Geochemical Evaluation and Spatiotemporal Distribution of Fluoride in Groundwater of the Sedimentary Alluvial Plain of Bankura District, West Bengal, India

The contamination of groundwater by geogenic sources is a major problem in many nations, especially those in the developing world. Fluoride (F) is one of the most pervasive and well-documented geogenic contaminants because of the severe health risks it poses due to its toxicity. F contamination in groundwater in India has been the subject of intense research over the past many decades. In this article, we describe the underlying geochemical process liable for F contamination as well as the factors controlling its spatiotemporal distribution in the Sedimentary Alluvial Plain (SAP) of Bankura District, West Bengal, India. To achieve the desired objective, representative groundwater samples were collected from tube wells and hand pumps at different locations of the study region during pre- and post-monsoon seasons. Collected samples were subjected to F and other hydrochemical analysis following standard test methods. Analysis shows that 37% of all groundwater samples collected during the pre-monsoon period have fluoride levels over 1.5 ppm (the limit specified by the World Health Organization, Geneva, 2004); however, the contamination level dropped to 30% during the post-monsoon period. The investigation of groundwater level changes indicates that, as water levels rise during the post-monsoon, F concentrations decrease due to the dilution effect. Piper trilinear diagram suggested Na-Ca-HCO₃ type of groundwater for both seasons. According to Gibbs diagrams, rock-water interactions (mineral dissolution) are responsible for major ion chemistry in groundwater samples. Factor analysis (FA) of hydrochemical parameters revealed that the occurrence of F in groundwater was due to the weathering and dissolution of fluoride-containing minerals. X-ray diffraction (XRD) analysis of SAP sediments further confirmed the presence of fluoride-bearing minerals (muscovite and fluorite) in the subsurface lithology of the region. A substantial positive loading (> 0.75) of F with pH and bicarbonate for FA demonstrates that F is being leached from the host material by an alkaline-dominated environment. To account for the spatial variability and seasonality to the spatial change of F concentration in groundwater of the SAP, geographical information systems tools and inverse distance weighting interpolation method were used. The results revealed that significant spatiotemporal variability of F contamination was mainly influenced by the recharging rainwater and the average recharge altitude of groundwater in the area under study. The contamination level is significant in the elevated region where replenishing rainwater is more likely to come into contact with fluoride-bearing minerals when it infiltrates and percolates through the vadose zone. This phenomenon increases the F leaching through chemical weathering along groundwater flow pathways. The findings of this study can serve as a scientific foundation for the efficient management of F-contaminated groundwater in the SAP.