Fluoride hydrogeochemistry and its occurrence in drinking water in Morappur region of Dharmapuri District,South India

Groundwater is the major source of freshwater in region devoid of surface water resources. Once such region is the Morappur area, Dharmapuri district of Tamilnadu, wherein groundwater is major source of water for all purposes. The area is reported to be severely affected by fluorosis due to excessive Fluoride in groundwater. The area comprises of rocks of Archaean age, namely Charnockite, Epidote Hornblende Gneiss and ultramafic rocks. The area has experienced numerous tectonic disturbances in which numbers of vertical joints have developed and are filled with quartz/feldspathic veins, and highly mineralised. Two aquifer systems have been identified in the area, namely the weathered aquifer and fractured aquifer. In order to understand the factors controlling high Fluoride concentration in groundwater, 149 groundwater samples were collected during pre and post-monsoon period. Analytical results indicate that 35% groundwater samples show Fluoride concentration more than 1.5 ppm (permissible limit). Results indicate that both the aquifer units are affected by high Fluoride and deeper aquifers are more affected. Biotite and Hornblende minerals present in the area and interact with groundwater to release Calcium, Magnesium along with Fluoride. Further chloro-alkaline indices indicate that calcium ions are replaced by sodium due to reverse ion exchange, leading to high concentration of Fluoride along with high concentration of Sodium. Government has taken measures to provide Fluoride-free drinking water from distant surface water sources. However, it is important to follow certain water management methods to improve the groundwater quality.