Hyporheic exchange and nitrogen cycle processes under the dual effects of flooding and heterogeneous streambed

In this study, we investigated the effects of media heterogeneity and flood processes on hyporheic exchange and nitrogen cycle processes in hyporheic zone adjacent to a natural in-stream gravel bar. We conducted field experiments and set a reactive transport model of surface-groundwater exchange based on the monitoring results. The flood process is obtained based on field monitoring, and the heterogeneous medium field is constructed using the SAR3DC method based on in-site experiment results. The results showed that both the heterogeneity of the medium and the flood process can enhance the spatial variability of velocity distribution, resulting in an increase of exchange flux, a decrease in lag time, and an extension of the migration path. For heterogeneous media, it is easier to form specific channels for high velocity flow, and the flow velocity within them is more sensitive to changes in media and hydrodynamic conditions, the direction of hyporheic exchange can be reversed in some specific channels. For the nitrogen cycle processes, the heterogeneous medium condition determines the source-sink functions and the direction of nitrate nitrogen reduction in hyporheic zone by providing a wider reaction space for reactants, reducing the lag time to 1/40 and increasing the migration path by 4 times. The flood process can increase the infiltration amounts of reactants by 5 times and reduce the lag time (time of particle entry into the hyporheic zone to migrate out) to 1/6. In general, under the effects of media heterogeneity and flood processes, the reaction rates and nitrate nitrogen reduction rates during the nitrogen cycle processes are decreasing and the hyporheic zone will face greater pollution risks.