GEOLOGICAL AND GEOCHEMICAL CONSTRAINTS OF THE PRESENCE OF GROUNDWATER FLUORIDE IN CHIHUAHUA, MEXICO

Chronic intake of fluoride (F^-) in drinking water causes fluorosis. At low levels (<1.5 mg/L) it causes discoloration of teeth (dental fluorosis) and at higher concentrations (>4 mg/L) a more serious condition affecting the bones (skeletal fluorosis). The north-central part of Mexico has been identified as a region with high concentrations of geogenic F^-, especially the states of Chihuahua, Durango and Zacatecas, with maximum concentrations of 16, 28, and 22 mg/L F^-, respectively, according to 2017-2019 data reported by Mexico’s water office CONAGUA. Chihuahua has a similar geologic setting as Durango and Zacatecas; however, groundwater quality is more sparsely reported and according to the fewer data available, the F^- levels are not as high as those of Durango and Zacatecas. Some inferences are tested to see if this difference in concentration is due to the density of sampling or if geochemical processes are responsible for the amelioration. Among the latter, presence of other ions (mainly Ca²⁺ and Na⁺), saturation conditions, and residence time were considered.