Model-based analysis of dissolved oxygen supply to aquifers within riparian zones during river level fluctuations: Dynamics and influencing factors

Abstract Dissolved oxygen (DO) largely impacts the environmental function of riparian aquifers. However, the dynamics and influencing factors of DO supply to riparian aquifers are not well understood. In this study, we developed a model considering air–liquid two-phase flow, air entrapment and multi-phase transport of O2(g) in a riparian aquifer to investigate the hydrologic exchange flows and DO supply during river level fluctuations. A sensitivity analysis was conducted to quantify the influences of hydrological conditions of river, hydrogeological conditions of riparian aquifer, DO consumption rate constant and DO concentration in the river water on DO supply to the riparian aquifer. Results show that the river, the entrapped air and the unsaturated zone all provide a significant amount of DO to the riparian aquifer, and there are also noticeable differences in the spatial distribution of DO from the three different sources. Results also indicate there are a DO-rich strip and a DO-rich plume occurring in the top of the riparian aquifer and the channel-aquifer interface, respectively. Both DO-rich areas are the research focus in evaluating the environmental function of riparian zones. Further, the DO supply amount shows a significant increase as the river amplitude and period increase, and a losing river and increasing hydraulic conductivity also leads to the increasing DO supply flux because these incidents promote infiltration of the river water. In contrast, the high DO concentration in the river water increases the DO flux from river but reduces the DO supply from the entrapped air and the unsaturated zone because of the competition in different supply pathways. Moreover, the model simulations agree with the measurements of DO source in a field that has been reported. This work provides a useful tool for estimating DO supply to riparian aquifers and improves our understanding of the environmental function of riparian zones.