Geochemical and Isotopic Composition of Natural Waters in the Central Main Ethiopian Rift: emphasis on the study of source and genesis of fluoride

In the Main Ethiopian Rift (MER), the supply of drinking water principally relies on groundwater wells, springs (including some hot springs), and rivers, and is characterized by a significant problem of fluoride (F¯) contamination. New analyses reveal that the F− geochemical anomaly is sometimes associated with hazardous content of other potentially toxic elements such as As, B, Mo, U, Al, Fe, and Mn. The F¯ content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/L) in many important wells (up to 20 mg/L), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 mg/L and 384 mg/L respectively) and is causing prevalent endemic fluorosis disease in the region. 87 % of the groundwater wells, 38 % of rivers and 100 % of hot springs and lakes show F¯ content above 1.5 mg/L. The groundwater and surface water from the highlands, typically characterized by low Total Dissolved Solids (TDS) and Ca2+ (Mg2+)-HCO3¯ hydrochemical facies, do not show high F¯ content. The subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the TDS and a variation of the chemical signature towards Na+-HCO3¯ compositions, with a parallel enrichment of F¯. The interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the glassy groundmass with other mineral phases, it appears that the former stores most of the total F¯ budget. This glassy material is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride. The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release of fluoride into the interacting water. This mainly occurs by a process of baseexchange softening with the neo-formed clay minerals (i.e. Ca-Mg uptake by the aquifer matrix, with release of Na+ into the groundwater). This is plausibly the main enrichment mechanism that explains the high F¯ content of the local groundwater, as evidenced by positive correlation between F¯, pH, and Na+, and inverse correlation between F¯ and Ca2+ (Mg2+). Saturation indices (SI) were calculated (using PHREEQC-2) for the different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride can not precipitate as CaF2, and so F¯ mobilizes freely without forming other complexes. On the other hand, 35 % of the 23 investigated groundwater wells and 70 % of the 12 hot springs (and deep geothermal wells) show Arsenic concentration above the recommended limit of 10μg/L (WHO 2006). The average concentration of Arsenic is 0.9μg/L in rivers, 39μg/L in hot springs, 236μg/L in deep geothermal wells, 21.4μg/L in groundwater wells, 77μg/L in lakes, whereas maximum concentrations reach up to 3μg/L, 156μg/L, 278μg/L, 157μg/L and 405 μg/L respectively. Arsenic in groundwater wells shows positive correlations with Na+ (R2=0.63) and HCO3 − (R2=0.70) as well as with other trace elements such as Mo (R2=0.79), U (R2=0.70), V (R2=0.68) whereas no correlations are observed with Fe and Mn. PHREEQC speciation modelling indicates that Fe and Al oxides and hydroxides are stable in the water systems, suggesting that Fe and Al mineral phases are potential adsorbents and thus influence the mobility of As. The oxidizing, high pH condition combined with Na+- HCO3¯ hydrochemical facies (competing effect of HCO3 − for adsorption sites) of the MER waters play an important role in the mobilization of arsenic. Chemical analyses of leachates from MER rhyolitic rocks and their weathered and reworked fluviolacustrine sediments were performed in order to evaluate their contribution as a source of the mentioned geochemical anomalies. The leachates were obtained from a one-year leaching experiment on powdered rocks and sediments mixed with distilled water (10g:50ml). The sediment leachates contain as much as 7.6 mg/L of F¯, 220 μg/L of As, 181 μg/L of Mo, 64 μg/L of U and 254 μg/L of V suggesting that the local sediments represent the main source and reservoir of toxic elements. Laboratory column experiment was also conducted in volcanic ash sample using synthetic rain water flushing, and the result showed that significant amount of F¯ were leached out over the duration of the experiments. This showed that these elements were originally present in the glassy portion of the MER rhyolitic rocks, were progressively concentrated in weathered and redeposited products. It further confirms that the pyroclastic materials are the major source and reservoir of many of the chemical elements (e.g. F¯, As). Therefore, together with the renowned F¯ problem, the possible presence of geochemical anomalies in As, B, Mo, V, U, Al, Fe, and Mn have to be taken into consideration in water quality issues and future works has to investigate their possible health impact on the population of MER and other sectors of the east African rift. The stable δ18O, δD and radiogenic (87Sr/86Sr) isotopic composition of waters and representative volcanic rocks (Ignimbrite and basalt) were carried out during this study. Different ranges of isotopic values were recorded for different water groups: 10 hot spring samples show δ18O value with in the range of (-3.36‰ – 3.69‰) and δD (-0.95‰ – 24.23‰) (VSMOW), 12 groundwater wells δ18O (- 3.99‰ – 5.14‰) and δD (-19.69‰ – 32.27‰) in contrast to the 5 Lakes δ18O (3.98‰ – 7.92‰) and δD (26.19‰ – 45.71‰). The 2 deep geothermal wells and 1 of the 2 river samples are depleted in stable isotopic values. 87Sr/86Sr values range from 0.7045 to 0.7076 in the hot springs, and the two deep geothermal wells have 0.7043 and 0.7054 values. These signatures are typical of water interacted with mantle derived materials (with a minor crustal contamination), similar to the rocks widely covering the study area. The Sr isotope values of the basalt and ignimbrite samples are 0.7063 and 0.7071 respectively. Generally, the result shows that there exists a complex surface water and groundwater interactions that is reflected on a diversity of the stable and Sr isotopic signature in waters. The preliminary results of the study has showed that there is a need for future extended works on the geochemistry of solid samples (rocks, sediments and soils) as well as in waters that investigate all the spectrum of chemical elements that are potentially detrimental to human health and environment. Furthermore, from water resource point of view, the following works must focus on a comprehensive study of various isotopes and geochemical data to constrain groundwater age dating, water-rock interaction and flow path and thus help to model and systematize the hydrologic cycles in the basin.