Hydrogeochemical processes, mobilization controls, soil-water-plant-rock fractionation and origin of fluoride around a hot spring affected tropical monsoonal belt of eastern Odisha, India

Abstract

Fluorosis, poor crop yield, and degraded soil fertility due to excess fluoride are mostly associated with igneous exposures and fertilizer use. Geochemical, petrographic, microprobe and mineralogical examinations revealed thermal spring origin of fluoride (up to 15.8 mg L−1) affecting agriculture and public health in eastern India. No fluoride was detected in plant leaves and exposed charnockite, but nearby soils and siderite precipitates are enriched in fluoride up to 3.4 g kg−1. Both geogenic hot spring and fertilizer origin are proposed with fluorosis severity found to be closer to the hot spring where the stream, pond, and well enrichments might have resulted upon mixing with high F− hot spring water. The pond, stream, bore well, hand pump, well and hot spring water in the region are mostly fresh water (dissolved solids <1000 mg L−1) and hard to very hard in nature with a pH of 5.7–9.7 and a temperature of 23–55 °C. Major water is of NaCl type with minor components of mixed CaMgCl, CaHCO3 and CaCl types enriched in the order Na+>Ca2+≈Mg2+>K+>Si4+>FeT > Al3+ and Cl−> HCO3− > NO3− > SO42−> PO43−. Stream, pond, and bore well waters exhibit better suitability for irrigation use compared to hot spring and well waters, while all the studied water resources are of poor to unsuitable quality for human consumption. Carbonate weathering dominated silicate weathering processes for the stream, pond, and bore well water, while the latter was for hot spring and well water. Reverse ion exchange is the major process that prevailed in the region where rock-water interaction and evaporation governed the regional hydrochemistry. Fluoride mobilization is favored under alkaline conditions and enhanced by competitive anions in the order NO3− > SO42−> PO43−. Acidic rainwater does not favor fluoride release from soil, and F-containing siderite is prone to release fluoride under ambient conditions.