Water pollution has many causes, including:

Sewage

Untreated sewage releases harmful chemicals and microorganisms into water bodies, which can contaminate the water and kill living organisms. 

Industrial waste

Industrial waste can be a major cause of water pollution, especially if industrial sites don't have proper waste management systems. 

Oil spills

Oil spills and leaks are a major cause of water pollution because oil doesn't dissolve in water. Oil spills are often caused by oil drilling operations or ships that transport oil. 

Radioactive waste

If dumped carelessly into water bodies, radioactive waste products from nuclear reactors can pollute water. 

Groundwater pollution

Contaminated groundwater can pollute surface water, such as streams, lakes, and coastal waters. 

Point source pollution

Point source pollution originates from a specific place, but it can affect miles of waterways and oceans. The EPA regulates point source pollution by establishing limits on what can be discharged into a body of water. 

Suspended matter

Suspended solids are a major source of water pollution. When these particles settle to the bottom of a body of water, they become sediments. 

Other causes of water pollution include:

Plastics and polythene bags

Pesticides and fertilizers

Population growth

Urbanization

Mining

Agro-chemical wastes

Thermal pollution