Several human activities leads to presence of several undesirable substances in water which causes water pollution.  Various sources of different water pollutants are shown below. Pollutant Source Micro-organisms Domestic sewage Organic wastes Domestic sewage, decaying animals and plants, animal excreta and waste, discharge from food processing industries. Plant nutrients Chemical fertilizers Toxic heavy metals Chemical factories and industries Sediments Strip mining and soil erosion Pesticides Chemical used for killing fungi, weed, insects Radioactive substances Mining of uranium-containing minerals Heat Water used for cooling in industries.