Safeguarding Groundwater Nitrate within Regional Boundaries in China.

Groundwater, essential for irrigation, industry, and drinking, plays a crucial role in environmental health and human well-being. A major threat to groundwater quality is nitrate pollution, primarily stemming from human activities. Safeguarding nitrogen levels in groundwater within regional thresholds remains a global challenge. By integrating 3,134 groundwater samples and nitrogen budget modeling, we found that China's national average nitrate concentration has risen by 29% since the 2000s, reaching 14 mg N L. The main sources of nitrate contamination are cropland, landfills, and wastewater disposal, with average annual nitrogen leaching of 1.91 ± 0.16, 0.86 ± 0.18, and 0.63 ± 0.17 million tonnes, respectively; these sources collectively account for 73% of the total nitrate leakage during 2000-2020. Current robust mitigation practices could reduce nitrogen leaching into groundwater by 45% (1.93 million tonnes N), delivering a net societal benefit of US$83 billion in China. Nevertheless, this reduction remains insufficient to meet the safe nitrogen boundary for all provinces, underscoring a compelling necessity for additional measures and policy guidance tailored to protect groundwater resources on a site-specific basis.