Origin and Hydrogeochemistry of Fluoride in the Context of the Yemen Regime

Abstract

Groundwater is a natural resource that is used in a variety of fields, which has an impact on its quality. In many places of the world, fluoride-enriched water has become a major public health concern. It is necessary to investigate the geochemical mechanism of fluoride enrichment in drinking water. In Yemen, groundwater is the only supply of water, and its quality is critical because it determines the groundwater’s usefulness for drinking and other domestic purposes. The primary goal of this chapter is to gain a better understanding of factors that influence high fluoride levels in groundwater and its impacts from selected parts of Yemen. The elevated ion concentrations in groundwater are most likely due to water-rock interaction, according to the regional hydrogeochemical investigation. The main findings of this review indicate that the children in the area who get their drinking water from wells with high fluoride levels are suffering from dental and skeletal fluorosis. The population in the research area is at high risk due to excessive fluoride intake, particularly in the absence of knowledge about quantity of fluoride consumption.

1. Introduction

Groundwater’s chemical composition is obtained from a variety of sources of solutes, including gases and atmospheric aerosols, below-surface replacement and precipitation reactions, weathering and erosional activities of soils and rocks, and other anthropogenic effects. The study of water chemistry can reveal a lot about the geological history of rocks as well as the velocity and direction of water flow [1]. Groundwater’s chemical, physical, and bacteriological properties determine its suitability for municipal, commercial, industrial, agricultural, and domestic use [2].

To understand an aquifer’s hydrogeochemistry, a detailed understanding of the rock-water interactions that influence groundwater chemical composition is required. The mineral composition of the rock is the primary component that governs a location’s water chemistry [3]. The local regime differs from other sites due to the continual interfacial reactivity of water with rocks.

Fluoride, currently considered a pollutant in several regions of the world, is frequently related with the dissolution of fluorine-containing minerals in rocks, as well as growing anthropogenic influences [4]. Groundwater chemistry is influenced by mineral water interfacial interactions such as carbonate weathering and dissolution, silicate weathering, and ion exchange activities. Groundwater composition in shallow alluvial aquifers is controlled by hydrogeochemical processes such as dissolution, cation exchange processes, calcite equilibrium, and residence period, as well as the flow channel. The hydrogeochemical fluctuations of groundwater from a semiarid sedimentary basin are caused by salt leaching from the surface, ion exchange processes, and residence time [5].

Fluoride ion concentrations in groundwater can alter owing to chemical processes including hydration and hydrolysis, weathering and deposition, ion exchange processes, oxidation and reduction that occur during mineral-water contact [6]. These interactions influenced the mobility of dissolved constituents and altered the pH of groundwater in diverse sites. Fluoride levels were found to be excessive (10 mg/l) in several areas of Yemen [7]. The very alkaline groundwater conditions were thought to be the primary cause of fluorite disintegration.

The primary aim of this chapter is to understand the influence of geochemical processes on fluoride enrichment in groundwater in Yemen regime, as well as its relationship with other major element concentrations and health implications. The primary goal of this chapter is to review and improve understanding of the factors that influence high fluoride levels in groundwater samples.

2. Rock-water interaction

Interactions between ground water and the minerals that make up the aquifer system control major-ion chemistry trends in the aquifer system to a large extent. Mineral dissolution and precipitation, oxidation and reduction, and ion exchange are all important geochemical reactions that can affect solute concentrations in groundwater systems. Evaporation and mixing of water from various sources are examples of additional processes that can affect solute concentrations [8].

Natural causes such as rock-water interactions while flowing are of specific groundwater quality concerns. Fluoride is one of the most common geogenic pollutants found in groundwater [9]. More than 260 million people are thought to be impacted by elevated fluoride levels in drinking water across the world. People in more than 230 districts across 20 states in India are experiencing health problems as a result of elevated fluoride levels in groundwater [10]. Many studies have found that high fluoride concentrations in groundwater are frequently linked to longer residence times in crystalline rocks in arid-semiarid climates with abundant Na-HCO₃ and low Ca, as well as alkaline pH [11].

Geographic Information System (GIS) has been used to map and evaluate groundwater quality all around the world [12, 13, 14]. Gibbs [15] used total dissolved solids (TDS) vs. Na/(Na + Ca) and TDS vs. Cl/(CI + HCO₃) to identify rock-water interaction. Minerals of various rock types, such as igneous, metamorphic, and sedimentary, entirely or partially dissolve in water depending on chemical weathering resistance. Chemical weathering resistance is high to extremely high in quartz-cemented sandstone, silt, slate, shale, schist, gneiss, and quartzite. Calcite cemented sandstone, limestone, rock salt, gypsum, marble, and basalt, on the other hand, have low to moderate chemical weathering resistance. Different minerals, such as halite, pyrite, gypsum, dolomite, and calcite, demonstrate good water dissolution as a result of these interactions. Because of their low resistance, olivine, pyroxene, hornblende, and biotite dissolve in water via oxidation-reduction and hydrolysis reactions. Feldspar, quartz, and clay dissolve slowly in groundwater due to their considerable resistance to weathering. Fluorite and fluorapatite, among other minerals, are regarded possible sources of fluoride as a groundwater contaminant [16].

3. Dissolution/precipitation of minerals

Fluoride in groundwater comes primarily from natural sources, with earth’s crust containing 0.32% fluoride. Weathering and dissolving of fluoride minerals are the primary controls on its concentration in groundwater, since lithology plays a vital impact in its occurrence [17]. The availability and solubility of fluoride minerals, pH, temperature, anion exchange capacity of aquifer materials, type of geological materials, residence time, porosity, structure, depth, groundwater age, and concentration of carbonates and bicarbonates in water all influence the fluoride contamination of groundwater [18].

The chemical study of groundwater provides insight into the geochemical processes that occur in that area. As previously stated, geological formations regulate water quality when they come into contact with flowing water. Several investigations have found that Na-rich, Ca-poor groundwater with an alkaline pH and high HCO⁻₃ can mobilize fluoride from fluoride-rich rock formations, resulting in higher F concentrations in groundwater. Ionic exchange between F and hydroxyl ions in fluorite minerals such as mica, amphiboles, illite, and others may occur at higher pH levels. As a result, the alkaline composition of groundwater promotes fluoride ion desorption and hence increases solubility [19].

4. Fluoride contamination and fluorosis in Yemen

Fluorosis is a major public health problem spot-wise all over the world, including Yemen. Endemic fluorosis has been nearly recognized as a major public health problem in six governorates in Yemen. Since the groundwater forms a major source of drinking water in rural areas, rural populations are facing a major health problem in these governorates. Fluorosis-affected areas in various parts of the country are being discovered on a regular basis. As a result, fluorosis remains an endemic problem in Yemen. Unfortunately, proper fluoride mapping has not been carried in Yemen so as to locate areas with normal, low, or high levels of fluoride. Where, the available report prepared by the General Authority of Rural Water Projects (GARWP) about the increasing fluoride content in groundwater (Between 2000 and 2006) in districts of some governorates (Sana’a, Ibb, Dhamar, Taiz, Al-Dhala, and Raimah) considered to be the first alarming report highlighted the problem of fluoride in Yemen [7].

A systematic study and delineation of fluoride contamination from Taiz and Al-Dahla governorates have been conducted by Alamry [7].

The iso-line contour map of fluoride ion concentration was created using the chemical analyses taken from wells and springs in the selected areas. As demonstrated in Figure 1, locations with a high fluoride concentration of more than 1.5 mg/l are labeled as fluoride-contaminated (bold green color).

Figure 1.

Iso-line contour map of fluoride ion concentration from Al-Dhala districts.

The fluoride concentrations map clearly shows that there are two significant areas with high fluoride concentrations. These two places are located in the upper portion of Wadi Tuban in the low land area, separated by the Jehaf district’s highlands plateau. The first is in the Al-Dhala Qat; abah Basin, stretching from Qarad to Qa’tabah and encompassing sections of the Al-Dhala, Qa’tabah, and Al-Husha districts. The second is located south of Al-Dhala city and stretches southwest along the Wadi Tabagyan catchment region in Al-Azerq district, which is a tributary of Wadi Tuban.

The Al-Dhala Qatabah Basin’s morphology ranges from flat plains to steep slopes and hills made up primarily of restricted volcanic rocks; sands and outwash sediments blanketed the wadi basin. The Al-Dhala Qatabah Basin is located between 1100 and 1800 meters above sea level and receives about 269 mm of rainfall per year, as well as significant recharge from nearby mountain drainage.

The delineation of fluoride contamination areas from Taiz governorate has been conducted, and the iso-line contour map of fluoride ion concentration from At Aaiziyah district and its surrounding villages is given in Figure 2.

Figure 2.

Iso-line contour map of fluoride ion concentration from Taiz districts.

It’s clearly observed that the villages of Jabal Sabir, Hawban, Hethran, and Al- Bryehey as well as Taiz City are the most affected areas by fluoride contamination in groundwater.

Some of Sana’a governorate districts, particularly Sanhan, had the highest fluoride concentrations in their drinking water (UNICEF, 2008). The majority of Yemenis living in rural areas rely on deep well water for drinking and cooking, and many of these wells are contaminated with fluoride in concentrations ranging from 2.5 to 32 mg/l. Fluorosis, particularly skeletal fluorosis, has never been seen in Yemen before, only about 8–10 years ago since it was first reported. Clinically, it develops as a result of the high fluoride concentration in bones. Dental fluorosis, on the other hand, is not a new phenomenon in Yemen, particularly in the Taiz governorate [7].

A regional hydrogeochemical study from different Yemeni terrains indicated that water-rock interaction was most likely the primary cause of high ion concentrations in groundwater. According to geochemical modeling, the main minerals controlling the aqueous geochemistry of elevated fluoride ion contamination are calcite and fluorite. The concentration of F⁻ in groundwater was positively correlated with the concentrations of HCO₃⁻ and Na⁺, indicating that groundwater with high concentrations of HCO₃⁻ and Na⁺ leads to the dissolution of some fluoride-rich minerals. This situation of fluoride solubility control at higher fluoride concentrations can be explained by the fact that fluoride ions in groundwater can be increased as a result of CaCO₃ precipitation at high pH, which removes Ca²⁺ from solution and allows more fluorite to dissolve [20].

The groundwater data from different Yemeni areas reveals the chemical reactions that take place in the aquifer system. The results of the linear regression study on the relationship between F and HCO₃ (total alkalinity) from published papers show a positive connection, which could be owing to the simultaneous release of hydroxyl and bicarbonate ions during the leaching and dissolution of fluoride containing minerals into groundwater. With higher levels of alkalinity, the rate of weathering and mineral leaching rises, resulting in higher fluoride ion concentrations. High amounts of fluoride are also linked to greater Na⁺ ion concentrations. This also favors that groundwater with high HCO₃⁻ and Na⁺ content is usually alkaline and has relatively high OH⁻ content, so the OH- can replace the exchangeable F⁻ of fluoride-bearing minerals, increasing the F- content in groundwater [20].

5. Types of fluorosis in Yemeni regime

Fluoride is one of the few chemicals that have been proven to have harmful effects on people when consumed through drinking water. Fluoride in drinking water has beneficial effects on teeth at low concentrations, but excessive exposure to fluoride in drinking water, alone or in combination with fluoride from other sources, can cause a variety of problems. As the level and duration of exposure increase, these range from mild dental fluorosis to crippling skeletal fluorosis.

The chemical characteristics of the drinking water from the study reflect high fluoride contamination above the permissible limits of Yemeni, and WHO standers and most of them are poor whoever, their nutritional status is expected to be very poor. These factors including the high concentration of Na- and HCO₃⁻ and low concentration of Ca²⁺ions will increase the severity of fluorosis.

The visual observations in some selected Yemeni villages identify that there are two types of fluorosis recognized, they are:

5.1 Dental fluorosis

Mottling of teeth is one of the most easily recognized symptoms from different Yemeni governorates especially Taiz and Al Dhalla. The teeth of the children in the affected areas lose their normal creamy white translucent color and become rough, opaque, and chalky white. Some of the local inhabitants indicated that their teeth have be extracted and replaced with dentures. The dental fluorosis ranges from mild to severe fluorosis. The photographs represent some of dental fluorosis from the different areas in Yemen. High percentage dental fluorosis among the children has been observed in Taiz and Al-Dhala basins (Figure 3A and B).

Figure 3.

Figures present severe dental fluorosis (A) and moderate dental fluorosis (B) in Yemeni children in the affected area, after Alamry (2009) [7].

Dental fluorosis is the most common fluoride ailment identified in the afflicted areas, according to visual observations from selected villages. Fluorosis in the teeth can range from mild to severe.

In general, there is a link between fluoride in the water and the occurrence of dental fluorosis in the Taiz and Al-Dhala regions. A published research paper concluded that there is a positive relationship between fluoride in water and the occurrence of dental fluorosis in Sanhan, Taiz, and Al-Dhala regions [7].

5.2 Skeletal fluorosis

Fluorapatite is 1000 times less soluble than hydroxyapatite, the most common mineral found in bone. The F⁻ ion aggressively substitutes for the OH⁻ ion, resulting in an accumulation of F⁻ in bone tissue and skeletal fluorosis [20].

The patient often complains of a vague discomfort in the limbs and trunk early on in the development of fluorotic changes in the skeleton. Back pain and stiffness, particularly in the lumbar region, follow. Fluorosis in the teeth is frequently visible with the naked eye and is easy to detect even by laypeople. On the other hand, even with the assistance of appropriate equipment, skeletal fluorosis and nonskeletal fluorosis are difficult to diagnose. Radiography is frequently used to detect symptoms such as joint enlargements or minor bone deformations.

There are no any publications presenting the problem of the skeletal fluorosis among children in the affected Yemeni places, while I have seen children in Al-Dhalla villages showing skeletal deformation, and these cases among children could be considered as cases of skeletal fluorosis unless proved otherwise, as it is shown in Figure 4.

Figure 4.

A group of children showing skeletal deformation from Al-Dhalla region Yemen.

6. Dietary practices of the children and fluorosis

The hydrogeochemical characteristics of the groundwater in the study area indicated that the volcanic and plutonic rocks are the primary sources of fluoride whoever, the food could be the secondary sources.

Fluoride intake during infancy and early childhood is mirrored in dental fluorosis patterns. In most cases, the fluoride content of drinking water is considered sufficient for determining the level of fluoride exposure in a given area. There has been evidence that fluoride uptake from other sources such as food, dust, and beverages is many times higher than that from water [21].

The percentage of children with fluorosis was found to be extremely high. Although high fluoride levels in drinking water may be to blame, various food habits (such as drinking black tea and chewing Gatof Catha edulis Forsk leaves (Khat)) indicated a high fluoride contribution to the diet. Some of the children used to chew Khat also, and the Khat is cultivated in the volcanic soil and irrigated by the high fluoride concentration water [22].

Cooking with fluoridated water raises fluoride levels significantly, particularly in dry foods such as maize flour, which absorbs a lot of water during cooking. It has been reported that the simultaneous intake of food and fluoride-containing compounds can affect fluoride availability in a positive or negative way, depending on the food type, mode of administration, and type of fluoride compound [21].

The diet consumed by the children was not balanced and lacked quality. It is composed of maize flour with milk and a few rare vegetables. Intake of milk and milk products is said to diminish the fluoride availability by 20–50% in humans. Although the area under study had children taking whole milk (boiled or fermented).

7. Conclusion

A high fluoride concentration has been reported in groundwater in tertiary volcanics, from Yemeni terrain. The major ion chemistry data from groundwater of this rock revealed that Na⁺ is the most predominant cationic constituent followed by Ca²⁺ and Mg²⁺, the HCO₃⁻ and SO⁴⁻ are found to be the most predominant anions followed by Cl⁻ and NO₃. High fluoride ion concentration in the Yemeni groundwater appears to be caused by high alkalinity due to HCO₃⁻ ions. Na⁺ has a positive correlation with F⁻, whereas Ca²⁺ has a negative correlation, resulting in an equilibrium condition in groundwater. CaCO₃ precipitation at high pH can increase fluoride ions in groundwater by removing Ca²⁺ from solution and allowing more fluorite to dissolve.

Dental fluorosis is the widely fluoride disease observed in the affected areas, whereas skeletal fluorosis is observed in some villages from Al-Dhalla region. The principal causes of fluorosis among the Yemeni population appear to be related to high fluoride concentration in drinking water and Khat chewing habits, which are cultivated in the volcanic soil and irrigated by the high fluoride concentration water.

Despite the high dental fluorosis prevalence in the affected areas, no restorative treatment is being carried out. Therefore, it is highly recommended to provide safe drinking water to fluoride-affected areas.

Acknowledgments

I would like to thank the National Water Resources Authority (NWRA) Yemen and UNDP for funding this research. Also I would like to thank the publisher and anonymous reviewers for valuable suggestions and comments that have enhanced the chapter quality.