THE ORIGIN OF HIGH BICARBONATE AND FLUORIDE CONCENTRATIONS IN WATERS OF THE MAIN ETHIOPIAN RIFT-VALLEY, EAST-AFRICAN RIFT SYSTEM

Thermal waters in the Main Ethiopian Rift Valley are characterized by high Na, bicarbonate and fluoride concentrations, and near-neutral to alkaline pH. Sodium, bicarbonate and fluoride are positively correlate d in the waters. The principal reason for the bicarbonate in the area is the high rate of carbon dioxide outgassing. This, combined with aci d volcanics, geothermal heating, low Ca and low salinity, is also one of the causes of high fluoride in this part of the active volcanic zon e of the East African Rift. Evaporative concentration is responsible f or the high salinity, alkalinity and fluoride in the closed-basin lake s of the region. The waters are undersaturated with respect to fluorit e and anhydrite. Calcium tends to be fixed in Ca bearing minerals such as calcite and epidote, which are abundant in the system. Hence, it a ppears that fluoride is a mobile component in add volcanic geothermal systems. Copyright (C) 1996 Elsevier Science Ltd