Stable Isotopic Composition of Geothermal Fields in Kenya; the Relationship Between Geothermal Field and Kenya Rift Lakes Waters

Abstract

The results of the review of the stable isotopic composition relationship between the geothermal fields and the Kenya rift lake waters are presented. The Kenya rift Lakes Include; Lake Baringo, Bogoria, Nakuru, Elementaita and Naivasha respectively. Geothermal fields considered for this study are Menengai and Olkaria. Menengai is a major Quaternary central volcano dominated mainly by trachytic rocks. It is located within the axis of the central segment and is host to one of the high temperature geothermal fields in the Kenya Rift while Olkaria is a rhyolitic volcanic complex located immediately to the south of Lake Naivasha in the Central Kenya Rift Valley Africa. It is geothermally active and is being used to generate growing quantities of clean electrical power. Data from various workers has been used to try and discern the differences between the waters with a view to understanding the origin and isotopic relationships between them. The results indicate that most of the Lake waters are varied in composition with some plotting on the Kenya Rift Meteoric water line (KRMWL) and some plotting slightly of this line on either side. Lake Baringo appears to be to be quite enriched in ∂18O and ∂2H isotopes which could be attributed to increased evaporation especially during the dry seasons, though some thermal input cannot be ignored. It may also be inferred that this waters have travelled long distances probable from the highlands to be discharged into the lake. One sample from Lake Nakuru is also very similar to one from Lake Baringo. The Olkaria geothermal field has been divided into four sectors which also show some varied compositions; Olkaria domes waters appear to be more enriched in both the ∂18O and ∂2H isotopes than the other fields this is attributed to high temperatures that may result in boiling or increased water interaction that results in rocks losing their isotopes to the geothermal wells. Olkaria East is more depleted in both isotopes and has fairly similar composition to Lake Naivasha Waters. It was noted however is that Menengai geothermal field well waters are very different from the surrounding lakes i.e. Lake Nakuru and Elementaita. They appear to be depleted with respect to both ∂18O and ∂2H. This is interesting and can be inferred to mean that either Menengai geothermal field is recharged mainly be rain water or by waters that have very little residence time.