Fluoride presence in drinking water along the southeastern part of El Bajío Guanajuatense, Guanajuato, Mexico: sources and health effects.

Drinking water with a high natural concentration of fluoride (F) has serious consequences for the health of the rural population in the state of Guanajuato, Mexico, where the water contains levels of F that are not allowed by national and international regulations (1.5 mg/L). This health problem is very common in multiple states throughout Mexico where drinking water is generally extracted from aquifers that are hosted in fractured volcanic rocks of the Tertiary. These aquifers show similar geological characteristics: deep basins that formed as a result of felsic eruptive events and the extensional deformation of the Basin and Range and are now filled with unconsolidated sediments. In this study, we assessed the occurrence of F in volcanic rocks collected at 11 sampling sites along the Sierra de Codornices in Guanajuato (ranging between 0.01299 and 0.146 wt%, average 0.039 wt%, and SD = 0.039 wt%; n = 10), a region where both rural and urban communities consume drinking water with a high F content (up to 7.1 (mg/L). The F content is dispersed in volcanic rocks, and the highest levels are present in felsic rocks. The statistical and hydrogeochemical results of a sampling campaign of 32 wells in the Juventino Rosas (JR) and Villagran (Vill) municipalities in 2019 suggest that F mobilization in groundwater is the product of silicate weathering and the dissolution of volcanic glass, alkaline desorption in the surfaces of F-containing minerals, and possibly ion exchange of minerals and clays or deep fluids enriched with F, in addition to the precipitation of carbonates that decrease the Ca concentration in groundwater. All of these processes can be accelerated by groundwater geothermal characteristics within the study area. The hydrogeochemical, fluoride exposure risk, and fluoride pollution index (FPI) results, as well as the epidemiological survey, indicate that teenagers and older adults from Praderas de la Venta are at risk of exposure to F due to the high concentrations ingested over a long period, the toxicity of the element, and its ability to accumulate in the bones. Extended exposure to elevated levels increases the risk. This work allows us to observe how the populations of JR and Vill can be exposed to high F contents in drinking water due to the geological characteristics of the region.