Recent Isotopic Measurements at Ohaaki

Abstract

Helium isotope measurements have recently been completed on a series of samples collected from a cross-section of wells in the production area of the Ohaaki geothermal field. The results obtained show relatively small changes in 3He/4He from measurements on the same wells in the late 1970's and mid-1980's. In particular the correlation with changes in B/Cl ratios, predicted by Hulston (1995), have not been in evidence. A correlation has however been observed between the 3He/4He and C0,/4He ratios on the repeat measurements of some wells. Some changes to the hydrogen and carbon isotopes of the methane with time may be evident, but are yet to be confmned. Although the information from this study is of value to the building up of our knowledge of geothermal systems, it is not yet able to provide an estimation of the recharge of geothermal systems and the sustainability of these systems.