Water–Rock Interaction and Lake Hydrochemistry in the Main Ethiopian Rift

Abstract

This study investigates the halogen budget of the Main Ethiopian Rift (MER) lithologies and water, and on the basis of new laboratory experiments gives insights on the water–rock interaction processes which ultimately mobilize fluoride in the environment. The halogen composition, and in particular, the chloride content of MER lakes is also taken into consideration to evaluate compositional variation occurred during the last 80 years that have to be mainly related evaporative effects. The evaporation trends are also investigated on the basis of new analyses of stable isotopes of oxygen and hydrogen that are compared with those available in the literature since the 1970s. In such complex scenario, although the average annual temperature increased ~1 °C in 30 years, we did not observe systematic trends valid for all the investigated lakes. The record defined for the last 30 years by δ18O–δD denote fluctuations of the climatic parameters with extreme evaporation preceding the year 2005, then declining to more “normal” conditions. The relation between the observed climatic parameters and the water isotopic composition suggests that the study lakes quickly respond to the environmental changes, possibly within one (or two) year(s). We therefore suggest to continue the data acquisition of climatic and hydrochemical parameters in order to implement the existing hydro-archive that could be useful to point out possible environmental changes.