Evaluation of the potential impact of fluorine-rich fertilizers on the Guarani Aquifer System, Rio Grande do Sul, Southern Brazil

It has been determined that the fluorine concentration reaches up to 11 mg L⁻¹ in the porous-confined Santa Maria Aquifer, Guarani Aquifer System, in the central region of the state of Rio Grande do Sul, southern Brazil. Due to the elevated fluorine content in the groundwater, which is the major supply for drinking water, several incidences of dental fluorosis occurred. This area is the largest tobacco producer in Brazil and has a long history of fertilizer application. The fluorine in the local groundwater is likely to have come from the use of apatite-based fertilizers. This hypothesis was tested by soil and water analyses. Fluorine concentration has been monitored in the field for 2 years. The data combined with laboratory experiments on the adsorption and leaching of fluorine in soils showed that fluorine is easily retained by adsorption on aluminum clay-rich soils (Udults). The local clay-rich soils present high cation exchange capacity, aluminum content, and low pH; they act as a trap for fluorine derived from fertilizer and play an important role in the mobility of fluorine.