The BOD (Biological Oxygen Demand) of a freshwater ecosystem decreases as you move further away from a point-source sewage spill.
Pollution originating from a single, identifiable source, such as a discharge pipe from a factory or sewage plant, is called point-source pollution. The term "biological oxygen demand" refers to the quantity of oxygen needed to allow aerobic bacteria to decompose waste organic matter in water.
The need for oxygen decreases as you travel away from the sewage spill and then increases. The need for oxygen decreases as there is more raw sewage present. Fish cannot survive in water with dissolved oxygen levels below 4 gm/liter, and the level will continue to drop.
Therefore, the BOD (Biological Oxygen Demand) of a freshwater ecosystem decreases as you move further away from a point-source sewage spill.
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