Water that falls on the Earth's surface moves through runoff (surface flow), cutoff (groundwater), and fly-off (atmospheric water). In each of these three levels (surface, ground, and atmosphere), water is being polluted by several agents. Water pollution means unfavorable alteration of water, largely by human actions through modification of energy and/or matter to such an extent that degradation disturbs the natural balance. Heat and chemicals alter the physical and chemical properties of water and thus contaminate surface water, ground water, and atmospheric water adversely. Most water pollutants fall into one of these three categories: liquid organic wastes, liquid inorganic wastes, and water-borne or water-based pathogens.