Hydrology and reservoir characteristics of three geothermal systems in western Uganda

Several sampling surveys have been carried out in three geothermal areas in western Uganda, known as Katwe-Kikorongo (Katwe), Buranga, and Kibiro. Sixty-three water samples from hot and cold springs, dug wells, rivers, and lakes, and 14 rock samples from surface outcrops have been collected and analyzed. They were then analyzed for chemistry and isotopes of hydrogen (δHH2O2, HH2O3), oxygen (δOH2O18, OSO418), carbon (δ13CDIC, 14CDIC), sulfur (δSSO434), and strontium (S87/86rH2O, 87/86SrRock). The results suggest a meteoric origin for the geothermal waters, with little secondary alteration. Based on isotope data, Katwe and Buranga are recharged from the Rwenzori Mountains, and Kibiro from high ground represented by the Mukihani-Waisembe Ridge in Kitoba Sub-county, 20 km to the southeast. Oxygen isotope geothermometry, based on aqueous sulfate and water equilibrium fractionation, indicates a subsurface temperature of 200 °C for Buranga, which is higher than that inferred from chemical geothermometry (160-170 °C), and lower temperatures (140-150 °C) for Katwe and Kibiro that are similar to the results of chemical geothermometry. Tritium concentrations indicate some involvement of modern cold water close to the surface at Kibiro but not at Buranga and Katwe, where hot springs discharge tritium-free waters. Sulfur isotope ratios (δSSO434) of hot water suggest magmatic contributions of sulfate in all three areas, confirming the results of earlier geochemical investigations. Strontium isotope ratios in water and rock samples (S87/86rH2O, 87/86SrRock) allow a preliminary identification of rock types that may have interacted with the thermal waters.