Surface Run-off - Causes of Water Pollution
The surface run-off from cultivated lands where inorganic fertilisers, pesticides, insecticides and rich manure are applied and solid waste disposal from urban and industrial sites, bring heavy loads of pollutants into natural water bodies.
Application of nitrogen fertilisers and nitrates and ammonia released from manure increase nitrate content of water bodies. As you know soil bacteria convert ammonium ions into nitrate which has high mobility because of its solubility in water and negative charge. It cannot bind to negatively charged clay and humic micells; therefore, nitrate ions gradually leach from the soil by percolating water and eventually find their way into ground water.