Geochemical assessment of soils in districts of fluoride-rich and fluoride-poor groundwater, north-central Sri Lanka

Abstract

High contents of fluoride in groundwater are a controversial issue in the dry zone of Sri Lanka. This study describes the geochemistry of residual soils from relatively fluoride-rich (<8mg/L; mean 2.0mg/L) and fluoride-poor (<1mg/L; mean 0.4mg/L) groundwater sites in the dry zone to identify possible sources for fluoride. Abundances of 22 major and trace elements were determined in 74 soil samples using X-ray fluorescence. The results show that soil fluoride is lower than average upper continental crust and basement rocks in both the fluoride-rich (<411mg/kg) and fluoride-poor (<277mg/kg) groundwater sites. Negative linear correlation exists between fluoride in the soil and the groundwaters, suggesting that fluoride is readily leached to water rather than being retained in the unconsolidated sandy clay loam soils. Weathering of heavy minerals such as zirconium, apatite, fluorite, monazite and garnet are the main source for the soil in the fluoride-rich groundwater districts. In these areas Zr, Nb and Th are immobile relative to the basement, and F, CaO and P2O5 are depleted, suggesting that the loss of CaO provides favorable conditions for the leaching of F to water. Conversely, soils in the relatively fluoride-poor district are enriched in TiO2, Fe2O3, MnO, Cr, V and Sc, denoting the weathering of biotite, hornblende, garnet and pyroxenes in the basement. Primary minerals present in the soils are the main cause for the enrichment of those elements. Further, fluoride levels in the soils and subsequently in the groundwaters show links with original magmatic contrast between the basement formations in each area. Soil geochemistry suggests that the meta-igneous rocks in the fluoride-rich districts may have been influenced by a fluoride-rich residual melt, whereas the fluoride-poor districts are associated with acidic meta-igneous rocks and meta-sedimentary rocks.