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Water quality pollutants
Non-persistent (degradable)
- domestic sewage
- fertilizers
- some industrial wastes
These compounds can be broken down by chemical reactions or by natural bacteria into simple, non-polluting substances such as carbon dioxide and nitrogen. The process can lead to low oxygen levels and eutrophication if the pollution load is high. But this damage is reversible.
Persistent (degrade slowly)
- some pesticides (e.g., DDT, dieldrin)
- some leachate components from landfill sites (municipal, industrial)
- petroleum and petroleum products
- PCBs, dioxins, polyaromatic hydrocarbons (PAHs)
- radioactive materials such as strontium-90, cesium-137, radium-226, and uranium
- metals such as lead, mercury, cadmium
This is the most rapidly growing type of pollution and includes substances that degrade very slowly or cannot be broken down at all; they may remain in the aquatic environment for years or longer periods of time. The damage they cause is either irreversible or reparable only over decades or centuries.
Other
- warm water from cooling towers (thermal pollution)
- floating debris
- garbage
- foam
These are examples not of chemical pollution, but of physical pollution which interferes mainly with the usability and/or aesthetic appeal of the water. In certain cases, thermal pollution can kill fish.
This diagram shows how toxic substances enter the aquatic environment: 
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