Impact of groundwater nitrogen legacy on water quality

The loss of agricultural nitrogen (N) is a leading cause of global eutrophication and freshwater and coastal hypoxia. Despite regulatory efforts, such as the European Union’s Nitrogen Directive, high concentrations of N persist in freshwaters. Excessive N leaching and accumulation in groundwater has created a substantial N reservoir as groundwater travel times are orders-of-magnitude slower than those of surface waters. In this study we reconstructed past and projected future N dynamics in groundwater for four major river basins, the Rhine, Mississippi, Yangtze and Pearl, showcasing different N trajectories. The Rhine and Mississippi river basins have accumulated N since the 1950s and although strategies to reduce excess agricultural N have worked well in the Rhine, groundwater legacy N persists in the Mississippi. The Yangtze and Pearl river basins entered the N accumulation phase in the 1970s and the accumulation is expected to continue until 2050. Policies to reduce N pollution from fertilizers have not halted N accumulation, highlighting the importance of accounting for the N legacy in groundwater. Restoring groundwater N storage to 1970 levels by diminishing N leaching will therefore take longer in the Yangtze and Pearl (>35 years) than in the Rhine (9 years) and Mississippi (15 years). Sustainable watershed management requires long-term strategies that address the impacts of legacy N and promote sustainable agricultural practices aligned with the Sustainable Development Goals to balance agricultural productivity with water conservation. National and international policies have aimed to protect the quality of freshwater by mitigating agricultural nitrogen emissions. However, the nitrogen legacy in groundwater must be accounted for when mitigating the impacts of nitrogen in watersheds.