MODELING BENTHIC OXYGEN-UPTAKE BY PUMPING

In porous-bed rivers, dunes and other irregularities cause an uneven p ressure distribution, which ''pumps'' small amounts of water through t he sediments. A simple analytical model is developed to estimate the c ontribution pumping makes to the benthic oxygen uptake rate (BUR) in a polluted river. The model predicts that pumping makes a significant c ontribution to deoxygenation in the Waiotapu River, New Zealand, where the gravel bed is highly permeable and the sediment microbial activit y is high, but not in the sand-bed Tarawera River. As particle size in creases, pumping flow increases and biological activity, residence tim e, and the dissolved oxygen (DO) drop along each streamline, decrease. There appears to be a critical particle size that maximizes the pumpi ng BUR, estimated to be 5 mm. The model shows that the kinetics of ben thic oxygen uptake vary depending on the river BOD and DO concentratio ns, and this has implications for the structure of river water quality models. The model could be extended to other benthic mass transfer pr oblems, including the deoxygenation of salmonid redds and benthic deni trification.