Anything found in water (including microorganisms, minerals, chemicals, radionuclides, etc.) which may be harmful to human health.

A microorganism commonly found in lakes and rivers which is highly resistant to disinfection. Cryptosporidium has caused several large outbreaks of gastrointestinal illness, with symptoms that include diarrhea, nausea, and/or stomach cramps. People with severely weakened immune systems are likely to have more severe and more persistent symptoms than healthy individuals.

Disinfectant Byproducts are chemicals that may form when disinfectants (such as chlorine), react with plant matter and other naturally occurring materials in the water. These byproducts may pose health risks in drinking water.

A microorganism frequently found in rivers and lakes, which, if not treated properly, may cause diarrhea, fatigue, and cramps after ingestion. People with severely weakened immune systems are likely to have more severe and more persistent symptoms than healthy individuals.

Mineral-based compounds such as metals, nitrates, and asbestos. These contaminants are naturally-occurring in some water, but can also get into water through farming, chemical manufacturing, and other human activities. US EPA has set legal limits on 16 inorganic contaminants.

The highest level of a contaminant that is allowed in drinking water. Maximum Contaminant Levels (MCLs) are set as close to the Maximum Contaminant Level Goal (MCLG) as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable standards.

The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety. MCLGs are nonenforceable health goals.

Tiny living organisms that can only be seen with the aid of a microscope. Some microbes can cause acute health problems when consumed (see pathogens).

Carbon-based chemicals, such as solvents and pesticides, which can get into water through runoff from cropland or discharge from factories. US EPA has set legal limits on 56 organic contaminants.