**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).

#### *Fluoride*

**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**.

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

*Origin and Hydrogeochemistry of Fluoride in the Context of the Yemen Regime DOI: http://dx.doi.org/10.5772/intechopen.104255*

**Figure 2.** *Iso-line contour map of fluoride ion concentration from Taiz districts.*

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<sup>+</sup> ion concentrations. This also favors that groundwater with high HCO3 − and Na<sup>+</sup> content is usually alkaline and has relatively high OH<sup>−</sup> content, so the OH- can replace the exchangeable F<sup>−</sup> of fluoride-bearing minerals, increasing the F- content in groundwater [20]**.**
