Recognition of the anthropogenic contribution to the input of fluoride in urban recharge

Fluoride is a common inorganic contaminant of geogenic origin around the world. Anthropogenic sources of fluoride could also contribute to increased fluoride concentrations in groundwater, although they have not been well-documented. This paper presents soil batches and unsaturated column experiments on fluoride-enriched recharge water percolating from a urine-polluted soil in an area where fluorosis is endemic. Results from the polluted soil collected near a main street in Addis Ababa (Ethiopia) are compared with an unpolluted soil collected in a garden inside the university campus, located 50 m from the polluted one. Soils were sampled at the end of the dry season and after the first intense rainfall at the beginning of the wet season. The objectives of the study were to evaluate the anthropogenic fluoride enrichment in urban recharge due to unmanaged human excretions along the streets and model recharge water fluxes and solutes leaching to evaluate the reactivity of dissolved species. Batch experiments found extremely high fluoride (42.1 ± 5.2 mg/kg) and urea (89.5 ± 8.1 mg/ kg) concentrations in polluted soil before the rainy season. The fluoride concentration in the polluted soil was more than one order of magnitude higher than that recorded in the uncontaminated soil. This study proved that in areas where the population is subject to a large fluoride daily intake, recharge waters extremely enriched in fluoride can be leached from urine-contaminated soils in urban environments. The proposed methodology could be applied for a first screening for the anthropogenic sources of fluoride in urban recharge.