Dissolution Of Fluorite Research Articles

Molten silicate interactions with thermal barrier coatings

The strain tolerance of thermal barrier coatings (TBCs) used in gas turbine engines is enhanced by incorporation of through-thickness pores within the coating. However, molten calcium–magnesium–alumino–silicate (CMAS) deposits are then able to permeate through the interconnected pore network resulting in dissolution of the coating in the silicate melt, precipitation of new phases, and the elimination of the pores needed for strain tolerance when silicate melt solidification occurs upon cooling. Here we used an electron beam directed vapor deposition method to deposit strain tolerant coatings containing a high volume fraction of inter-columnar pores to assess the effect of increasing the pore volume fraction upon silicate melt infiltration in a 7wt.% yttria stabilized zirconia (7YSZ) TBC. We then explore two potential mitigation approaches. One investigated the infiltration of a samarium zirconate (SZO) coating deposited on a 7wt.% yttria stabilized zirconia (7YSZ) buffer layer since recent work has indicated the related gadolinium zirconate (GZO) system was much more CMAS resistant than 7YSZ. The second explored the effects of an embedded platinum layer located near the outer surface of the SZO layer. The response of both systems to attack by a model 33CaO–9MgO–13AlO1.5–45SiO2 CMAS melt at 1250°C has been compared to that of the 7YSZ composition coating deposited by the same process. The 7YSZ specimens were fully infiltrated by CMAS in less than 1min and significantly dissolved and re-precipitated a globular phase after 4h of exposure. The penetration rate in SZO coatings was reduced by about a factor of five, but the coatings were still completely penetrated after a 4hour exposure. Significant SZO dissolution and precipitation of both globular fluorite and faceted apatite phases were observed after the 4h of exposure, with concomitant debonding at the SZO/YSZ interface. The insertion of a 5μm thick Pt layer about 30μm below the SZO surface arrested permeation of CMAS for about 16h and might provide a promising strategy for controlling CMAS degradation in TBCs without reducing the thermo-cyclic delamination resistance.

Groundwater fluoride contamination: A reappraisal

Dissolution of fluorite (CaF2) and/or fluorapatite (FAP) [Ca5(PO4)3F], pulled by calcite precipitation, is thought to be the dominant mechanism responsible for groundwater fluoride (F−) contamination. Here, one dimensional reactive–transport models are developed to test this mechanism using the published dissolution and precipitation rate kinetics for the mineral pair FAP and calcite. Simulation results correctly show positive correlation between the aqueous concentrations of F− and CO32− and negative correlation between F− and Ca2+. Results also show that precipitation of calcite, contrary to the present understanding, slows down the FAP dissolution by 106 orders of magnitude compared to the FAP dissolution by hydrolysis. For appreciable amount of fluoride contamination rock–water interaction time must be long and of order 106 years.

Importance of surface structure on dissolution of fluorite: Implications for surface dynamics and dissolution rates

Dissolution rates are usually calculated as a function of surface area, which is assumed to remain constant ignoring the changes occurring on the surface during dissolution. Here we present a study of how topography of natural fluorite surfaces with different orientation changes during up to 3200h of dissolution. Results are analyzed in terms of changes in surface area, surface reactivity and dissolution rates.All surfaces studied present fast changes in topography during the initial 200h of dissolution. The controlling factors that cause the development of topography are the stability of the step edges forming the initial surface and its inclination to the closest stable planes, which are specific for each surface orientation. During an initial dissolution regime dissolution rates decrease significantly, even though the total surface area increases. During a second dissolution regime, some surfaces continue to present significant changes in topography, while for others the topography tends to remain approximately constant. The observed variation of dissolution rates are attributed to a decrease of the density of step edges on the surface and the continuous increase in exposure of more stable surfaces. Calculations of dissolution rates, which assume that dissolution rates are directly proportional to surface area, are not valid for the type of surfaces studied. Instead, to develop accurate kinetic dissolution models and more realistic stochastic dissolution simulations the surface reactivity, determined by the relative stability of the planes and type of edges that constitute a surface needs to be considered. Significant differences between dissolution rates calculated based on surface area alone, and based on surface reactivity are expected for materials with the fluorite structure.

Geochemical Modelling of Fluoride Concentration in Hard Rock Terrain of Madhya Pradesh, India

Abstract:The aim of the present study is to locate and decipher the groundwater quality, types, and hydrogeochemical reactions, which are responsible for elevated concentration of fluoride in the Chhindwara district in Madhya Pradesh, India. Groundwater samples, quality data and other ancillary information were collected for 26 villages in the Chhindwara District, M.P. India during May 2006. The saturation index was computed for the selected samples in the region, which suggest that generally most of the minerals are saturated with respect to water. The concentration of fluoride in the region varies from 0.6 to 4.74 mg/l, which is much higher as per the national and international water quality standards. The study also reveals that the fluoride bearing rock formations are the main source of the higher concentration of fluoride in groundwater along with the conjuncture of land use change. Moreover, the area is a hard rock terrain and consists of fractured granites and amygdaloidal and highly jointed compact basalt acting as good aquifer, which is probably enriching the high content of fluoride in groundwater. High concentration of fluoride is found in deeper level of groundwater and it is possible due to rock‐water interaction, which requires further detailed investigation. The highly alkaline conditions indicate fluorite dissolution, which works as a major process for higher concentration of fluoride in the study area. The results of this study will ultimately help in the identification of risk areas and taking measures to mitigate negative impacts related to fluoride pollution and toxicity.

Hydrochemical Characteristics and Evolution Laws of Groundwater in Ligezhuang Area, Qingdao

This paper takes the Ligezhuang Area in the downstream of the Dagu River as the typical study case, and the groundwater samples are analyzed scientifically and systemically, then the descriptive statistics, the piper diagram and the mass balance simulation are used to gain the better understanding of the hydrochemistry characteristics and evolution laws of groundwater. The results show that (1) With the increase of the mineralization degree in the research area, the groundwater chemical types change from Ca Na Mg-HCO3 SO4 Cl type to NaMg-Cl SO4 and Na-Cl type.(2) When the groundwater flows from low mineralization area to high mineralization area, it mainly occurs the precipitation of calcite and montmorillonite; the dissolution of dolomite, gypsum, halite, CO2, chalcedony, fluorite and illite; meanwhile the cation exchange effect. The results will provide a better understanding of the groundwater resources in the region and help the government develop suitable utilization strategies for the groundwater resources.

Removal of fluoride and arsenate from aqueous solution by hydrocalumite via precipitation and anion exchange

Hydrocalumite was used to treat fluoride and arsenate solutions with a wide range of initial concentrations. A marked decrease in solution fluoride and arsenate to below 0.05 and 0.005mmol/L was observed when their initial concentrations were not higher than 30 and 5mmol/L respectively. However, a sharp increase occurred to equilibrium F and As concentrations over these critical initial concentrations. The mechanisms for fluoride and arsenate removal were investigated by comparison between measured chemical compositions of solution samples and their PHREEQC simulation along with XRD, SEM and EDX analyses for solid samples. The solution defluoridation was primarily a result of hydrocalumite dissolution and subsequent precipitation of fluorite and fluoride-bearing bayerite, although the anion exchange between fluoride in solution and chloride in interlayers of hydrocalumite functioned as well provided that not all hydrocalumite added to solution was dissolved. In contrast, the principal approach for arsenate removal was through the formation of johnbaumite and other arsenic-bearing minerals or solid–solution series. Solution pH value has little effect on defluoridation and dearsenication by hydrocalumite due to its pH buffering effect, while the coexistence of competitive anions and the variation of reaction temperature affected the fluoride and arsenate removal significantly. The maximum uptake capacities of hydrocalumite for fluoride and arsenate are 719.1 and 361.7mg/g respectively, much better than other types of layered double hydroxides. It is promising for treatment of not only naturally occurring poor-quality groundwater with not very high fluoride and arsenic concentrations but also industrial wastewater enriched highly in fluoride or arsenic.

Geochemical simulations to assess the fluorine origin in Sierra de Gador groundwater (SE Spain)

AbstractSierra de Gádor is a complex, Triassic age aquifer system formed by dolostones and limestones with interbedded gypsum. On top, a thick Neogene‐Quaternary series gives rise to shallow aquifers. F‐Pb‐Zn stratabound deposits are found within the Triassic carbonate formations. Their mineral paragenesis includes sphalerite, galena, pyrite, marcasite, fluorite and barite. The Sierra de Gádor groundwater is the principal water resource for this semiarid environment, and the population is potentially exposed to hazardous elements related to mining activities and wastes. In October 1966, an intensive precipitation event (190 mm day−1) broke several old tailing structures, flooding part of the Berja village with polluted sludge. Fluorine values in some boreholes and springs were found higher than 1.5 mg l−1, the maximum value permitted by Spanish regulation for human consumption. Hydrogeochemistry data, statistical tools and geochemical simulations were used in this study to assess the physicochemical processes and the fluorine sources within this carbonate aquifer. Most of the analysed water samples are close to equilibrium with respect to carbonate minerals but are undersaturated with respect to fluorite and gypsum. About 39% of the samples have fluorine concentration higher than the regulatory limit. A statistical analysis indicates that fluorine has a low correlation with other variables. Inverse and mixing models, performed with the code PHREEQC, differentiate the predominance of water–rock interaction processes within the carbonate aquifer and water mixing between the carbonate and shallow aquifers. The results indicate that some fluorite dissolution must occur in the carbonate Triassic aquifer, whereas the shallow aquifers leach fluorine from mining wastes. Both types of aquifers are interconnected, and their waters mix below the flooded area. There, the high fluorine values appear to have two sources: (i) the regional fluorine enrichment due to the mineralization; and (ii) the polluted 1966 flood that reached the deep carbonates via the shallow aquifer.

Geochemical processes regulating groundwater chemistry with special reference to nitrate and fluoride enrichment in Chhatarpur area, Madhya Pradesh, India

The present study focuses on the hydrogeochemical composition of groundwater in Chhatarpur area with special focus on nitrate and fluoride contamination, considering the fact that groundwater is the only major source of drinking water here. Carbonate and silicate mineral weathering followed by ground water–surface water interactions, ion exchange and anthropogenic activities are mainly responsible for high concentrations of cations and anions in the groundwater in the region. The average concentration of nitrate and fluoride found in 27 samples is 1.08 and 61.4 mg/L, respectively. Nitrate enrichment mainly occurs in areas occupied with intense fertilizer practice in agricultural fields. Since the area is not dominated by industrialization, the possibility of anthropogenic input of fluoride is almost negligible, thus the enrichment of fluoride in groundwater is only possible due to rock–water interaction. The highly alkaline conditions, which favor the fluorite dissolution, are the main process responsible for high concentration of fluoride.

Water deep inside the mountains: Unique water samples from the Gotthard rail base tunnel, Switzerland

The construction of the new 56km long Gotthard railway base tunnel in Switzerland opened a large number water conducting fractures and 122 water samples were collected along the Amsteg section of the tunnel and then chemically analyzed. The Amsteg section of the tunnel cuts across granites, gneisses and metarhyolites of the Alpine basement. The overburden of basement rocks above the tunnel axis is up to 2200m thick with rock temperature reaching 45°C. Sodium is the prime cation of most waters; calcium, potassium and magnesium are low to very low in concentration. The associated anions are carbonate/bicarbonate, sulfate and chloride in widely varying concentrations. High to ultra-high fluoride concentrations of up to 29mg/L F and high pH values of up to 10.4 are significant properties of the waters. Six chemical types of water can be distinguished reflecting the different types of fractured rock present in the section. Most of the waters can be viewed as ternary mixtures of dissolved natrite (Na2CO3), thenardite (Na2SO4) and halite (NaCl).The geological and hydrogeological context suggests a meteoric origin of the waters. The unique water compositions and characteristics result from interaction with continental basement rocks along the flow path. The assemblage of major minerals of the different rocks is generally very similar, but the rocks differ with respect to minor minerals and accessories. Derived rock data suggest that dissolution and precipitation of calcite and fluorite, albite dissolution, leaching of salty inclusions, sulfide oxidation by CO2, alteration of biotite to chlorite, precipitation of stilbite and hematite are the important processes that create the observed water compositions. Although fracture water acquires its composition exclusively by chemical interaction with crystalline basement rocks with very similar mineral assemblages, the resulting waters are of great chemical variability.

Multivariate statistical analysis for fluoride occurrence in groundwater in the Northern region of Ghana

The presence of excess fluoride in groundwater in the Northern region of Ghana has resulted in the closure of many boreholes for drinking water supply to avoid the incidence of fluorosis and other related health effects. The fluoride concentration in 357 groundwater samples from the area ranged between 0.0 and 11.6mg/L, with a mean value of 1.13mg/L. Piper graphical classification, correlation coefficients, principal component analysis (PCA) and thermodynamic calculations were used as an approach to gain insight into the groundwater chemical composition and to help understand the dominant mechanisms influencing the occurrence of high fluoride waters. Spatial join procedure was used to examine the relationship between the underlying geology of the study area and fluoride distribution. Six groundwater types were identified for the area: Ca–Mg–HCO3, Ca–Mg–SO4, Na–Cl, Na–SO4, Na–HCO3 and mixed water type. PCA performed on the groundwater chemical data resulted in 4 principal components (PCs) explaining 72% of the data variance. The PCs represented the predominant processes controlling the groundwater chemistry in the study area which include; mineral dissolution reactions, ion exchange processes and evapotranspiration processes. PHREEQC calculations for saturation indices for the groundwater samples indicated they were largely saturated with respect to calcite and under-saturated with respect to fluorite, suggesting that dissolution of fluorite may be occurring in the areas where it is present. A review of the PCA results and an evaluation of the equilibrium state of the groundwater based on the saturation indices, suggest that some of the processes controlling the overall groundwater chemistry in the study area also influenced the fluoride enrichment. These predominant processes include the dissolution of the mineral fluorite, anion exchange processes (F−/OH−) involving clay minerals and evapotranspiration processes. Elevated fluoride levels in the study area were found to occur predominantly in the Saboba and Cheriponi districts and also in the Yendi, Nanumba North and South districts. These areas are underlain by the Middle Voltain formation (Obossom and Oti beds), comprising mainly of sandstone, limestone, conglomerate, shale, arkose and mudstone. Results of the hydrochemical analysis show that aside from the boreholes with elevated concentrations of fluoride (beyond 1.5mg/L), groundwater in the study area based on the parameters analyzed is generally chemically acceptable and suitable for domestic use.

Fluoride enrichment in aquifers of the Thar Desert: controlling factors and its geochemical modelling

AbstractThe groundwater is the only source of drinking water in the Jaisalmer district of Rajasthan, India. The study area is a part of the Thar Desert. It has low and scattered population and no industries; hence, the possibility of anthropogenic input of fluoride is almost negligible. Thus, the enrichment of fluoride is only possible due to geochemical processes taking place in the groundwater of the region. A total of 100 groundwater samples, 34 samples from Jaisalmer and 66 samples from the Pokharan administrative blocks, were collected. It was observed that the concentration of fluoride ranged from 0.08 mg/l to 4.56 mg/l in the groundwater of Jaisalmer and from 0.56 mg/l to 6.60 mg/l in the samples of the Pokharan block. The alkaline condition (average pH, 7.7 ± 0.22 and 8.01 ± 0.25 in Jaisalmer and the Pokharan administrative block, respectively) in the region favours fluorite dissolution. Ion exchange, dissolution of calcite, semi‐arid climate, alkaline conditions and weathering are responsible for fluoride enrichment in the groundwater of the study area. Copyright © 2012 John Wiley & Sons, Ltd.

Hydrogeochemical processes controlling the high fluoride concentration in groundwater: a case study at the Boden block area, Orissa, India

The present investigation reports the assessment of hydrochemical/geochemical processes controlling the concentration of fluoride in groundwater of a village in India (Boden block, Orissa). Boden block is one of the severely affected fluoride-contaminated areas in the state of Orissa (India). The sampling and subsequent analysis of water samples of the study area was carried out following standard prescribed methods. The results of the analysis indicate that 36.60% groundwater F(-) concentration exceeds the limit prescribed by the World Health Organization for drinking water. The rock interaction with groundwater containing high concentration of HCO(3)(-) and Na(+) at a higher pH value of the medium could be one of the important reasons for the release of F(-) from the aquatic matrix into groundwater. Geochemical classification of groundwater based on Chadha rectangular diagram shows that most of the groundwater samples having fluoride concentration more than 1.5 mg L(-1) belongs to the Na-K-HCO(3) type. The saturation index values evaluated for the groundwater of the study area indicated that it is oversaturated with respect to calcite, whereas the same is undersaturated with respect to fluorite content. The deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess of fluoride concentration. The risk index was calculated as a function of fluoride level in drinking water and morbidity of fluorosis categorizes high risk for villages of Amera and Karlakote panchayat of Boden block.

Geochemical Modeling of High Fluoride Concentration in Groundwater of Pokhran Area of Rajasthan, India

The groundwater is the only major source of drinking water in western part of Rajasthan, India. The study was carried out to locate and decipher hydrogeochemical reactions responsible for elevated concentration of fluoride. The concentration of fluoride ranged from 0.6 to 4.74ppm in groundwater of study area. Since the area is a desertic terrain and no industries are present thus possibility of anthropogenic input of fluoride is all most negligible thus the enrichment of fluoride in groundwater is only possible due to rock-water interaction. The highly alkaline conditions indicated fluorite dissolution as major process responsible for high concentration of fluoride in Pokhran.

Hydrochemical Formation Mechanisms and Quality Assessment of Groundwater with Improved TOPSIS Method in Pengyang County Northwest China

Inverse geochemical modeling was used in this paper to quantitatively study the formation mechanisms of groundwater in Pengyang County, China. An improved TOPSIS method based on entropy weight was used to perform groundwater quality assessment in this area. The assessment results show that the groundwater in the study area is fit for human consumption and the high concentrations of some elements can be attributed to the strong water‐rock interactions. The inverse geochemical modeling reveals that the dominant reactions in different parts of the study area are different. In the south part of the study area, the precipitation of sodium montmorillonite, calcite and the dissolution of gypsum, fluorite, halite, albite and dolomite as well as CO2 dissolution and cation exchange are the major water‐rock interactions, while in the north part, the leading reactions are the precipitation of gypsum, dolomite, sodium montmorillonite, fluorite, the dissolution of calcite and albite and the CO2 emission and cation exchange are also important. All these reactions are influenced by the initial aquatic environment and hydrodynamic conditions of the flow path.

Fluorite dissolution at acidic pH: In situ AFM and ex situ VSI experiments and Monte Carlo simulations

Dissolution of the fluorite (1 1 1) cleavage surface was investigated by means of in situ atomic force microscopy (AFM) and ex situ vertical scanning interferometry (VSI) experiments at pH range 1–3 in HCl solutions. Surface retreat was quantified at different pH values, yielding dissolution rates that were used to derive an empirical rate law for fluorite dissolution: Rate fluorite ( mol m - 2 s - 1 ) = 2.0 ± 0.4 × 10 - 6 · ( a H + ) 0.38 ± 0.07 where a H + is the proton activity. The influence of Δ G on fluorite dissolution rate at pH 2 was investigated by means of AFM and VSI surface measurements and flow-through experiments with powdered fluorite. The fluorite dissolution rate decreases non-linearly with increasing Gibbs energy (Δ G) and a dissolution plateau is obtained at Δ G ⩽ −7 kcal mol −1. This Δ G effect can be expressed with a rate law of the form Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp - 4.7 × 10 - 2 · | Δ G | RT 1.41 ± 0.24 An alternative form based on a formulation making use of a Temkin number is also possible Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp Δ G 8 RT Dissolution proceeds by formation of equilateral triangular etch pits with trigonal pyramidal morphology and emanation of stepwaves that are responsible for the surface retreat. The results of Monte Carlo simulations are consistent with this reaction mechanism.

Geochemistry of fluoride rich groundwater in Kolar and Tumkur Districts of Karnataka

Groundwater is a significant water resource in India for domestic, irrigation, and industrial needs. By far the most serious natural groundwater-quality problem in India, in terms of public health, derives from high fluoride, arsenic, and iron concentrations. Hydrogeochemical investigation of fluoride contaminated groundwater samples from Kolar and Tumkur Districts in Karnataka are undertaken to understand the quality and potability of groundwater from the study area, the level of fluoride contamination, the origin and geochemical mechanisms driving the fluoride enrichment. Majority of the groundwater samples did not meet the potable water criteria as they contained excess (>1.5 mg/L) fluoride, dissolved salts (>500 mg/L) and total hardness (75-924 mg/L). Hydrogeochemical facies of the groundwater samples suggest that rock weathering and evaporation-crystallization control the groundwater composition in the study area with 50-67% of samples belonging to the Ca-HCO3 type and the remaining falling into the mixed Ca-Na-HCO3 or Ca-Mg-Cl type. The saturation index values indicated that the groundwater in the study area is oversaturated with respect to calcite and under-saturated with respect to fluorite. The deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration.

Hydrogeochemical genesis of groundwaters with abnormal fluoride concentrations from Zhongxiang City, Hubei Province, central China

The groundwaters from Zhongxiang City, Hubei Province of central China, have high fluoride concentration up to 3.67 mg/L, and cases of dental fluorosis have been found in this region. To delineate the nature and extent of high fluoride groundwaters and to assess the major geochemical factors controlling the fluoride enrichment in groundwater, 14 groundwater samples and 5 Quaternary sediment samples were collected and their chemistry were determined in this study. Some water samples from fissured hard rock aquifers and Quaternary aquifers have high fluoride concentrations, whereas all karst water samples contain fluoride less than 1.5 mg/L due to their high Ca/Na ratios. For the high fluoride groundwaters in the fissured hard rocks, high HCO3− concentration and alkaline condition favor dissolution of fluorite and anion exchange between OH− in groundwater and exchangeable F− in some fluoride-bearing minerals. For fluoride enrichment in groundwaters of Quaternary aquifers, high contents of fluoride in the aquifer sediments and evapotranspiration are important controls.

Hydrochemistry and source of high fluoride in groundwater of the Nairobi area, Kenya / Hydrochimie et origine des fortes concentrations en fluorure dans l'eau souterraine de la région de Nairobi, au Kenya

This study aims to identify the hydrogeochemical processes influencing the high fluoride concentrations in groundwater of the Nairobi area, Kenya. For this purpose 16 groundwater samples were collected and analysed. Fluoride concentrations above the WHO standard are found in the downstream areas. The high F− concentrations are correlated with high sodium and pH and low Ca2+ concentrations. Weathering of sodium-rich alkaline igneous rocks causes a pH increase resulting in an increase in HCO3 − and CO3 2- by dissolution of CO2. Groundwater becomes oversaturated compared to calcite and calcite precipitation occurs, leading to a decrease in Ca2+. This causes a sub-saturation with respect to fluorite and dissolution of fluorite increases the F− concentration. These reactions were modelled using the PHREEQC model and the results showed a good agreement with the measured groundwater quality, indicating that the proposed reactions are plausible for explaining the observed concentrations in groundwater.

Hydrogeochemical controls and usability of groundwater in the semi-arid Mayo Tsanaga River Basin: far north province, Cameroon

Deuterium, δ 18O, major ions and dissolved silica in groundwater from semi-arid Mayo-Tsanaga river basin in the Far North Province, Cameroon were used to trace hydrogeochemical processes that control their concentrations and to explore for usability of the water. Electrical conductivity ranges from 57–2,581 μs/cm with alternating low and high values along the hydraulic gradient. Waters from piedmont alluvium show low concentrations in major cations, which peak in Mg within basalt, Na within plain alluvium, and Ca within basalt and the sandy Limani-Yagoua ridge. The initial dominant groundwater composition is CaHCO3, which did not evolve within the basalt and piedmont alluvium, but evolved to NaHCO3 in the granite and plain alluvium. The main processes controlling the major ions composition include the following: (1) dissolution of silicates and fluorite; (2) precipitation of fluorite and carbonate; (3) cation exchange of Ca in water for Na in clay; (4) and anthropogenic activities. The δD and δ 18O ratios vary from −35 to 0.7 and −5.3 to 1.1‰, respectively. The lowest and highest isotope ratios are observed in groundwater within the downstream sandy Limani-Yagoua ridge and the upstream graintes respectively. Variation in isotope ratios depends on altitude effect of −0.48‰ per 100 m between 600 and 850 m asl, and on evaporation, which had insignificant effect on the water salinity. Seventy percent of the groundwater shows poor drinking quality and 90% is suitable for irrigation.

The origin of fluoride in the granitic aquifer of Porto Alegre, Southern Brazil

The Porto Alegre region, Southern Brazil, comprises a fractured aquifer system constituted by pre-Cambrian granitic and gneissic rocks, and a porous aquifer system formed by Cenozoic muddy–sandy to sandy sediments. A model is presented for the origin of the fluoride in the groundwater of the fractured aquifer system, based on ionic interrelations, a statistical analysis of physicochemical parameters and geochemical modeling. The fluoride is present at levels of up to 6.13 mg/L in groundwater and it arises due to the dissolution of secondary fluorite filling fractures in the granitic and gneissic rocks. The dissolution of fluorite occurs at the same time as calcite and dolomite solubilization. The statistical analysis identified three chemically distinct groups (named 1, 2 and 3) and two subgroups (1A and 1B) of groundwater in the fractured aquifer. The most significant differences between these groups are the different concentration ranges of fluoride, calcium, bicarbonate, magnesium, total dissolved solids and chloride and pH values, as well as the correlations between them. The compositional evolution of each groundwater group is governed mainly by how much high-salinity groundwater from the porous aquifer system is mixed with them and the different thermodynamic equilibrium conditions of calcite, dolomite and fluorite.