Dissolved Oxygen Demand at the Sediment-Water Interface of a Stream: Near-Bed Turbulence and Pore Water Flow Effects

A microbial dissolved oxygen (DO) uptake model was developed for a stream bed, including the effect of turbulence in the flow over the bed and pore water flow in the porous bed. The fine-grained sediment bed has hydraulic conductivities $0.01\le k\le 1\mathrm{cm}/\mathrm{s}$, i.e., sediment particle diameter $0.006\le d_s\le 0.06\mathrm{cm}$. The pore water flow is driven by pressure fluctuations at the sediment-water interface, mostly attributable to near-bed coherent motions in the turbulent boundary layer above the sediment bed. An effective mass transfer coefficient ($D_e$) coupled to a pore water flow model was used in the DO transport and DO uptake model. DO flux across the sediment-water interface and into the sediment, i.e., sedimentary oxygen demand (SOD), was related to hydraulic conductivity and microbial oxygen uptake rate in the sediment and shear velocity at the sediment-water interface. Simulated SOD values were validated against experimental data. For hydraulic conductivities of the sediment bed up to $k\approx 0.01\mathrm{cm}/\mathrm{s}$, the pore water flow effect on SOD was found negligible. Above this threshold, the effective mass (DO) transfer coefficient in the sediment bed ($D_e$) becomes larger as the hydraulic conductivity ($k$) becomes larger as the interstitial flow velocities increase; consequently, DO penetration depth increases with larger hydraulic conductivity of the sediment bed ($k$), and SOD increases as well. The enhancement of vertical DO transport into the sediment bed is strongest near the sediment-water interface, and rapidly diminishes with depth into the sediment layer. An increase in shear velocity at the sediment-water interface also enhances DO transfer. Shear velocity increases at the sediment-water interface will raise SOD regardless of the maximum oxidation rate if the hydraulic conductivity is above the threshold of $k\approx 1\mathrm{cm}/\mathrm{s}$. The relationship is nearly linear when $U_{*}<0.8\mathrm{cm}/\mathrm{s}$. At shear velocity $U_{*}=1.6\mathrm{cm}/\mathrm{s}$, SOD for oxidation rates $\mu =1000$ and $2000\mathrm{mg}{\mathrm{l}}^{-1}{\mathrm{d}}^{-1}$ are almost five times larger than those with no pore water flow. When pore water transport of DO is not limiting, SOD is a linear function of oxygen demand rate $\mu$ in the sediment when $0\le \mu \le 200\mathrm{mg}{\mathrm{l}}^{-1}{\mathrm{d}}^{-1}$.