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OXYGEN BALANCE IN RIVERS
Oxygen is critical for aquatic life. For fish, salmonid species need oxygen concentrations greater than 5 mg L-1; cyprinids (carp family) need more than 2 mg L-1. Oxygen is a key factor in the biogeochemical processes that modify and reduce pollutant loads in rivers.Dissolved oxygen in natural running waters should be close to 100 percent saturation, that is, between 9 and 11 mg L-1, depending on temperature. Oxygen depletion is usually caused by bacterial degradation of organic matter. The potential for oxygen consumption is measured as BOD5 (Biological Oxygen Demand over a five day period) and/or by the oxidization of chemical bonds (Chemical Oxygen Demand -- COD). Oxygenation of water occurs naturally through turbulence of river waters and by vertical mixing in lakes.
Global BOD5 levels (see figure) measured at GEMS/WATER stations average 2 mg L-1 which indicates a limited degree of organic pollution. Much higher observed values are usually associated with discharge of municipal waste water, and wastes from agro- food and some types of industrial effluents. In contrast with most North American stations, South American stations show the lowest BOD levels. An urban water body from Djakarta City has a high degree of organic pollution and can be virtually devoid of oxygen. The relationship between BOD and COD for this canal is typical of the poor state of water quality found in rivers running through many fast-growing megacities in Asia and Latin America. The Espierre River that flows from France to Belgium is actually an urban and industrial sewer.
The Xi Jiang (Pearl River) shows natural variability of BOD5 ranging from 0.3 to 1.2 mg L-1 for 12 years of record. Such low levels are rarely found in the GEMS/WATER database. The Xi Jiang also demonstrates how higher values of BOD5 are associated with low flow of a river when effluents are least diluted.
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