THE WAITOA LOW ENTHALPY GEOTHERMAL SYSTEM, NEW ZEALAND

Abstract

Waitoa is a small locality situated about 30 km from the city of Hamilton, New Zealand (Figure 1). It is in the heart of the Waikato dairy land which generally has low lying rolling landscapes, making it ideal for dairy farming. Geologically, Waitoa is located in the Hauraki Rift in the North Island of New Zealand. It is a broad rift valley bounded on the western side by Jurassic greywackes (forming the Hunua Range and its southern continuation, the Hapuakohe Range), and on the eastern side by the tertiary volcanic of rock of the Coromandel Range.
Thermal activity and manifestations are moderate in Waitoa. The spring temperatures rarely exceed 60 oC and the normal range is from 30-40 oC. Most of the springs in Waitoa area are associated with the active Kerepehi Fault and its associated structures. During a feasibility study of the area three springs and two boreholes were sampled. Data from previous studies in the area were also used to complement this study. The study has revealed that the thermal fluids have a common origin. Using the Cl-SO4-HCO3- ternary diagram, we were able to classify the fluids as peripheral waters. The Na-K-Mg diagram revealed that most of the waters are immature, therefore, not suitable for solute geothermometry. Nonetheless reservoir temperatures were estimated using the SiO2 vs Log K2/Mg and the Log K2/Mg vs K2/Ca plots, which indicated reservoir temperatures ranging from 80-100 oC. The temperatures pointed out a low enthalpy resource that could be developed for direct use applications. Temperature as high as 200 oC could be encountered, if a deep (>3 km) exploration well is drilled in the field; this should be sufficient to run a binary power plant.