Origin of Fluoride and Arsenic in the Main Ethiopian Rift Waters

Abstract

In the Main Ethiopian Rift (MER) area, rural populations often use water that exceeds the World Health Organization thresholds for fluoride (F–) and arsenic (As), two elements that are hazardous for human health. In this study, twenty-nine water samples were collected from lakes and hot and cold springs in southern MER to investigate source(s) and health-risk of the F– and As contamination. According to major ion and trace element analyses, only cold spring water is safe for consumption, whereas hot spring water is the most contaminated. Leaching tests performed with the MER rhyolitic volcanic rocks and their weathered products (fluvio-lacustrine sediments) demonstrate that the main cause of the F– and As release is geogenic, i.e., not related to anthropogenic activities. The weathering of volcanic glass and minerals (apatites, clays, hydro-oxides) by CO2-bearing alkaline water induces the mobilisation of F– and As from solid to liquid phase. This process is particularly fast, when fluvio-lacustrine sediments are involved, and can be further enhanced by hot groundwater leaching. This study, investigating the distribution, sources, and mechanisms of F– and As release in MER water, could be of interest also for other sectors of the East African Rift and other similar volcano-tectonic settings.

Application of multi hydrochemical indices for spatial groundwater quality assessment: Ziway Lake Basin of the Ethiopian Rift Valley

“…To assess the involvement of cation exchange reactions in ZLB, the two Chloro-Alkaline Indices (CAIs), CAI -I and CAI -II were applied as recommended by Schoeller (1965) These two indices are expressed as Equations ( 5) and ( 6) (the concentration of the ions are expressed in meq/l). As suggested by several authors (Gizaw 1996;Yirgu et al 1999;Rango et al 2009;Haji et al 2018;Bianchini et al 2020), the above-mentioned process, silicate weathering and cation exchange, are also responsible for the high content of Fexceeding the permissible limit in the rift sector of the study area (Fig. 4).…”

Groundwater Resources in the Main Ethiopian Rift Valley: An Overview for a Sustainable Development

“…During magmatic processes, fluoride does not easily fit into the crystal structure of minerals and is preferentially partitioned into melt, remaining in the residual magma until its crystallization. Consequently, fluoride is more concentrated in evolved sialic rocks, such as the bedrock of the lowland Oromia Region, rather than in primitive basic rocks (e.g., basalts) [22]. The increase in pH and temperature favors the interaction between sialic rock and water because they influence the reactions of dissolution and adsorption-desorption, which contribute to the mobility of the fluoride contained in the primary mineral phases (such as fluoride, apatite, amphibole, biotite, and volcanic glass) or secondary minerals (such as Fe − , Al − and Mn − oxides and hydroxides, and clay minerals), particularly in arid or semi-arid climates [21,57,58].…”

“…Natural sources of elevated fluoride concentrations in groundwater and lakes in the MER have been reported in the scientific literature [20][21][22][23] and, therefore, water is not always suitable for human consumption. Indeed, prolonged ingestion of drinking water containing F − ions that exceed the tolerance limits can cause dental and skeletal fluorosis, with negative effects on children and young people [24].…”

“…The concentration and mobilization of F − ions and other elements in groundwater are dependent on the chemical and physical processes that occur between the water and the geological environment. The occurrence of F − in water is generally associated with volcanic rocks, particularly in the high-temperature geothermal settings that are common at convergent plate boundaries, as well as in intraplate areas characterized by extensional tectonic activity, such as the African Rift [22].…”

Simultaneous Determination of Fluorine and Chlorine in Marine and Stream Sediment by Ion Chromatography Combined with Alkaline Digestion in a Bomb

“…Moreover, the detection limits of the determination by INAA or X-ray Fluorescence Spectrometry (XRFS) are generally too high, relative to usual abundances [29]. In contrast, ion chromatography, a compact and inexpensive instrument, is commonly used in many laboratories and is the most suitable method for sensitive and simultaneous determination of fluorine and chlorine [5,6,[28][29][30][31][32][33][34][35]. However, it is difficult to quantitatively extract fluorine and chlorine from geological materials for ion chromatography analysis.…”