Abstract
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.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 3225-3242 |
| Number of pages | 18 |
| Journal | Water Resources Research |
| Volume | 54 |
| Issue number | 5 |
| DOIs | |
| State | Published - May 2018 |
Funding
We thank Marcelo García for use of facilities at the Ven Te Chow Hydrosystems Laboratory and Scott Simpson, Nick Marchuk, Michael Peshkin, Andrea Salus, Duanding Yuan, Wu Heng, Carlo Zuniga Zamalloa, and Diogo Bolster for assistance with experiments and analysis. KRR was supported by an NSF Graduate Research fellowship. We also thank Stanley Grant and two anonymous reviewers for constructive comments that helped improve the manuscript. The research effort was supported by National Science Foundation grant EAR-1215898 and the Department of the Army, U.S. Army Research Office grant W911NF-15-1-0569. Supporting data is available at DOI: 10.6084/ m9.figshare.4244405. The authors declare no conflicting interests.
Keywords
- hyporheic exchange
- rivers
- solute transport
- streams
- transition layer
- turbulence
ASJC Scopus subject areas
- Water Science and Technology