Influence of geological and hydrogeological factors on the high natural fluoride concentrations in groundwaters in Bambui Group (Neoproterozoic), northern of Minas Gerais state, Brazil

In the northern semi-arid and semi-humid region of Minas Gerais State, in the southeast Brazil, substantial part of the water supplying is produced by means of deep wells. In several localities the fluoride concentrations are higher than 1.5 mg/L, maximum acceptable limit for drinking water, endangering the public supply and the public health. Endemies of dental fluorosis have been detected since the decade of 1990. The cause of this problem has been associated to the consumption of local drinking water. Current studies indicate the occurrences of fluorite in the limestones of the Bambui Group as the main mineral-source for the fluoride content in the groundwater, however without identifying the specific contribution of each hydrostratigraphic unit. This work shows the results of stratigraphic studies and hydrochemical analysis of 144 deep wells and the influence of the carbonatic facies in the fluoride concentrations of the water. Fluoride concentrations are related to fluorite occurrences/mineralizations (lithostratigraphic control) and to circulation depth of groundwater (geochemical control), newly distinguishing three hydrostratigraphic units. The correlation analysis of hydrogeochemic parameters (F-, Mg2+, SO42- and electric conductivity) corroborates these grouping into three units. The probability of the contamination by fluoride, as a function of the hidroestratigraphic units, in decreasing order, seems to be: top level of the Sete Lagoas Formation; Lagoa do Jacare Formation; and botton and middle levels of the Sete Lagoas Formation.