Answer. Water pollution due to agricultural waste is because of use of fertilizers and pesticides. Fertilizers are used to make up the deficiency of nitrogen, phosphorus, etc., of the soil because of intensive cultivation of crops in the recent years.
On the other hand, pesticides are used either directly to kill or control the growth of pests. Pests may be weeds, herbs, insects, fungi, viruses, etc. They all damage crops and transmit diseases both to human beings and animals.
Agricultural effluents have dual effects:
(i) Intensive cultivation of crops causes these chemicals from fertilizer and pesticides to seep into the ground water commonly called leaching process. The high nitrate contents in ground water are mainly because of irrigation run-off from agricultural fields.
(ii) Sun-off from the agricultural land (where fertilizer and pesticides have been used), enters into ponds, streams or fivers. This water contains nitrate (NO3-) and phosphate (PO43-) salts. These substances results in a rapid growth of algae, floating over the surface of water. They prevent the sunlight and air (oxygen) to reach up to aquatic life. When algae dies, and decompose bacteria consume oxygen of the water for decomposition. As a result oxygen depletes in the water. Aquatic animals feel suffocation and ultimately die due to insufficient supply of oxygen.