Coastal groundwater-level trends reveal global susceptibility to seawater intrusion

Abstract Coastal groundwater is a vital freshwater source threatened by overabstraction and sea-level rise, yet global patterns of declining groundwater levels and susceptibility to seawater intrusion (SWI) remain poorly constrained. Here we present a global assessment based on in situ observations from ~480,000 coastal monitoring locations. From 1990 to 2024, 21% of gridded coastal areas show statistically detectable rising or falling groundwater-level trends with magnitudes ≥0.1 m yr −1 and with declines becoming more frequent in the last 9 years. More pronounced changes are observed for deeper water tables ( ρ s = 0.63), in arid settings ( ρ s = 0.56) and in some rural areas. Seawater intrusion susceptibility is higher where seaward freshwater discharge is weak or where hydraulic gradients reverse landward, leaving limited hydraulic resistance to seawater intrusion. Extrapolating observed trends suggests that these conditions mostly persist (93.4%), while 3.5% newly emerge and 3.1% stabilize (gradients strengthen seaward). The results provide global evidence for prioritizing monitoring and management of coastal groundwater at risk of salinization.