The environmental impact of any geothermal development will depend on the particular characteristics of the field, especially its size, and the way it is developed. Adverse environmental impacts of geothermal development may include:
- changes in surface geothermal activity, including hydrothermal eruptions
- increased microseismic activity
- effects on ecosystems, flora and fauna.
Unconstrained development and human interference with natural geothermal systems has resulted in damage to surface features. Of five major New Zealand geyser fields in existence a century ago (Rotomahana, Whakarewarewa, Orakeikorako, Wairakei, and Tauhara) only Whakarewarewa still has a significant number of active geysers. The Orakei Korako field was largely drowned when the Waikato River was raised for hydro-electricity. Development of the Wairakei field for geothermal energy resulted in a decline of ground water levels at both the Wairakei and nearby Tauhara fields and the loss of geysers and alkaline springs. Some land at Wairakei has subsided by 15 m. The Rotomahana field was destroyed by the 1886 Tarawera eruption. Of more than 200 geysers active in the central North Island in the 1950s, only about 40 remain.
Because of marked decline of geyser activity in the Rotorua field, a ban was placed on geothermal aquifer extraction within 1.5 km of the main geothermal area and restrictions were placed on other users to encourage more efficient use. The previously unsustainable draw down by a large number of small users is now mitigated under the Rotorua Geothermal Management Plan. The plan is designed to limit fluid extraction from the field, ensure fluid taken from existing bores is re-injected and end inefficient resource use.
Modern geothermal field management practise is to develop the reservoir in stages to avoid large-scale and irreversible effects on surface features and resource sustainability. While many risks are therefore manageable, the potential for hydrothermal eruptions and subsidence may prevent future development in sensitive areas such as Wairakei-Tauhara and Rotorua.
While geothermal subsidence may sound dramatic, it generally is not. Precise levelling surveys of benchmarks are normally required to detect its existence with subsidence rates being measured in mm/year. Occasionally there are local “bowls” of subsidence for which tensional cracks may be evident at the surface. Geothermal subsidence occurs due to depressurisation of a compressible body of rock. However, in a geothermal area there may be a range of causes of subsidence, many of which are unrelated to geothermal development. Non-geothermal causes include effects of heavy transport or uncompacted fill, decay of buried vegetation, and collapse of tomos. There can be regional subsidence due to tectonic deformation, again unrelated to geothermal development.
A localised subsidence bowl at Wairakei has affected drains in the steamfield and caused cracking in the nearby road. Claims of subsidence-induced damage to several houses at Tauhara have now been refuted by independent assessments. Subsidence at Ohaaki is threatening to eventually inundate the Marae and other sacred sites under Lake Ohakuri unless action is taken, and potential solutions have been investigated.
However, these adverse factors need to be balanced against the advantages of geothermal energy over fossil fuels. It is also useful to consider that geothermal systems are subject to natural change, and that there is a continuum between natural and human influences on thermal features e.g. the “Craters of the Moon” thermal area at Wairakei is a natural feature where thermal activity has dramatically increased as a result of pressure drawdown of the geothermal system by the generating plant.