Differences of denitrification potential between straight and meandering pool-riffle streams with riparian zones

Abstract

The hyporheic zone (HZ), an interstitial space immediately beneath and adjacent to streams, is characterized by frequent exchanges between groundwater and surface water, and by transformations of energy, organics, and solutes. Our previous studies on HZ (Huang and Chui, 2021, https://doi.org/10.1029/2020wr029182, 2022, https://doi.org/10.1029/2022WR032221) have shown that stream morphology, such as meanders and pool-riffle bedforms, significantly complicates subsurface flow patterns and increases the flowrate and scale of the hyporheic exchange (HE), leading to a hypothesis of a more dynamic and complex biogeochemical regime in both vertical and lateral extents of the HZ. In this study, we focused on the biogeochemical reactions associated with stream nitrogen cycling, including aerobic respiration (AR) and denitrification (DN) in both straight and meandering pool-riffle streams — due to their significant effects on stream ecology improvement. We also compared how stream morphology influences redox zonation and denitrification potential on a spatial-temporal scale. Using the physical pool-riffle stream model built in our previous study, we conducted tracer experiments with the conservative material NaCl. A three-dimensional (3D) model that couples with hyporheic flow and biogeochemical reactions was developed to investigate AR and DN processes. The groundwater reaction model was calibrated using the measured concentration curves at downwelling and upwelling regions of the HZ, assuming reaction rate was zero. Our preliminary results from the numerical simulation showed that 1) both straight and meandering pool-riffle streams with riparian zones exhibit significant nitrate transfer zones in the anoxic regions beneath the streambed and in the riparian area, which is dominated by advection. 2) the interbank of meandering streams may contain multiple DN hotspots, which is significantly affected by the local HEs. Ignoring the riparian zone may lead to an underestimation of the denitrification potential in the HZ. Further studies are needed to comprehensively investigate biogeochemical reactions in 3D meandering streams and their responses to morphologic factors (e.g., sinuosity), hydrological factors (e.g. stream discharge and groundwater flow) and environmental factors (e.g. diurnal and seasonal temperature variations).