Excerpts:
Abstract
In central Benin, fluoride concentrations in groundwater generally exceed the national and WHO guideline values (1.5 mg/L) and are locally above 7 mg/L (max. 7.19 mg/L). In this area, aquifers are found in a Precambrian bedrock made of migmatitic gneiss and granites. Recent hydrogeochemical studies have shown that the occurrence of fluoride in groundwater in this area is of geogenic origin. The aims of this investigation are to determine the sources of fluorine in the bedrock and to assess the role of geology on the mineralization and high fluoride concentrations in groundwater.
Thirty-five rock samples were collected in different areas that display contrasting concentrations of fluoride in groundwater. We carried out analyses of petrology, mineralogy and geochemistry on the samples. The results show that fluorine concentrations vary between 100 and 2900 ppm. Several fluorine-bearing minerals such as (biotite, muscovite, amphibole), titanite, fluorite, fluor-apatite, fluor-allanite, epidote and chlorite were identified. Groundwaters with elevated concentrations of fluoride are found in geological aquifers units and watershed rocks that contain fluorine bearing minerals. Biotite occurring in the granitic formations has the highest fluorine concentrations and appears as the most important contributor to the total fluorine from the bedrock to the groundwater.
Introduction
In Benin, as in many other countries around the world, groundwater is the main source of drinking water supply and groundwater quality must conform to national and international standards. In central Benin, the geological formations are mainly of Precambrian age and consist primarily of granite and migmatitic gneiss (Boukari, 1982; Bigioggero et al., 1988). Important aquifers are located in the fractured parts of these magmatic and metamorphic rocks. In these aquifers, abnormally high concentrations of fluoride (max. 7.19 mg/L) exceeding the national and the World Health Organization (WHO) drinking water guidelines value (1.5 mg/L) have been recorded (Dovonon et al., 2011). High concentrations of fluoride in drinking water causes dental fluorosis and, in extreme cases sekeletal fluorosis (Fawell et al., 2006; WHO, 2017; WHO, 2018). Around Central Benin, many cases of dental fluorosis are recorded (Dovonon et al., 2011). Recent hydrogeochemical and investigations (Tossou, 2016; Tossou et al., 2017) have demonstrated that the presence of fluoride in groundwater is mainly due to water-rock interactions (geogenic origin) and that fluorine concentrations in the main fertilizers used in the area is rather low (under detection limit, 0.1 ppm). Furthermore, a geostatistical study in the area supports a strong correlation between the spatial differentiation of the petrographic and textural characteristics of the basement rocks and the spatial distribution of groundwater mineralization (Tossou et al., 2019).
Fluorine occurs in many types of rocks (sedimentary, metamorphic and agneous). Fluoride is included in a variety of silicates or non-silicates minerals (Lahermo et al., 2000; Edmunds et al., 2005). The principal minerals of igneous rocks that contain fluorine are listed in Table 1, together with their chemical formulae (Handa, 1975; Josephus Thomas et al., 1977; Susheela et al., 1999; Edmunds and Smedley, 2005). Minerals such as chlorite and clays could also contain fluorine in their structures (Edmunds and Smedley, 2005).
The average concentration of fluorine in the earth’s crust ranges from 500 to 1000 ppm, however, some rocks like the Rapakivi granites in Finland may show fluorine concentrations of up to 5000 ppm (Bell et al., 1988). As in Finland, several other regions worldwide have geological formations with high fluorine concentrations.
The chemistry of groundwaters and specifically their fluoride concentration largely depends on the composition of the host rocks in the aquifers and watershed (Hem, 1985; Edmunds and Smedley, 2005; Karro et al., 2013a, 2013b). Therefore, it is important to investigate the mineralogy and geochemistry of the aquifer bedrock materials in which groundwater flows.
The aim of this study is to test the hypothesis that the mineralogy of the crystalline bedrock exerts a first order control on the fluorine content in groundwater of the study area. First, the geochemistry of fluorine and other major elements is investigated in the different geological formations of Central Benin. The second goal is to identify the type of geology and the main minerals that mostly contribute to groundwater mineralization and particularly, the main sources of high fluoride concentrations.
Figures
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Location of the Department of Collines in Benin and Sub-Saharian Africa together with the elevations and hydrographic networks of the studied area. The inserts show the location of Benin in Africa. -
Geology of the study area and sampling locations for rock samples. Inv. Zone in the map refers to the Investigations Zones (see section on Methodology). To make it short, “IZ” will be used instead of …
Section snippets
Location, relief, hydrography and climate
The study area is located in central Benin and corresponds to the so-called “Département des Collines” (Department of Collines) administrative region. This department (Fig. 1) is situated between latitudes 7°27? and 8°46?N and longitudes 1°39? and 2°44?E (WGS 84/UTM zone 31N coordinate and projection system). The study area covers approximately 14 000 km2 and includes six administrative districts (so-called communes): Bantè, Dassa-Zoumé, Glazoué, Ouessè, Savalou and Savè. According to data from
Field observations and rock samples collection
The rock samples were taken from three sites that were selected based on previous hydrogeochemical characterizations (Tossou et al., 2016). These sites are referred to as “Investigation Zone (Inv. Zone or IZ)” and shown in Fig. 2.
Investigation Zone 1 (IZ1) is located in the southern part of the region (Dassa-Zoumé and surroundings) and is characterized by the highest mineralized waters and fluoride concentrations (up to 7 mg/L). Twenty-four (24) rock samples were taken from various dominant
Field and petrographic data
We will first describe the different geological facies that were recognized during our field campaigns. From one investigation zone to another, large variability and heterogeneity can be observed when considering the morphology and the grain-size of the geological formations (Fig. 3). The IZ1 (Dassa-Zoumé and vicinity) is characterized by long chain hills with heavily fractured outcrops (Fig. 3a–d). By contrast, in IZ2 and IZ3, there are fewer outcrops and they are often present as domes with
Petrography and mineralogy impact on the spatial differentiation of groundwater mineralization and its fluoride concentrations in the region
The petrographic analysis shows that the geological formations in the study area exhibit strong spatial variability in their textures and mineralogy. In IZ1 and especially in the areas closed to the Dassa-Zoumé pluton, rock materials show large grain sizes with clear indications of advanced physical and chemical weathering. In particular, the outcrops show fractures and large quantities of clay minerals are observed around the foothills. In contrast, outcrops in IZ2 and IZ3 display fine grain
Conclusions and perspectives
Results of this study have highlighted the significant control of the bedrock mineralogy and texture on the F concentrations of groundwaters in the Department of “Collines” in central Benin. Biotite is the most contributing mineral to elevated fluoride concentrations in groundwater, mainly in the granitic formations. Other fluorinated minerals such as fluorite, fluorapatite, amphiboles, etc. exist and can contribute much smaller proportion of fluoride than biotite.
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