ORIGINAL PAPER A study on the status of fluoride ion in groundwater of coastal hard rock aquifers of south India C. Singaraja & S. Chidambaram & P. Anandhan & M. V. Prasanna & C. Thivya & R. Thilagavathi Received: 22 February 2012 / Accepted: 22 August 2012 # Saudi Society for Geosciences 2012 Abstract India has an increasing incidence of fluorosis, dental and skeletal, with nearly about 62 million people at risk. High fluoride groundwaters are present espe- cially in the hard rock areas of the country. This paper analyzes the most extensive database on fluoride and other chemical constituent distribution in the coastal hard rock aquifers of Thoothukudi district. A total of 135 samples were collected and analyzed for major cations and anions to assess the geochemical process. The fluoride concentration in drinking waters varied from BDL to 3.2 mgl -1 in the study area. Majority of the samples do not comply with WHO standards for most of the water quality parameters. The saturation index of fluorite saturation index was used to correlate with F - to identify their relationship to increase of fluoride levels. The correlation between the F - concen- tration and the water type was also attempted. Spatial distribution of fluoride in groundwater was studied to understand the influencing factors. The relationship of F - with HCO - 3, Na + and pH concentrations were studied and found that HCO - 3 , has good correlation with F - than the other parameters. Keywords Fluorosis . Saturation index . Hard rock . Spatial distribution . Correlation Introduction The presence of different hazardous contaminants like fluo- ride, arsenic, nitrate, sulfate, pesticides, other heavy metals, etc. in groundwater has been reported from different parts of India (Liu et al. 2005; Mulligan et al. 2001). In many cases, the water sources have been rendered unsafe not only for human consumption but also for other activities such as irrigation and industrial needs. Therefore, now there is a need to focus greater attention on the future impact of water resources planning and development taking into consider- ation all the related issues. In India, fluoride is the major inorganic pollutant of natural origin found in groundwater. Fluorine (F) is one of the important trace elements for human health, which is obtained mostly by drinking water. Fluorine in natural waters originates dominantly from natu- ral processes and occurs mainly as a free ion (F - ). Excessive intake of fluoride greater than 1 mgl -1 can cause dental, skeletal, and non-skeletal forms of fluorosis. Skeletal and dental problems can be prevented by maintaining fluoride concentration of about 1 mgl -1 from dietary intake (Sujana et al. 1998). A concentration of fluoride less than 0.8 mgl -1 in drinking water may result in dental caries, while high concentrations above 1.5 mgl -1 may cause an endemic disease called dental fluorosis according to Handa (1975); WHO (1984a); USPHS (1987); and Ripa (1993). In India, it was first detected in Nellore district of Andhra Pradesh in 1937 (Short et al 1937). Presently, 17 states in India have been identified as endemic for fluorosis, and Tamil Nadu is one of them. It has been reported that the solubility of F - varies with the soil type as well as pH and has the tendency to be higher in the alkaline pH range (Larsen and Widdowson 1971; Gilpin and Johnson 1980), leading to higher bio-availability. Although high F - concen- trations in groundwater were previously considered to be a problem mainly in hard rock areas (Handa 1975), they have now been discovered in other environments as well (Chadha and Tamta 1999). C. Singaraja (*) : S. Chidambaram : P. Anandhan : C. Thivya : R. Thilagavathi Department of Earth Sciences, Annamalai University, Annamalai Nagar 608002, India e-mail: geolionking@gmail.com M. V. Prasanna Department of Applied Geology, School of Engineering and Science, Curtin University, CDT 250, 98009 Miri, Sarawak, Malaysia Arab J Geosci DOI 10.1007/s12517-012-0675-6