Fluoride

MCL: 4 ppm

Source: Occurs naturally in rocks and minerals. It may be added to water to increase tooth enamel, and it is used in fertilizers.

Water Treatment: https://tdb.epa.gov/tdb/contaminant?id=10700

Summary: Fluoride exists as an anion of fluorine, a halogen and element 9 on the periodic table. Fluorine is the most electronegative element in the periodic table, and as such is highly reactive. Fluoride occurs naturally in rocks and minerals. The chemical is used in fertilizers and is may be added to water by municipal water treatment facilities to strengthen tooth enamel.