Ammonium (NH₄⁺) transport processes in the riverbank under varying hydrologic conditions

Attenuation of groundwater ammonium (NH₄⁺) is expected to occur through redox reaction and adsorption of the riverbank. Previous studies determined that NH₄⁺ mostly degraded through nitrification along subsurface flow, however, the adsorption capacities of riverbanks were always ignored in the NH₄⁺ reduction processes. In this study, field experiments were conducted in the Fuliji section of the Xiaosuixin River, China, to understand NH₄⁺ transport and attenuation under rainfall events-induced river and groundwater interactions. The results indicated that the NH₄⁺ concentration in river water increased significantly after heavy rainfall events and reached a peak of about 5.88 mg L^-1, and the lag time was more than 2 weeks compared with the river peak stage. Adsorption plays a dominant role in attenuation of NH₄⁺ in riverbank with high amounts of organic materials and clay minerals, reducing its concentration to less than 0.05 mg L^-1. A two-dimensional lateral exchange and transport model of NH₄⁺ was developed and calibrated against observations in the aquifer, and an exponential reduction pattern of NH₄⁺ was identified. The model's possible implications about the effects of varying hydrologic changes (i.e., peak stage and lag time differences between river and groundwater) on NH₄⁺ transport were also discussed. Thus, the effects of river-groundwater interactions on nitrogen pollution should be taken into consideration in river regulation strategies in order to ensure proper hydrogeochemical functioning of river-aquifer interfaces and related ecosystems.