Water quality pollutant criteria would be the classification of the origin of the specific pollutants found in the water. There are several origins of pollutants that may be found in drinking water available for human consumption. Contaminants may be from microorganisms found living in the water, such as viruses and bacteria. The water could possess inorganic contaminants, such as salts and metals. The water could also have organic contaminants from industrial processes and petroleum use. The water could also have radioactive activity in it, if it was released from a nuclear facility.
Maximum contaminant levels would be the greatest amount of that contaminant allowed by law. The standards seem to vary from country to country, and it should be noted most all samples of drinking water are not altogether pure. Most water available for human consumption has a minor allowance of some type of contaminant not considered dangerous enough to make humans sick. People who want pure water either buy bottled water or have their water run through a home-purification system. People who get their drinking water from a well, or some other natural system are not regulated by the maximum levels in the law.