The long term success of any geothermal energy utilization depends on understanding ground water movement and recharge areas. Water and steam condensate from sub-fields in the Olkaria geothermal field have been studied using the stable isotope ratios of hydrogen and oxygen to gain information on thermal fluid flow and trace the origin of thermal waters. Isotope fractionation factors and mass balance equations were used to characterize reservoir fluid composition of wells in the geothermal field assuming single stage steam separation. Comparison of the two methods yields coherent results for the oxygen-18 isotope ratio but the deuterium isotope ratio varies to some extent. The reservoir fluid isotope composition of the thermal waters suggests three recharge zones for the field, one from groundwater from the eastern Rift wall with äD about = -24‰, groundwater from the western Rift flank with äD about = -30‰ and the other from an evapo-concentrated source with about äD=36‰. Qualitative interpretation of the available data suggests that the evapo-concentrated source largely contributes to recharge of the East field, North East field and a part of the Domes field. Groundwater from the western rift wall recharges the West field of the area and the eastern Rift wall groundwater contributes to the southeast part of the Domes field. The isotopic composition of some thermal waters in the Olkaria geothermal field indicates that there could be mixing between the waters from the flanks and the evapo-concentrated source.

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