**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.

**Keywords:** hydrogeochemistry, fluorosis, fluoride contamination, volcanic rocks, rock-water interaction, fluorite, Yemen

### **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 [12–14]. 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 quartzcemented 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,

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

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].
