Vadose zone nitrogen legacy threatens achievement of global groundwater quality goals

Groundwater nitrate (NO₃^-) pollution jeopardizes drinking-water safety under the UN’s Sustainable Development Goal 6. Recovery lags behind policy ambitions because legacy nitrogen (N) stored in the vadose zone creates a “long tail” of persistent contamination, where NO₃^- continues to leach for decades. Here, we integrate the “long tail” mechanism into a global N-balance model, and couple this enhanced model with 0.5° × 0.5° simulations to project centennial-scale NO₃^- dynamics (1961–2100) in groundwater systems worldwide. Our modeling suggests that by 2020, shallow aquifers had accumulated 162 ± 6 Tg N of NO₃^-, contaminating approximately 10% of global land area above the WHO drinking water safety limit (11.3 mg N L^-1). More critically, a vast NO₃^- reservoir (4,037 ± 214 Tg N) in vadose zones sustains this long tail, which may generate new contamination hotspots over the next 80 years. Even with 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, from “no additional action” to “multi-generational remediation”, and propose tailored strategies balancing water quality, food security, and soil sustainability. Our findings redefine the temporal scope of environmental governance, highlight priority regions for targeted interventions, and provide a science-based roadmap for achieving safe groundwater within realistic socioeconomic constraints.