The long tail of nitrate pollution in groundwater challenges governance of global water quality

Groundwater nitrate pollution threatens drinking water safety. Legacy nitrogen (N) in the vadose zone creates a “long tail” of leaching that can persist for decades after policy actions. Here we model global nitrate dynamics in groundwater from 1961 to 2100. By 2020, shallow aquifers had accumulated 162 ± 6 Tg N as nitrate, exceeding the WHO drinking water limit across 10% of global land area. This contamination is sustained by a large nitrate reservoir of 4037 ± 214 Tg N in the vadose zone, which will continue to generate new contamination hotspots for decades. Even under an immediate transition to zero-N-surplus, 4% of affected regions are projected to remain above the WHO limit beyond 2100. To guide effective governance, we classify global croplands into four management archetypes and identify tailored strategies that balance water quality, food security, and soil sustainability. These findings redefine the temporal scope of governance, identify priority regions, and provide a science-based roadmap towards safe groundwater. Groundwater nitrate pollution persists for decades due to a large legacy nitrogen reservoir in the vadose zone. Even with immediate emission cuts, 4% of affected regions will exceed WHO limits beyond 2100, requiring tailored governance strategies.