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

The geochemical and isotopic fingerprint of natural water is strictly related to the nature of the outcropping lithologies and to the extent of their weathering, which is in turn related to the climatic conditions. This study investigates the halogens 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 seventies. In such complex scenario, although the average annual temperature increased 1 °C in 30 years, we didn’t observe systematic trends valid for all the investigated lakes. The only two lakes that show a coherent increase of salinization (coupled with a decrease in the surface extension) are Lakes Abijata and Lake Chamo. Other lakes show nearly constant areal extension and steady hydrochemistry, with second order fluctuations that are difficult to be decoded in terms of climatic changes. This incongruence probably suggests that the physiographic data reported in the literature are scarcely constrained and have to be taken with caution. The record defined for the last 30 years by delta18O-deltaD 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 unravel possible environmental changes.