Chemical weathering of the Indo-Gangetic alluvium with special reference to release of fluoride in the groundwater, Unnao district, Uttar Pradesh

In the central part of Indo-Gangetic alluvium in the Unnao district, Uttar Pradesh there are many pockets where groundwater shows high fluoride content. Drinking of fluorinated ground water has effected a large population and in many villages more than 80% of the population is suffering from fluorosis. The source of this fluoride appears to be the alluvial sediments deposited in the geological past as no hard rock terrain is present in the nearby areas. The area is dominantly made up of mud with pockets of sand. The sand fraction is made up of quartz, plagioclase, microcline, muscovite and biotite along with some accessory minerals like garnet, epidote, chlorite, tourmaline, hornblende, kyanite and a few opaque minerals. Moreover, the fluoride content in the groundwater varies both spatially and with depth indicating a sporadic occurrence. The surface water is devoid of high content of fluoride but is reported in hand pumps and in the dug wells. This paper deals with the geochemical study of the sediments up to a depth of 45m as most of the hand pumps are up to this depth to understand the source of fluoride. C-14 dates of calcretes have suggested that the 45 m thick succession must have been formed in about 45000 years. Two different location sites were selected; one showing higher concentration of fluoride (Marksnagar village) while at other site which is about 4 km east of Marksnagar, the fluoride content was minimal (Durgajkhera village). Major elements and 24 trace elements were determined using XRF and it was found that when major elements are normalized with respect to upper continental crust (UCC) there is an enrichment of Si in all the samples. Na shows depletion where as Ti and K show enrichment. Fe and Mn show enrichment probably due to the formation of clay minerals. Si, and K enrichment is due to weathering of feldspar while Mg, Fe and K may have been released by the weathering of biotite. The CIA for the ancient sediments ranges from 54-64 while for the modern sediments of the Ganga River it varies from 50-64 indicating that there is no change in the rate of weathering in both modern and ancient sediments. The rate of weathering at all the sample locations was compared with that of UCC. The CIA values also suggest that there is an incipient weathering and indicate that the weathering of biotite is more progressive than muscovite. There is also a positive correlation between CIA values and the fluoride content in the ground water. Higher percentage of biotite and chlorite (altered biotite) was found at Marksnagar in comparison to Durgajkhera. It appears that the fluoride content in the ground water is due to dissociation/alteration of mica minerals mainly biotite.