Effect of groundwater residence time on geogenic fluoride release into groundwater in the Mt. Meru slope area, Tanzania, the Great Rift Valley, East Africa.

Fluoride is one of the most ubiquitous elements in geological formations. Since high levels of fluoride cause dental and skeletal fluorosis, the World Health Organization (WHO) limits fluoride in drinking water to 1.5 mg/L. Fuge (2019) reviewed geogenic and anthropogenic sources of environmental fluoride. Ali et al. (2016) reviewed worldwide contamination of water by fluoride. Rasool et al. (2018) reviewed the global outlook on fluoride contamination in groundwater. On the geogenic fluoride contamination in groundwater, the two specific geochemical environments alkalinization (e.g., Degnan et al., 2020; Luo et al., 2018; Zabala et al., 2016), and increased weathering under oxic conditions (e.g., Chae et al., 2006) have been studied. On anthropogenic fluoride contamination in groundwater, the fluoride enrichment in groundwater has been reported to be mainly derived from fertilizers, pesticides, household wastes, and coal-combusted atmospheric pollutants under alkaline conditions (e.g., Farooqi et al., 2007; Luo et al., 2018).

In most countries using groundwater for drinking, groundwater contamination by high fluoride is a crucial problem. Especially in China and India, which have large populations, and in developing countries of Africa with a high population growth rate, drinking geogenic contaminated groundwater with high fluoride is inevitable and it is extremely difficult to limit the damage to human health.

In the north, northeast, and northwest parts of China, high fluoride groundwater is widely distributed mainly in the shallow aquifer (e.g., He et al., 2020). Li et al. (2014) reported the enrichment of fluoride is controlled by geologic and hydrogeological conditions, fluorine-bearing minerals presented in the alluvial aquifer of Weihe River, China, the warm semi-humid continental monsoon climate zone. Similarly, many researchers have reported the natural and human factors of fluoride contamination including health risks in groundwater in China (e.g., Li et al., 2019; Liu et al., 2022; Wu et al., 2015). High fluoride level was observed in deep groundwater areas characterized by semiarid conditions (Fuhong and Shuqin, 1988). Luo et al. (2018) reported the enrichment of fluoride in groundwater due to cation exchange (Na/Ca), alkalinization, anthropogenic contamination from pesticides, fertilizer use and industrial waste discharge, and evaporation and ion effects in the Yuncheng Basin.

In India, highly to moderately contaminated water with fluoride has spread to almost every state (Ali et al., 2016) and approximately 62 million people are affected by dental and skeletal fluorosis (Susheela, 1999). Most of the high fluoride contamination of groundwater was formed by weathering fluorine-bearing minerals in granite and granitic gneisses accompanied by groundwater evaporation (e.g., Brindha et al., 2011; Sharma et al., 2011; Singh et al., 2011).

In Africa, groundwater with a very high level of fluoride concentration has been found in a part of the Great Rift Valley (GRV) Africa extending from the Jordan valley down through Sudan, Ethiopia, Uganda, and Kenya to Tanzania (Ali et al., 2016). In the GRV in East Africa (Fig. 1), fluoride contamination occurs widely in natural water systems and is associated with alkaline volcanic rocks and ash (e.g., Bosshard-Stadlin et al., 2017; Kut et al., 2016). Tanzanian people living in the GRV have actively utilized groundwater since the 1960s because of a shortage of surface water, which dries up during the dry season. As a result, they suffer from both dental and skeletal fluorosis. The level of fluoride in the groundwater in the GRV, which averages 4.5 mg/L, exceeds the upper limit (1.5 mg/L) prescribed by the WHO (Kaseva, 2006; Kitalika et al., 2018; Kut et al., 2016; Mbabaye et al., 2018; Vuhahula et al., 2009). Tanzanian national and local governments and communities in villages supply drinking and cooking water to people mainly via groundwater from wells. The Mt. Meru watershed (MMW) and the Mt. Kilimanjaro watershed (MKW) are the most important headwater areas of the Pangani River Basin in the northeastern part of Tanzania, and the shortage of groundwater and its contamination with fluoride are the most serious issues that threaten the water resources in this basin. Factors affecting the concentration of fluoride in groundwater in Africa are complex, and the concentration varies depending on local aquifer conditions (Malago et al., 2017).

Chacha et al. (2018a), who reported on groundwater age data obtained by application of the radioisotope ¹⁴C dating technique and the recharge mechanism of the Arusha aquifer, northern Tanzania, concluded that the presence of modern groundwater suggests that shallow aquifers are actively recharged and respond positively to seasonal variations. Mckenzie et al. (2010) reported on the dating of hydrogeologic systems in the Kilimanjaro area based on a hydrogeochemical survey that included tritium values. These authors concluded that there is a mixture of old groundwater and young surface water in groundwater in the MKW. However, dating using tritium or ¹⁴C is not sufficiently precise to evaluate variations in groundwater residence time in mountain slope areas. Lugodisha et al. (2020) also reported that there is a mixture of old groundwater and young local recharge water in groundwater based on the application of oxygen and hydrogen isotope tracers in the southwestern part of the MMW (Fig. 2). However, their research was limited to Ngaramtoni small watershed in the MMW, did not cover the Themi River watershed, including the Arusha city, the capital of Arusha Region, and did not address dating of groundwater. Mbabaye et al. (2018) reported that the fluoride concentration in well water increases exponentially as the depth of the well decreases for wells with a depth of 5 m to 100 m in the eastern part of the MMW (Meru district in the Arusha region). Malago et al. (2017) noted that no study has reported the long-term trend of fluoride concentration in water over time. In East African regions such as Tanzania, no previous research has reported on the geogenic release of fluoride into groundwater and its dependence on the length of the contact time between minerals in an aquifer and groundwater based on high-precision dating. This study aims to demonstrate the effect of groundwater residence time on geogenic fluoride release into a natural groundwater system in an important headwater area in the GRV based on high-precision dating using sulfur hexafluoride, SF₆, in groundwater. Especially groundwater in the southern slope area of Mt. Meru supplying drinking water to Arusha City, which has a particularly high rate of population growth, will be highlighted.