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Water pollution arises as a result of several human activities, which leads to the presence of several undesirable substances in water. Major water pollutants with their sources have been tabulated as follows:

PollutantSource
Micro-organismsDomestic sewage
Organic wastesDomestic sewage, decaying animals and plants, animal
excreta and waste, discharge from food processing
industries
Plant nutrientsChemical fertilizers
Toxic heavy metalsChemical factories and industries
SedimentsStrip mining and soil erosion
PesticidesChemicals used for killing fungi, weed, insects
Radioactive substancesMining of uranium-containing minerals
HeatWater used for cooling in industries

Roles played by major pollutants are: 

1. Pathogens: These water pollutants include bacteria and other organisms. They enter water from animal excreta and domestic sewage. Bacteria present in human excreta (for example, Escherichia coli and Streptococcus faecalis) cause gastrointestinal diseases. 

2. Organic wastes: These are biodegradable wastes that pollute water as a result of run off. The presence of excess organic wastes in water decreases the amount of oxygen held by water. This decrease in the amount of dissolved oxygen inhibits aquatic life. 

3. Chemical pollutants: These are water soluble chemicals like heavy metals such as cadmium, mercury, nickel, etc. The presence of these chemicals (above the tolerance limit) can damage the kidneys, central nervous system, and liver.

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