Geochemistry of Fluoride Rich Groundwater in a Weathered Granitic Rock Region, Southern India by K. Brindha, L. Elango

Groundwater used for drinking should contain all ions within the prescribed limits for drinking purpose proposed by various health organisations. Keeping this in mind, this study was carried out with an objective to understand the present status of fluoride occurrence in groundwater, its spatiotemporal variation and sources in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected and monitored for fluoride concentration in forty-five wells from March 2008 to January 2010. The concentration of fluoride in groundwater varied from 0.07 to 8.8 mg/l. The spatial variation in groundwater level and fluoride concentration indicated that fluoride concentration increases along the flow direction. The vertical variation in fluoride concentration indicated that the fluoride concentration was relatively higher in wells having a depth ranging from 5.1 to 10 m, i.e. wells penetrating up to the weathered part of the formation had more concentration of fluoride. Granitic rocks rich in fluoride and application of fertilisers containing fluoride were the main causes for fluoride contaminated groundwater in this area. This two-year study carried out by regular collection of samples once every two months showed that only 55 km² of the total 724 km² considered for the study had groundwater suitable for drinking and domestic purpose with respect to fluoride. It is very important to adopt groundwater management measures at the earliest to improve the groundwater quality in this area as the local people depend on groundwater for their everyday needs.

Link : https://www.researchgate.net/publication/256605786_Geochemistry_of_Fluoride_Rich_Groundwater_in_a_Weathered_Granitic_Rock_Region_Southern_India

Simultaneously Evaluate The Toxic Levels Of Fluoride And Arsenic Species In Underground Water Of Tharparkar And Possible Contaminant Sources: A Multi Variate Study by Kapil Dev Brahman, Tasneem GulKazi , Hassan Imran Afridi, Shahid Naseem, Sadia Sadaf Arain, Sham Kumar Wadhwa, Faheem Shah

The present study investigated total arsenic (tAs), inorganic arsenic (iAs) species and fluoride ion (F⁻) contamination in underground water of Mithi and Nangarparkar subdistricts of Tharparkar, Pakistan. Statistical parameters, principal component analysis, cluster analysis, sodium absorption ratio and saturation indices (SI) were used to detect interrelation and sources of concentration of tAs, iAs species (As³⁺ and As⁵⁺), F⁻ and others physicochemical parameters. The concentration of As³⁺ was measured by cloud point extraction using ammonium pyrrolidinedithiocarbamate (APDC) as complexing reagent, while inorganic arsenic (iAs) was determined by solid phase extraction, using titanium dioxide. The positive correlation was observed between F⁻ contents with As species and other major ions, found in the underground water of the study area. The resulted data indicated that underground water samples of two areas of Tharparkar were severely contaminated with arsenic (0.100–3.83 mg/L) and fluoride ion (13.8–49.3 mg/L), which were exceeded the World Health Organization provisional guideline values, 0.01 mg/L and 1.5 mg/L, respectively. The SI of fluorite and calcite in the underground water samples showed that most of the samples were oversaturated with respect to calcite and fluoride.

Evaluation Of Physical Chemical Characteristics Of Ground Water At Alirajpur, Madhya Pradesh, India By Dilip Kumar and Iqbal Sanjeeda

Ground water is one of the major sources of drinking water in many parts of Alirajpur, Madhya Pradesh,India. Ground water contain of fluoride 0.5 to1.00 mg/l. our study shows that pH 7.23 to 8.42 Electrical conductivity 946 to 978 mg/l, Total Alkalinity 142 to 178 mg/l, Total Hardness 624 to 699mg/l, Chloride 15.4 to 51.41mg/l. and Fluoride 2.93 to 8.55mg/l. is present which is than permissible limit of APHA (American Public Health Association)high Fluoride in ground water at various selected sampling sites our research shows that high level of the groundwater of Alirajpur District of Madhya Pradesh, India. In present study it is found that is the ground water Alirajpur district is affected by high fluoride concentration.

Fluoride In Natural Waters By W Mike Edmunds And Pauline L Smedley

The element fluorine has long been recognised to have benefits for dental health: low-fluoride intake has been linked to development of dental caries and the use of fluoride toothpastes and mouthwashes is widely advocated in mitigating dental health problems. Fluoridation of water supplies to augment naturally low fluoride concentrations is also undertaken in some countries. However, despite the benefits, optimal doses of fluoride appear to fall within a narrow range. The detrimental effects of ingestion of excessive doses of fluoride are also well documented. Chronic ingestion of high doses has been linked to the development of dental fluorosis, and in extreme cases, skeletal fluorosis. High doses have also been linked to cancer (Marshall, 1990), although the association is not well-established (Hamilton, 1992).

Simulating fluoride evolution in groundwater using a reactive multicomponent transient transport model: Application to a crystalline aquifer of Southern India by Marie Pettenati, Jerome Perrin, Helene Pauwels Shakeel Ahmed

Overexploitation of crystalline aquifers in a semi-arid climate leads to a degradation of water quality, with the main processes responsible for the observed salt loads probably being irrigation return flow (IRF) and a high evaporation rate. The present study has focused on modelling the F accumulation caused by IRF below rice paddy fields in the small endorheic Maheshwaram watershed (Andhra Pradesh, Southern India). The transient simulation was performed with a 1D reactive transport PHREEQC column and took into account IRF evaporation, kinetically controlled mineral dissolution/precipitation, ion adsorption on Fe hydroxides, and mixing with fresh groundwater. The results revealed the role of cationic exchange capacity (CEC) in Ca/Na exchange and calcite precipitation, both favouring a decrease of the Ca²⁺ activity that prevents fluorite precipitation. Iron hydroxide precipitation offers a not inconsiderable adsorption capacity for F immobilization. The principal sources of F are fluorapatite dissolution and, to a lesser extent, allanite and biotite dissolution. Anthropogenic sources of F , such as fertilizers, are probably very limited. After simulating an entire dry-season irrigation cycle (120 days), the results are in good agreement with the observed overall increase of Cl⁻ in the Maheshwaram groundwater. The model enables one to decipher the processes responsible for water-resource degradation through progressive salinization. It shows that F enrichment of the groundwater is likely to continue in the future if groundwater overexploitation is not controlled.

Principal component analysis of fluoride geochemistry of groundwater in Shanxi and Inner Mongolia, China By Shan Hu, Ting Luo, Chuanyong Jing

Insightful knowledge of geochemical processes controlling fluoride (F) mobility is fundamental to understand the occurrence of elevated F in groundwater. Principal component analysis (PCA) was applied to explore the dominating factors in the F geochemistry in Shanxi and Inner Mongolia, two severely F-affected areas in China. Field sampling results of 111 tubewells showed that 26.1% of drinking water wells, with F concentrations in the range 0.3–5.6 mg/L, exceeded WHO standards of 1.5 mg/L. PCA with 16 geochemical parameters demonstrated that F occurrence in Shanxi could be the result of mineral weathering and water–rock interactions in the aquifer. Groundwater F concentrations increased with TDS in Shanxi, but not in Inner Mongolia. In agreement with our PCA, the occurrence of F in Inner Mongolia may be attributed to multiple processes including agriculture and mining activities, and water–rock interaction processes in the aquifer. Calcium is the scavenger of fluoride in Shanxi and Inner Mongolia. The results of this study further our understanding of the similarities and differences in the F occurrence and mobility at various locations.

High-Fluoride Groundwater By N Subba Rao

Fluoride  is essential for normal bone growth, but its higher concentration in the drinking water poses great health problems and fluorosis is common in many parts of India. The present paper deals with the aim of establishment of facts of the chemical characteristics responsible for the higher concentration of F⁻ in the groundwater, after understanding the chemical behavior of F⁻ in relation to pH, total alkalinity (TA), total hardness (TH), carbonate hardness (CH), non-carbonate hardness (NCH), and excess alkalinity (EA) in the groundwater observed from the known areas of endemic fluorosis zones of Andhra Pradesh that have abundant sources of F⁻-bearing minerals of the Precambrians. The chemical data of the groundwater shows that the pH increases with increase F⁻; the concentration of TH is more than the concentration of TA at low F⁻ groundwater, the resulting water is represented by NCH; the TH has less concentration compared to TA at high F⁻ groundwater, causing the water that is characterized by EA; and the water of both low and high concentrations of F⁻ has CH. As a result, the F⁻ has a positive relation with pH and TA, and a negative relation with TH. The operating mechanism derived from these observations is that the F⁻ is released from the source into the groundwater by geochemical reactions and that the groundwater in its flowpath is subjected to evapotranspiration due to the influence of dry climate, which accelerates a precipitation of CaCO₃ and a reduction of TH, and thereby a dissolution of F⁻. Furthermore, the EA in the water activates the alkalinity in the areas of alkaline soils, leading to enrichment of F. Therefore, the alkaline condition, with high pH and EA, and low TH, is a more conducive environment for the higher concentration of F in the groundwater.

A Comparison Of Different Rule-Based Statistical Models For Modeling Geogenic Groundwater Contamination By Manouchehr Amini, Karim C. Abbaspour, C. Annette Johnson

There is an increasing interest in modeling groundwater contamination, particularly geogenic contaminant, on a large scale both from the researcher’s as well as policy maker’s point of view. However, modeling large scale groundwater contamination is very challenging due to the incomplete understanding of geochemical and hydrological processes in the aquifer. Despite the incomplete understanding, existing knowledge provides sufficient hints to develop predictive models of geogenic contamination. In this study we used a global database of fluoride measurements (>60,000 entities), as well as global-scale information relevant to soil, geology, elevation, climate, and hydrology to evaluate several hybrid methods. The hybrid methods were developed by combining two classification techniques including classification and regression tree (CART) and “knowledge based clustering” (KBC) and three predictive techniques including multiple linear regression (MLR), adoptive neuro-fuzzy inference system (ANFIS) and logistic regression (LR). The results indicated that combination of classification techniques and nonlinear predictive method (ANFIS and LR) were more reliable than others and provided a better prediction capability. Among the different hybrid procedures, combination of KBC-ANFIS and also CART-ANFIS resulted in larger true positive rates and smaller false negative rates for both training and test data sets. However, as the CART classifier is very unstable and very sensitive to resampling, the combination of KBC and ANFIS is preferred as it not only is more robust but also is flexible enough to account for geohydrological conditions.

Fluoride dynamics in the granitic aquifer of the Wailapally watershed, Nalgonda District by D.V. Reddy, P. Nagabhushanam, B.S. Sukhija, A.G.S. Reddy and Smedley, P.L.

High concentrations of fluoride (up to 7.6 mg/L) are a recognized feature of the Wailapally granitic aquifer of Nalgonda District, Andhra Pradesh, India. The basement rocks provide abundant sources of F in the form of amphibole, biotite, fluorite and apatite and whole-rock concentrations of F in the aquifer are in the range 240–990 mg/kg. Calcretes from the shallow weathered horizons also contain comparably high concentrations of F, in the range 635–950 mg/kg. The concentrations of water-soluble F in the granitic rocks and the calcretes are usually low (1% of the total or less) but broadly correlate with the concentrations observed in groundwaters in the local vicinity. The water-soluble fraction of calcretes is relatively high in weathered calcretes compared to fresh samples. Groundwater major-ion composition shows a well-defined trend with flow downgradient in the Wailapally aquifer, from Na-Ca-HCO3-dominated waters in the recharge area at the upper part of the catchment, through to Na-Mg-HCO3 and ultimately to Na-HCO3 and Na-HCO3-Cl types in the discharge area in the lowest part. The evolution occurs over a reach spanning some 17 km. Groundwater chemistry evolves by silicate weathering reactions, although groundwaters rapidly reach equilibrium with carbonate minerals, favouring precipitation of calcite, and ultimately dolomite in the lower parts of the watershed. This precipitation is also aided by evapotranspiration. Decreasing Ca activity downgradient leads to a dominance of fluorite-undersaturated conditions and consequently to mobilisation of F. Despite the clear downgradient evolution of major-ion chemistry, concentrations of F remain relatively uniform in the fluorite-undersaturated groundwaters, most being in the range 3.0–7.6 mg/L. The rather narrow range is attributed to a mechanism of co-precipitation with and/or adsorption to calcrete in the lower sections of the aquifer. The model may find application in other high-F groundwaters from granitic aquifers of semi- arid regions.

Modeling Large Scale Geogenic Contamination Of Groundwater, Combination Of Geochemical Expertise And Statistical Techniques by Amini .M , A. Johnson, K.C. Abbaspour and K. Mueller

There is an increasing interest in modeling groundwater contamination, particularly geogenic contaminant, on a large scale both from the researcher’s as well as policy maker’s point of view. However, modeling large scale groundwater contamination is very challenging due to the incomplete understanding of geochemical and hydrological processes in the aquifer. Despite the incomplete understanding, existing knowledge provides sufficient hints to develop predictive models of geogenic contamination. In this study we used a global database of fluoride measurements (>60,000 entities), as well as global-scale information relevant to soil, geology, elevation, climate, and hydrology to evaluate several hybrid methods. The hybrid methods were developed by combining two classification techniques including classification tree (CART) and knowledge based clustering (KBC) and three predictive techniques including multiple linear regression (MLR), adoptive neuro-fuzzy inference system (ANFIS) and logistic regression (LR). The results indicated that combination of classification techniques and nonlinear predictive method (ANFIS and LR) were more reliable than others and provided a better prediction capability. Among the different hybrid procedures, combination of KBC-ANFIS and also CART - ANFIS resulted in larger sensitivities and smaller false negative rates for both training and test data sets. However, as the CART classifier is very unstable and very sensitive to re-sampling, the combination of KBC and ANFIS or LR is preferred.

Link : http://www.mssanz.org.au/modsim09/I16/amini.pdf

Geochemical factors controlling the occurrence of high fluoride groundwater in the Nagar Parkar area, Sindh, Pakistan by Tahir Rafique, Shahid Naseemb , Tanzil Haider Usmani , Erum Bashir, Farooque Ahmed Khana , Muhammad Iqbal Bhanger

Fluoride concentrations in groundwater near Nagar Parkar in the Thar Desert of southeastern Pakistanrange from 1.13 to 7.85 mg/l, and roughly 78% of the samples contain fluoride in concentrations thatexceed the drinking water standard of 1.5 mg/l set by WHO. The groundwater is alkaline (pH 7.1–8.4),brackish (TDS 449–15,933 mg/l), and classified as Na–Cl type water. This prevailing chemical characterreflects the influence of saltwater intrusion, high evaporation rates, and ion exchange. Groundwater is alsosupersaturated with respect to calcite, which promotes the removal of Ca2+ and HCO3− fromsolution. As aresult, groundwater is generally undersaturated with respect to fluorite, the mineral that typically controlsthe upper limit of fluoride concentrations. This study examines a number of geochemical parameters in aneffort to discover the controls on fluoride concentrations in groundwater. High fluoride concentrations areassociated with high TDS, high pH, high Na concentrations, and high sodium absorption ratios (SAR). Thissuggests that elevated fluoride levels are the result of enhanced fluorite solubility due to Ca depletion andhigh ionic strength and the release of fluoride from colloid surfaces under high pH conditions. Contraryto what has been found in other studies, sample depth and water temperature do not appear to have asignificant role in the distribution of fluoride within the groundwater.

Inventories and Mobilization of Unsaturated Zone Sulfate, Fluoride, and Chloride Related to Land Use Change in Semiarid Regions, Southwestern United States and Australia by Bridget R. Scanlon, David A. Stonestrom, Robert C. Reedy, Fred W. Leaney, John B. Gates, and Richard G. Cresswell

Unsaturated zone salt reservoirs are potentially mobilized by increased groundwater recharge as semiarid lands are cultivated. This study explores the amounts of pore water sulfate and fluoride relative to chloride in unsaturated zone profiles, evaluates their sources, estimates mobilization due to past land use change, and assesses the impacts on groundwater quality. Inventories of water-extractable chloride, sulfate, and fluoride were determined from borehole samples of soils and sediments collected beneath natural ecosystems (N = 4), nonirrigated ("rain-fed") croplands (N = 18), and irrigated croplands (N = 6) in the southwestern United States and in the Murray Basin, Australia. Natural ecosystems contain generally large sulfate inventories (7800-120,000 kg/ha) and lower fluoride inventories (630-3900 kg/ha) relative to chloride inventories (6600-41,000 kg/ha). Order-of-magnitude higher chloride concentrations in precipitation and generally longer accumulation times result in much larger chloride inventories in the Murray Basin than in the southwestern United States. Atmospheric deposition during the current dry interglacial climatic regime accounts for most of the measured sulfate in both U.S. and Australian regions. Fluoride inventories are greater than can be accounted for by atmospheric deposition in most cases, suggesting that fluoride may accumulate across glacial/ interglacial climatic cycles. Chemical modeling indicates that fluorite controls fluoride mobility and suggests that water-extractable fluoride may include some fluoride from mineral dissolution. Increased groundwater drainage/recharge following land use change readily mobilized chloride. Sulfate displacement fronts matched or lagged chloride fronts by up to 4 m. In contrast, fluoride mobilization was minimal in all regions. Understanding linkages between salt inventories, increased recharge, and groundwater quality is important for quantifying impacts of anthropogenic activities on groundwater quality and is required for remediating salinity problems.

Link : http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1172&context=geosciencefacpub

Statistical Modeling of Global Geogenic Fluoride Contamination in Groundwaters by Manouchehr Amini, Kim Mueller, Karim C. Abbaspour, Thomas Rosenberg, Majid Afyuni, Klaus N. Moller, Mamadou Sarr and C. Annette Johnson

The use of groundwater with high fluoride concentrations poses a health threat to millions of people around the world. This study aims at providing a global overview of potentially fluoride-rich groundwaters by modeling fluoride concentration. A large database of worldwide fluoride concentrations as well as available information on related environmental factors such as soil properties, geological settings, and climatic and topographical information on a global scale have all been used in the model. The modeling approach combines geochemical knowledge with statistical methods to devise a rule-based statistical procedure, which divides the world into 8 different “process regions”. For each region a separate predictive model was constructed. The end result is a global probability map of fluoride concentration in the groundwater. Comparisons of the modeled and measured data indicate that 60−70% of the fluoride variation could be explained by the models in six process regions, while in two process regions only 30% of the variation in the measured data was explained. Furthermore, the global probability map corresponded well with fluorotic areas described in the international literature. Although the probability map should not replace fluoride testing, it can give a first indication of possible contamination and thus may support the planning process of new drinking water projects.

Link : https://www.researchgate.net/publication/5309965_Statistical_Modeling_of_Global_Geogenic_Fluoride_Contamination_in_Groundwaters

Distribution Of Highly Arsenic And Fluoride Contaminated Groundwater From East Punjab, Pakistan, And The Controlling Role Of Anthropogenic Pollutants In The Natural Hydrological Cycle by Abida Farooqi, Harue Masuda, Minoru Kusakabe, Muhammad Naseem and Nousheen Firdous

This paper reports the extended study from a previously-described study on As and F contaminated groundwater from a small village, Kalalanwala, in east Punjab, Pakistan (Farooqi et al., 2007). Of the 147 groundwater samples investigated, 91% exceeded the WHO standard (10 μg/L) for As and 75% exceeded the WHO standard (1.5 mg/L) for F^-. The highly contaminated As (max. 2400 μg/L) and F^- (max. 22.8 mg/L) groundwaters were found from shallow depths down to 30 m from the surface. The contaminated groundwaters are characterized by high pH (max. 8.8), alkalinity (HCO₃^- up to 1281 mg/L), SO₄^2- (max. 960 mg/L), Na⁺ (max. 1058 mg/L) and maximum electric conductivity >4.6 mS/cm. Fluoride concentrations showed positive correlations with those of Na⁺ and HCO₃^- and negative ones with Ca²⁺ and Mg²⁺. The alkaline waters were saturated with calcite in spite of the low Ca²⁺ concentrations. Fluoride concentration is governed by fluorite solubility. Speciation analysis showed As is mostly in the form of As^V. There was a positive correlation between As and pH, while there is no relationship between As vs. Fe and F^-. Thus, the fluoride and As contamination occurred in the oxidizing and alkaline conditions of the groundwater. However, F^- and As are derived from two or more sources. Suspected contaminant sources in the study area contained considerable amounts of F^- and As; fertilizers (DAP, n = 5) contained leachable F^- ranging from 53-255 mg/kg, and As 5-10 mg/kg, and coals (n = 8) contained F^- ranging from 5-20 mg/kg. Sulfur isotopic ratios indicated that the high SO₄^2- in groundwater (3.2-7.0‰, CDT) is mainly derived from coal combusted atmospheric pollutants, fertilizers and household wastes. Nitrogen isotope data (8-30‰, Air) showed that NO₃^--N is attributed to animal waste distributed in the study area. The major chemical characteristics of the groundwaters are related with anthropogenic activities on the ground surface. The resultant major chemistry, especially highly alkaline and low Ca²⁺ and Mg²⁺ concentrations, must promote the high concentrations of F^- and As in the studied groundwaters. 

Link : https://www.terrapub.co.jp/journals/GJ/pdf/4104/41040213.pdf

Study of quaternary aquifers in Ganga Plain, India: Focus on groundwater salinity, fluoride and fluorosis by Anil Kumar Misra, Ajai Mishra

In marginal and central alluvial plains (Ganga Plain) of India, the inland salinity is continuously increasing, canal network and arid to semi-aridclimatic conditions that led to excessive evapotranspiration concentrates the salt in soil and thereby escalating the groundwater salinity. In MatTahsil, Mathura district (Ganga Plain) study on shallow and deep aquifer salinity and fluoride was carried out in August 2001 and 2004.Groundwater salinity in some parts is more then 4000−1/cm. This region is severely affected by endemic fluorosis due to consumption offluoride-contaminated water. Analysis of F−, Na+, K+, Cl− and HCO3− was carried out at 30 sites of dugwells and borewells. Result shows thatthere is a variation and continuous escalation in the groundwater salinity and fluoride concentration in deep and shallow aquifers on the basis ofanalysis. Classification of salinity levels was carried out in 2001 and 2004. The deep aquifers (borewells) are found more saline as compare tothe shallow aquifers (dugwells) while F−, Na+, K+, Cl− and HCO3− shows high concentration in shallow aquifers. The fluoride concentration inthe groundwater of these villages showed values from 0.1 to 2.5 mg/l, severe enough to cause dental and skeletal fluorosis among the inhabitants,especially children of these villages. One of the major effects of inland salinity in this region is from saline groundwater, which is reaching theland surface and causing soil salinisations and water logging in the NE and SE parts of Mat block.

Factors influencing natural occurrence of fluoride-rich groundwaters: a case study in the southeastern part of the Korean Peninsula By Kangjoo Kim , Gi Young Jeong

Factors regulating the occurrence of fluoride in groundwater were investigated using natural isotope tracers and geo-chemical data in the southeastern part of Korea where about 10% of the total public water-supply wells (n = 422) inspected in this study had fluoride levels exceeding the drinking water limit of Korea (1.5mg/l). The F-rich public wells are mostly distributed along the major faults, especially in the terrain of the F-rich granitic rocks. The stable isotope  analysis results provide substantial information for the relative ages of groundwaters. It is revealed that the F-rich groundwaters are deeply circulating paleogroundwaters and occur along the faults due to upward flow along the fault plane. While reacting with granitic rocks for a prolonged period, the F concentrations of groundwater are continuously enriched even after the groundwater reaches an equilibrium state with respect to fluorite (CaF2) due to removal of Ca by precipitation of calcite (CaCO3). These observations reflect that rock chemistry, groundwater age, well depth, and geologic structure are the important factors controlling the occurrence of high F groundwaters. However, high F ground-waters are rarely observed in the fault zones where the associated fractures are widely developed. Isotopic signature provides an evidence for deep penetration of recently recharged groundwater into the wide fault zone, indicating that the hydrologic condition of the fault is also an important factor controlling the occurrence of high F groundwaters.

Controls On The Genesis Of Some High-Fluoride Groundwaters In India by Gunnar Jacks , Prosun Bhattacharya , Vikas Chaudhary, K.P. Singh

India has an increasing incidence of fluorosis, dental and skeletal, with some 62 million people at risk. High fluoride groundwaters are present especially in the hard rock areas south of the Ganges valley and in the arid north-western part of the country. The phenomenon is related to groundwater with residual alkalinity $({\mathrm{Ca}}^{2+}<{\mathrm{HCO}}_3^{-})$. Fluoride concentrations are governed by adsorption equilibria and by fluorite solubility. Evapotranspiration leads to a precipitation of calcite, a lowering of Ca activity and increase in Na/Ca ratios, and this allows an increase in F⁻ levels. In southern India, Mg seems to be controlled by dolomite, while sepiolite and palygorskite are Mg sinks in Rajasthan but may then release F⁻ under alkaline conditions. The latter two minerals are probably also important sources and sinks for F⁻ in the hydroxy-positions. The increase in the extent of sodic soils as a result of irrigation is a contributing factor to the increasing incidence of fluorosis. Remedial measures including addition of gypsum and rainwater harvesting are needed even in areas where the sodicity does not cause structural problems in the soil.

Link : https://www.researchgate.net/publication/223541978_Controls_on_the_genesis_of_some_high-fluoride_groundwater_in_India_Appl_Geochem