Role of weathering of ferromagnesian minerals and surface water irrigation in evolving and modifying chemistry of groundwater in Palakkad district, Kerala, with special reference to its fluoride content

The chemical characteristics of groundwater in parts of Palakkad district, Kerala show the role of slow weathering of rocks rich in ferromagnesian minerals in increasing the residual alkalinity of water, and promoting calcium carbonate precipitation and dissolution of fluoride from minerals like biotite and hornblende. Alkaline clayey soils having high cation exchange capacity have a significant role in the process. Leaching of fluoride from soil and weathered rock in the vadose zone appears to be a major cause for high fluoride in groundwater. No relation is seen between fluoride contents in rocks and soils, and in groundwater, while the degree of weathering is a probable controlling factor. The study indicates that, while alkaline pH promotes dissolution of F−, a further linear increase in pH is not required for more increase in dissolution of F−. Similarly, the correlation of F− with HCO3−/Ca2+ ratio may be a more reliable indicator than the correlation of F− with Ca2+. Surface water irrigation has influenced groundwater quality in a large tract. Dug well and bore well waters belong to the same chemical type indicating connectivity between the two aquifers. There is evidence for influent flow from streams into the fractured rock aquifer in some areas. The study was conducted between November 2009 and May 2011.