Influence of point bars on nitrogen transport and reaction in riparian zones

Abstract

Point bars formed by riverbank deposition are an important part of the hyporheic zone, where surface water and groundwater are actively exchanged; they play a key role in the nitrogen biogeochemical processes of river ecosystems. In this study, we aimed to investigate the effects of point bars on nitrogen transport and reactions in riparian zones. A two-dimensional (2D) coupled flow-reactive transport model was developed, and three key chemical reactions, namely, aerobic respiration, nitrification, and denitrification, were considered to study the effects of the curvature and size of a point bar on the nitrogen distribution and chemical reaction rate in the hyporheic zone. The results indicated that the point bar altered the groundwater velocity field, resulting in higher flow velocity and shorter subsurface residence time, which significantly increased the depth of the four key species [dissolved oxygen (DO), nitrate (NO3−), dissolved organic carbon (DOC), and ammonium (NH4+)] that are generally transported to riparian zones. This phenomenon played a significant role in increasing the nitrification and denitrification rates. In the scenario wherein a point bar was formed, the amount of nitrogen removed was 4.29 times higher than that in the scenario wherein no point bar was formed. Furthermore, the transfer depths of DO, NO3−, DOC, and NH4+ and the amount of nitrogen removed increased with the curvature/area of the bar. This study indicates that the presence of a point bar can accelerate the nitrogen cycle and increase the nitrogen removal capacity in riparian zones.