TRACING NATURAL SOURCES OF F- IN GROUNDWATER: CASE NORTHERN MEXICO

Groundwater in arid and semi-arid areas of northern Mexico contain high concentrations of fluoride (F), a toxic element that causes dental and skeletal fluorosis and bone fractures, among other health problems. A previous (2020) study identified Durango, Zacatecas and Chihuahua as the Mexican states with highest F concentrations. The source and the factors that cause its enrichment were investigated here. To this purpose, F and related water parameters (As, TDS, T, U) were acquired from groundwater quality databases of samples collected in 2019. A map showing the sites with highest concentrations in the region (north-central Mexico and bordering region in the USA) was constructed. The analysis shows that the origin of F is mostly natural, likely caused by weathering of silica-rich volcanic rocks contained in the alluvial fill. High alkalinity and evaporation concentrate F further. The chemical species under alkaline conditions and present solutes are F-, MgF⁺, and AlF₂⁺. F showed a strong correlation to As and correlated weakly with TDS. Both temperature and U data were too fragmented to show a trend.