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 source of freshwater that is threatened by overabstraction and rising sea levels. Yet, our understanding of where global coastal groundwater levels (GWLs) are declining and what regions are susceptible to future seawater intrusion (SWI) remains limited. Here, we present the first global, observation-based assessment of coastal GWL trends, using more than 550,000 in-situ well records. From 1990 to 2024, 21% of coastal wells had significant GWL trends (±0.1 m year⁻¹), rising to 28% in the last 9 years, when declines became more frequent. More pronounced changes are observed in deeper (s_rho=0.61), arid (s_rho=0.53), and rural wells (s_rho=0.23). Because declining GWLs weaken freshwater-seawater gradients that resist SWI, we identify susceptible coastal hotspots with stabilizing (3.7%), persistent (92.0%), and emerging (4.3%) conditions, with emerging hotspots in Australia, Europe, West India, and North America. Our findings emphasize the need to understand SWI intrusion across coastal landscapes to ensure safe drinking water for coastal communities.