Fluoride in Water
Fluorine is never found free in nature because it is the most reactive element of the nonmetals. Fluorine is the most electronegative of all elements.
In water solutions, it forms fluoride F- ions. Other oxidation states are not found in natural aqueous systems. In contrast to chlorine, fluorine is almost all tied up in rock minerals with only very small percentage of the total contained in sea water.
Fluorides in high concentrations are not a common constituent of natural surface waters owing to their origin only being in certain types of rocks and which are found only in a few regions.
However, fluorides may occur in detrimental concentrations in ground waters. The sources of fluoride in water are primarily from calcium fluoride (fluorite CaF2) which is a common fluoride mineral. This mineral has a rather low solubility and occurs both in igneous and sedimentary rock.
Apatite commonly contains fluoride, amphiboles such as hornblende and some of the micas may contain fluoride replacing part of the hydroxide. Rocks rich in alkali metals and also obsidian are a class higher in fluoride content than most other igneous rocks. Fluoride often is associated with volcanic or fumarole gasses and in some areas these may be important sources of fluoride for natural water. Mineral particles that contain fluoride are wide-spread constituents of resistate sediments.
Ground water containing fluorite concentrations exceeding 1 mg/L are found in many places in the U.S. and is obtained from a wide variety of geologic terrains.
Fluorides in sufficient quantity are toxic to humans with doses of 250 to 450 mg giving severe symptoms and 4 grams causing death. The generalization is that water containing less than .9 to 1.0 mg/L of fluoride will seldom cause mottled enamel in children and for adults concentrations of less than 3 or 4 mg/L are not likely to cause endemic cumulative fluorosis and skeletal effects.
Concentrations of fluoride likely to be found in natural waters or in polluted streams apparently will have no detrimental effects on plants.
The use of fluoride bearing insecticides appears also to cause no harmful concentrations of fluoride in the soil. However, fluoride ions appear to have direct toxic properties towards aquatic life.