The Tuia Mātauranga Pōkai Whenua GeoTour follows the footsteps of early explorers of Aotearoa New Zealand taking you to places where leaders of the past searched for food, resources and ways to adapt and survive in this new land.
Use the Pōkai Whenua GeoTour as your classroom to explore the stories of the past, in the present, to preserve what is unique in Aotearoa New Zealand for the future.
Collect the codewords to get the Geocoin puzzle piece.
To be able to complete this GeoTour and receive your special Geocoin collectable, remember to take a note of the codeword placed in the cache. This will need to be recorded in your passport which can be downloaded here.
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"63 of the 150 Pōkai Whenua GeoTour caches will contain a randomly placed special FTF token (a replica of the Tuia Mātauranga GeoTour commemorative coin). This is yours to keep! If you find more than one, you might consider leaving it for the next person who finds the cache."
THE IMPACTS OF DEVELOMENT: Historically, there were alkaline hot springs and bathing pools at Ōhaaki. Some of these springs and a sacred cave were flooded when the Waikato River level was raised to fill nearby Lake Ōhakuri for the hydro-electric power scheme in 1961. This development resulted in the displacement of the Ngāti Tahu people. They were forced to leave the original pā at Ōrakei Korako, about 20km west of Ōhaaki when the hydro-power dam was constructed. Ngāti Tahu chose Ōhaaki, on the banks of the Waikato River, to rebuild their marae.
The Ōhaaki Ngawha (boiling pool) is the dominant remaining natural feature of the field. Before the area was developed, the large Ōhaaki Ngawha with its clear, pale, turquoise-blue water and extensive white sinter terrace was described as "the most handsome pool in the whole thermal area".
When the power station commenced, the extraction of geothermal liquid caused water levels in the Ōhaaki Ngawha to drop. This resulted in a partial collapse of the delicate sinter edge and the white silica formations weathered to a dull dirty grey. The sinter terrace has cracking and plants growing through it. The Ngawha is now fed by geothermal bore water, which contains chemicals added to prevent silica depositing in the bore pipes.
Most of the other flowing surface features at Ōhaaki have dried up because of the extraction of geothermal fluid. Before development Ōhaaki had several mud pools heated by steam, hot pools isolated from ground water by a layer of mineralised earth, also heated by steam, and hot springs producing chloride water and depositing sinter. Very few of these features now remain as anything other than dry hot ground. Most of the springs and pools have dried up and the mud pools have cooled.
Subsidence caused by geothermal development has been a major concern for locals. A nearby road all but disappeared into the Waikato River, and the Ōhaaki Marae faced a similar fate. According to estimates, by 2013, land around the marae had sunk by 3.7 metres. Once again, Ngāti Tahu felt aggrieved by the loss of lands and geothermal resources used for the national good.
Today, Contact Energy works in partnership with Ngāti Tahu and Ōhaaki Marae to address the impact of geothermal generation activities on the marae and surrounding whenua. When the power station was opened in 1988, it produced 108 megawatts of electricity. Because of cooling of the resource as a result of development, it now produces only 45 megawatts.
POWER GENERATION: An integral part of the Wairakei Power Scheme, Ōhaaki is notable for its 105 metre (m) high cooling tower and the re-injection of water into the steam field. Developed in harmony with Māori tribal values, the Ōhaaki power station is an integral part of the Wairakei system. Ōhaaki uses tower cooling instead of river water and conserves the borefield by condensate reinjection. Located on the Broadlands geothermal field, about 25 kilometres (km) north of Wairakei, and opened in 1989, Ōhaaki added 108 megawatts (MW) generating capacity to the national grid and contributes annually about 750 gigawatt hours to the country’s energy resources – approximately three per cent of total requirements.
The geothermal fluid is fed to five separation plants where the steam and water are separated. To avoid any contamination of the Waikato River, this hot separated geothermal water is reinjected back into the ground at 150 degrees Celsius. This also prevents silica build-up in the pipelines and plant. The steam is piped to the power station and passes through the turbines to generate electricity. The steam then passes to the condenser where circulating water is sprayed through the steam to cool it.
The resultant warm water is then passed through a natural draught cooling tower, a concrete structure which is a first for New Zealand. Incorporating state-of-the-art design, its thin shell and special foundations are designed to resist wind and earthquakes. It is 70 m diameter at the base, and 40 m at the throat. The tower will remove 420 MW of heat from the 20,000 tonnes per hour of circulating water as it falls through an induced flow of air. The evaporation of a small fraction of water provides the necessary cooling of the circulating water. In this way the circulating water system continuously gains condensed steam, which is balanced by evaporating losses from the cooling tower and by discharge of condensate which is reinjected into the ground.
Reinjection has not been carried out previously in New Zealand on such a large commercial scale. The non-condensable gases, mainly carbon dioxide, are extracted from the condenser by gas exhausters and are dispersed from the cooling tower. A system continuously monitors the concentration of hydrogen sulphide in the area surrounding the plant to ensure that there is no build-up of gas.
Some of the hot water separated from the geothermal fluid is used in nearby wood-drying kilns.
THE CACHE: The cache container is 100% recycled. It is large enough for small swaps and trinkets. Please bring your own pen/cil. Take care when accessing the cache. It is hidden from view by vegetation. Please avoid stomping around and unneccessarily trampling the plants.