Quantifying the effect of the nitrogen biogeochemical processes on the distribution of ammonium in the riverbank filtration system

Ammonium (NH4+) enrichment of riverbank filtration (RBF) systems is gaining popularity. However, most previous research has concentrated on NO3− removal efficiencies, while the mechanisms of NH4+ enrichment remain unknown. A nitrogen biogeochemical process model was developed for the quantitative analysis of NH4+ enrichment in the Kaladian well field in northwest Songyuan City, NE China. Data from laboratory experiments and in-situ monitoring were used to determine initial values and calibrate the thermodynamic/kinetic parameters representing nitrogen (N) biogeochemical reactions. (1) The NO3− from river was subjected to denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) within 10–14 m of the shore, whereas the NH4+ in groundwater was caused by DNRA, organic nitrogen mineralization (MIN), and mixing with laterally recharged high NH4+ groundwater. (2) DNF and DNRA were regulated by hydrodynamic processes, with the ranges of these processes being more significant in the wet season due to a higher hydraulic gradient. MIN occurred widely throughout the water flow path, with temperature primarily controlling the rates of the three reactions. (3) DNRA activity was relatively higher in the wet season when the water temperature was higher within 10–14 m of the shore. In the wet season, DNRA contributed 25%–30% to NO3− reduction, which was higher than in the dry season (5%–10%). DNRA contributed at least 40% and 15% to NH4+ enrichment in the wet and dry seasons, respectively. (4). Organic N in media gradually released NH4+ into groundwater via MIN and desorption across the entire flow path, with contributions to NH4+ enrichment reaching 75% and 85%, respectively, in the wet and dry seasons. •Spatiotemporal variations of N biogeochemical reactions in RBF were identified.•Conditions for denitrification, DNRA, and mineralization in RBF were identified.•Simulation parameters were refined through multi-environmental N experiments.•Contributions of DNRA to NO3− reduction and NH4+ enrichment were quantified.•Introduction.