Unsteady diffusional mass transfer at the sediment/water interface: Theory and significance for SOD measurement

Abstract

Dissolved oxygen uptake at a sediment/water interface (SOD) is controlled by mass transport and/or biochemical reactions in two adjacent boundary layers: the diffusive boundary layer δ D in the water and the penetration depth δ in the sediment. Either one of those boundary layers or both can be controlling. The transition from sediment control to water control is a function of shear velocity at the sediment/water interface ( U *) and biochemical activity rate ( μ 0) in the sediment. A model was developed for the unsteady response of SOD and DO profiles near the sediment/water interface. Michaelis-Menten kinetics were used initially, but zero order kinetics work just as well when the half saturation coefficient K O 2 is small as was suggested by field data. Beginning with zero DO in the sediments the times required to reach steady state DO profiles and SOD was on the order of minutes to hours, faster where biochemical activity is strong. The values of SOD estimated by the model were compared with experimental data to verify the reliability of the model. The model can reproduce observed penetration depths and diffusive boundary layer thickness. Values of SOD estimated by the model were of same magnitude as observed data. The unsteady DO uptake model can be used to provide guidance for field measurements of SOD. Placing a chamber (with a stirrer) into the sediments disturbs the DO equilibrium at the sediment/water interface. A new equilibrium will be reached within a time that can be measured in terms of cumulative DO consumption in the chamber (SOD exerted). Upper bounds for (SOD exerted) are larger when biochemical activity in the sediments is smaller. Values of SOD exerted are less than 0.1 gm −2 when μ 0 is less than 50 mg l −1 d −1 and U *>0.1 cm/s. In other words, steady state conditions are easier to reach for high SOD values. Actual times required to reach steady state can be from minutes to hours. If flow conditions in the chamber and at the natural sediment/water interface are much different, measured SOD values have to be adjusted. A procedure for the adjustments, which can be substantial, has been developed.