Effects of natural streambed sediment on the riverbed exchange flows and microbial respiration

<p>Exchange flows at the water-sediment interface control river water quality and carbon cycling through microbial respiration. However, accurate quantification of these exchange flows and microbial respiration is still challenging in field surveys due in part to the dynamic turbulence generated by streambed topography. Using a framework that combines Structure-from-Motion (SfM) photogrammetry with a fully-coupled surface-subsurface computational fluid dynamics (CFD) model, this work studies the effects of streambed sediment structure on riverbed turbulence and its impact on exchange flows and microbial respiration. Specifically, the SfM photogrammetry is first applied to obtain mm- to cm-scale resolution riverbed topography over a meter scale domain at four sites; these high-resolution riverbed topography data are then used to generate meshes for use in hyporheicFoam, a fully coupled surface-subsurface model developed in OpenFOAM. Simulated time series of water depth and average flow velocity from a previously-developed 30-kilometer scale CFD model will be used to set the water depth and mean flow velocity conditions for high-resolution CFD models of the SfM-characterized locations. The modeling results will be used to investigate the dependence of riverbed exchange flows, concentration gradients, and the concentration profile from the water surface to riverbed on water depth, mean velocity, roughness size, sediment distribution, bed porosity, and subsurface permeability. The relative importance of flow advection, turbulence dispersion, and microbial reaction in both streambed and surface water will also be evaluated.</p>

Use of Sequential Extraction and Mercury Stable Isotope Analysis to Assess Remobilization of Sediment-Bound Legacy Mercury

The goal of this project was to assess how anthropogenic legacy mercury (Hg) retained in streambed sediment may be remobilized to stream water. To do this, we performed sequential extractions...

Effects of spatial scale on fingerprinting properties and sediment contributions in the black soil region of Northeast China

<p>Sediment provenance is an important factor in understanding soil erosion processes or assessing the ecological effects of soil and water conservation measures. Sediment fingerprinting is an effective technique used globally for identifying sediment sources. Few studies have examined sediment sources at different spatial scales. In this study, sediment fingerprinting was used with a Bayesian mixing model to quantify the relative contributions of different sediments to streambeds in the Hebei catchment (ca. 28.0 km<sup>2</sup>) and its sub-catchment (ca. 3.5 km<sup>2</sup>) in the black soil region of Northeast China. Three potential sediment sources were identified: cultivated topsoil, uncultivated topsoil, and gullies. A similar number of sediment samples were collected for each source in both catchments: 71 and 69 sediment samples from the sub-catchment and Hebei catchment, respectively. Five uniformly distributed streambed sediment samples were collected from each catchment. The results showed a significant difference in the spatial variability of fingerprinting properties between the two catchments (p < 0.01). The spatial variability in fingerprint properties of cultivated topsoil and gully soil was more sensitive to scale than that of uncultivated topsoil. The optimum composite fingerprint that was used to discriminate potential sediment sources differed between the sub-catchment and Hebei catchment. Cultivated topsoil and gully soil were the main sediment sources, comprising more than 95% of the streambed sediment. There were significant differences (p < 0.01) in the contributions of cultivated topsoil and gully soil at different spatial scales. Cultivated topsoil contributed 47.8% and 42.0% in the sub-catchment and Hebei catchment, respectively, whereas gully soil contributed 49.6% and 55.3% (mean absolute fit >0.95). The upper stream segment mainly received sediment from the gullies (>60%) and the contribution from cultivated topsoil gradually increased downstream.</p>