Fluoride, a negative ion, can exist as a free ion or as a compound to make salts with other minerals. Fluoride can frequently be found originating from the common mineral fluorite (CaF  ), but is also present in the common minerals amphibole, biotite, and apatite. Fluoride in groundwater is often found associated with granites and gneisses or volcanic rocks. Fluoride can also become concentrated in shallow aquifers in arid regions.

Rural areas that rely on groundwater are therefore greatly at risk for overexposure to fluoride. If the wells are near high fluoride bedrocks, the fluoride can leach into the water over time accumulating to unsafe levels.

Some studies have already attempted to link geology to groundwater fluoride...