Identification of salinization by bromide and fluoride concentration in coastal aquifers near Chennai, southern India

This study aims to use fluoride as an additional indicator for seawater intrusion apart from the known effective geochemical indicators such as EC, TDS, sodium, chloride and bromide. Groundwater in a coastal part of Arani-Koratalaiyar river basin located north of Chennai, Tamil Nadu, India was assessed to determine its suitability for domestic purpose and to delineate the region affected by seawater intrusion. Fluoride concentration in groundwater samples from forty nine locations varied from 0.02 mg/l to 2.9 mg/l with an average of 0.5 mg/l. As per the Bureau of Indian Standards, the permissible range of fluoride in drinking water is 0.6-1.5 mg/l. Fluoride concentration in groundwater of this area exceeds 1.5 mg/l in 6% of the groundwater samples and is below 0.6 mg/l in 74% of the groundwater samples. There is no known geological source for fluoride in this area. Spatial variation in fluoride concentration in groundwater indicates that the coastal areas have comparatively high fluoride due to seawater intrusion which was similar in comparison with EC and bromide concentration measured in groundwater. This study indicates that fluoride can be used as an indicator of seawater intrusion in coastal areas.