The pollution of rivers and streams with chemical contaminants has become one of the most critical environmental problems of the century. Chemical pollution entering rivers and streams can be classified according to the nature of its sources: point pollution and nonpoint pollution. Point pollution comes from factories, refineries, or outfall pipes. Nonpoint pollution comes from sources that cannot be precisely identified, such as runoff from agricultural or mining operations or seepage from septic tanks or sewage drain fields. It is estimated that each year 10 million people die from drinking contaminated water.