Fluorine is a halogen that naturally exists in the form of fluoride (F?) due to its high reactivity and electronegativity (Ganyaglo et al., 2019; Zhang et al., 2020; Murray et al., 2021). It constitutes about 0.06–0.09% of the earth’s crust, been the 13th most abundant element (Rusiniak et al., 2021).
Fluoride (F–) occurs in waters in varied concentrations (Jacks et al., 2005; Brindha and Elango, 2011; Dar et al., 2011; Vithanage and Bhattacharya, 2015a, b). Its natural concentration in waters depends largely on the nature of the geologic formations; fluoride-bearing minerals, anion exchange capacity of aquifer materials (OH– for F–), pH, alkalinity, temperature and residence time of waters within a particular formation (Saxena and Ahmed, 2001; Sunkari et al., 2018; Sunkari et al., 2019; Zango et al., 2019). Some of the minerals that have the greatest impact on the hydrogeochemistry of fluoride are: fluorite, apatite, mica, amphiboles, some clay minerals (such as illite, montmorillinite and kaolinite) and villiamite (Dar et al., 2011; Sunkari et al., 2018). Fluoride is colourless, tasteless or scentless when dissolved in water thereby making it difficult to be determined through physical examination. Its concentrations in waters can only be determined by chemical analysis (Brindha and Elango, 2011).
Fluoride causes health problems in people all over the world. A concentration of at least 0.6 mg/L promotes growth of human teeth and bones (Narbutait? et al., 2007). Intake of groundwater with fluoride levels exceeding 1.5 mg/L will eventually lead to dental fluorosis (Sunkari et al., 2019; Kimambo et al., 2019; Aravinthasamy et al., 2020; Zango et al., 2021; Ijumulana et al., 2021). A further consumption of higher concentration will result in skeletal fluorosis (Brindha and Elango, 2011; Ijumulana et al., 2020, 2021). There are several ways of mitigating fluoride contamination in groundwater. These methods include: dilution of the groundwater contaminated with fluoride, adsorption, ion exchange, and precipitation among others. Preference of any of these methods depends on the: affordability, magnitude of the contamination of groundwater as well as the origin of the contaminants; whether it is natural or anthropogenic (Bannerman and Ayibotele, 1984; Apambire et al., 1997; Abugri and Pelig-Ba, 2011; Brindha and Elango, 2011; Kimambo et al., 2019).
Ghana is one of the fluorosis endemic countries in West Africa. The highly fluoridated parts of Ghana are mainly restricted to the northern fringes of the country, which are largely dominated by varying lithologies involving rocks belonging to the Birimian Supergroup and the Voltaian Supergroup (Fig. 1). These rocks are mainly volcanic and sedimentary rocks that are intruded by granitoids (Fig. 1). The fluorosis endemic parts of Ghana have been the focus of many researchers in the past decades (Apambire et al., 1997; Anku et al., 2009; Atipoka, 2009; Salifu et al., 2012; Yidana et al., 2012; Firempong et al., 2013; Alfredo et al., 2014; Craig et al., 2015, 2018; Anornu et al., 2017; Tay, 2017; Sunkari et al., 2018; Donzagla et al., 2019; Ganyaglo et al., 2019; Sunkari et al., 2019; Zango et al., 2019, 2021; Zakaria et al., 2021). Most of these studies are localized to specific communities, districts, municipalities and districts in northern Ghana. However, there is no comprehensive review on the fluoride menace to the groundwater resources in the entire country. This study aims at reviewing all studies previously conducted in Ghana, especially in the highly fluoridated parts of northern Ghana to elucidate the: groundwater fluoride levels, sources of fluoride enrichment in groundwater, extent and severity of fluorosis, and sustainable mitigation measures of high groundwater fluoride.
Mineralogy and geochemistry of fluorine
Fluorine (F) has a higher concentration of about 850–1200 mg/L in intermediate igneous rocks under terrestrial conditions (Kabat-Pendias and Pendias, 1992; Abugri and Pelig-Ba, 2011; Vithanage and Bhattacharya, 2015a, b). The commonest F-bearing minerals involved in the mechanism of F– enrichment in water are: fluorite (CaF2), fluoroapatite (Ca5(PO4)3F), biotite (KMg3[AlSi3O10]F2), muscovite (KAl2[AlSi3O10]F2), cryolite (Na3AlF6), villiaumite (NaF), amphiboles, (Ca,Na,K)0,1(Ca,Fe,Li,Mg,Mn,Na)2
Mechanism of fluoride concentration and enrichment in groundwater
The mechanism of fluoride concentration and enrichment in groundwater is explicitly explained by Rao (2011) and Malago et al. (2017) below:
Precipitation waters get enriched in CO2 from the air and soil as well as biochemical reactions of bacteria and organic matter during infiltration (Equations (4), (5))), resulting in an increase in pH and subsequent over-saturation of CaCO3.CO2 + H2O ? H2CO3H2CO3 ? HCO3– + H+
The onset of evapotranspiration coupled with dry climatic conditions lead to the
Sources of fluoride in groundwater
High concentrations of fluoride in groundwater are derived from two main sources: natural (lithogenic/geogenic) and anthropogenic (Kim et al., 2010; Brindha and Elango, 2011, Rao, 2011; Ali et al., 2016). Geogenic sources include: the nature of aquifer material and volcanic ash (Brindha and Elango, 2011; Ijumulana et al., 2020, 2021). Anthropogenic sources include phosphate fertilizers and fly ash (Kim et al., 2010; Brindha and Elango, 2011).
Socio-economic and health implications of high groundwater fluoride
Water for domestic purposes, especially drinking water is the major source of F– intake in humans (Antwi et al., 2011). Most of the people affected by highly fluoridated groundwater are low income earners and they largely live in the tropical countries (arid and semi-arid regions), where the per capita consumption of water is high due the dry climatic conditions (Kim et al., 2010). In Ghana for instance, the daily consumption of water is about 3–4 L/day per adult, which is higher than the WHO
Groundwater fluoride hotspots in Ghana
A summary of the groundwater F– concentrations in northern Ghana is given in Table 1. The occurrence of high F– concentrations in groundwater has been reported in parts of the Savelugu Municipality (Tay, 2017) and parts of the Gushiegu Municipality (Maxwell et al., 2012) both in the Northern Region of Ghana (Fig. 1; Table 1). The Bongo District and its surrounding areas (parts of the Bolgatanga Municipality) in the Upper East Region of Ghana (northeastern Ghana) are of epidemic levels (Apambire
Human health risks associated with fluoridated groundwater in Ghana
Studies on human health risk assessment of fluoridated groundwater in Ghana are not readily available. Most of the studies only predict the presence of dental fluorosis without a detailed health risk assessment. However, at the time of writing this review, only Ganyaglo et al. (2019) and Zango et al. (2019) conducted human health risk assessment of high groundwater fluoride in the fluorosis endemic parts of Ghana. Whilst Ganyaglo et al. (2019) studied the Upper East Region, Zango et al. (2019)
Sustainable mitigation measures
Various measures proposed by researchers for mitigation of F– contamination in water are summarised in a schematic chart in Fig. 4. They are classified into in-situ and ex-situ measures (Fig. 4). In-situ methods aim at mitigating the concentration of F– in the aquifer through artificial recharge (Bhagavan and Raghu, 2005; Brindha and Elango, 2011). The ex-situ methods aim at the decontamination of water at the household or community level (Brindha and Elango, 2011).
Conclusion, recommendations and research gaps
The presence of F–-bearing minerals in the host rocks and water-rock interactions in an alkaline environment under dry climatic conditions have been established to be the main causes of F– enrichment and contamination in groundwater. Chemical processes such as decomposition, dissociation, and dissolution are also responsible for the mobility and transport of fluoride in groundwater. Chemical weathering under arid to semi-arid conditions with relatively high alkalinity and long residence time
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors thank everyone who contributed to improving the quality of this paper, especially the Editor and the three anonymous Reviewers.