The study examines the impact of groundwater nitrogen (N) legacy on water quality in four major river basins: the Rhine, Mississippi, Yangtze, and Pearl. Despite regulatory efforts, high N concentrations persist in freshwaters due to excessive N leaching and accumulation in groundwater. The Rhine and Mississippi basins have accumulated N since the 1950s, while the Yangtze and Pearl basins entered the N accumulation phase in the 1970s. Strategies to reduce N pollution have had limited success, highlighting the importance of addressing N legacy in groundwater. Restoring groundwater N storage to 1970 levels will take longer in the Yangtze and Pearl basins (>35 years) compared to the Rhine (9 years) and Mississippi (15 years). Sustainable watershed management requires long-term strategies that address N legacy and promote sustainable agricultural practices aligned with the Sustainable Development Goals. The study uses the Integrated Model to Assess the Global Environment–Global Nutrient Model (IMAGE-GNM) to analyze N dynamics and future scenarios under different socioeconomic and climatic conditions. The results show that the effect of reducing N surpluses in agriculture will be delayed due to the large amounts of N stored in groundwater, emphasizing the need for long-term strategies to manage N legacy.The study examines the impact of groundwater nitrogen (N) legacy on water quality in four major river basins: the Rhine, Mississippi, Yangtze, and Pearl. Despite regulatory efforts, high N concentrations persist in freshwaters due to excessive N leaching and accumulation in groundwater. The Rhine and Mississippi basins have accumulated N since the 1950s, while the Yangtze and Pearl basins entered the N accumulation phase in the 1970s. Strategies to reduce N pollution have had limited success, highlighting the importance of addressing N legacy in groundwater. Restoring groundwater N storage to 1970 levels will take longer in the Yangtze and Pearl basins (>35 years) compared to the Rhine (9 years) and Mississippi (15 years). Sustainable watershed management requires long-term strategies that address N legacy and promote sustainable agricultural practices aligned with the Sustainable Development Goals. The study uses the Integrated Model to Assess the Global Environment–Global Nutrient Model (IMAGE-GNM) to analyze N dynamics and future scenarios under different socioeconomic and climatic conditions. The results show that the effect of reducing N surpluses in agriculture will be delayed due to the large amounts of N stored in groundwater, emphasizing the need for long-term strategies to manage N legacy.