EXPLAINING THE PRESENCE OF FLUORIDE IN GROUNDWATER OF NORTH-CENTRAL MEXICO

High concentrations of fluoride in groundwater in north-central Mexico has been a concern since groundwater replaced surface water as the main source of drinking water. The connection between dental fluorosis and groundwater was reported in 1982 for the state of Durango. After this finding, multiple studies have tried to narrow down the causes (if natural or human related) and conditions (pH, alkalinity, evaporation, residence time) responsible for its occurrence. The results agree on a natural source (geogenic) of the contaminant and an association with primarily felsic volcanic rocks (rhyolite, tuff, ignimbrite), followed by other volcanic rocks (andesite, basalt) and secondary minerals (clays, iron oxyhydroxides). As for the processes responsible for enrichment, increasing residence time, alkalinity, and evaporation, in this order, all exert an influence, however under certain limits. For example, groundwater saturated with calcium incorporates fluoride into precipitating calcite, resulting in a decrease in fluoride concentration in areas of high evaporation.