For those who want to know…. WHY?
The Ngawha geothermal field is located in one of two inter-plate volcanic zones of New Zealand. It covers an area of between 25 to 50 km2 and is centered on the town of Ngawha Springs. This area is the only high temperature geothermal field in New Zealand outside of the Taupo Volcanic Zone.
The major rock units below Ngawha are present as layers. The surface that we can see is essentially a basin with overlying ridges of the lava. The springs and gases emerge through the old lake sediments and around the edges of the lava flows. The sandy lake sediments are full of cracks and the fluid flows through these.
Below this shallow layer is a thick layer (about 500m) of sedimentary rocks of a very mixed and confused character, in a matrix of clays and mudstone. This layer to a large extent prevents the flow of water and gas through it from below, except in small quantities, and it is thought that these flows occur in fault zones, where there is much cracking and perhaps movement to keep the cracks open. While this “caprock” limits the quantity of deep water that can flow upwards through it, it also contains within it groundwater which mixes with deeper fluid.
Beneath the “caprock” is a great thickness of the rock known as greywacke. The base of the greywacke has not been intercepted by drillholes (one of which was drilled for 3,300 m). It is thought that within this area there is a productive reservoir of the permeable volume which is more than 1,000m thick. The water comes into the reservoir from the northeast, get heated and acquire dissolved components somewhere below the Ngawha area. The greywacke is faulted over large distances, and this is thought to allow water circulate quite slowly in the cracks and faults in the greywacke, and drain away to the southeast, at some depth (perhaps 1000m or more). Underneath the greywacke there is a heat source, a hot magma body, which provides the heat for the geothermal system.
Because the water passing through different faults and roots acquires different chemical composition, origins of water in each spring can be defined. Tests revealed that gas and water in some springs gets through the relatively impermeable caprock because in these springs waters and gases, diluted and modified to some extent, are similar to those from the wells which take water from below the caprock.
So there are three types of water in the Ngawha pools:
- that from within the greywacke, which is characterised by being dominated by borate and chloride as dissolved components;
- that from the caprock, which has more bicarbonate and less of the borate and chloride;
- that from the surface, including rainwater, or near surface, which has more sulphate in it than the others and contains little dissolved matter.
Deep water is saturated with the gas carbon dioxide, and as the water rises in the crust, the pressure lessens and the gas comes out of solution, so a liquid and a gas phase are both present. In Ngawha pools gases reach the surface in unusually large quantities. The gas is mostly carbon dioxide, but can contain a few percent of hydrogen sulphide, and traces of other gases. At the surface, the gases affect ponded waters by depositing sulphur causing the suspended white matter and sulphur muds, and forming sulphate when in contact with air.
Given its water chemistry, and its geology, the Ngawha geothermal field is quite different to other geothermal fields in New Zealand.