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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
- subsidence
- 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.
See Also Other Environmental
Aspects
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