Coastal groundwater level trends reveal global susceptibility to seawater intrusion

Annika Nolte, Steffen Bender, Jens Hartmann, Stefan Baltruschat, Nils Moosdorf, Robert Reinecke

Coastal groundwater is a vital freshwater source threatened by overabstraction and sea-level rise, yet global patterns of declining groundwater levels (GWLs) and susceptibility to seawater intrusion (SWI) remain poorly constrained. Here, we present the first global assessment based on in-situ observations from ~480,000 coastal monitoring locations. From 1990 to 2024, 21% of grid-cell aggregates had significant rising or falling GWL 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 (ρs=0.63), in arid settings (ρs=0.56), and in some rural areas. SWI susceptibility is higher where seaward freshwater discharge is weak or gradients reverse landward, leaving limited hydraulic resistance to SWI. Decadal trend extrapolation 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.