Abstract

Geochemical investigation of the power potential of Songwe geothermal prospect in south-western Tanzania within the East African Rift Valley was carried out. The thermal geochemistry was studied by collecting and analysing samples from 14 hotsprings. The maximum discharge of thermal water occurs at the northern part of the prospect, at the bottom of a travertine cliff, 35 km above River Songwe level, at a rate of 26 kg/s (T = 80°C, pH = 6.7). The southern springs are discharging between 30 to 50 m above river level. Degassed thermal water (CO₂ and H₂S) discharges at the river level and cause an increase of about 0.2 to 2°C above the ambient river water temperature (25°C in November), with a maximum spring temperature of 81°C (pH = 6.9). The lowest is Bwana Hutu (3 to 5 m above river level) thermal area where springs are degassing carbon dioxide gas with maximum temperature of 81°C (pH = 7.6). All springs in Songwe are depositing travertine except one spring degassing at (46.7°C, pH = 6.8). The total discharge rate of thermal fluid is between 45 to 75 kg/s with a median temperature of 60.5°C, and an estimated heat transfer of 10 MW. The results showed that the waters are of sodium-bicarbonate type, formed by steam heating below water table at the periphery of a geothermal system. The low predicted subsurface temperature of maximum 152°C (SiO₂ no steam loss geothermometer), compared to the earlier prediction of 270°C (unknown geothermometer) is due to great dilution of the deep fluid by percolating rainwater and encroaching ground water (July). This was indicated by high magnesium concentration in the samples of over 8 mg/l compared to deep fluid magnesium level of less than 1 mg/l and also by the chloride-enthalpy plot in which most of the data plotted along the dilution line. The total usable power potential is estimated to be 107 MWe.

Key words: Geochemistry, geothermometer, Mbeya prospect, geothermal power potential.