Pollutants in water include a wide spectrum of chemicals, pathogens, and physical chemistry or sensory changes. Many of the chemical substances are toxic or even carcinogenic. Pathogens can obviously produce waterborne diseases in either human or animal hosts. Alteration of water's physical chemistry include acidity, conductivity, temperature, and excessive nutrient loading (eutrophication). Even many of the municipal water supplies in developed countries can present health risks. In the U.S. in 1972 stringent federal laws were enacted setting specific discharge limitations.
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