Question

define water pollution . how it is caused ? what are the causes? in more than 200 word ...

Answer 1

Water pollution: The undesirable changes in water caused by human activities. 
 How it is caused?
 - The addition of undesirable substances to water-bodies.   Fertilizers and pesticides used in farming.  Poisonous substances like mercury and lead used by industries and found in domestic waste. Disease-causing organisms like the Salmonella typhus, Vibrio cholerae.
 - The removal of desirable substances from water-bodies.  Depleted Oxygen levels. Increased BOD (Biological Oxygen Demand). Depleted Nutrient Levels
 - Undesirable change in temperature.   Sudden release of water from large dams disturb the temperature of water downstream the river.  Release of hot water from industries and Nuclear plants. 
 What it causes?
 - The addition of undesirable substances to water-bodies cause  Accumulation of fertilizers and pesticides inside the ecosystem including us.  Poisoning of humans like minamata (mercury), and lead poisoning.  Water contamination and hence water borne diseases like typhoid, cholera.
 - The removal of desirable substances from water-bodies cause  Death of desirable fishes and damage of water ecosystems
 - Undesirable change in temperature causes  Loss of ecological balance and fisheries as fishes need certain temperature to breed.  

Answer 2

WATER IS REGARDED AS POLLUTED WHEN THERE IS CHANGE IN ITS EQUALITY AND COMPOSITION DIRECTLY OR INDIRECTLY BY PEOPLES ACTIVITY. 
CAUSES - IT CAUSES LARGE NO. OF DISEASE  LIKE CHOLERA , DYSENTERY , etc.  IT IS ALSO HARMFUL FOR AQUATIC LIFE . SEVERAL CHEMICALS WHICH ARE CAUSED TO KILL INSECTS .FLOW WITH WATER DUE TO WHICH SEVERAL LIVING ORGANISM DIE . .
 
HOW IT IS CAUSE -- IT CAUSES BY THE DOMESTIC SEWAGE LIKE FROM KITCHEN , TOILETS , etc.  WHICH DISPOSED INTO PONDS, RIVERS, etc. 
                            -- BY INDUSTRIAL DISCHARGES INDUSTRIAL WASTES , ACIDS , etc. ARE MIXED UP WITH RIVERS .
                            -- IT IS ALSO CAUSED BY THE AGRICULTURAL SOURCES  BY THE PESTICIDES ,, HERBICIDES , etc. ARE CARRIED TO THE LAKES FROM AGRICULTURAL FIELD IN RAINY SEASONS .