HYDROGEOCHEMISTRY OF SPRINGS AT AMBO! CENTRAL ETHIOPIA

Abstract

The Ambo hydrothermal springs are low temperature hydrothermal systems located in
central Ethiopia. The area is characterized by C02 dominated thermal activity with temperature that
varies between 30 and 40"C. The chemical composition of 17 water samples over an area that includes
lakes, boreholes, cold and thermal springs were compared. The hydrogeochemical investigation
indicates that the most plausible mechanism for the formation of travertine deposit is the release of CO:>
due to pressure relief when the thermal water emerges at the surface. The sources of C02 are the
degassing of a magma chamber from deeper sources and decarbonation of the Mesozoic sediments.
The shallow acidic magma chamber of Dandi and Wanchi could act as a source of temperature and
additional C02 input. Silica sinters associated with travertine imply the thermal origin of the deposits as
well as initial high temperature of the formation. From the positive Saturation Indexes (51) with. respect
to carbonate and silica and from the general geology of the region it is expected that the parent
bedrocks could be limestones. and/ or calcareous sandstones that has controlled the hydrochemiStry of
deep regional groundwater. The hydrolysis of silicates and the chemistry of the outcropping rocks play
additiomil role in controlling the hydrochemical evolution of shallow groundwater