Question

why fertilizers have become source of water pollution

Answer 1

INCREASING OF ALGAE
LESS NUTRIENTS IN WATER -death of water animals 
loss of certain breed

Answer 2

When we use chemical fertilisers only 40% is soaked and rest 60% is left out , when it rains it takes the fertiliser to underground water and as it mixes with it , it gets polluted. Then as per the water cycle it reaches to every source of water and eventually chemical fertilisers have become source of water pollution.