Contaminant

"An undesirable substance not normally present, or an usually high concentration of a naturally- occurring substance, in water, soil, or other environ- mental medium. In more restricted usage, a substance in water that may be harmful to human health." (Virginia Polytechnic Institute and State University, n.y.)

Sources

Virginia Polytechnic Institute and State University. "A Glossary  of Water-Related Terms", by Brian Benham. Publication 442-758