Climate Change Accelerates Nitrate Delivery to Groundwater

Rising precipitation under climate change can increase groundwater recharge in many regions and accelerate the mobilization of legacy nitrate stored in the vadose zone. We couple a machine learning emulator of a global hydrological model with the nitrate time bomb (NTB) model to quantify nitrate migration from 1958 to 2100. Nitrate migration velocity increases across all climate zones except the tropics, with the most pronounced gains in the cold (+0.20/+0.25 m year –1 under SSP2–4.5/SSP5–8.5) and temperate (+0.17/+0.15 m year –1 ) zones. Groundwater table shallowing further shortens transport distance; in the arid zone, climate change advances nitrate peak arrival by about 6/8 years (locally >20 years). Among grid cells where nitrate peaks reach groundwater before 2100, more than 60% show shortened NTB countdowns under both scenarios. After the first global nitrate accumulation peak, depth-projected leaching shows renewed increase by 2019, suggesting a second phase of nitrate loading to the vadose zone. Integrating legacy nitrate mass with climate-adjusted transport identifies the North China Plain, South Asia, and Western Europe as NTB hotspots. These results show that climate forcing can accelerate the release of long buried agricultural pollutants, underscoring the urgency of proactive groundwater protection.