Application of stable isotope geochemistry to tracing recharge and flow systems of fluids in the Olkaria geothermal field, Kenya

Abstract

The long term success of any geothermal energy utilization depends on understanding groundwater movements and recharge areas. Water and steam condensates from subfields 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 to trace the origin of thermal waters. Isotope fractionation factors and mass balance equations were used to characterize reservoir fluid composition of wells in the area, 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 δD about = -24‰; groundwater from the western Rift flank with δD about = -30‰; and the other from an evapo-concentrated source with δD about = 36‰.

Qualitative interpretation of the available data suggests that the evapo-concentrated source largely contributes to the 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. 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 groundwater mixing between the waters from the flanks and the evapo-concentrated source. The chloride content of the recharge water cannot be used to determine the degree of mixing between these recharge zones because of the similar content of chloride in both types of water. Evolution of the water isotope composition of the well fluids over time indicates possible temporal variations related to the recharge zones. More data from boreholes and springs in the area must be collected to monitor the variations and evolution of water isotope composition in the field.