Nitrate removal mechanism in riparian groundwater in an intensified agricultural catchment

Nitrate contamination in ground and surface water is a persistent problem in agricultural countries. The transition zone between rivers and riparian aquifers plays an important role in mediating riverine nitrate export, as it promotes intensive denitrification, resulting in permanent nitrate removal from aquatic systems. However, the underlying mechanisms controlling riparian denitrification are not well understood. In this study, we present an assessment of water chemistry and isotope data from a two-year river and groundwater sampling period along a coastal watershed in Southeast China. The results showed that the highest oxygen depletion and denitrification rates in the riparian aquifer occurred in the infiltrated river proportions with higher groundwater temperatures. Heterotrophic microbial responses in the riparian zone may have been driven by bioavailable organic carbon from the river water in the riparian aquifer. For the underlying processes of NO3- removal efficiency from riparian groundwater, the additional processes substantially surpassed denitrification (mean of 12.1% vs. 6.6%), particularly at the furthest distance from the river in winter. The mean proportional contribution of manure and sewage (54.5%) was the highest for nitrate contamination, followed by soil N fertilizer (23.7%), chemical fertilization (18.5%), and atmospheric deposition (3.3%). This study provides valuable guidance for agricultural water management based on the key finding that high connectivity between rivers and groundwater might improve the NO3- removal potential.