Influence of Bedform Migration on Nitrate Reduction in Hyporheic Zones of Heterogeneous Sediments

AbstractThe transport of sediments induced by river flow leads to the formation and movement of bedforms like ripples, and the exchange flux caused by the interaction between stream and bedforms significantly influences nitrogen dynamics. Previous studies have studied the role of bedform migration on nitrogen cycling in homogeneous sediments, however, how bedform migration affects nitrate reduction in heterogeneous streambeds is still unknown. In this study, we established a series of two‐dimensional numerical models to investigate the role of bedform migration on nitrate removal in physically (permeability) and chemically (particulate organic carbon; POC) heterogeneous sediments. Results from mobile bedforms were compared with matching immobile bedforms and equivalently homogeneous mobile ones. Mobile bedforms create periodic exchange flux fluctuations, reduce solutes penetration depths and residence time, and result in decreased denitrification rate. Both a larger bedform celerity and a smaller silt fraction lead to a lower NO3− removal efficiency. Thus, the bedforms are typically assumed to be immobile potentially making an overestimation of NO3− removal. Moreover, sediment heterogeneity cannot enhance NO3− removal in mobile bedforms, where the penetration depth and the residence timescale of NO3− are suppressed, and the formation of anoxic microzone is inhibited. The finding is significantly different from the previous understanding about sediment heterogeneity that can obviously enhance NO3− removal. This study is useful for understanding nitrate processing in streambed sediments and benefits to the management of watershed nutrients.