Geochemistry and water quality assessment of central Main Ethiopian Rift natural waters with emphasis on source and occurrence of fluoride and arsenic

摘要

Drinking water supply for the Main Ethiopian Rift (MER) area principally relies on groundwater wells and springs and is characterized by natural source of elevated fluoride concentration. New analyses reveal that the F~- geochemical anomaly is associated with other potentially toxic elements such as As, U, Mo and B. Particularly, 35% of the 23 investigated groundwater wells and 70% of the 14 hot springs (and geo-thermal wells) show arsenic concentrations above the recommended limit of 10μg/L (WHO, 2006). Arsenic in groundwater wells has a positive correlation with Na~+ (R~2 = 0.63) and alkalinity (HCO_3~-;R~2 = 0.70) as well as with trace elements such as U (R~2 = 0.70), Mo (R~2 = 0.79) and V (R~2 - 0.68). PHREEQC speciation modelling indicates that Fe oxides and hydroxides are stable in water systems, suggesting their role as potential adsorbents that could influence the mobility of arsenic.rnChemical analyses of leachates from MER rhyolitic rocks and their weathered and re-worked fluvio-lacustrine sediments were performed to evaluate their contribution as a source of the mentioned geochemical anomalies. These leachates were obtained from a 1-year leaching experiment on powdered rocks and sediments mixed with distilled water (10 g:50 ml). They contain as much as 220 μg/L of As, 7.6 mg/L of F~-, 181 μg/L of Mo, 64 μg/L of U and 254 ug/L of V suggesting that the local sediments represent the main source and reservoir of toxic elements.rnThese elements, originally present in the glassy portion of the MER rhyolitic rocks were progressively concentrated in weathered and re-deposited products.rnTherefore, together with the renowned F~- problem, the possible presence of further geochemical anomalies have to be considered in water quality issues and future work has to investigate their possible health impact on the population of MER and other sectors of the East African Rift.