Geothermal field studies using stable isotope hydrology : case studies in Uganda and Iceland

Abstract

The first part of this report is a review of isotopic studies that have been carried out in the geothermal areas of Uganda for reservoir evaluation. Some new isotopic data from the Kibiro areas are also included. The second part of the report is a review of isotopic monitoring in the Svartsengi geothermal field, Iceland. Both case studies demonstrate the potential of isotopes to evaluate groundwater systems in combination with conventional geochemical methods. Isotopic analysis indicates that Uganda's geothermal water at Kibiro and Buranga is meteoric water that probably circulates to depth through fault systems, being heated and rising again. The origin of hot spring water at Buranga is at a higher altitude than the cold water and the whole area seems to have a single hot water reservoir. Relatively shallow and fast infiltration along permeable faults and fissures is inferred in this area. Very good agreement is observed for the two data sets (from 1994 and 2000) available for the isotopic composition of the water within the Kibiro area. It is suggested that the thermal water at Kibiro and the cold groundwater have a common origin, possibly local precipitation. An oxygen shift of about 1‰ is observed for the thermal water. Isotopic composition of Lake Albert is different from that of the hot springs, ruling out the possibility of direct recharge from the lake despite its close proximity. The geothermal water in Katwe is thought to be local and mixing is suggested between dilute cold groundwater and saline lake or dilute lake water.