Oxygen transfer from flowing water to microbes in an organic sediment bed

Abstract

Mass transfer of dissolved oxygen (DO) across a sediment–water interface was investigated using laboratory experiments and a numerical simulation model. DO concentration profiles and velocity profiles were measured with high resolution in a recirculating flume with water flowing at cross-sectional average velocities from 3.5 to 11.5 cm/s over a flat and hydrodynamically smooth organic sediment bed. Parameters extracted from the measurements included (1) the DO penetration depth, (2) the effective diffusion coefficient in the sediment layer, (3) the thickness of the turbulent diffusive boundary layer, and (4) the diffusion coefficients in the diffusive boundary layer. DO penetration depths were less than 1 mm, and diffusive boundary layer thicknesses were less than 8 mm. Diffusion in the sediment porewater system was shown to be essentially molecular. The laboratory data were compared to results from a deterministic simulation model. The model included explicit descriptions of (1) mass transfer through the diffusive boundary layer above the sediment/water interface, (2) the boundary layer development over the sediment bed of finite length, (3) diffusive transfer in the sediment porewater system, and (4) microbial uptake of DO in the sediment. The model included both water-side and sediment-side mass transport limitations. The control of DO flux could alternate between water-side and sediment-side without discontinuity. Monod-type kinetics was adopted for DO uptake in the sediment. Organic substrate availability in the sediment did not vary over the course of an experiment. A kinetic limitation for organic matter (substrate) was not considered, and microbial activity in the sediment was parameterized by biomass density. Measured and simulated DO concentration profiles showed satisfactory agreement, with some discrepancies at the interface caused by roughness and porosity effects of the sediment surface.