Sources of Water Pollution

Although water bodies can occasionally be polluted by natural causes, water pollution is overwhelmingly the result of human activity, and it is usefully classified by how it reaches the water. Point sources discharge pollutants at specific, identifiable locations through pipelines or sewers directly into a water body; factory effluents, sewage outfalls, underground mines, oil wells, oil tankers, and agricultural discharge points are common examples. Non-point sources cannot be traced back to a single discharge site -- acid rain falling over a wide area, plastic dumped into water bodies at many scattered points, and agricultural chemical run-off spread across whole fields are typical examples. Oil leaks and spills form a third category, occurring mainly through ship collisions, offshore oil rig accidents, and leakages during transport or discharge into the sea. Sources are also classified by sector into three types: municipal wastewater from homes and commercial establishments; industrial discharge (effluents), which may carry heavy metals such as cadmium, chromium, and lead along with organic and inorganic chemicals in sometimes toxic concentrations, and which can also raise water temperature and lower dissolved oxygen; and agricultural wastes, including fertilizer and pesticide run-off, food-processing waste, sawdu …