Fluoride hydrogeochemistry in alluvial aquifer: an implication to chemical weathering and ion-exchange phenomena

Fluoride (F-) concentration in some parts of the quaternary alluvial aquifer of Birbhum district, West Bengal, India varies from 0.31 to 13.61 mg/L with an average of 3.13 mg/L. High F- groundwater mostly belongs to Na-HCO3 type. Isotopic signature suggests that most of the groundwater having fast recharge component either from flowing canals or rainfall, contains relatively less F- (< 2 mg/L) and relatively depleted delta O-18 content (delta O-18: -5 to -4 aEuro degrees). On the other hand, some of the groundwaters having relatively depleted delta O-18 (-4 to -3 aEuro degrees) do exhibit F- in the range of 4-13 mg/L. Average ratio of Na+/Na+ + K+ (0.69) and Cl-/Cl- + HCO3 (-) (0.25) with average TDS (267.5 mg/L) represents that the weathering of aquifer sediments or sediment-water interaction primarily controls the major ion chemistry particularly F- concentration of groundwater in this region. This idea is further strengthened by the relationship between Na+ + K+ and total cations (TZ(+)) and Na/Cl molar ratio. Moreover, groundwater is under saturated with respect to amorphous silica (average SISiO2(a) = -0.64), while all water samples are either saturated or super saturated with respect to quartz (average SIquartz = 0.66). Mineral stability diagrams for the NaO-H2O-Al2O3-SiO2 and K2O-H2O-Al2O3-SiO2 systems suggest that most stable silicate mineral phase in the aquifers is kaolinite, suggesting that semi-arid nature of the study area and intermediated flow condition may give rise to kaolinite. Sample falling in kaolinite stability field indicates that the hydrolysis of some alumino-silicate minerals, such as albite and biotite, is responsible for the formation of HCO3 (-), Na+ and K+ which are dominant in the groundwaters. This ultimately leads to higher rate of ion exchange of F- from biotite- and muscovite-rich aquifer matrix to groundwater of the study area.