Abstract
Groundwater samples were evaluated throughout Turkana County (Kenya, East Africa) while looking for drinking water sources. Some samples showed high concentrations of fluoride with values in the range of 0.15–5.87 mg/L. Almost 50% of the samples exceeded the WHO and Kenyan potable water standard guideline value of 1.5 mg/L for drinking water quality. The hydrogeochemical studies revealed that the dominant cation in water is Na+ and the dominant anion is HCO3− resulting in Na-HCO3 type of groundwater, followed by Ca/Mg-HCO3 or Na-SO4 and Na-Cl in a few cases. Speciation modelling revealed that the groundwater is undersaturated with respect to gypsum and anhydrite, mostly undersaturated with respect to fluorite (6 samples are at equilibrium), and supersaturated or at equilibrium with respect to calcite (CaCO3). Precipitation of calcite favours the dissolution of F-rich minerals in the alkaline medium. Simultaneously, groundwater is enriched with sodium and bicarbonate, probably as a result of chemical weathering of Na-feldspar. Investigated groundwater can be presumably used for drinking purposes from 17 wells, but a detailed investigation of other trace element concentrations is necessary.
Highlights
Fluorine (F) belongs to the halogen group (17th group in the periodic table), its prevalence in the Earth’s crust reaches 0.06%–0.09% and classifies it at the 13th place in terms of occurrence among other elements (Singh et al 2018)
The hydrogeochemical characteristics including chemical analyses, calculating the charge balance, the mineralization based on the major ions
This correlation has been observed in fluoride ion (F-)rich groundwaters in Ethiopia (Bretzler et al 2011) and Namibia (Sracek et al 2015)
Summary
Fluorine (F) belongs to the halogen group (17th group in the periodic table), its prevalence in the Earth’s crust reaches 0.06%–0.09% and classifies it at the 13th place in terms of occurrence among other elements (Singh et al 2018). Fluoride ions in the human organism can co-precipitate with calcium ions as insoluble CaF2 (fluorite) in the human body leading to hypocalcaemia (Pettifor et al 1989; Fordyce 2011; Khairnar et al 2015; Singh et al 2018) In water, it occurs predominantly as fluoride ion (F-), and its main source is minerals containing fluorine i.e., fluorite (CaF2), cryolite (Na3AlF6), fluorapatite [Ca5(PO4)3F], and rock phosphate. The general mechanism controlling the high concentration of F- ions in groundwater has geogenic sources that are: long residence-time favouring water–rock interaction processes, evaporation, hydrogeochemical type changes during water flow (from Ca-HCO3 to Na-HCO3), weathering or dissolution of F-bearing minerals, geological setting i.e., presence of crystalline basement rocks within the wells or salt-rich geological formations, sorption and ion-exchange processes. The objective is to identify wells with high fluoride concentrations and to determine processes responsible for the fluoride groundwater enrichment
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