Water pollution can occur via several mechanisms, but the most common causes are dumping industrial waste and sewage into bodies of water, along with the use of toxic substances in agriculture that runs off into rivers. There are also less common causes of water pollution like accidental oil spills.
Water pollution is broadly defined as any toxic substance being introduced into a body of water. A substance classified as toxic has negative health effects on the surrounding ecosystem, both wildlife in the water and surrounding plant life. Clean water is essential to human, plant and other animal life, which is why water pollution is important to study and try to reduce.