2- 17% ENTHALPY CHANGE OVER 51 YEARS FOR HOT SPRINGS ALONG THE ALBERTINE-RHINO GRABEN, UGANDA

Geochemical investigations of hot spring major ion, stable isotope, and silica (SiO₂) concentrations are used to understand rock-water interaction, water source, reservoir temperatures, and to calculate the enthalpy of geothermal fluids. This study reports new geochemistry data for 26 hot springs along the 600 km Albertine-Rhino Graben (ARG) and compares with previous geochemical data collected and published in reports. The ARG is a rift segment located along the magma-poor Western Branch of the East African Rift System with three main prospective geothermal fields. However, possible changes in the enthalpies in these geothermal fields over time have not been investigated. Our objectives are to (1) estimate the reservoir temperature and enthalpy and (2) quantify the potential temporal amount of enthalpy loss of geothermal reservoirs associated with hot springs along the ARG over periods between 17 and 51 years. The hot spring geochemistry along the ARG was analyzed and classified as Na-SO₄, Na-HCO₃, Ca-HCO₃, and Na-Cl water types controlled by the mineralogy of the host rock. The δ¹⁸O and δ²H relationship of the hot springs suggests a meteoric water source that flows through faults, gets heated, and ascends to the earth’s surface. The conductive quartz geothermometer indicates reservoir temperatures that range from 53 ⁰C to 140 ⁰C and enthalpies using the silica geothermometry that ranges from 221 kJ/kg to 589 kJ/kg. Temporal investigations of the geochemistry of Kibiro, Nyasimbe, Rwagimba, Muhokya, and Kibenge hot springs show decreases in the SiO₂ concentration and enthalpy. The Kibiro hot spring found in the Kibiro main geothermal fields shows a 17% enthalpy decrease over a 51-year interval, while Nyasimbe hot spring in the Buranga main geothermal field, shows a 3.4% enthalpy decrease over a 17-year interval. Over a period of 17 years, the enthalpy loss for Rwagimba, Kibenge, and Muhokya geothermal were calculated to be 0.68%, 1.19%, and 2.89% respectively. We suggest that the enthalpy loss is due to SiO₂ precipitation that leads to an enthalpy decrease along fault walls as the geothermal fluids ascends to the Earth’s surface rather than a decline in the energy content of the geothermal reservoir. We conclude that the use of silica geothermometry should be studied over long periods of time to fully understand the enthalpy state of geothermal reservoirs.