Fluoride (F -) contamination in groundwater has been recognized as a serious problem worldwide. It is estimated that more than 200 million people from 25 countries are under the dreadful fate of fluorosis, among them India and China are the two worst affected countries In India 62 million people (including 6 million children), suffer from dental, skeletal or non-skeletal fluorosis because of consumption of fluoridecontaminated water. Rajasthan is one of the most affected states with fluoride concentration in the range of 0.1-14 mg L -1 . The source of fluoride in soil and water is predominantly geogenic and released by weathering of fluoride containing minerals such as fluorite and fluorapatite, while anthropogenic sources include brick industry, aluminium smelters and use of phosphatic fertilisers. The geochemistry of fluoride is mainly governed by two processes i.e. enrichment and leaching. Fluoride mobility in soil is highly dependent on the soil's sorption capacity, which varies with pH, salinity and types of sorbents present in soil. In general, fluorine present in soil as minerals, adsorbed on clay and oxy-hydroxides, while a few amount is present as dissolved form in soil solution. Fluoride normally enters the human body through water, food, industrial exposure, drugs, cosmetics etc. The fluoride levels of food depend upon the nature of soil and quality of water used for irrigation and thus varies from place to place. Prosopis julifora is a hyper-accumulator of Fand has potential to remediate fluoride contaminated soils (Baunthiyal and Sharma, 2012). Fluoride ion concentration in drinking water can be easily detected by ion selective electrode. Various defluoridation techniques have been developed to reduce the fluoride content to the desired level including principally membrane and adsorption processes (Amor et al., 2001). Different bio-sorbents such as chitosan coated silica (Krishnaiah et al., 2009) stalks of sorghum and canola are efficient in removal of fluoride from water. Apart from these technical approaches awareness among the people needs to be created for prevention from fluoride health hazards.

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High fluoride concentration in groundwater occurs in majority of the dry and semidry regions of the Indian subcontinent. Mahendergarh district of Haryana state has been identified as Red Alert zone of high fluoride by Geological Survey of India (GSI). Most of the residents of Mahendergarh district are using ground water for drinking and irrigational purposes. The main purpose of the present study was to analyze the level of fluoride in ground water in all the eight blocks, i.e., Satnali, Sihma, Narnaul, Mahendergarh, Nizampur, Ateli, Kanina and Nangal Chaudhary of Mahendergarh district. Water samples were collected from 355 villages of the study area during December 2016 to May-June 2017 and were analyzed to find out the fluoride concentration with ion-selective electrode method. The fluoride distribution in Mahendergarh district ranged from 0.3 to 16.0 mg/l in ground water samples. Lowest value of 0.3 mg/l was reported in Dalanwas village of Satnali block and highest value of 16.0 mg/l in Narnaul city of Narnaul block. The outcome of the study revealed that out of 355 villages of the district, the fluoride level in 205 villages was found within the upper acceptable limit prescribed by WHO, i.e., 1.5 mg/l. The fluoride concentration in 150 villages was above permissible limit that may cause fluorosis among residents of the district. The fluoride distribution pattern showed that the high fluoride level was observed in the central part and the lowest in northwest part of the study area.

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