| Abstract |
The study focuses mainly on the Geochemical monitoring of the Langano, Shalla and Wondo Genet areas that are all located in the Lakes District areas, Ethiopian Rift Valley. The main objective of the work is to monitor the changes of concentration of chemical constituents of hot springs present in the Lakes District area, through time, and to assess the stability condition of the geothermal system in the Lakes District area. Evaluation of change of concentrations of elements (various cations and anions including silica) through time have demonstrated the state of stable condition of the thermal system in the Lakes District area (Langano and Shalla) for the last thirty years. A plot of Cl-HCO3-SO4 has indicated that water samples collected from Langano, Shalla and Wondo Genet hot springs are all characterized by near neutral bicarbonate waters. However, water samples from the Bole spring belong to the water type of near neutral chloride sulfate water. Evaluation of water-rock equilibria based on the Na-K-Mg content of thermal fluid indicates that the waters from Langano, Shalla and Belle spring of Wondo-Genet area get plotted in the partial-equilibrium regions along the dilution line indicating that the waters are the result of mixing of the meteoric and/or shallow ground water and deep reservoir fluid that is in equilibrium with the reservoir rock at a Na-K temperature range of 180-220 ?C. The water from Giorgis spring of Wondo Genet area falls in the region of immature water, indicating that this water can not be used for cation geothermometers. Evaluation of solution-mineral equilibria based on saturation indices has demonstrated that most of the thermal waters from Langano area are over-saturated with respect to calcite, quartz and fluorite within the various range of temperatures. With the exception of thermal water of Bole spring, all waters from the lakes District area are under-saturated with respect to anhydrite. In Shalla area, there is a tendency of precipitation of travertine associated with hot springs. The super-saturation condition of Calcite in Wondo Genet hot springs is also confirmed by the occurrence of significant hydrothermal deposits of travertine associated with the hot springs. Application of various cation empirical geothermometers to the waters of the Langano thermal system and Belle spring of Wondo Genet area has documented a sub-surface temperature ranging from 128-221?C. The Na-K-Mg diagram is also supports this evidence where the Langano waters are plotted in the partially-equilibrated region along the dilution line as a result of mixing of the shallow groundwater and the deep reservoir fluid which is in equilibrium with the reservoir rock at a temperature range of 160-200 ?C. This range of temperature is also supported by the equilibrium temperature (140-200?C) of Na-K minerals (Albite & Adularia) with respect to the deep fluid of the Langano, thermal system. The range of estimated sub-surface temperatures of Langano thermal system using the silica geothermometers (139-193?C) is in close agreement with the range of equilibration temperatures of quartz with respect to the deep thermal water of the Langano area (100-200?C). Na-K geothermometers failed to work for the Shalla hot springs due to mixing of the Na rich Lake Shalla water with the thermal water of the hot springs discharged at the eastern and southwestern shore of the lake. |