Abstract
A high concentration of fluoride (F−) in drinking water is harmful and is a serious concern worldwide due to its toxicity and accumulation in the human body. There are various sources of fluoride (F−) and divergent pathways to enter into groundwater sources. High F− incidence in groundwater was reported in Raigarh district of Central India in a sedimentary (Gondwana) aquifer system. The present study investigates the hydrogeochemistry of groundwater in the Tamnar area of Raigarh district to understand the plausible cause(s) of high F− concentration, especially the source(s) and underlying geochemical processes. Groundwater samples, representing pre-monsoon (N = 83), monsoon (N = 20), and post-monsoon (N = 81) seasons, and rock samples (N = 4) were collected and analyzed. The study revealed that (i) groundwater with high F− concentration occurs in the Barakar Formation, which has a litho-assemblage of feldspathic sandstones, shales, and coal, (ii) high F− concentration is mainly associated with Na-Ca-HCO3, Na-Ca-Mg-HCO3, and Na-Mg-Ca-HCO3 types of groundwater, (iii) the F− concentration increases as the ratio of Na+ and Ca2+ increases (Na+: Ca2+, concentration in meq/l), (iv) F− has significant positive correlation with Na+ and SiO2, and significant negative correlation with Ca2+, Mg2+, HCO3−, and TH, and (v) high F− concentration in groundwater is found in deeper wells. Micas and clay minerals, occurring in the feldspathic sandstones and intercalated shale/clay/coal beds, possibly form an additional source for releasing F− in groundwater. Feldspar dissolution coupled with anion (OH− or F−) and cation (Ca2+ for Na+) exchange are probably the dominant geochemical processes taking place in the study area. The higher residence time and temperature of groundwater in deeper aquifers also play a role in enhancing the dissolution of fluorine-bearing minerals. Systematic hydrogeochemical investigations are recommended in the surrounding area having a similar geologic setting in view of the potential health risk to a large population.