Modeling Benthic Oxygen Uptake by Pumping

Abstract

In porous-bed rivers, dunes and other irregularities cause an uneven pressure distribution, which “pumps” small amounts of water through the sediments. A simple analytical model is developed to estimate the contribution pumping makes to the benthic oxygen uptake rate (BUR) in a polluted river. The model predicts that pumping makes a significant contribution to deoxygenation in the Waiotapu River, New Zealand, where the gravel bed is highly permeable and the sediment microbial activity is high, but not in the sand-bed Tarawera River. As particle size increases, pumping flow increases and biological activity, residence time, and the dissolved oxygen (DO) drop along each streamline, decrease. There appears to be a critical particle size that maximizes the pumping BUR, estimated to be 5 mm. The model shows that the kinetics of benthic oxygen uptake vary depending on the river BOD and DO concentrations, and this has implications for the structure of river water quality models. The model could be extended to other benthic mass transfer problems, including the deoxygenation of salmonid redds and benthic denitrification.