Geogenic Fluoride Contamination in Two Diverse Geological Settings in West Bengal

Fluoride contamination in groundwater has been studied in two important geological settings of West Bengal, viz. (i) alluvium of Rampurhat–Nalhati area of Birbhum district, and (ii) consolidated formation of Barabazar block of Purulia district. Rampurhat–Nalhati area in Birbhum district is located in ‘basin margin’ area represented by Tertiary sediments overlain by Quaternary deposits. A two aquifer system exists in the area: (i) shallow aquifer in the Pleistocene Older Alluvium in Quaternaries, and (ii) deeper aquifer within the Tertiary sediments. The shallow aquifer has high groundwater potentiality with low fluoride content, while the deeper aquifer system is characterized by comparatively low yield and high fluoride concentration. Maximum concentration of fluoride of 10.7 mg/L has been encountered at Junudpur in Birbhum district. At places, only one continuous aquifer system exists where the groundwater potentiality is high with low fluoride content. The fluoride content of groundwater has, in general, an inverse relationship with calcium and magnesium content and positive relationship with sodium concentration. Barabazar block in Purulia district is situated in the eastern fringe of Chhotanagpur Gneissic Complex, occupied mostly by meta-sedimentaries and a few thin linear patches of alluvium along narrow courses of stream. Groundwater prospect is better in northern parts underlain by gneissic rocks than in the south, which is mainly underlain by phyllites and schists. No relationship could be established between the fluoride content of groundwater with the concentrations of bicarbonate, calcium and sodium. Groundwater is generally potable, except at places near ‘south shear zone’, where groundwater with high concentration of fluoride and iron has been encountered. The concentration of fluoride increases with depth as well as in proximity to shear zone. In both cases, fluoride concentration in groundwater increases with depth.