Sources of water pollution in lakes, streams, rivers, oceans, and other water bodies include municipal sewage, industrial waste, agricultural runoff, and oil spills, among others. Waste may be dumped in water intentionally or accidentally. Although water pollution is usually caused by human activity, some natural phenomena—like volcanoes, storms, and earthquakes—cause changes in water chemistry. Sometimes, high saline or mineral content of water makes it unfit for certain uses, even though the water is not polluted in the traditional definition of pollution.

Many toxic synthetic chemicals cannot be broken down by natural processes and cause serious harm. These substances may be dissolved or suspended in water or deposited in sediments, but they do not go away. This results in the pollution of water—the quality of the water ...