Dissolved oxygen downstream of an effluent outfall in an ice-covered river: Natural and artificial aeration

In ice-covered rivers, dissolved oxygen (DO) might fall below critical levels for aquatic biota in the absence of surface aeration, combined with low winter flow conditions and reduced photosynthesis rates. Open-water zones, however, can be created downstream of a diffuser by warm effluent discharges, resulting in an increase in surface aeration. In this study, we modeled the behavior of the effluent plume and the resulting open-water lead development in the Athabasca River, Alberta, Canada downstream of a pulp mill diffuser. The DO was found to increase by 0.26 mg/L due to surface aeration of an open-water lead of 6.07 km. We also evaluated oxygen injection into the effluent pipeline to increase the DO in the river. At an injection rate of 3,500 and 5,000 Ib/day of liquid oxygen, the DO was increased by 0.16 and 0.21 mg/L, which corresponded to an absorption efficiency of about 50%. The artificial aeration technique evaluated here appears to be an effective alternative to increase DO levels in ice-covered rivers. The results of this study are important in developing accurate DO models for ice-covered rivers and in evaluating oxygen injection systems.