Effect of bioclogging on the nitrate source and sink function of a hyporheic zone

Nitrogen dynamic in a hyporheic zone (HZ) is mediated by microbes in the form of biofilms attached to sediment grains. Previous studies have neglected the influence of microbial effects on hyporheic nitrate function despite the fact that biofilm-induced clogging can significantly reduce sediment permeability and affect both residence time distribution and biochemical reaction rates. In this study, to assess how the development of bioclogging affects the nitrate source and sink function of HZs, we established a numerical model by coupling water flow, reactive solute transport, and microbial metabolism to simulate nitrogen cycling in a dune-shaped streambed. The classic Damköhler number (DaO2) with an average reaction timescale was not appropriate to determine hyporheic function for the context of biofilm growth induced streambed permeability and biogeochemical reaction rates changing with time. We proposed a new Damköhler number (DaO2∗) with a correction factor (the ratio between current bacteria and initial bacteria) to identify hyporheic function while accounting for the dynamic process of microbial growth. Our results demonstrated that bioclogging layers with aerobic bacteria aggregation acted as a nitrate source system with DaO2∗ ≫ 1, whereas deeper streambed with negligible microbiota survival severed as a nitrate sink with DaO2∗ < 1 when VO2 = 1 h−1. The threshold value of net nitrate source or sink system was influenced by maximum reaction rate of oxygen, and it was in a general range of 1–50 (the range of VO2 was in 1–3 h−1). Threshold transition times from nitrate source to sink were evaluated from the spectrum of flow paths and had strong spatiotemporal variability due to microbial growth. The threshold times decreased over time and stabilized between 1.0 and 3.7 h (for VO2 = 1 h−1) in a dune-shaped streambed when biofilm layers formed.