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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
Practical methods of minimizing or mitigating
environmental effects from integrated geothermal
developments; recent examples from New Zealand
Chris Bromley
Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Taupo
Email: c.bromley@gns.cri.nz
Abstract
Monitoring of the environmental effects of geothermal resource utilisation in New
Zealand has confirmed the benefits of appropriate management in terms of production
and reinjection strategies. Such strategies can minimise, reverse or mitigate the effects on
surface thermal activity. This applies to direct use of low enthalpy resources as well as
integrated use of high enthalpy resources. At Rotokawa, a strategy of deep production
and total shallow reinjection for an integrated steam turbine/binary power plant has
resulted in a gradual enhancement of several chloride springs, with no significant
detrimental effects. At Wairakei, less than 50% of the waste hot water is reinjected, but
several users are able to take advantage of the separated hot water in a way that mitigates
for the historic loss of geysers at Wairakei Valley. These include tourist facilities based
on a geothermally-heated prawn farm, and hot stream restoration with an artificial
geyser/silica terrace that was developed by local Maori. At Mokai, several years of
production history from a binary/steam turbine, with shallow reinjection of brine and
steam condensate, has not caused any significant environment effects on surface thermal
features. At Rotorua, management of extraction and reinjection from numerous domestic
bores has achieved a significant recovery in hot spring and geyser activity. Users of
many other hot spring areas in New Zealand are also managed by application of
regulatory control through policies and plans under the Resource Management Act.
These plans are presently undergoing a process of industry-wide review and
improvement, by addressing changes in the philosophy of environmental management.
Keywords: New Zealand, environmental, Rotokawa, Wairakei, Rotorua, Mokai.
1 Introduction
New Zealand, like Iceland, is a country that has pioneered the sustainable use of its
indigenous geothermal resources, reducing the need to burn hydrocarbons, and
thereby reducing CO2 emissions. With declining natural gas reserves, N.Z. energy
planners are increasingly looking to fill the gap in future energy supplies by increased
geothermal utilisation, as a renewable energy source, rather than using coal. A key
factor in achieving this goal is the management of environmental effects, through
appropriate regulation. More practical methods of minimizing or mitigating such
effects are needed, along with more integrated or “cascaded” uses, and examples of
more-efficient and economic direct geothermal energy use, to encourage greater
uptake of geothermal technology. Examples of such methods from recent geothermal
developments in New Zealand (summarised in Thain and Dunstall, 2000) are given in
this paper, together with a discussion of appropriate and practical geothermal system
management policies.
2 Rotokawa
An integrated steam turbine and binary power plant at Rotokawa, with an installed
capacity of about 25 MWe has been operating successfully since July 1997, utilizing 2
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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
production and 3 reinjection wells. Confidence in the resource performance led to an
increase in December 2002 of 5 MWe, and plans for a second stage (nominally 30
MWe) are well advanced. A strategy of deep production (1500-2500 m) and shallow
injection (300-600 m) was adopted for the first stage based on limited vertical
connection between these aquifers. Over 5 years, 20 Mtonnes has been produced. Full
reinjection is practised, including steam condensate, but excluding non-condensable
gases. Gravity and pressure monitoring has shown that the injection aquifer
(originally 2-phase) has been re-saturating within a few hundred meters of the
injection wells, and pressures have risen by a few bars. Production wells have shown
no significant changes in enthalpy or output, although RK9 was shut down after
problems in 2002-3 associated with casing damage. Deep pressures have declined by
at least 12 bars.
Environmental monitoring, established under conditions associated with the
original resource consent, has included gases (H2S, Hg), groundwater, and surface
thermal features. Over the 5 years, there have been no significant changes in gas
emissions or ground water levels. Groundwater chemical monitoring has shown a
gradual rise in chloride concentration of up to 5% per year. Average chloride flux,
through surface discharges into the Parariki Stream, has also increased by about 8%
per year, but remains within the wide range of natural fluctuations (+/-50%) caused by
rainfall on Lake Rotokawa, the source of the stream. In December 2001, a new high-
chloride discharging spring (“Ed’s spring”) appeared from an area of near-boiling hot
pools about 300m southeast of the power station. This feature now has occasional
periods of vigorous boiling and eruption, discharges about 2 l/s, deposits sinter, and is
evolving an associated thermophilic ecosystem. Although its chemistry is distinctly
different from that of the reinjected fluid, precluding the possibility of a direct fluid
connection, the small pressure rise that stimulated its activity is probably related to
increased pressures in the underlying injection aquifer. It is therefore considered an
indirect effect of development, and an enhancement to the thermal feature
environment at Rotokawa.
3 Mokai
At Mokai, a nominal 57 MWe integrated power plant (steam turbine and binary),
began commercial operation in February 2000. Full reinjection (excluding gases) is
also practised here, with production from 4 deep wells and injection into 2 shallow
wells. Changes in the reservoir pressures have generally been as expected, and there
has been no indication of premature reinjection returns or unexpected chemical
changes. Improvements in direct use include a large glasshouse-heating project
currently under construction.
A comprehensive environmental monitoring programme covering springs,
streams, and groundwater, has shown no significant post-production changes to water
chemistry due to abstraction or injection of fluids at Mokai. Temperatures and water
levels in groundwater monitor bores have also shown no changes that could be
attributed to reservoir pressure drawdown or reinjection returns. Monitored
ecosystems, consisting of rare thermal ferns and aquatic invertebrates associated with
hot spring discharges, have not been affected. A small increase in thermal activity was
observed in March 2000 associated with a line of existing thermal craters near the
reinjection area. These craters contain steam-heated mud pools. The increase in steam
activity was local, and did not directly include reinjected chloride fluid, but may have
been related to a local pressure increase in the underlying aquifer. A nearby hot spring
used for bathing was not affected. Within a year, the expanded area of steam-heated
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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
ground was populated by thermally tolerant plants such as club mosses, leading to an
overall enhancement of the local thermal ecosystem. The only adverse effect was the
cost of re-fencing the thermal area to keep out stock.
4 Wairakei
Wairakei Power Station has been producing about 160 MWe for 45 years. Contact
Energy is presently applying for renewal of Resource Consents to maintain full
production for a further 25 years. In recent years, changes have included the purchase
by Contact of the nearby 55 MWe Poihipi Road Power Station. This generates, from
steam wells, a load-following output averaging 24 MWe (limited by Consents since
1997, to avoid interference). Historically, all the separated liquid at Wairakei was
discharged into the Waikato River, but since 1997, 30-50% (about 13MT/yr) has been
reinjected. This has caused a small (2 bar) pressure rise in the production area, and
future plans are to reinject more fluid outside the resistivity boundary of the field to
avoid premature cooling. A 15 MWe binary plant proposal to extract more energy
from 130oC reinjection fluids is currently undergoing detailed commercial
consideration. Small quantities of steam are provided to Wairakei businesses for
direct heating purposes. These include the Geotherm Exports orchid glasshouse at
Poihipi, the Wairakei Resort Hotel (7.45 kT/yr), and Century Resources /IGNS
offices. Increased direct use of waste hot water for tourist facilities has also been
achieved at the nearby Prawn Farm (0.71 MT/yr) and the Wairakei Terraces (1.46
MT/yr), where new artificial silica terraces, a geyser, and alum pools have been
constructed. The adjacent Te Kiri O Hinekai thermal stream, with its historic
“Honeymoon Pool”, has been re-established by diverting hot water from the main
Wairakei drain. In conjunction with a Maori ‘living village’ and animal park, this is
now a popular tourist facility. “Craters of the Moon” (Karapiti) is another very
popular Wairakei Tourist Park facility, freely accessible to the public and maintained
by the Department of Conservation. This steam-heated thermal area expanded
dramatically during the early days of Wairakei pressure drawdown, when boiling
created more upwardly-mobile steam. The heat output increased 10 fold, from 40 MW
in 1952, and then settled to a relatively stable 200 MW. Ongoing intermittent
hydrothermal eruptions (about 1/yr) are an exciting reminder of the natural transience
of these steam-heated features. All these environmental and amenity benefits are
considered to partially mitigate for historic adverse effects, such as the loss of geysers
at Wairakei Geyser Valley and Spa Park (Taupo), when reservoir pressures initially
declined in the 1960s. Other environmental effects at Wairakei have included gradual
subsidence (broad bowls up to 15m deep beneath the Wairakei Stream, and 3m at Spa
Hotel, Taupo), and local drainage of groundwater aquifers in the Eastern Borefield
(1980s) and Alum Lakes area (since 1997). These effects have been due to a steady
decline (by over 60%) in the shallow steam zone pressures, which has caused
drainage of some overlying compressible mudstones, and induced down-flows of
groundwater through local fractures. The main consequences have included remedial
adjustments to fixed structures such as pipelines, drains and transmission lines to
accommodate strain accumulation, and some cooling and dilution of deep production
fluids by down-flowing acidic groundwater.
5 Rotorua
Records of thermal feature changes at Rotorua go back more than 150 years. They
have demonstrated a high degree of natural variability in geyser and hot spring
discharges (Scott and Cody, 2000). Exploitation of the thermal aquifer beneath the
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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
city started in the 1920s but greatly expanded between 1967 and 1986. Natural surface
activity declined noticeably from the 1970s, and despite the previous evidence for
natural variability, this decline was attributed to pressure drawdown from excessive
fluid extraction. To counteract this, the government implemented in 1987 a control
program that included closures of many wells (within 1.5 km of the centre of
Whakarewarewa thermal area), and punitive royalty charges with provisions to
encourage reinjection. These measures have been very successful in reversing the
decline. Aquifer water levels have risen by 2-3 m, and many thermal features have
been rejuvenated. Spring discharge flows have increased, and geysers have resumed
stronger or longer duration eruptions. The pressure rise has also stimulated some
recent hydrothermal eruptions in Kuirau Park, including a dormant vent that had
previously been buried and built upon.
6 Regulatory control through geothermal plans
The environmental management of geothermal resources in New Zealand is
administered by Regional Councils under the Resource Management Act. The
Councils have formulated geothermal policies and plans, and, in the case of Waikato
Region, these are presently under review. The definition and use of terms in these
documents can be a source of debate and confusion. Examples, in connection with
thermal features, are: “significant”, “sinter deposition”, “protection/preservation”,
“natural/artificial”, “interference”, and “reversible /recoverable”. In connection with
resource use, issues such as “renewable/sustainable utilisation” and
“adverse/beneficial effects” also cause concern. The following comments on these
issues are intended to provide useful and practical guidance for managing such
environmental concerns.
6.1 Significant or sinter depositing features
It is usually accepted that there will be some risk of losses of individual features in
systems identified for development. The purpose of ranking surface geothermal
features in a region is to identify, for protection, geothermal systems exhibiting
“outstanding” features that could be seriously affected by resource utilisation, and to
ensure that a representative range of features is protected. However, it is inappropriate
to apply the term “significant” to all identified natural geothermal features. Some
thermal areas are many square kilometres in size, containing dispersed weak steam
vents and large portions of non-thermal ground. Application of rules to such features
could place undue constraints on the owners of these properties.
The term “sinter depositing” can also be used inappropriately with regard to a
means of classifying or ranking thermal features. It apparently provides a means of
visually identifying springs that could be susceptible to deep reservoir pressure
drawdown associated with fluid extraction. Highly mineralised hot springs and
geysers, feeding from deep reservoir fluids, often do deposit large quantities of silica
sinter. However, the term “sinter” covers a wide range of deposits that form in springs
(e.g. amorphous silica, travertine, calcite) and these are not all diagnostic of a direct
plumbing connection between the spring and a high temperature geothermal reservoir.
Sinters can also form from acidic steam-heated groundwater, which is not directly
connected to deep reservoir liquid. Indeed, deep pressure drawdown is likely to
enhance such features through additional upward steam flow. Therefore, the term
“sinter depositing” should not be used to rank features for protection on the basis of
resilience or rarity, because the term is simply not a useful discriminator and hot
spring “sinters”, in the broadest definition of the term, are relatively common.
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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
6.2 Protection/preservation
Management plans are sometimes premised by an underlying simplistic assumption
that protection of natural geothermal features from change is, a) achievable, and b)
guaranteed by excluding large-scale resource utilisation. However, observations show
that nearly all geothermal features vary naturally (cyclically, randomly or
intermittently) over timescales that can range from minutes to decades. It is not
possible to guarantee their preservation in terms of maintaining a constant discharge
temperature, flowrate or heatflow. Furthermore, recent experience (e.g. at Rotokawa
and Mokai) has demonstrated that large-scale resource development does not
necessarily result in loss of surface geothermal features. Indeed, with innovative
resource management strategies (e.g. shallow injection, when appropriate) discharge
from thermal features of many types can often be enhanced rather than reduced. The
principal aim of geothermal management plans and policies should be to encourage
efficient integrated use, while protecting the diversity of thermal features in the region
(rather than specific individual features). This can be achieved (as proposed in the
Waikato Region) by designating several geothermal systems to remain undeveloped
(except for tourism facilities), as a kind of environmental insurance policy. However,
properly managed development of all other geothermal resources for sustainable
energy utilisation should be facilitated, with reasonable conditions imposed, in a
balanced manner. Conditions should encourage enhancement of any type of surface
thermal feature, by way of mitigation for unavoidable and adverse changes to other
thermal features. This replicates the sort of variation behaviour that occurs naturally.
Geysers and fumaroles, for example, are both naturally transient features. So newly
created steam vents compensate for the loss of chloride springs, or vice-versa.
An issue commonly faced by direct users of low enthalpy resources is the “buffer
zone” distance from significant thermal features, and other users, that a new user
should respect in order to avoid interference effects. A distance of 20 m is considered
reasonable in New Zealand for relatively small amounts of fluid extraction and
injection (<1 kg/s). There should also be some regulatory incentive for the use of
down-hole heat exchangers or ground-source heat pumps, rather than direct fluid
extraction, because of the relative benefit to the environment, in that pressure
interference is no longer an issue.
6.3 Natural/artificial
A common misperception regarding geothermal features is to regard them in ‘black-
and-white’ terms as being either natural or artificial. This can lead to a pedantic
application of rules designed to preserve natural features and discourage artificial
features. In fact, there is a continuum of natural to human influences on thermal
features (that is, many ‘shades-of- grey’). At one end of the spectrum, for example,
the artificial geyser and silica terrace at Wairakei Terraces, which uses water from the
reinjection pipeline, is indisputably man-made. The Lady Knox “geyser” at Waiotapu
is artificial, in the sense of being stimulated daily by soap to erupt through a hidden
pipe (installed in 1906), but has a very natural appearance and is highly valued. The
“Healy 2 Bore” at Tokaanu is another example of a geysering spring, sinter-cone and
terraces, with an associated highly valued ecosystem, that has evolved over 50 years
from an abandoned bore. Although it was initially created by human activity, it now
appears totally natural. The “Craters of the Moon” thermal area at Wairakei has
always existed as a natural feature, but the intensity of thermal activity increased
dramatically in response to Wairakei pressure drawdown, so it has been indirectly
affected by human activity. The same could be said of existing geysers and
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International Geothermal Conference, Reykjavík, Sept. 2003 Session #12
discharging hot springs at Orakei-Korako that are indirectly supported by raised
groundwater levels in response to the artificial filling of Lake Ohakuri in 1961.
Several hydrothermal eruptions at Kuirau Park, Rotorua, were stimulated by pressure
recovery related to the bore closure programme. These examples illustrate the point
that rules need to be made flexible enough to cater for a wide spectrum of scenarios
when considering the desirability of human influences on geothermal features.
6.4 Reversible/recoverable development effects
Many of the past assumptions of the likely effects from new, large-scale geothermal
energy developments are outdated. The modern philosophy is to develop new fields in
stages, big enough to create measurable effects on the resource, but not big enough to
create large irreversible effects on surface thermal features or resource sustainability.
Stages are typically about 5 years in duration, and up to 2 times the previous level of
utilisation. Monitoring, and predictions based on regularly updated reservoir models,
provides confidence of the probable effects (out to about 50 years) for each stage.
Hence the risks are minimised for the regulator, the owner, the developer, and the
investor. Historically, of the 7 geothermal fields developed for power generation in
N.Z., only 2 of the earlier developments have directly resulted in significant loss of
surface geothermal features. At Rotorua, a change of bore management policy to raise
pressure has caused a significant recovery of geysers and springs. This demonstrates
that such features can be recovered, and are not necessarily lost irretrievably when
pressures decline.
6.5 Sustainable/renewable
An issue for sustainable utilisation is the duration of “reasonably foreseeable use” (eg
1-4 generations, or 25-100 years). Most reservoir modellers would not be confident
about predicting geothermal reservoir behaviour beyond about 50 years, and this is
probably a reasonable period to choose. Within that time, it is expected that
technological advances will have provided access to far greater heat resources deeper
within the earths crust. Furthermore, a long-term strategy of cyclic use of existing
geothermal reservoirs would have the advantage of encouraging natural recharge of
fluids and heat during a “fallow” period of recovery in between periods of heat
extraction. Thus the concepts of renewable and sustainable geothermal energy use can
be upheld whilst undertaking cyclic extraction of heat by drawing down reservoir
pressure. This is analogous to hydroelectric lake storage management, but on a longer
scale.
7 Conclusions
When considering the induced effects of geothermal development on the
environment, a balanced view is to weigh up the adverse effects against the beneficial
effects to determine a net effect that may be mitigated for. Examples of beneficial
effects that are often overlooked include: subsidence induced wetlands; thermal
ecosystems associated with increased areas of steam-heated ground and surface-
disposal of hot water; and reduced gas emissions relative to fossil fuel alternatives.
Geothermal plans should recognize the modern approach to utilisation of new
resources, by allowing staged development of all but a few “protected” systems, in a
manner that minimizes risk, and allows for recovery by adjustments to field
management. Optimum size increments should be established by considering the
resource knowledge acquired during each stage. Monitoring can provide early
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warning of adverse effects, and remedial measures can be implemented. If adverse
effects on thermal features occur, they can often be reversed by locally managing the
subsurface pressures.
8 References
Scott, B.J., Cody, A.D. (2000). Response of the Rotorua geothermal system to
exploitation and varying management regimes. Geothermics 29, 539-556.
Thain, I.A., Dunstall, M. (2000). 1995-2000 Update report on the existing and
planned use of geothermal energy for electricity generation and direct use in New
Zealand. Proc.World Geothermal Congress 2000, Kyushu-Tohuku, Japan, 481-489.
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