11.3 The East African Rift Valley

The highest concentrations of F in groundwater worldwide are known to occur in the East African Rift Valley. The Rift Valley extends through Eritrea, Djibouti, Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi, Malawi, with a western branch that extends through Zambia and is thought to terminate in Botswana or Namibia (McCarthy, 2013; McFarlane and Eckardt, 2007). Fluoride occurs in association with alkaline, hyper-alkaline and silicic volcanic rocks and with associated hydrothermal fluids. High concentrations are found in groundwater used for drinking, as well as some river waters, but can be extremely high in hot springs and alkaline lakes. Cases of both dental and skeletal fluorosis from chronic exposure via drinking water, cooking water and food are well-documented across the region (Ayenew, 2008; Bugaisa, 1971; Gaciri and Davies, 1993; Mabelya et al., 1997; Nanyaro et al., 1984; Rango et al., 2014; Tekle-Haimanot, 2005; Tekle-Haimanot et al., 1995).

Alkaline volcanic rocks including nephelinites and carbonatites, silicic rocks including rhyolites and ignimbrites, associated ashes and reworked lacustrine sediments derived from them, are all capable of bearing large F contents. Volcanic ashes and reworked ash-bearing sediments are particularly reactive. Weathering of the silicate minerals in the lavas, ashes and volcanogenic sediments produces Ca-poor, Na-HCO₃⁻-enriched groundwater compositions (Ayenew, 2008; Jones et al., 1977). These are commonly undersaturated with respect to fluorite (Rango et al., 2009) and in such conditions, F concentrations are not constrained by fluorite precipitation.

Water sources close to active and dormant volcanoes have especially high F concentrations. In Tanzania, concentrations in the range 12 to 76 mg/L were recorded for rivers and 15 to 63 mg/L for springs draining Mount Meru (Nanyaro et al., 1984). The extremes were attributed to weathering of fluorine-rich alkaline igneous rocks, hydrothermal inputs from fumaroles and cycling of F-rich trona (Na₂CO₃ · NaHCO₃ · 2H₂O), a seasonal evaporitic encrustation. Around the Oldoinyo Lengai volcano, high F concentrations can be attributed to nephelinitic tephra deposits containing the highly soluble mineral villiaumite (NaF) (Bosshard-Stadlin et al., 2017).

Rift Valley alkaline and crater lakes have some of the highest F concentrations; many are also brackish. In Ethiopia, Tekle-Haimanot and others (2006) observed F maxima of 264 mg/L and 202 mg/L in Lakes Shala and Abijata respectively. Nanyaro and others (1984) found concentrations up to 690 mg/L in the alkaline Momella Lakes Group of Mount Meru, Tanzania. In Kenyan Lake Magadi, Jones and others (1977) reported F concentrations up to 1,980 mg/L in surface brine. High F concentrations are achieved by extreme evaporation, associated calcite precipitation (loss of Ca) and hydrothermal inputs (Jones et al., 1977; Kilham and Hecky, 1973). Lake Magadi waters appear to be largely saturated with respect to fluorite and the mineral is an abundant accessory authigenic phase in many of the Magadi lake sediments (Jones et al., 1977). Jones and others (1977) reported extremely high fluoride concentrations (up to 2,170 mg/L) in saline groundwaters from boreholes in Magadi lake sediments.

In a study of groundwater and surface water from the main Rift Valley of Ethiopia (Reimann et al., 2003; Reimann et al., 2002), 31 percent of analyzed groundwater samples from deep boreholes and 38 percent from shallow boreholes had F concentrations greater than the WHO guideline value of 1.5 mg/L (Figure 16). Hot springs also had relatively high concentrations, but the highest (up to 175 mg/L) were observed in samples from alkaline lakes. High F concentrations are associated with low Ca and high alkalinity values, reflecting the F mobilization processes outlined in Section 9. Most groundwaters in the study were undersaturated with respect to fluorite (Figure 16).

Mitigation of fluoride problems in the Rift Valley has long been difficult because of factors including the widespread scale of the problem, occurrence in rural areas, limited testing, availability and cost of many raw materials for water treatment, practicality of water treatment, and above all, scarce supplies of water.

Figure 16 – Variation of a) fluoride with calcium; b) fluoride with alkalinity; and, saturation indices for c) calcite and d) fluorite in water samples from the Ethiopian Rift Valley (n=148) (data from Reimann et al., 2003; Reimann et al., 2002); WHO GV: Guideline value from World Health Organization.