Turbulence Links Momentum and Solute Exchange in Coarse-Grained Streambeds

The exchange of solutes between surface and pore waters is an important control over stream ecology and biogeochemistry. Free-stream turbulence is known to enhance transport across the sediment-water interface (SWI), but the link between turbulent momentum and solute transport within the hyporheic zone remains undetermined due to a lack of in situ observations. Here, we relate turbulent momentum and solute transport using measurements within a streambed with 0.04 m diameter sediment. Pore water velocities were measured using endoscopic particle image velocimetry and used to generate depth profiles of turbulence statistics. Solute transport was observed directly within the hyporheic zone using an array of microsensors. Solute injection experiments were used to assess turbulent fluxes across the SWI and patterns of hyporheic mixing. Depth profiles of fluctuations in solute concentration were compared with profiles of turbulence statistics, and profiles of mean solute concentration were compared to an effective dispersion model. Fluorescent visualization experiments at a Reynolds number of Re ≥ 27,000 revealed the presence of large-scale motions that ejected tracer from the pore waters, and that these events were not present at Re = 13,000. Turbulent shear stresses and high-frequency concentration fluctuations decayed greatly within 1–2 grain diameters below the SWI. However, low-frequency concentration fluctuations penetrated to greater depths than high-frequency fluctuations. Comparison with a constant-coefficient dispersion model showed that hyporheic mixing was enhanced in regions where turbulent stresses were observed. Together, these results show that the penetration of turbulence into the bed directly controls both interfacial exchange and mixing within a transition layer below the SWI.