Kinds and Sources of Water Pollutants:
Pollutants entering water sources are classified broadly into following categories: domestic sewage and oxygen-demanding wastes; infectious agents; plant nutrients; chemicals such as insecticides, herbicides, and detergents; other minerals and chemicals; sediment from land erosions; radioactive substances; and heat from power and industrial plants.
These aquatic pollutants come from many sources. Excessive nutrients, such as nitrates and phosphates, commonly originate in domestic sewage, run-off from agricultural fertilizer, wastes materials from animal feed lots, packing plants, etc.
These nutrients cause pollution primarily because they stimulate the growth of micro-organisms which often increase the biological oxygen demand (BOD) of the water and reduce the amount of dissolved oxygen available for fish, higher animals and other aquatic animals.
Toxic chemicals as agents of water pollution originate in industrial operations, acid mine drainage, surface erosion from strip mines, washing of herbicides and insecticides, radioactive fallout from atomic explosion, and commercial accidents such as oil spills or the rupture of chemical tanks.
Besides the pollutants which come from point sources such as sewage, factory or industry, there are many pollutants which come from watershed run-off. Urban and sub-urban run-off, for example, contains many pollutants such as oil, pesticides, radioactive dust, salt, fertilizers, miscellaneous chemicals, and nematodes, pathogenic protozoans, bacteria and viruses. However, Kimball (1975) recognized only three major sources of pollutions: domestic, industrial and agricultural.