Genesis and geochemistry of high fluoride bearing groundwater from a semi-arid terrain of NW India

Occurrence of fluoride in natural water resources and the associated health hazards have been reported from many parts of India during the last decade. With the objective of organizing a systematic scientific programme to understand the geochemical behaviour of fluoride in natural water resources in relation to the local hydrogeological, climatic conditions and agricultural use, a typical semi-arid terrain in the northwestern part of India was chosen for detailed study. The present work is an attempt to study the fluoride contamination in groundwater in parts of Jaipur district, Rajasthan state, and to assess the origin and genesis of fluoride in groundwater. The area, falling under the semi-arid tract of central Rajasthan, is geologically occupied by alluvial formations and Precambrian rocks (gneisses and schists) where groundwater occurs under unconfined to semi-confined conditions. A total of 66 groundwater samples was collected during the pre-monsoon period and analysed, which gave a concentration of fluoride in water in the range of 0.35 to 9.3 mg/l with 62 % of the samples in excess of permissible limit of 1.5 mg/l. As compared to alluvial formations, fluoride enrichment in groundwater is higher in hard rock areas. The samples collected from the granite gneissic terrain show higher values when compared to those in the schist. The groundwater of the shallow aquifers (soil-clay complex weathered zone) has a higher concentration of fluoride than the deep (fractured hard rock) aquifer does. The possible sources of fluoride are chemical weathering and leaching of fluorine-bearing minerals present in hard rocks and alluvial formations under the alkaline environment in arid to semi-arid conditions. High pH as well as alkalinity and low levels of Ca, Mg and total hardness suggest favourable chemical conditions for the fluoride dissolution process. A higher rate of evapotranspiration, intensive and long-term irrigation and heavy use of fertilizers are the supplementary factors that further increase the fluoride content in the groundwater. A groundwater management programme is suggested.