attributed to human activities and climate change, the role of paleo-salt water remains poorly quantified. This continental-scale investigation of seawater intrusion in China reveals that paleo-salt water, not modern seawater, dominates salinity patterns in large sedimentary basins. Analysis of over 2100 water samples shows a strong correlation between intrusion and tectonic subsidence zones, with paleo-salt water affecting 66% of the observed salinized areas. In northern coastal plains, paleo-salt water trapped in confined aquifers is characterized by salinity levels exceeding 144.6 grams per liter, whereas modern seawater intrusion (34%) occurs in hilly regions with thinner sediments. Isotopic signatures (δ18O) trace paleo-salt water intrusion to Quaternary sea level changes. These findings improve seawater intrusion source identification and inform global salinity management.