Projected expansion of potential seawater intrusion into coastal aquifers of Aotearoa New Zealand to 2150

Seawater intrusion into aquifers is an escalating global threat to coastal freshwater security, agriculture, and infrastructure. Seawater intrusion is generally more likely where the water table lies below sea level. In Aotearoa New Zealand (henceforth ‘Aotearoa’) this threat is increasing. Here, we analysed 1.14 million coastal groundwater level observations (2000–2023) from 46,677 wells and developed a random forest model to estimate water table elevations relative to sea level across Aotearoa’s usable coastal aquifer areas. We also incorporated projections of sea-level rise (under three Shared Socioeconomic Pathways (SSPs)), and vertical land motion to provide a national-scale screening assessment of the potential scope for seawater intrusion to the year 2150.Currently, 1.9% of Aotearoa’s useable coastal aquifer area exhibits negative groundwater levels relative to sea level. Under all SSPs, these areas expand nationwide, from 4.4% (SSP1-1.9) to 8.3% (SSP5-8.5) by 2150. Areas of coastal aquifers with groundwater below sea level are projected to expand up to 8.3% under future sea level rise in Aotearoa New Zealand, according to a random forest analysis of 1.14 million groundwater observations and projections of sea level and vertical land motion.