IN THE ENVIRONMENT COURTAT AUCKLAND

IN THE MATTER of the Resource Management Act 1991

AND

IN THE MATTER of appeals under section 120 of the Act

BETWEEN ROTOKAWA JOINT VENTURE LIMITEDand MIGHTY RIVER POWER LIMITEDENV-2006-AKL-000685 (formerly ENV A0535/04)ENV-2006-313-000061 (topic reference)

AND TAUPO DISTRICT COUNCILENV-2006-AKL-000691 (formerly ENV A0542/04)ENV-2006-313-000061 (topic reference)

AND CONTACT ENERGY LIMITED (ENV-2006-AKL-000692 (formerly ENV A 0543/04)ENV-2006-313-000061 (topic reference)

AND NZ PRAWNS LIMITEDENV-2006-AKL-000695 (formerly ENV A0547/04)ENV-2006-313-000061 (topic reference)

Appellants

AND WAIKATO REGIONAL COUNCIL

Respondent

AND CONTACT ENERGY LIMITED

Applicant

EVIDENCE OF CHRISTOPHER JOHN BROMLEY

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1. INTRODUCTION

1.1 My name is Christopher John Bromley. I am a geothermal scientist with an MSc (Hons)

degree in geophysics from Victoria University. I am employed by GNS Sciences, at the

Wairakei Research Centre, as a geothermal consultant and research scientist.

1.2 I am involved in a wide range of geoscientific exploration and environmental monitoring

projects, and I have published 60 papers and produced more than 60 technical reports

related to geothermal projects. I have 27 years’ experience in geothermal scientific

research and consulting, specialising in geophysics, geothermal resource assessments

and environmental impacts. In that time, I was employed for 3 years by GENZL (now PB

Power), for 7 years by KRTA (now SKM) and for 16 years by GNS Sciences (formerly

DSIR Geophysics Division). I have been a resident of Taupo for 16 years and I own a

shallow geothermal bore with a downhole heat exchanger for home heating.

1.3 My relevant geothermal work experience includes:

(a) Published research into interpretation of specialised geothermal resistivity,

hydrological and heat-flow techniques, hydrothermal eruptions, induced

earthquake and subsidence mechanisms, and hazard assessments;

(b) Scientific resource assessments and video infrared surveys of thermal ground at

twelve New Zealand geothermal fields;

(c) Assessment of impacts on hot springs and groundwater at Wairakei, Tauhara,

Ohaaki, Reporoa, and Orakei Korako;

(d) Expert witness for geothermal consent hearings for Wairakei, Tauhara and

Reporoa, geothermal aspects of the Mighty River Power (“MRP”) Waikato River

hydro-dam consent hearing, and Waikato Regional Council (“EW”) Geothermal

Policy and Plan hearing, appeal, and technical facilitation meetings related to

issues of subsidence, geothermal field inter-connections, and development

effects on thermal features.

(e) Member of EW Peer Review Panels for geothermal developments at Mokai

since 1999, Rotokawa since 1997, and Wairakei from 1997 to 2003.

(f) International geothermal consulting, including: geophysical exploration in Iran,

Kenya, Indonesia, and the Philippines; a review of worldwide injection

experience; induced micro-earthquakes studies at Palinpinon (Philippines);

reservoir measurements at Kamojang (Indonesia); resource assessments of

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Ulumbu and Wayang Windu (Indonesia), Sumikawa (Japan), Bacon-Manito,

Biliran, Northern Negros (Philippines), and Olkaria (Kenya).

(g) New Zealand representative, (since 2002), and leader of the Environmental

Task (Annex 1), of the International Energy Association, Geothermal

Implementing Agreement, undertaking collaborative geothermal environmental

effects research with USA, European Union, Japan, Italy, Mexico, and Iceland.

1.4 My evidence presents scientific information and interpretation associated with matters

under appeal by Contact Energy Limited (“Contact”) and other parties, regarding the

granting of resource consents for operation of the Wairakei Geothermal Field.

1.5 Specifically, my evidence deals with the following matters:

(a) Geophysical techniques for identifying geothermal system characteristics

(b) Description of changes in surface geothermal activity

(c) Description of shallow groundwater aquifers

(d) Characteristics of the shallow Wairakei-Tauhara hydrothermal regime

(e) Effects of development on thermal features

(f) Changes in shallow aquifers post development

(g) Strategies for management of the productive potential of the resource

(h) Strategies for management of thermal features

(i) Infield and outfield injection strategy

(j) Proposed outfield injection areas

(k) Effects of targeted injection on subsidence

(l) Subsidence mechanisms

(m) Hydrothermal eruptions

(n) Mitigation of hydrothermal eruptions through injection

(o) Peer review management mechanisms

1.6 I have been provided with a copy of the Code of Conduct for Expert Witnesses in the

Environment Court Practice Note (31 March 2005). I have read and agree to comply

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with that Code. Except where I state that I am relying upon the specified evidence of

another person, my evidence in this statement is within my area of expertise. I have not

omitted to consider material facts known to me that might alter or detract from the

opinions which I express.

2. GEOPHYSICAL TECHNIQUES FOR IDENTIFYING GEOTHERMAL SYSTEM

CHARACTERISTICS

2.1 A range of geophysical techniques can be employed to assist with the identification of

geothermal system characteristics.

2.2 The boundaries of the Wairakei-Tauhara geothermal system were initially determined

using the resistivity method. Maps of low resistivity values helped delineate the extent

of the geothermal reservoir in the absence of local borehole information. However,

resistivity signatures need to be interpreted with caution because they can be strongly

influenced by rock type and clay alteration type as well as geothermal fluid temperature,

salinity, and rock porosity.

2.3 The resistivity boundaries at Wairakei-Tauhara are indicative of the approximate extent

of the geothermal system. Productive areas lie within the low resistivity boundaries.

Practical experience from boreholes suggests that the Wairakei resistivity boundary

zone corresponds approximately with the 150 degree isotherm at about 500m to 1000m

depth. The lowest resistivity layer within the system boundary usually coincides with a

capping layer of intensely altered rock, mostly smectite, a swelling clay which has

relatively low permeability and forms at temperatures of about 100 to 180 oC at the top

of the reservoir. The extent and depth of the geothermal resource at temperatures

suitable for large scale electricity development (>180 oC) is not, however, solely

determined by resistivity boundaries or interpreted resistivity layers. Additional

information provided by boreholes would be necessary to refine the understanding of

the hydrology of the Wairakei-Tauhara geothermal system and the boundary zones in

particular.

2.4 There are uncertainties as to the specific location of any geothermal fluid connections

beyond the boundary of the Wairakei-Tauhara low resistivity anomaly. Information from

boreholes and future resistivity surveys (including deeper-penetrating magneto-telluric

surveys) will progressively change our knowledge of these system boundaries.

Accordingly, the map of the Wairakei-Tauhara system boundary may need to be

modified as new information is collected. The system extent, as currently defined in the

Proposed Waikato Regional Geothermal Plan (Variation 2) documents, is shown in

exhibit CJB 1. It is based on the outer extent of the resistivity boundary transition zone

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shown in exhibit CJB 2, and includes hot spring discharges along the foreshore of Lake

Taupo.

2.5 Other geophysical methods have helped delineate the physical properties of the

reservoir. For example, airborne magnetic surveys, which map the intensity of the

earth’s magnetic field, show a large magnetic low anomaly over most of the geothermal

system, where geothermal-alteration of rocks has converted magnetic minerals into

non-magnetic minerals. Magnetic high anomalies often reveal the presence of relatively

unaltered volcanic formations, such as buried rhyolite domes or lava flows. Such

information has been useful for mapping potential sub-horizontal permeable zones,

such as the fractured surfaces of lava flows, particularly in the Poihipi West and Aratiatia

out-field injection sectors. Mr Rosenberg has discussed rhyolite bodies in his evidence.

2.6 Seismic reflection surveys have revealed reflecting layers within the Huka Falls

sedimentary formation. This has assisted with interpretation of fault displacements and

the overall shape of a depression containing Huka Falls Formation in the vicinity of the

Wairakei Power Station, on the eastern boundary of the field. Also changes in seismic

reflectivity mark the western boundary of the system near the Poihipi Power Station.

2.7 Gravity surveys have also helped interpret and extrapolate geological structure beyond

the constraints of borehole information. This is because formations of different densities

produce gravitational anomalies at the surface. Monitoring of minute changes in gravity

at benchmarks has also helped map the effects of subsurface density changes with

time, normally caused by variations in mass of liquids. At Wairakei and Tauhara, the

creation and “dry-out” of steam zones originally caused a local mass deficit and gravity

decline. Subsequent gradual liquid resaturation of the steam zones caused by

groundwater inflow or liquid injection has created an increase in gravity in some places.

These include the Wairakei Eastern Borefield and an area near TH4 of Northern

Tauhara (exhibit CJB2 for location of TH4).

2.8 Geophysical monitoring activities have also included micro-seismic monitoring for

induced earthquakes. Continuous monitoring since 1996 at a site near Poihipi Power

Station has revealed a background of intermittent natural swarms of small earthquakes

in the Wairakei area. These are mostly related to activity along the nearby Taupo Fault

belt. There is no evidence of any increase in earthquake activity related to low pressure

injection commencing in 1997. From December 2004 to February 2006, a 14 month

microseismic array study was undertaken using 4 portable seismometers installed

around out-field injection wells WK305 and WK307 at Aratiatia (see exhibit CJB 2 for

well location). No earthquake events large enough to be felt (or magnitude>2) were

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located in the vicinity of the outfield injection wells during pumped (high pressure)

injection.

2.9 Ultimately, it is boundary delineation wells that provide the best information on actual

temperatures, pressures, permeability, geology and fluid chemistry with depth. Such

wells help reduce the uncertainties regarding hydrological processes occurring at these

boundaries. Peripheral wells are relatively few in number (see exhibit CJB 1) and the

resource boundary has generally been left to be inferred from the geophysics data.

Accordingly, there are practical difficulties involved in relying on hydrological criteria to

define the limits of the Wairakei-Tauhara geothermal system. In its interim decision on

policy matters, the Environment Court adopted the pragmatic position of using the

resistivity based Plan boundaries as the default where insufficient hydrological data is

available.

2.10 Where information is available from wells located near the Wairakei field boundary the

chemistry of the fluids encountered indicates a natural chemical diffusion process

through a broad mixing zone. This is the case at Wairakei East (near the Power Station

and the Aratiatia outfield injection sector). Wood et al (1997) describe the situation

where cooler fluids at one depth are diffusing inwards, while hotter fluids at another

depth are diffusing outwards. Marked temperature peaks and inversions with depth are

often observed in such wells. Furthermore, lateral flows through the apparent field

boundary vary with time in response to other hydrological factors, especially those that

influence pressures or permeabilities. Examples are earthquake-generated fracturing

and self-sealing by mineral deposition.

2.11 Mr Clotworthy discusses in his evidence the pressure data which has been obtained

from peripheral wells and the extent to which, by applying a criterion of 15% pressure

response, the “hydrological boundary” can be identified. In summary, for Wairakei-

Tauhara, there are only a few areas where the location of a pressure boundary can be

identified with any certainty.

2.12 There would be some benefit to resource management from new hydrological

information obtained by drilling wells near the boundaries. A practical reason for drilling

such wells would be exploration for suitable outfield injection sites.

3. DESCRIPTION OF CHANGES IN SURFACE GEOTHERMAL ACTIVITY

3.1 In a table, (exhibit CJB 3), and a map (exhibit CJB 4), I summarize the main natural

geothermal surface discharge areas, listed in order from northern Wairakei to southern

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Tauhara. Most features now consist of areas of steaming ground or steam-heated

groundwater discharge.

3.2 In Variation 2 of the Waikato Regional Plan “Significant Geothermal Features” (SGFs)

at Wairakei-Tauhara were identified and mapped, although these maps rely strongly on

visual observation of the extent of thermal vegetation rather than identification of actual

surface discharges of hot water or steam, and the Plan gives no ranking of relative

significance. I have indicated my view of the relative importance of the geothermal

areas in a regional context, by ranking as A (outstanding), B (significant), C (moderately

significant) and D (common, or low rank). The rankings take into account attributes such

as relative vulnerability, viability, rarity and resilience, as well as intrinsic, aesthetic and

scientific values. The table also identifies the dominant thermal feature types,

characteristic vegetation type, and surface area.

3.3 Artificial features have also been created at Wairakei Terraces, a tourist facility operated

by Netcor adjacent to the Wairakei Information Centre. These include hot pools, sinter

terraces (with microbial habitat) and a geyser, using hot brine from the injection pipeline.

They display most of the same highly valued characteristics of other A-ranked features,

that is, geysering, vigorous sinter deposition and high aesthetic value. The nearby

Kiriohineki Stream has also been re-established and enhanced by diverting separated

geothermal water from the main Wairakei geothermal drain. Because this involved

restoration of a natural hot stream, and it is providing habitat for thermal vegetation

(ferns) as well as aesthetic enhancement, I would currently rank this feature relatively

highly (B). Its continued presence would be dependant on the granting of consents to

discharge separated water into surface waterways.

3.4 Monitoring of changes in the extent and intensity of surface geothermal features has

been undertaken at Wairakei and Tauhara using ground-based heat loss assessments

supplemented by aerial photography, and aerial infrared surveys. Repeat infrared

surveys, recorded on videotape and conducted by helicopter at night, map changes in

surface temperature, at a ground resolution of about 1m and a temperature resolution of

about 0.2 degrees. The most recent surveys were conducted over Tauhara thermal

areas in March 2006, over Karapiti (‘Craters of the Moon’) in May 2000, and over the

rest of Wairakei thermal areas in 1997. Exhibit CJB 5 is a composite of thermal infrared

images of steaming ground and craters at Karapiti. Comparison of thermal imagery from

repeat surveys has confirmed that significant local variations in the intensity of thermal

activity occur from time to time in some areas of vigorously steaming ground.

3.5 In the first decade of extraction at Wairakei, total natural heat flow from all the Wairakei

thermal features doubled from an estimated initial pre-development field-wide value of

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about 400 MWt (note t=”thermal”) to a maximum of about 800 MWt. This was despite a

decline of 46 MWt in heatflow from Geyser Valley hot springs discharging into the

Wairakei Stream. Later, the heatflow from steam features gradually declined and

stabilized at a total value of about 400 MWt by the 1990’s. So there is little net change

in MWt, between current discharge and pre-development discharge. A similar change

occurred at Tauhara from an original field-wide heat flow of about 100 MWt to a peak of

about 200 MWt during the 1970’s, followed by a gradual decline back to an estimated

140 MWt by 2000. Exhibit CJB 6 illustrates changes in heat flow with time as measured

at Karapiti (Craters of the Moon).

3.6 The main difference between the pre-development and present day heat flow is the

form of heat discharge. Pre-development, the heat flow had a contribution from the

discharging chloride springs. The present day heat flow is dominated by steam vents

and steam-heated groundwater discharges.

3.7 The location of the principal areas of thermal activity has not changed significantly with

time. There is no evidence of substantial migration of the activity away from the

originally mapped thermal areas, although many areas of steaming ground did expand

in extent and intensity during the 1960’s and 1970’s. An example is Craters of the Moon

which has approximately doubled in area. Locations where the intensity of thermal

activity has decreased are along the Waikato River (at Spa Park and Huka Falls),

Geyser Valley (along the Wairakei Stream) and lower Waiora Valley. Some small areas

of steaming ground have cooled off since the 1980’s.

3.8 The dominant overall effect has been a large net increase in steaming ground and

associated increase in geothermal habitat for thermally tolerant plants. The habitat

effects are discussed more fully in the evidence of Dr Bruce Burns. The induced

increase in steaming ground and associated habitat at Wairakei-Tauhara represents

about 10% of the known total area of thermal ground in the Waikato Region. On the flip

side, there has been some loss in the length of the Wairakei stream that used to

discharge high temperature chloride water (between Geyser Valley and Main Drain

discharge structure). This has meant a partial loss of thermal microclimate habitat for

some tropical fern species where these grew along the stream banks. Fern habitat has

increased, however, along the Otumuheke Stream (in Spa Park, northern Tauhara),

which is discharging steam-heated water at source temperatures that steadily increased

from 40 oC to 95 oC over a period of 32 years.

3.9 The southern area of Tauhara Geothermal Field (including springs at Waipahihi and the

foreshore of Lake Taupo) has shown no evidence of changes that can be linked to

Wairakei pressure drawdown. The chemistry of these southern springs reveals a 10-

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20% contribution from deep chloride waters, and there is no evidence of increased

temperatures from steam heating. Temperatures between 1964 and 2006 have

remained relatively stable at about 65 (+/-4) oC. Flowrates vary with long-term rainfall

(e.g. Waipahihi Source Spring) or changes in lake-level (foreshore springs).

3.10 Plots of chloride concentration and flowrate of Waipahihi Source Spring show a simple

linear dilution trend (exhibit CJB 7), implying that the upflow of chloride water has not

changed. Therefore, the observed changes in spring flow rate and chloride

concentration result from changes in shallow groundwater level and rainfall dilution.

Accordingly, I infer that the pressure drawdown measured in deep wells in North

Tauhara (resulting in turn in the formation of a steam zone in that area) has not

expanded into the southern Tauhara area, so as to influence these springs. I also

conclude that the Onekeneke Stream (Waipahihi valley) and lake–side hot springs have

not been affected by Wairakei pressure drawdown.

4. DESCRIPTION OF SHALLOW GROUNDWATER AQUIFERS

4.1 Surface thermal features are supplied with heated fluid (water or steam) from

underground aquifers. The Wairakei-Tauhara system contains many aquifers of

mineralised hot water and mixtures of water and steam. Aquifers occur at a range of

depths and temperatures. Aquifer fluids are chemically diverse, and contain a range of

dissolved chemicals resulting from various mixtures of groundwater, acidic steam

condensate, deep-sourced mineralised geothermal water and magmatic gases.

Interaction between different mixing aquifers is the result of natural convection, rock

fracturing and chemical deposition processes, as well as pressure or temperature

changes induced by fluid production and injection.

4.2 The Wairakei-Tauhara aquifers are vertically separated by mudstone layers of low

permeability (Upper and Lower Huka Falls formation, and Post Oruanui Sediments in

parts of Tauhara). Mr Rosenberg discusses the geology of the system in his evidence.

The deepest aquifers receive upflows of high-temperature fluid (>260oC), with high

chloride concentrations (~1500 mg/kg). Deep upflows are located at Te Mihi (Wairakei),

and near Mt Tauhara. The Tauhara upflow may be split into two, one providing

recharge fluids to the northern reservoir linked to Wairakei, and the other providing

recharge to shallow aquifers discharging chloride fluids at Waipahihi, southern Tauhara.

This would explain the apparent pressure difference between the drawn-down deep

aquifers of North Tauhara and the aquifers in South Tauhara that have not been

affected by Wairakei production.

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4.3 In the pre-development state, the deep Te Mihi and northern Tauhara chloride fluids

also passed into shallower aquifers, mixed with groundwater, and discharged to the

surface. The chloride concentration was highest in relatively undiluted discharges from

the middle level Huka-2 aquifer (1000-1500 mg/kg), resulting in high-chloride springs at

Geyser Valley, Lower Waiora Valley, Spa Sights and along the Waikato River, near

Huka Falls. Groundwater mixing created more dilute discharges (100-600 mg/kg

chloride) from higher level aquifers at places such as Upper Waiora Valley, and

Waipahihi. Discharges from the perched upper-most aquifers often contained very low

chloride (e.g. 8 mg/kg at AC Baths spring) because these waters consisted of rainfall

infiltration, heated by steam.

4.4 Some pockets of steam formed naturally as a result of subsurface boiling. The rising

steam either discharged to the atmosphere as fumaroles or condensed into

groundwater creating an acid-sulphate steam-condensate. This contributed to some hot

spring discharges, such as in the Alum Lakes area. After 1952, pressure drawdown of

deep fluids from Wairakei production formed larger steam zones. The drawdown has

isolated the deeper aquifers from the overlying groundwater aquifers. The groundwater

has consequently become more “perched” and less saline. Accordingly, the

concentration of elements such as chloride, arsenic and boron in the thermal

groundwater has also reduced.

4.5 The water-level surface of the shallowest groundwater aquifer, as mapped by the green

contour lines in exhibit CJB 8, follows a trend of downward sloping gradients (averaging

2.2%) across Wairakei and Tauhara, indicating gradual horizontal flow towards Lake

Taupo and the Waikato River. Contours of the water level surface also reveal some

regions where the lateral gradient is much steeper than normal. These sometimes

indicate regions of overlap between groundwater aquifers at different levels, where the

upper level aquifer pinches out along the edge of an underlying aquiclude. The

aquiclude can be a layer of mudstone or impermeable clays.

4.6 One such zone, trending north-northwest, and lying between Spa Park thermal area and

Waipahihi Source Spring, occurs in Taupo (exhibit CJB 8). To the east of this lineation,

the water levels of the shallowest aquifer are at about 400 to 410 m-asl, while to the

west they are at about lake level (360 to 370 m-asl). The upper aquifer is generally

steam-heated fresh groundwater, and strongly influenced by changes in rainfall, varying

in level by 1-2 m over periods of 2 to 6 years. A large part of this steam-heated upper

aquifer has also steadily declined in water level by up to 8 meters. I will discuss possible

reasons for this later. The lower aquifer is influenced by seasonal lake level variations

(about 1m), especially within a few hundred meters of the lake edge, and by changes in

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recharge from the upper aquifer. To the south, at Waipahihi, the lake level aquifer also

contains a proportion (about 10 to 20%) of deep chloride water.

4.7 At Wairakei, another north-trending lineation in water level contours passes through the

upper Waiora Valley and west of Karapiti. The western (higher) part of the groundwater

aquifer responds mostly to changes in rainfall recharge. The eastern (lower) part of the

aquifer has declined in pressure in the vicinity of the Eastern Borefield, and is now partly

controlled by the water level in the ponded Wairakei Stream.

5. SHALLOW GROUNDWATER BOREHOLES

5.1 Comprehensive Information about the shallow thermal and non-thermal groundwater at

Wairakei and Tauhara has been collected from the monitoring of water-levels,

temperatures, flowrates and chemistry from a large number of shallow boreholes over

many decades. Exhibit CJB 8 shows the locations of many of these monitored bores

(Black symbols on the exhibit). Exhibit CJB 9 provides a selection of well data over the

long record of Wairakei monitor bore measurements, many of them dating from the

1950’s.

5.2 A large number of shallow domestic and commercial bores have also been drilled in the

Taupo urban area to utilize geothermal heat. DSIR had recorded, by 1984, a total of 431

bores. Curtis (1988) reported that in excess of 500 thermal bores existed in Taupo in

1987 of which approximately 90% were in use. He also reported that heat exchangers

were installed in about 45% of the bores, while 48% abstracted water. Total heat

consumption was estimated as 8 MWt, (less than the heat output of the largest fumarole

at Karapiti). Approximately 85% of the geothermal heat consumption in Taupo came

from a few large commercial users, including Taupo District Council bores at AC Baths.

Since 1987, some Taupo thermal bores have been abandoned, but more than 50 new

bores have been drilled suggesting that the total of about 500 bores is still

approximately valid. Those bores without downhole heat exchangers are pumped either

continuously or on demand, to provide heat for direct use in buildings or for bathing.

Once used, the waters are disposed of into injection boreholes, shallow soakage pits or

geothermal streams. Current commercial bore users that are extracting hot water

include the AC Baths, Taupo Hot Springs, Care and Independence Hospital, Outrigger

Terraces Hotel, Copthorne Manuels Hotel, and several lakeside motels.

5.3 About half of the domestic and commercial bores in and around Taupo were drilled into

or through the steam-heated Upper Aquifer north-east of a line between Cherry Island

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(on the Waikato River) and Taupo Hot Springs (Waipahihi Spring in exhibit CJB 8).

Many of these bores were drilled deep enough to penetrate into the underlying aquifer

(where the water level is at about lake level), and flows from the upper aquifer down to

the lower lake level aquifer developed within the bores (when not pumped). These wells

became drainage points for the upper aquifer and even when abandoned, such bores

can still provide a conduit for downflows. Old Crown-owned Tauhara bores, such as

TH4, may also have developed internal downflows through corroded or damaged

casing. Such continuous downflows could amount to a significant component of shallow

aquifer drainage.

5.4 Many borehole owners in Taupo benefited from the increased temperature of the

steam-heated groundwater that resulted in the 1960’s and 1970’s from the deeper liquid

pressure drawdown at Wairakei. For example, a monitor bore at Taupo Hospital

increased in temperature by 50 oC at 65m depth between 1966 and 1980. This increase

in the temperature of the groundwater provided heat which has been widely utilised for

residential and commercial purposes. The peak of this temperature enhancement effect

occurred about 1980. Temperatures have since declined by 10 oC to 30 oC in some

boreholes, but they have generally remained above pre-development groundwater

temperatures.

5.5 Continued domestic utilisation of the shallow upper aquifer for heating is dependant on

a continued upflow of steam from underlying steam zones. This is at risk of reducing

and failing if, in the future, the steam zones are quenched by water.

6. CHARACTERISTICS OF SHALLOW WAIRAKEI-TAUHARA HYDROTHERMAL

REGIME

6.1 Transient reservoir behaviour causes changes in surface thermal features and

groundwater. The time scales of transient effects depend on properties of the aquifers

such as rock porosity and degree of fracturing. High porosity rock (e.g. pumice) leads to

greater fluid storage, and high permeability rock (e.g. fractured rhyolite lava) leads to

more rapid propagation of pressure changes. High permeability also allows for greater

extraction rates of mass and energy. At Wairakei, there is a high permeability

connection between the production aquifers and the underlying recharge zone. As

discussed in Mr Clotworthy’s evidence, this means that the initial production pressure

drawdown of 25 bars stimulated a large increase in flow-rate of hot recharge fluid,

stabilizing liquid pressures, and sustaining energy extraction rates.

6.2 The aquifers can be thought of as reservoir “tanks” with various degrees of

interconnection, usually along permeable paths. In some cases fluid interconnection is

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rapid. A small pressure gradient (such as between a production and injection well) can

cause relatively fast subsurface flows of fluid over long distances. At Wairakei, known

examples of rapid connection (based on tracer tests) exist along several north-east

oriented faults passing through the production borefield area. These have also been

discussed in the evidence of Mr Clotworthy. In other cases the aquifers are relatively

weakly linked, and pressure changes can take a long time to propagate between them.

In general, the longer the time delay, the more effective the conductive gain or loss of

heat energy that occurs between the surrounding rocks and the fluid. This heat-sweep

effect is achieved by fluid flowing through fractures and diffusing slowly in and out of the

porous rock matrix.

6.3 At Wairakei and Tauhara, some aquifers are separated or capped by

porous but impermeable aquicludes, such as hydrothermal clays, mudflow deposits,

and the Huka Falls Formation of lake-deposited siltstones and mudstones. These

aquicludes deflect vertical flow into horizontal flow. As a result, lateral outflows of hot

liquid or steam sometimes produce surface discharges several kilometres from the

upflow source. Along these subterranean outflow pathways, the hot water and steam

thermally and chemically alter the rock matrix into clays. They may also deposit silica.

Therefore, the rock properties (such as permeability and compressibility) of the

subsurface formations in these outflow areas can vary depending on location, duration

and type of hot fluid interaction.

6.4 As pressures have reduced across the Wairakei-Tauhara system, the mudstone

aquicludes or thermal clay deposits have, in some places, deformed into local

subsidence anomalies. Parts of these formations are likely to have compressibility and

permeability properties that are locally anomalous. Despite their high porosity (about

60%), the relatively low permeability of these mudstone or clay formations results in

slow diffusion of pressure. This, in turn, causes a substantial delay between the aquifer

pressure decline, and the compaction of the adjacent aquiclude.

6.5 Another potential explanation for the time delay is a process called “creep”. It is likely

that much of the anomalous compaction originates from clays, such as smectite, inter-

layered smectite-illite and kaolinite. These characteristically deform in an in-elastic (or

plastic) manner, resulting in “creep” (that is, ongoing compaction under constant load).

Analysis of core samples from the centres of subsidence bowls at Ohaaki (reported by

Read et al, 2001) and Crown Rd Tauhara (as discussed in the evidence of Mr

Rosenberg), included petrological, geomechanical and scanning electron microscope

studies. These have revealed some cores that contain up to 85% clay, that continue to

compact under constant load in laboratory tests for at least a month, and that show

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clear microscopic evidence of plastic deformation rather than brittle failure when

compressed.

6.6 As a result of creep, the compaction process is delayed when the clay-rich material is

subjected to pressure reduction. It is also mostly permanent. Rising pressures will not

“reinflate” the ground surface to any significant degree. With creep behaviour,

subsidence rates generally peak and then gradually tail off with time under constant

load, that is, at constant pressure. The effective instantaneous compressibility (the ratio

of rock compaction to applied load at a point in time) is not a constant property. It is a

variable which depends on both load history and time. The net result is that although

subsidence may be initially triggered by underlying fluid pressure drawdown,

subsequent subsidence rates may not be a simple function of pressure alone.

6.7 Geothermal systems are naturally dynamic and this includes the variation in surface

thermal features. Variations in fluid discharges at surface thermal areas are a natural

occurrence and potentially valued characteristic, some of which can be very large. In

my view, natural variability should be taken into consideration in planning the monitoring

and mitigation program for Wairakei-Tauhara.

6.8 Geothermal features have varied ecological and geological characteristics. These are

partly governed by the previous history of changes in thermal activity. They are also

dependant on other surface environmental factors (such as stock access, weathering

and weeds). Some geothermal resource management practices may affect future

discharges from thermal features (either active or dormant).

6.9 To summarize this section, there are complications that arise from transient reservoir

behaviour, that need to be considered when developing the optimum injection and

production strategy. Fluid changes affect formation properties and vice versa.

Consequently, it is my opinion that future field management should be approached in a

flexible and adaptive manner, rather than a prescriptive manner. Such an approach

would have better chances of successfully minimizing or mitigating environmental

effects.

7. EFFECTS OF DEVELOPMENT ON THERMAL FEATURES

7.1 Historical fluid extraction and pressure drawdown at Wairakei has had effects on the

hydrological aspects of some shallow aquifers and resulting surface thermal features.

The thermally tolerant ecosystems associated with these thermal features are

discussed by Dr Burns in his evidence, where he concludes that some significant

increases have occurred in specific steam-heated habitats. In this section I will

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comment on the likely causes and consequences of the physical effects and the

predicted effects of future Wairakei development scenarios.

7.2 Historical geysers and hot springs along a part of Wairakei Thermal Valley (Geyser

Valley) steadily declined in discharge during the early years of development (1950’s and

1960’s), as the source aquifer of deep mineralised chloride water was drawn down in

pressure. The type of thermal discharge at Wairakei Thermal Valley changed through

time from dominantly liquid to dominantly diffuse steam discharges. This physical

change has created an increased habitat for steam tolerant vegetation growing amongst

the weakly steaming geyser vents and old sinter deposits.

7.3 Along the Waikato River bank at Spa Park, Tauhara (formerly known as Spa Sights),

historical geysers declined in activity between 1930 and 1950, initially as a result of the

Napier earthquake, and later in response to nearby river level changes when the river

bed was modified in conjunction with construction of the control gates at Lake Taupo

outlet. After 1959 the remaining chloride hot spring discharges also gradually declined.

This was in response to deep liquid pressure decline propagating from Wairakei.

7.4 In contrast to hot chloride spring declines, areas of steaming ground and fumaroles at

Wairakei and Tauhara have increased in intensity and extent, particularly during the

initial stage of Wairakei pressure drawdown. This was caused by an expanding

subsurface steam zone which caused increased quantities of steam to reach surface

features. Karapiti thermal area of steaming ground (“Craters of the Moon”) discharges

about 50% of the total natural heat-flow at Wairakei (see exhibit CJB 6). In a 1946 aerial

photo, Karapiti is shown to consist of an area of about 0.17 km2 of weakly-steaming and

bare thermal ground, containing several inactive eruption craters and a large fumarole,

known as the “Karapiti blowhole”. By 1964, thermal activity had increased 10 fold, and

the area had doubled. Thermal activity has since declined and levelled off at a value of

about 200 MW, 5 times the pre-development heat-flow value. The area of thermally-

tolerant vegetation associated with various grades of the diffusely steaming ground has

at least doubled in size over the same period. The Karapiti blowhole ceased

discharging in 1987 due to a natural vent collapse. The ‘Craters of the Moon’ thermal

area has become a well-known tourist attraction. It is a good example of enhanced

steaming ground, associated thermal vegetation, intermittent steam eruptions and

variations in diffuse steam flux.

7.5 I will now describe the mechanisms driving changes in surface steam discharge at

Karapiti and elsewhere at Wairakei-Tauhara. Steam (in a 2-phase zone) has been

created by subsurface boiling as the deeper liquid reservoir pressures declined. Steam,

being more mobile than water, rose to the surface, leading to an initial increase in

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steam-heated surface features, such as heated groundwater, springs or fumaroles. In

places, steam pressures then declined due to natural processes involving pressure

reduction and cooling, combined with steam extraction, and the flow of steam to the

surface also declined. However, the surface steam flow-rates are hard to predict

accurately because there is an interaction between the steam pressure driving the flow,

and the “dry-out” effect from boiling of residual water which increases steam mobility.

The presence of groundwater quenching the upflowing steam is also significant. In

addition, an impermeable layer separating a newly-developed steam zone from

overlying groundwater can delay the effect of increased heating (through heat

conduction). It follows that establishing the contributions to cause and effect of the

interactions of long-term and short-term influences on surface steam flow can be

difficult.

8. CHANGES IN SHALLOW AQUIFERS POST DEVELOPMENT

8.1 Exhibits CJB 9, 10 and 11, illustrate a range of groundwater responses that have

occurred at Wairakei and Tauhara. The plots show water level changes in

representative shallow groundwater bores and lakes.

8.2 To summarize the changes at Wairakei, since 1960, downflows of groundwater in an

area between the Eastern and Western borefields have caused water level declines of

up to 40m (equivalent to about 4 bars) before stabilizing at the level of the ponded

Wairakei Stream. This is illustrated in exhibit CJB 9 by the purple, pink and blue data

plotted at the bottom of CJB 9.

8.3 Water levels in the Alum Lakes area have reduced locally, since 1997, by an estimated

10m, and have also recently been levelling off (exhibit CJB 10). The black data points

and lines are the data for the changes in water level in this figure.

8.4 Water levels in shallow bores in a part of northern Tauhara have declined by up to 8m in

the past decade (exhibit CJB 11). This is shown in the data at the top of the exhibit

labelled Tauhara Upper Aquifer. This is in contrast to the lake level aquifer where, as

this exhibit shows the water levels have been relatively stable for the last 25 years.

8.5 Shallow groundwater aquifers interact with deep geothermal aquifers in a complex way.

At Wairakei and Tauhara some shallow aquifers have provided avenues of surface fluid

discharge and others sources of near-surface fluid recharge. In some places,

groundwater has mixed with deep chloride water or steam condensate and dispersed as

fluid outflows. Some subsurface outflows have extended beyond the apparent

“boundaries” of the deep system. The chemistry of some peripheral or out-field wells at

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Wairakei shows that some outfield aquifers are chemically and thermally influenced,

despite the large induced pressure gradient which should be drawing most fluids

towards the centre of the field.

8.6 Pressure drawdown has caused some cool recharging groundwater to percolate down

into the system, heating up as it goes. The inflows are generally slow, but if too rapid,

they could prematurely cool the reservoir. Pressure drawdown has also caused rising

steam to boil the groundwater which then discharges as secondary steam to the surface

(e.g. at ‘Craters of the Moon’ and at Broadlands Rd Reserve). Groundwater aquifer

responses to reservoir pressure changes are therefore variable.

8.7 As noted previously, the increased upflow of heat at northern Tauhara caused by

Wairakei development, created steam-heated shallow aquifers, which proved valuable

for direct use by Taupo domestic or commercial interests. In southern Tauhara

(Waipahihi) the heated groundwater contains chloride water of deep origin, but the

source pressures driving deep chloride waters up to the shallow groundwater aquifers

have not diminished with time. There is no evidence of any pressure drawdown effects

from Wairakei production here, and historic direct use of the hot water aquifer has also

continued.

8.8 To summarize the previous two sections, the diverse responses of geothermal aquifers

to Wairakei-induced pressure or temperature changes leads to a range of effects on

surface thermal manifestations and associated thermal ecosystems. Some effects can

be considered beneficial, others may be adverse. Examples of past effects at Wairakei-

Tauhara include: areas of expanded and intensified steaming ground, more fumaroles,

increased areas of thermal vegetation, increased steam-heating of groundwater

aquifers, increased temperature of steam-heated springs, and thermal ecosystems

associated with hot water discharge channels. Examples of past adverse effects include

the production-induced decline in chloride springs and geysers at Geyser Valley and

Spa Park.

9. STRATEGIES FOR MANAGEMENT OF THE PRODUCTIVE POTENTIAL OF THE

RESOURCE

9.1 In the natural state, geothermal systems usually approach a dynamic equilibrium where

the deep inflows of energy and mass approximately balance the outflows. At Wairakei,

the initial induced pressure draw-down increased the amount of hot recharge into the

base of the production aquifer (possibly by more than 100%). With time, a new pressure

regime was established where the increased mass recharge again approximately

balanced the net discharge. This mass equilibrium has been maintained, presumably

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because the deeper recharge source is very large. Over the 48 years of development

reservoir temperatures have declined, however, so energy inflows are less than

outflows. Mr Clotworthy discusses this in his evidence.

9.2 Some of the increased induced mass recharge also consists of cooler fluids from the

reservoir edges and the overlying groundwater which, if inflowing too rapidly, this can

prematurely cool down the production aquifer. Cooler fluids that are injected directly

through wells also have the potential to cool the reservoir. The relative benefits or

adverse effects of in-field injection partly depend on the temperature of the recharge

fluid that it displaces. At Wairakei, the injectate (~90 oC) will be cooler than both the

inflowing groundwater recharge (which is heated up to about 150 oC along its inflow-

path to the production aquifer) and the deep recharge (about 260 oC). In terms of the

energy balance the net effect of infield injection is detrimental.

9.3 Reservoir management at Wairakei will involve countering the adverse effects of

premature temperature decline with appropriate and flexible production and injection

strategies. Such strategies will in all likelihood need to be adjusted to achieve the

correct balance. Flexibility in locating and utilising future injection wells both inside and

outside the hydrological edges of the system will, in my view, be the key to achieving a

successful outcome.

9.4 Geothermal resources are renewable when viewed over an appropriate timeframe.

With appropriate management the Wairakei-Tauhara geothermal system can be utilised

over a long term (~100 years). If it is then retired, pressure and temperature recovery

will follow, the former occurring more quickly than the later. In my opinion, the ability to

use the system over a long term without prejudicing its ability to recover constitutes a

sound long term resource utilisation strategy.

10. STRATEGIES FOR MANAGEMENT OF THERMAL FEATURES

10.1 Recovery of flows to individual dormant geothermal features is a potential consequence

of changes in field management. There are examples at Bao-Banati Springs, in

Tongonan, Philippines, (Bollanos and Parilla, 2000), and Kuirau Park, at Rotorua, (Scott

et al, 2005) where recovery of mineralised hot springs has occurred in response to

reservoir pressure increases through production/injection management. To sustain or

recover fumarolic or steam-heated activity requires maintenance or recovery of

pressures in the underlying steam zone feeding the features.

10.2 Mineralised springs are likely to resume flowing when source aquifer pressures are

increased, such as during a total production shut-down and recovery period, or as a

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result of targeted injection. The predicted recovery process, following a modelled shut

down of Wairakei in 2053 was published by O’Sullivan and Mannington (2005). This

revealed an approximate symmetry between the pressure draw-down rate and the

pressure recovery rate. In the case of Wairakei, pressures dropped by about 20 bars

over 10 years. They are expected to recover upon shut down at about the same rate.

So the timing of recovery in mineralised spring flow-rate (which is dependant on

pressure recovery) should be of the order of 10 years.

10.3 From the reservoir model, plots of temperature recovery rates in the production aquifers

are also similar to temperature decline rates. It is reasonable to conclude, in the

Wairakei case, that recovery to over 90% of predevelopment conditions is likely within a

period of time that is comparable to the resource utilisation period. The main factor that

would affect the ratio of recovery time to energy extraction time will be the ratio of net

energy extraction rate to energy recharge rate (natural plus induced) during utilisation. If

these remain balanced, then the recovery time should be similar to the extraction time.

10.4 The recovery of Geyser Valley and Spa Park spring temperatures (and therefore heat

flow and boiling effects such as geysering) will take longer, however, because of the

conductive heating and cooling effects along the fluid flow-path. Prediction of the timing

of full temperature recovery of springs is difficult because of the lack of detailed physical

knowledge of the parameters of the interacting aquifers and conduits feeding an

individual spring. The best available tools for this purpose are still well calibrated 3D

reservoir models, although in reality, the individual conduits will probably reheat more

quickly than the large scale block models would predict. The model predictions, in terms

of temperature recovery timing for hot springs are therefore conservative.

10.5 Despite the ability to manage thermal feature discharges by subsurface pressure

control, natural changes in shallow vent structure and interactions with cooling

groundwater can modify discharge patterns of individual features. An overall increase in

surface discharges is likely to be achievable with pressure and temperature recovery,

but the character of the resulting discharges cannot always be precisely predicted.

Geysers and sinter-depositing springs will return, but they may be different in character.

10.6 Recovery of some dormant thermal features may not be convenient where urban

development has expanded across historically active thermal areas. The history of

expansion of Taupo residential and industrial properties over the known steam-heated

thermal ground to the east of Taupo is an example where benefits and concerns

interact. This area increased in thermal activity during the 1960’s and 1970’s due to

Wairakei production. At the time, ground heating around buildings and services caused

concern before the heat flow started to decline again. Many property owners also

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enjoyed use of the shallow resource for heating purposes and development of this area

has continued to expand. Future thermal activity changes could potentially be seen as

both adverse and beneficial by different parties in the same area.

11. INFIELD AND OUTFIELD INJECTION STRATEGY

11.1 Criteria for differentiating “in-field” from “out-field” injection locations at Wairakei are

difficult to apply. Pressure response criteria are impractical to apply everywhere

because there are insufficient deep monitor wells, and only a few have a long history of

pressure monitoring. Depending on the objective, another criterion could include

evidence of geothermal influence.

11.2 One method of determining geothermal influence is to measure the subsurface

temperatures with depth. If these are greater than the normal regional temperature

gradient (about 50oC per vertical kilometre) then the aquifers are likely to be

geothermally affected. Another method is to take downhole fluid samples to determine

directly if aquifers are chemically affected. Geothermally influenced peripheral aquifers

are not usually suitable for use as potable fresh water resources. If the focus on

injection is avoidance of contamination of fresh surface water and groundwater

resources they could be considered suitable recipient aquifers for injection of

geothermal fluids.

11.3 There are several examples of peripheral wells at Wairakei where a deep aquifer (below

300m depth) has moderate temperatures of about 90 to 160 oC, and high sodium-

bicarbonate levels, indicating steam heating. These include: WK305, WK307, and

WK231. Exhibit CJB 1 shows the locations of these wells with respect to the nominal

field boundary (as defined in the Regional Plan). Other peripheral wells (WK227,

WK224, WK32, and WK33) have pressure histories that reveal drawdown of up to 10%

of the in-field pressure change, implying a weak hydrological connection. Another

peripheral well (WK223) has a pressure profile consistent with full hydrological

connection, but relatively low temperatures (100 oC) and dilute chloride chemistry,

suggesting mixing with fluid from external aquifers.

11.4 Some deep peripheral aquifers, beyond the nominal hydrological boundary, are natural

sources of cold water recharge into the geothermal system, through a low permeability

connection. Separated geothermal water injected into these recharge pathways will

preferentially flow towards the production wells, albeit rather slowly. The net effect of

replacing cold recharge with hotter fluid should be beneficial to the utilisation of the

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geothermal resource. Impacts on surrounding deep groundwater resources should be

limited in extent because of the preferred flow direction towards the geothermal field.

11.5 Some shallow groundwater aquifers within the boundaries of the deep geothermal

system are high quality potable water sources. An example is the shallow groundwater

at Taupo Gliding Club on Centennial Drive, within the north-eastern part of Tauhara

geothermal field (Bromley et al 2000). These aquifers are fed by rainfall recharge, and

are preserved from natural geothermal chemical influences by an effective cap over the

top of the geothermal aquifers. In my view, continued preservation of the quality of these

aquifers can be achieved through site specific management, particularly of the depth of

injection. The same approach should preserve shallow potable groundwaters in outfield

sectors by demonstrating that they are isolated from deeper peripheral aquifers by a cap

or aquiclude.

11.6 I consider that a site-specific but flexible injection management plan is the preferred

method of catering for all of the complications. Prescriptive regulation of the relative

amounts of infield and outfield injection could be counter-productive to optimizing both

environmental and sustainable geothermal resource utilisation outcomes.

11.7 The strategy for Wairakei-Tauhara I prefer is to set up an adaptive framework for

injection, governing injection both within and outside the nominal boundary of the

system. This would provide the consent holder with options for targeting aquifers

located within and around the periphery of the system. An objective of the framework

would be to select injection target aquifers which avoid rapid returns of injectate to the

production aquifers. This would reduce the risk of damaging the long term productivity of

the geothermal aquifers by premature cooling. Another objective would be to prioritize

for outfield injection deep peripheral aquifers that have previously been affected by

diffusion of geothermal fluid or gas through the field boundary zone. The purpose would

be to avoid compromising the future availability of peripheral fresh water aquifers for

other purposes. A third objective would be to achieve a dynamic balance between

injection into aquifers in the “waist” area, between Wairakei and Tauhara, for the

purposes of maintaining in-field Waiora formation pressures at Tauhara, with injection

into hydrologically more distant peripheral aquifers where the pressure and temperature

response on the resource would be more muted. A fourth objective would be to identify

deeper injection aquifers that are hydrologically isolated from shallow aquifers

containing high quality potable groundwaters.

11.8 To summarize, strategies for disposal of geothermal fluid by injection should, in my

opinion, remain as flexible as possible. Reasons for flexibility include: a) the dynamic

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behaviour of hydrological flow regimes, b) the variability and uncertainty of reservoir

parameters, and c) the site-specific nature of effects on groundwater aquifers.

12. PROPOSED OUTFIELD INJECTION AREAS

12.1 Three out-field injection areas are proposed for the Wairakei operation. These areas are

Aratiatia, Poihipi West, and Te Mihi North. They are shown on the map attached as

exhibit CJB 13. The potential for adverse effects on groundwater, on Aratiatia Dam,

and on Rotokawa Geothermal Field have all been considered. In a technical report

addressing the options for outfield injection of separated water (Bromley, Reeves and

Rosen, 2000), we undertook a survey of groundwater users, and considered the issues,

future needs and possible impacts on them. We also determined the chemical quality of

shallow groundwaters, and made assessments of the likely hydrological isolation of

shallow aquifers from deep peripheral aquifers that we considered to be potentially

suitable for injection.

12.2 The recommended strategy for outfield injection is to target deep aquifers where the

injection would not influence the shallow groundwater accessed and used by property

owners. Suitable aquifers are probably hosted within permeable Waiora or rhyolite

formations beneath the Huka Falls Formation. Positive magnetic anomalies suggest the

presence of buried rhyolite lavas in all three target areas.

12.3 In the proposed Aratiatia outfield injection sector good permeability has been

demonstrated outside the eastern field boundary at depths of 300m to 600m, below the

capping laying of the Huka Falls Formation. During 2005-2006, pumped injection has

been undertaken into WK305 (Exhibit CJB1) at well head pressures of about 25 bars.

No earthquakes large enough to be felt were induced by the injection. No other adverse

effects were detected. Pressure response was muted (<1%) in a deep monitor bore

(WK227), located towards Rotokawa, about 1 km from the injection zone. I therefore

consider that the potential for adverse affects on Aratiatia dam from induced seismicity

or ground uplift, or on the Rotokawa geothermal system as a result of fluid migration, is

low. These risks can appropriately be monitored by the ongoing operation of the

existing seismic and pressure response monitoring.

12.4 In the proposed Poihipi West outfield injection sector an existing injection well (WK680)

located outside the western field boundary has been successfully accepting about 2000

tonnes/day of steam condensate since 1997. Effects have been minor. A small pressure

rise of about 0.5 bars has gradually occurred in the local overlying groundwater. This

can be attributed to a small amount of subsurface leakage around the relatively shallow

casing of WK680. Future outfield injection of separated water in this sector should be

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undertaken using boreholes with deeper casings cemented into the bottom of the Huka

Falls formation to avoid such leakage. Pressure changes recorded in nearby WK224

(Exhibit CJB1) show a small pressure decline below the Huka falls formation of about 2

bars since 1968 in response to Wairakei pressure drawdown of 25 bars. This implies a

hydrological connection, with a subsurface flow gradient towards the field. A proportion

of any injected hot water would replace cold lateral recharge from this direction.

12.5 Te Mihi North is a sector that looks suitable for injection from the geological point of

view, in that there are probably some deep peripheral aquifers existing here below a

capping structure. However, the area needs further investigation with drillholes, and

possibly injection tests, to provide more information on the permeability structure and

the extent of separation of shallow and deep aquifers.

12.6 The results of further drilling and testing of these three areas will dictate the final design

of the future outfield injection scheme. Sufficient allowance should be made for backup

options and to maximize flexibility in future injection management, which will include a

monitoring programme designed to address possible associated effects. I recommend

that monitoring should include pressure, temperature and chemistry of fluids from

suitable monitor bores, and microseismic monitoring for induced earthquakes.

12.7 The consent conditions before the court contain a detailed monitoring regime governing

outfield injection including monitoring in the shallow groundwater aquifers. This regime

reflects my advice to Contact which was worked into conditions as part of the first

instance hearing process.

13. EFFECTS OF TARGETED INJECTION ON SUBSIDENCE

13.1 The principal cause of subsidence bowls in the Wairakei-Tauhara system is thought to

be pressure decline within the Waiora, Mid Huka or overlying groundwater aquifers.

Declining pore pressure has the effect of increasing the load by reducing support, and

causes compaction of highly compressible formations such as mudstones (parts of the

Huka Falls formation), or hydrothermally altered clay deposits.

13.2 At Wairakei, the relationship between aquifer pressure decline and subsidence is not

simple. As I described previously, there is no direct correlation between them.

Subsidence in the Wairakei anomaly had commenced by 1956 when pressures had just

begun to decline, but then continued for decades after the reservoir pressures stabilised

in the early 1980’s. Even the effects of injection after 1997, which raised deep Waiora

liquid pressures by about 2 bars, has not stopped the on-going subsidence, although

the rates have been declining since 1980. One possible explanation for the delayed

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response is the slow diffusion of pressure change through the impermeable mudstone.

An additional explanation is that the anomalous compaction is occurring as plastic

deformation. Subsidence continues but tails off in an exponential manner with time,

even after pressures have stabilised or slightly increased. The result is mostly

permanent (or irreversible) deformation.

13.3 If the deformation were simply elastic and linearly correlated with pressure, an

avoidance/mitigation option would be to undertake targeted injection into the

compacting formations to arrest the pressure decline and reduce the subsidence. This

option has been observed in one instance at Wairakei (outside the area of the main

bowl), where injection in 1993 into a liquid aquifer in the middle of the mudstone

formation at well WK303 (near the power station) caused a pressure rise, and the local

mudstone compaction and subsidence ceased for a while (Allis, 2000).

13.4 However, such a method has limited applicability. It works when the compacting layer

contains a permeable embedded sub-layer, or adjacent layer, that contains liquid and

can be re-pressurised. Without a permeable horizon adjacent to the compacting layer, it

will be very difficult to inject sufficient fluid to have an effect on formation pressures. It

could take a lengthy period of time (months or even years) for the injected fluid to

diffuse through the compacting layer and have the desired effect. Furthermore, if the

sub-layer used for injection is very permeable then fluid will readily flow to other parts of

the system and pressures will not increase sufficiently.

13.5 Even if there exists an adjacent permeable layer available to receive injection water,

injection will not necessarily be effective in reducing subsidence. If the targeted

injection aquifer adjacent to the compacting layer is two-phase (steam and water) the

cooler injected liquid will condense the steam, and pressures adjacent to the

compacting layer could fall. This could initially increase the rate of subsidence. The

length of time this will take will depend on the injection rate, the size of the steam zone,

and the formation porosity and permeability. Eventually, local re-saturation of the steam

zone will be complete, liquid pressures will start to rise, and the desired effect may then

be achieved.

13.6 In addition, cool liquid sinks because it is denser than hot liquid. So, if a good

permeable connection to the underlying production aquifer exists, quenching of the

shallow steam zone with cooler water is also likely to produce the adverse effect of

prematurely cooling the underlying production aquifer.

13.7 Finally, as I have previously noted, it is likely that the anomalous subsidence in the main

bowl areas is plastic (or visco-elastic) in origin. Targeted injection to increase pressures

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may have limited immediate effect on an existing irreversible consolidation process that

was initiated many years previously by the initial pressure decline.

13.8 Because of the risks of unintended consequences, any targeted infield injection to

reduce subsidence should be undertaken cautiously, and in small increments.

Monitoring would be required to understand the effects. In my opinion, it is not a

procedure that should be adopted routinely, and should only be used when subsidence

effects at the surface are potentially severe, and cannot be dealt with in any other

practical way.

14. SUBSIDENCE MECHANISMS

14.1 Causes for subsidence other than geothermal reservoir pressure decline are common in

the Taupo Volcanic Zone. They can include deep-seated tectonic movement. At

shallow levels, consolidation of clays can be caused by a groundwater pressure decline,

which may in turn be caused by rainfall decline or increased drainage through shallow

boreholes into deeper aquifers (Bromley et al 2004). Slumping into in-filled gullies may

occur as buried vegetation slowly decays. Collapse pits or tomos may form as a result

of sub-surface soil erosion from storm water inflows.

14.2 Tectonic deformation in the Taupo Volcanic Zone can explain changes in level, both up

and down, of up to +/- 20 mm/yr. The intervals between changes in direction can be

periods of several months or years. For example, exhibit CJB 14 shows a plot of Taupo

Airport levels from continuous GPS monitoring over the past 4 years. This site is outside

the Tauhara Geothermal Field. During 2005 the average rate was uplift of +15 mm/yr.

At another continuous GPS site near Rotorua the ground was subsiding over the same

time period by about -10 mm/yr.

14.3 There is an implied expectation that tectonic movement involves simultaneous

movement of relatively large areas of land. But these GPS results and repeat levelling

surveys have revealed that the anomalous rates of tectonic movement vary from place

to place. For example, Taupo Fundamental benchmark (near the Taupo Harbour) has

risen by 160 mm in 55 years relative to the Aratiatia dam benchmark (A93), about 10

km away. Although the long term rates of movement at this site average +3 mm/yr, the

rates between individual levelling surveys, several years apart, has varied between + 9

mm/yr and – 4 mm/yr, with the latest rate between 2001 and 2004 being + 8 mm/yr.

Higher rates of relative inflation (10 to 20 mm/yr) have been measured using

conventional surveying over areas of urban Taupo during 2004 and 2005.

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14.4 Another perspective on variations in surface deformation rates, both geographic and

time-related, comes from analysis of matching pairs of satellite-based differential radar

images (INSAR). An example is given in exhibit CJB 15. Despite some coherency

problems in places, due to vegetation growth, these images show anomalous

deformation rates that are largely consistent with conventional levelling discussed in the

evidence of Mr Currie.

14.5 In addition, they reveal rate changes outside of the geothermal fields that are probably

of tectonic origin. A typical width of such anomalies is about 5-10 km. This suggests a

source depth in the mid to lower crust, where viscous creep deformation caused by

elevated temperatures is likely. In the more brittle upper crust, strain relief by swarms of

small earthquakes is also common, particularly along the Taupo Fault Belt (north-west

of Taupo). This will also have an influence on local vertical deformation rates.

14.6 I conclude, from these observations, that monitoring for subsidence of geothermal origin

should take into account other probable causes of subsidence and inflation. If natural

tectonic rates can vary in time by +/- 20 mm/yr, across horizontal distances of 5 to 10

km, then this sets an appropriate baseline threshold. Satellite INSAR images provide a

general picture of the wider deformation environment, particularly over urban Taupo

where coherency between repeat images is good.

14.7 The shape of the edges of a subsidence bowl often gives an indication of its depth of

origin. The broader the shape, the deeper the source, because overlying formations

usually deform like a thick blanket, rather than collapsing in vertically at the edges. I

believe the analogy to be valid in the Taupo area, because the overburden is mostly

Taupo Pumice and Oruanui ignimbrite, which have relatively uniform geomechanical

properties. As discussed by Dr Allis in his evidence, this allows a “Geertsma” approach

to analytical modelling of the depth and horizontal extent of the top of the compacting

unit (approximated as a cylindrical body). At Wairakei, the broad shape implies a

compaction depth of about 100m, consistent with the probable presence at these

depths of compressible Upper Huka Falls mudstones.

14.8 Modelling the shape of the subsidence curve with time can also be undertaken using

analytical functions. An example of a function that I have fitted to the Wairakei

subsidence history at P128, the centre of the main bowl, is given in exhibit CJB 14.

Extrapolation of this “Boltzmann” function, assuming status quo conditions, suggests

that the Wairakei subsidence event is nearly complete, with only 0.5m predicted

additional subsidence to go, on top of the existing 15m. The RMS average difference

between calculated and observed levels is only 0.07m.

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14.9 At Spa Park (Taupo) the best fit to the available data suggests a depth of about 50 to

80m, also consistent with the probable presence of Upper Huka Falls mudstones.

These outcrop at the Waikato River to the northwest, and are at 120 m depth in THM4

to the southeast. The Lower Huka Falls mudstone formation may not be a significant

contributor to subsidence at Spa Park because, despite its presence in THM4, it is

absent in wells recently drilled nearby to the north (as discussed by Mr Rosenberg).

Pressure decline in the mid-Huka formation at this location is inferred from the historic

decline in hot chloride water discharges at the nearby Spa Sights area along the

Waikato River.

14.10 At Crown Road, Taupo, a smaller bowl (~0.5m deep, and about 300m across)

developed in the late 1990s, with a shallower origin depth of 30m to 60m. Based on

borehole information, discussed by Mr Rosenberg, I interpret this to have been caused

by collapse of thermal clays within a zone of shallow boiling groundwater (Bromley,

2004). The clays were originally bridged and protected from compaction by a strong

silicified layer that eventually failed under groundwater pressure decline. The

mechanism is therefore quite different from the other well-known Wairakei-Tauhara

subsidence bowls.

14.11 The cause of the upper groundwater level decline in this area is not known for certain.

Exhibit CJB 11 shows that water levels in the shallow aquifer are locally declining in the

Crown Road bowl (THM11 shown in orange on CJB 11), and have been declining in

nearby THM1 (Blue) and THM5 (Purple star) for at least 10 years. But there is no

evidence to date of declines (since 1980) in the nearby underlying “lake-level” aquifer

pressures (THM2 (Pink), Hospital bore (Purple box)). Also, the deep Tauhara bores that

intersect the Waiora reservoir aquifer north of Crown Road (e.g. TH3) have not shown

decreases in pressure since about 1980. Indeed, the deep liquid pressures have

actually risen in recent years by a small amount, possibly in response to Wairakei

injection.

14.12 It is inferred from this data that the perched upper aquifer is probably draining into the

lower aquifer, probably because of increased downflows within disused boreholes rather

than because of any deeper pressure drawdown.

14.13 An alternative explanation is possible. An underlying steam zone local to the Crown

Road area, and isolated from the nearby ‘Lake level’ aquifer at THM2 or Waiora deep

liquid aquifer at TH3, could have been declining in pressure with time and thereby

influencing the rate of downflow from overlying groundwaters. The presence of such a

steam zone might be linked to past pressure drawdown caused by past Wairakei

development. If subsidence is related to the pressure reduction in that steam zone, that

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would suggest a need to exercise caution when considering future reservoir

management actions.

14.14 In addition, attempts to mitigate the Crown Road subsidence anomaly with targeted

shallow injection could be detrimental for the users of nearby shallow steam bores

(domestic and commercial) because the shallow steam zone may collapse. This could

also cause the natural discharge of steam to numerous thermal features in the vicinity of

Crown Park to cease.

14.15 As discussed in evidence by Mr Booth and Mr Currie, the ground tilt and curvature

effects on nearby structures of the 0.5m Crown Road bowl have so far been very small.

Average annual subsidence rates have levelled off, and compaction rates are expected

to gradually diminish as the thin local layer of compressible clays reaches its

consolidation limit. My prediction of subsidence behaviour at the centre of the Crown

Road bowl, based on the Boltzmann function fit is illustrated in exhibit CJB 17. The

uncertainties are greater here, than at Wairakei, because of the limited duration of the

subsidence event.

14.16 As discussed earlier in my evidence, experience at Wairakei suggests that the

relationship between pressure decline and subsidence is not a simple one. Other

parameters probably play an important role such as effective formation compressibility

as a function of load and duration of compaction. Clays can deform in a plastic manner,

producing the phenomenon of creep. Chemical effects such as silica deposition or rock

dissolution could also be important in the way they affect changes in the rock strength

or compressibility.

14.17 Injection may seem a simple means of raising pressure and mitigating subsidence. But

in some places injection may not have the desired beneficial effect and could even have

adverse effects.

14.18 At Mokai, Rotokawa and Kawerau, relatively shallow injection (about 200m to 500m

deep) has been practiced for more than 5 years. Subsidence levelling has

demonstrated at these locations that locally rising pressure through injection has not

avoided or suppressed subsidence. In fact, cooling by injected fluids has probably

caused some thermal contraction and minor amounts of subsidence of the land

surrounding the injection wells. At Mokai for instance subsidence rates up to -20 mm/yr

have been reported around the injection wells.

14.19 I conclude that subsidence mechanisms at the different Wairakei-Tauhara bowls are not

necessarily the same. Infield injection may not work as a means of reducing

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subsidence. Ongoing creep may be a factor that delays or diminishes any response to

injection. Adverse effects of targeted injection may outweigh the potential benefits.

15. HYDROTHERMAL ERUPTIONS

15.1 Hydrothermal eruptions are natural events. In April 2005 and 1948, eruptions occurred

at remote hot springs in undeveloped Ngatamariki geothermal field. Eruptions can occur

in boiling springs through natural triggering processes, such as atmospheric pressure

changes, local earthquakes or shallow fluid flow changes as chemical deposition or

dissolution modifies spring vents. Many other geothermal fields contain small craters

that are the remnants of previous hydrothermal eruptions.

15.2 The primary cause of most hydrothermal eruptions in geothermal fields is rapid boiling

and ejection of near-surface liquids in a vent. This results in expansion forces that erode

material into the eruption column. The removal of rock causes a further pressure drop,

and even more rapid boiling. The process continues downwards until the boiling front

reaches rock that is too strong to be excavated by flashing fluid, or temperatures are

cooled by inflowing groundwater. After several hours or days, the eruption typically

collapses into a vigorous fumarole at the bottom of a crater, or a heated lake.

15.3 An alternative explanation for eruptions is the catastrophic failure of a shallow capping

structure which has been trapping accumulations of high pressure gas or steam. This

mechanism is thought to be much less common.

15.4 Some historical examples of steam eruptions near Wairakei (at Craters of the Moon)

and Tauhara (at the Pony Club-Broadlands Road site), are attributed to an increase in

steam flux into shallow boiling groundwater. This increase was a consequence of

Wairakei production-induced pressure drawdown and expansion of steam zones. Early

(pre-1950) aerial photographs show that these thermally active areas were also the

sites of natural (pre-development) hydrothermal eruptions.

15.5 An eruption in March 2001 at Alum Lakes, Wairakei has been attributed to rapidly

declining groundwater levels causing a boiling-temperature spring vent to erupt

(Bromley 2001). It was triggered by a relatively rapid local pressure decline. During the

late 1990’s there was an increase in the rate of groundwater level decline in this small

area. As groundwater drained into the underlying steam zone, water levels in several

nearby lakes were locally declining at about 1.4 cm/day. The water level was declining

because the underlying steam zone pressure had declined to a value (about 7 bars) that

balanced the overlying head of groundwater (about 70m). There was therefore

insufficient upflow of steam through connecting conduits (fractures that pass through the

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mudstone aquiclude) to suppress the downflow of water. The anomalous drainage was

a local event. It was not related to any changes in the Eastern Borefield or the edge of

the Wairakei subsidence bowl, about 2 km to the east. A map illustrating the relative

locations of these features and groundwater bores is shown in exhibit CJB 18. A plot

showing changes in water level of selected Alum Lake features is shown in CJB 10, and

in surrounding groundwater bores (202/0 and 229/0), in exhibit CJB 9.

15.6 The eruption event at Alum Lakes was not in any way related to subsidence related

events such as postulated fracturing of the mudstone aquiclude at the edge of a

subsidence bowl. Plots of subsidence data through the Alum Lakes area show relatively

uniform subsidence tilts, and relatively uniform subsidence rates with time (about 30 +/-

5 mm/yr since the 1960’s). These small variations with time can be readily explained as

tectonic in origin because similar variations in rate occur outside the geothermal system

boundary.

15.7 In addition, the groundwater aquifer did not drain uniformly across the Alum Lakes area,

nor was there a consistent decline between the Alum Lakes and the Wairakei

subsidence bowl. It formed a local cone or depression in the water table, implying local

downflow, not lateral flow to the east as far as the edge of the Wairakei subsidence

bowl where it has previously been postulated that fracturing of the mudstone aquiclude

might have occurred through shear failure. The groundwater bores surrounding Alum

Lakes declined in water level over 5 years by 2 to 3 m, (see exhibit CJB 9) whereas the

central Alum lakes declined by more than 10m. Furthermore, the timing of the water

level declines radiated outwards from the central Alum lakes area. This is evidence that

the event was locally focussed.

15.8 It is my opinion, based on all the data available, that there is no evidence of a causative

link between subsidence and hydrothermal eruptions. Such a link has not been

demonstrated. Accordingly, although it remains a potential mechanism, the link has not

been proven, so, there is no valid basis for concern that ongoing subsidence through

aquiclude fracturing could create hydrothermal eruptions in new or unexpected locations

16. MITIGATION OF HYDROTHERMAL ERUPTIONS THROUGH INJECTION

16.1 Hydrothermal eruptions are a natural phenomenon and, on appropriate sites, they can

become valued thermal features in their own right because of their dramatic displays

(e.g. at Craters of the Moon, and Ngatamariki). All thermal features where liquid

temperatures are close to boiling point can erupt. The triggers could be of natural origin

(such as atmospheric pressure drop) or artificial origin (such as excavation of

overburden, or rapid dewatering). Because eruptions occur intermittently and

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unpredictably, it could be difficult to differentiate natural from man-made causes. The

risks of an eruption increase when shallow temperatures are rising, water levels are

dropping, and when boiling groundwater is present close to the surface.

16.2 Water injection could reduce the chances of hydrothermal eruptions occurring in

Wairakei-Tauhara thermal areas. If quenching of a near-surface boiling aquifer with

cooler water reduces the temperature to below the boiling point, this will reduce the risk

of a local hydrothermal eruption. Such a practice is used by drilling companies to control

steam pressures in steam wells when they are being worked on. A similar technique of

targeted shallow injection of cold water may also be useful in a situation where boiling

water or vigorous steam vents were threatening surface infrastructure.

16.3 Injection of high temperature water (>100 oC) has the potential to increase or decrease

the risk of hydrothermal eruptions. If the injected liquid condenses steam, then the

steam upflow into heated groundwater will be reduced, and steam-heated thermal

features (including hydrothermal eruptions) will diminish. However, the injected fluid

may cause a pressure rise in other, more distant, parts of an aquifer. These may not yet

have been cooled by the flowing injectate. A possibility then exists for an increased

upflow of boiling liquid into shallow groundwater. Existing thermal features may increase

in activity, and associated hydrothermal eruptions could be triggered.

16.4 To summarize, hydrothermal eruptions are rare but they occasionally occur in existing

thermal areas where boiling liquids are close to the surface. They can be triggered by a

variety of rapid pressure reduction mechanisms. Injection and production strategies

have the potential to increase or decrease the chances of hydrothermal eruptions

occurring. In terms of risk management, where appropriate, shallow injection of cold

water into or beneath a thermal vent area might be a way of quenching steam flow and

reducing the chances of an eruption.

17. PEER REVIEW MANAGEMENT MECHANISMS

17.1 I have had experience on peer review panels reporting to Waikato Regional Council in

relation to three geothermal systems. The way these panels have historically operated

is that they bring together a range of independent technical expertise. The panels meet

regularly (generally annually, but more frequently as required) to review resource data

compiled in the draft annual monitoring reports. Panels meet less frequently for a more

general review of field management in respect of those systems where an Operational

or System Management Plan is required.

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17.2 I believe the process has worked well, adding a fresh perspective to the views of the

consent holder and its advisers, as well as technical input to the regulatory arm of the

Regional Council.

17.3 Over time, the role and functions of the peer review panel have expanded. In the

context of the flexible and adaptive management that I believe is the best way to

achieve optimum environmental and production outcomes for geothermal resource

management, the peer review panel will continue to have an important role assessing

data as it comes to hand, and reviewing proposed management strategies.

REFERENCES

1. Allis, R.G. 2000. Review of subsidence at Wairakei field, New Zealand. Geothermics vol.

29, p455-478.

2. Bollanos G.T. and Parilla E.V., 2000. Response of Bao-Banati thermal area to

development of Tongonan geothermal field, Philippines. Geothermics v 29 No 4/5 p499-

508

3. Bromley, C.J., Reeves R.R., Rosen, M.R. 2000. Wairakei geothermal field- assessment

of deep outfield injection beneath shallow groundwaters. GNS report 2000/111 in

technical reports submitted to EW with Wairakei Resource Consents Application March

2001, Contact Energy.

4. Bromley, C.J., 2001. Water level changes in Alum Lakes area, Upper Waiora Valley,

Wairakei. GNS report 2001/43; in Appendix 2 of Contact Ltd, Wairakei Geothermal Power

Plant, Application for Resource Consents, Provision of Further Information, September

2001.

5. Bromley, C.J.; Manville, V.R.; Currie, S.; Allis, R. 2004 Subsidence at Crown Road,

Taupo, latest findings. p. 12-13 In: Manville, V.R.; Tilyard, D. (eds) Geological Society of

New Zealand/New Zealand Geophysical Society/26th New Zealand Geothermal

Workshop, 6th-9th December 2004, Taupo.

6. Curtis R.J. 1988 Use of the shallow hydrothermal resource at Tauhara Geothermal Field,

Taupo. Proc. 10th NZ Geothermal Workshop, Auck. Univ. P31.

7. Merrett M.F. and Burns B.R. 1998. Thermotolerant vegetation of the Wairakei

geothermal field. Landcare report LC9798/119 in Technical reports submitted to EW with

Wairakei Resource Consent application, Contact Energy, March 2001.

8. O’Sullivan M.O., Mannington W. 2005. Renewability of the Wairakei-Tauhara

Geothermal Resource. Proc World Geothermal Congress 2005, Antalya, Turkey.

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9. Read S.A.L., Barker P.R., Reyes A.G. 2001. Consolidation properties of Huka Falls

formation – linkages to subsidence at Ohaaki and Wairakei. Proc. 23rd NZ Geothermal

Workshop. p57-67. Auckland Univ.

10. Scott B.J. Gordon, D.A., Cody, A.D., 2005. Recovery of Rotorua geothermal field, New

Zealand: progress, issues and consequences. Geothermics 34 159-183.

11. Wood C.P., Mroczek E.K., Carey B.S. 1997. The boundary of Wairakei Geothermal Field:

geology and chemistry. Proc 19th NZ Geothermal Workshop, Auck. Univ.

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