New insights into nitrate sources and transformations in riparian groundwater of a sluice-controlled river: An integrated approach using major ions, stable isotopes and microbial gene methods.

Nitrate pollution in water environment is a serious problem worldwide. Identifying nitrate sources and transformations in the riparian aquifer is critical for effectively controlling and mitigating nitrate contamination, especially in sluice-controlled rivers. This study employs an integrated approach combining hydrochemical analysis, isotopes (δO-HO, δH-HO, δN-NO and δO-NO), quantification of nitrogen (N) functional genes and a Bayesian mixing model (MixSIAR) to comprehensively investigates nitrate sources and transformation processes in the riparian groundwater of a sluice-controlled Shaying River, China. Results revealed severe nitrate contamination in both the river (mean: 2.33-5.25 mg/L) and the riparian groundwater (mean: 0.42-24.46 mg/L). Manure and sewage were the primary sources (66.20-91.20 %) of nitrate contamination in both river and riparian groundwater. Key processes influencing nitrate dynamics in riparian groundwater included mixing with river water, external N supply, and transformation processes such as nitrification, vegetation uptake and anammox. We found that when sluices are closed, the nitrate concentration in riparian groundwater decreases. In contrast, during the flood season with sluices open, the nitrate concentration in the river water increases. This study also developed the first conceptual model illustrating the impact of sluice regulation on riparian nitrate dynamics, highlighting the complex interplay between sluice operations, hydrological conditions, and biogeochemical processes that govern nitrate behavior. These findings provide valuable insights into nitrate dynamics in riparian aquifers of sluice-controlled rivers, offering a robust scientific foundation for targeted nutrient management strategies in the Shaying River Basin and similar regulated environments globally.