What are the major causes of water pollution? Explain

Pollution of water originates from human activities. The different causes of water pollution are :
(i) Pathogens : The most serious water pollutants
are the disease causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contains bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.
(ii) Organic waste: The other major water pollutants are organic matters such as leaves, grass, trash etc. The large population of bacteria decomposes organic matter in water. The amount of oxygen which water can hold is consumed by bacteria. The concentration of dissolved oxygen in water is very important for aquatic l#e. The lack of oxygen in water results in oxygen dependent aquatic life to die. Thus anaerobic bacteria (which do not require oxygen) being to break down the organic waste, producing chemicals that have a foul smell and are harmful to human life. Aerobic (oxygen requiring) bacteria degrade these organic wastes and keep the water depleted in dissolved oxygen.
(iii) Chemical pollutants: Water being an excellent solvent dissolves inorganic chemicals that include heavy metals like cadmium, mercury, nickel etc. which constitutes an important class of pollutants. These chemicals are extremely dangerous to human because our body cannot excrete them. Organic chemicals present in petroleum products pollute many sources of water, e.g., major oil spills in ocean. Various industrial effluents like polly chlorinated biphenyls (PCBs) which are used as cleaning solvents, detergents and fertilizers add to the list of water pollutants. There is algae growth due to the presence of phosphates present in fertilizers. These algae growth often release, toxins in water.