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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.

Plain Language Summary Streams and rivers continuously exchange water with their underlying sediments in a region called the hyporheic zone. This zone is a hotspot of transformation for many societally relevant chemicals, including carbon, nutrients, and contaminants. Accurate predictions for how much transformation occurs in the hyporheic zone requires an improved understanding of how reactive chemicals are transported into, and within, this region of a riverbed. Although fluid turbulence can be the dominant process controlling surface-subsurface exchange in gravel-bed streams, its influence is poorly understood due to the difficulty of measuring turbulent fluid velocities and concentrations within the streambed. In this experimental study, we show that turbulence strongly couples surface waters with hyporheic waters in a thin layer where the water column and stream sediments meet. As a result, fluid transport and mixing are enhanced several centimeters into the hyporheic zone of gravel-bed streams. These findings support recent theoretical arguments that surface and subsurface waters are not independent and must instead be treated as a single unit to accurately model solute, particulate and pollutant transport in streams and rivers.