IN THE ENVIRONMENT COURT AT AUCKLAND IN THE … - Wairakei... · in the environment court at...

IN THE ENVIRONMENT COURTAT AUCKLAND

IN THE MATTER of the Resource Management Act1991

AND

IN THE MATTER of appeals under section 120 of theAct

BETWEEN ROTOKAWA JOINT VENTURELIMITED and MIGHTY RIVERPOWER LIMITEDENV-2006-AKL-000685 (formerlyENV A 0535/04)ENV-2006-313-000061 (topicreference)

AND TAUPO DISTRICT COUNCILENV-2006-AKL-000691 (formerlyENV A 0542/04)ENV-2006-313-000061 (topicreference)

AND CONTACT ENERGY LIMITED(ENV-2006-AKL-000692 (formerlyENV A 0543/04)ENV-2006-313-000061 (topicreference)

AND NZ PRAWNS LIMITEDENV-2006-AKL-000695 (formerlyENV A 0547/04)ENV-2006-313-000061 (topicreference)

Appellants

AND WAIKATO REGIONAL COUNCIL

Respondent

AND CONTACT ENERGY LIMITED

Applicant

EVIDENCE OF MICHAEL HORACE TIMPERLEY

AK778684 FINAL Page 2

1. INTRODUCTION

1.1 My name is Michael Horace Timperley. I hold a MSc (Hons) degree in

chemistry from the University of Auckland and a PhD degree from Massey

University, Palmerston North. I am employed by the Auckland Regional

Council as a Stormwater Technical Specialist and I also practise as a private

consultant. Prior to May last year I was Regional Manager Auckland for the

National Institute of Water and Atmospheric Research (NIWA) and Principal

Scientist for aquatic chemistry. I also lead a team of chemistry and hydrology

specialists. I have worked on environmental chemistry issues for more than

30 years, firstly for the New Zealand DSIR, then for Environmental Science

and Research Ltd and subsequently for NIWA, both Crown Research

Institutes. I have published and presented research papers on the influence

of geothermal fluids on the chemistry of the Waikato River and lakes in the

Taupo and Rotorua areas. I have also investigated the actual and potential

environmental effects on receiving waters of geothermal fluid discharges

from commercial utilisation of the geothermal resources at Wairakei, Ohaaki,

Mokai and Ngawha.

1.2 I confirm that I have been supplied 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 with that code. Except where I state that I

am relying on the specific evidence of another person, my evidence 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.

1.3 In my evidence I will primarily address:

• Chemicals in geothermal fluid from the Wairakei/Tauhara Geothermal

Field.

• The discharge of geothermal fluid and arsenic to Lake Taupo and the

Waikato River from the Wairakei/Tauhara Geothermal Field before and

after development of the Wairakei Power Station.

• The effects of arsenic in the Wairakei Power Station separated

geothermal water discharge on Waikato River water and sediment

quality.

• Hydrogen sulphide and mercury in geothermal steam and in the

Wairakei Power Station cooling water discharge


• The effects of hydrogen sulphide in the Wairakei Power Station cooling

water discharge on the quality of Waikato River water.

• The effects of mercury in the Wairakei Power Station discharges on the

water, sediments, aquatic animals and plants of the Waikato River.

2. CHEMICALS IN GEOTHERMAL FLUID FROM THE WAIRAKEI/TAUHARA

GEOTHERMAL FIELD.

2.1 Steam is produced to feed the Wairakei Power Station by drawing

geothermal fluid from deep within the reservoir of the Wairakei/Tauhara

Geothermal Field. Some of this fluid is separated into steam and water in

separation plants at ground level and some of the fluid is drawn from the

reservoir entirely as steam without a water phase (sometimes referred to as

“dry steam”).

2.2 At the Wairakei Power Station the steam passes through the turbines and is

then condensed back to liquid water by cooling with a spray of Waikato River

water. Approximately 0.4 tonne s-1 of steam is condensed with approximately

16.8 cumecs of river water. The resulting mixture, the “cooling water”, is

discharged to the Waikato River.

2.3 During the boiling and separation of steam and water in the separation

plants, the carbon dioxide, hydrogen sulphide and mercury contained in the

fluid mostly transfer to the steam phase. Dry steam also contains these

substances. No other chemicals occur in the steam or, as a result of the

condensing process in the Station, in the cooling water in sufficient quantities

to be of environmental significance.

2.4 Of the chemicals added to the cooling water by the steam:

(1) Hydrogen sulphide can be toxic to aquatic life.

(2) Carbon dioxide in the cooling water lowers the Waikato River water pH,

that is, it makes the water more acidic. The lower pH increases the

concentration of hydrogen sulphide as I will explain later.

(3) About half of the mercury in the cooling water has accumulated in the

bed sediments of Lake Ohakuri and lakes further downstream as I will

discuss in more detail later.

2.5 The geothermal water remaining after the separation of steam is referred to

here as “separated geothermal water”. The separated geothermal water from


the Wairakei bore field is discharged into the Waikato River a short distance

upstream of the Wairakei Power Station. This water contains quantities of all

the chemicals in the original geothermal fluid. The dominant chemical is

sodium chloride which is of no environmental concern in the Waikato River.

In addition to the other chemicals commonly found in fresh and salt waters

such as calcium, potassium, magnesium, sulphate and bicarbonate,

geothermal water also contains mercury, sulphide, ammonia, lithium,

caesium and rubidium. On the basis of existing knowledge, the quantities of

these elements in the separated geothermal water from Wairakei are, by

themselves, too small to cause adverse effects in the river, although mercury

in the separated water adds to the effects of the mercury in the cooling water.

3. THE DISCHARGE OF GEOTHERMAL FLUID AND ARSENIC TO LAKE

TAUPO AND THE WAIKATO RIVER FROM THE WAIRAKEI/TAUHARA

GEOTHERMAL FIELD BEFORE AND AFTER DEVELOPMENT OF THE

WAIRAKEI POWER STATION.

3.1 I will now describe the procedure I used to estimate the amount of

geothermal fluid and the arsenic it contained that discharged naturally from

the Wairakei/Tauhara Geothermal Field before development of the Field. I

will also compare this pre-development discharge with the present discharge.

3.2 Very few measurements of arsenic were made in the geothermal fluid from

the Wairakei/Tauhara Geothermal Field before field development. Some

measurements of chloride ion concentrations were made, however, and

these chloride ion measurements can be used to estimate the arsenic

concentration in the pre-development fluid as I will now explain.

3.3 The procedure I used was to firstly estimate the amount of chloride ion

discharged naturally from the Field before field development and then to

multiply this by the ratio of the arsenic concentration to the chloride

concentration in the fluid. This was possible because the arsenic to chloride

ion concentration ratio in the geothermal fluids from the Wairakei/Tauhara

Geothermal Field is known and this ratio has not changed appreciably over

time.

3.4 My best current estimates of the chloride ion mass loads into Lake Taupo

and into the Waikato River between Taupo Control Gates (TCG) and the

Aratiatia dam before and after the Wairakei/Tauhara Geothermal Field was

developed are given in Table 1. I made these estimates from published data

as I will now explain.


3.5 For estimating these mass flows I assumed that neither the flows of

freshwater nor its typical chloride concentration of about 3 g m-3 (Timperley,

1983) have changed since before the development of the Wairakei/Tauhara

Field. In my opinion this is a valid assumption.

Table 1: Natural mass flows (g s-1) of chloride ion from the Wairakei/Tauhara

geothermal field before and after development of the field.

Lake andriversection

Source ofgeothermalwater

Inflow pathway Pre-development

mass flow

Post-development

mass flow

Lake Taupo Tauhara Waipahihi Stream 9k

Tauhara Groundwater 0.1j

Tauhara Total inflows toLake Taupo

70g 9

TaupoGates toHuka Falls

Tauhara OtumuhekeStream

included in 6 4.4d

Wairakei WaipouweraweraStream

21c 0.5d

Tauhara Huka Falls Creek included in 6 3.2d

Wairakei/Tauhara

Other inflowsincludinggroundwater

159c 37

Wairakei/Tauhara

Total inflows forTaupo Gates toHuka Falls

180 45f

Huka Fallsto Aratiatia

Wairakei Dry Gully Creek included in 10 0.2e

Wairakei Kiriohineki Stream(Alum Lakes)

included in 10 0h

Wairakei Kiriohineki Stream(Waiora Valley)

9b <9

Wairakei Wairakei Stream 240a 3.8e*Wairakei Other inflows

includinggroundwater

89c <89

Wairakei Total for HukaFalls to Wairakei

338 <102

Total fromWairakei/Tauharafield

588 <156


a Glover (1998)b Glover (1998) from Ellis and Wilson (1955)c Ellis and Wilson (1955)d Gibbs (1987)e Environment Waikato geothermal monitoring programme (Timperley, 1994).f Huser (1989)g Allis (1988)h Contact Energy (flow from Alum Lakes now ceased)k Gibbs (1979), Timperley unpublished data.j see text

* This value is for a site immediately upstream of the original Geyser Valley and

so may be an under-estimate of the present-day natural flow from the valley.

Pre-development chloride ion mass flows from the Tauhara part of the Field

3.6 The pre-development flow of geothermal fluid from the Tauhara part of the

Field was estimated by Allis (1988) from the heat output of the inferred area

of deep geothermal resource and the surface drainage pattern. Because the

flow of chloride ion is directly related to the heat flow, Allis (1988) was able to

estimate the chloride ion mass flow.

3.7 This estimate of the chloride ion mass flow made by Allis (1988) was 140 g s-

1 and based on the heat flow, Allis (1988) estimated that about half of the

chloride ion could have been entering Lake Taupo. The other half would have

been entering the Waikato River.

Pre-development chloride ion mass flows from the Wairakei part of the field

3.8 The chloride ion mass flows in the Waikato River were measured in 1954 by

Ellis and Wilson (1955) at the outlet of Lake Taupo, at Huka Falls, at the

Aratiatia Rapids, in the Waiora Stream (Kiriohineki Stream) and at two sites

in the Wairakei Stream. From these data they estimated that 180 g s-1 of

chloride from geothermal fluids entered the river between Taupo Control

Gates and Huka Falls. A chloride ion mass flow of 21 g s-1 was estimated at

that time by Healy (pers.com. to Ellis and Wilson) for the Waipouwerawera

Stream which is one of the geothermal fluid flows into this section of the river.

3.9 In my opinion, these are reliable estimates of pre-development mass flows

because they are unlikely to have been greatly affected by field development

during the relatively short time between the start of development in about

1950 and the time that the measurements were made by Ellis and Wilson in

1954.


3.10 The balance of 159 g s-1 originated from the other chloride ion mass flows

into this section of the river. These other mass flows included 70 g s-1 from

the Tauhara part of the Field and 89 g s-1 from the Wairakei part of the Field.

3.11 By 1954 there were 20 bores extracting geothermal fluid from the Wairakei

part of the Field. These bores would have affected the natural geothermal

flows from Geyser Valley and this was taken into account by Glover (1998) in

deriving the estimate of 240 g s-1 given in Table 1 for the Wairakei Stream.

3.12 Glover (1998) considered that the bores were unlikely to have similarly

affected the flows from Waiora Valley so I consider that the estimate of 9 g s-1

made by Ellis and Wilson (1955) is a reliable estimate of the pre-

development mass flow from this area.

3.13 Other flows of geothermal water were estimated by Ellis and Wilson (1955) to

add a further 89 g s-1 of chloride ion to the river between Huka Falls and the

Aratiatia dam.

3.14 Thus, my best estimate of the total pre-development mass flow of chloride

ion into Lake Taupo and into the Waikato River from the Wairakei/Tauhara

Geothermal Field is 588 g s-1.

Post-development chloride ion mass flows from the Wairakei/Tauhara

Geothermal Field

3.15 On two occasions in 1978 the chloride ion concentration in the Waipahihi

Stream was 280 g m-3 (Timperley unpublished data) and the flow at the time

was about 0.031 cumecs (Gibbs, 1979). This gives a chloride ion mass flow

of about 9 g s-1 for this stream.

3.16 Gibbs (1979) estimated that the flow of groundwater through the Taupo

beach front in 1979 was about 0.5 L s-1. The chloride ion concentration in this

groundwater has not been measured but even if all of this flow is assumed to

be undiluted geothermal fluid with a chloride ion concentration of 1600 g m-3,

which is typical for the geothermal water from the Tauhara part of the Field,

the chloride ion mass flow in this groundwater would be only about 1 g s-1.

This is certainly an overestimate because warm groundwater occurs over

only a few short sections of the beach at its eastern end and the cold

groundwater has a chloride concentration of about 3 g m-3. In my opinion,

the average chloride ion concentration for groundwater over the whole length

of the 3 km beach is unlikely to be greater than 150 g m-3. At this


concentration the chloride ion mass flow with this groundwater flow would be

about 0.1 g s-1.

3.17 The chloride ion mass flows calculated by Gibbs (1987) from water flows and

chloride concentrations measured between 1978 and 1982 for the

Otumuheke and Waipouwerawera Streams and Huka Falls Creek were 4.4,

0.53 and 3.2 g s-1 respectively.

3.18 The total chloride ion mass flow into the Waikato River between Taupo Gates

and Huka Falls (including Huka Falls Creek) in 1989 was estimated by

Waikato Regional Council (“Environment Waikato”) to be about 45 g s-1

(Timperley, 1994).

3.19 Subtracting the chloride ion mass flows for the Otumuheke and

Waipouwerawera Streams and Huka Falls Creek of about 8 g s-1 from the

Environment Waikato estimate of 45 g s-1 implies a mass load of about 37 g

s-1 from groundwater and riverbed springs.

3.20 The total chloride ion mass flow in Dry Gully Creek and the Kiriohineki

Stream determined by the Environment Waikato geothermal monitoring

programme between 1986 and 1992 was about 9 g s-1. Contact has advised,

however, that flow from the Alum Lakes into the Kiriohineki (Waiora) Stream

has now ceased which means that the chloride ion mass flow from this

stream is now less than 9 g s-1.

3.21 The best measurement of the natural chloride ion mass flow in the Wairakei

Stream of about 3.8 g s-1 was made by the Environment Waikato geothermal

monitoring programme between 1986 and 1992. This value is, however, for a

site immediately upstream of the original Geyser Valley and so the value

might be slightly less than the present-day natural flow from the Valley.

3.22 The other pre-development mass flow of about 89 g s-1 into this part of the

river has not been measured post-development but there is no doubt that this

has decreased.

3.23 The possible size of this decrease can be estimated from the decline in the

chloride ion mass flow in the Waipouwerawera Stream which is the only

natural geothermal inflow to the Waikato River for which there are reliable

pre- and post-development chloride ion mass flows.

3.24 The 1954 estimate for this stream was 21 g s-1 (Healy, pers. com. to Ellis and

Wilson) and the measured value during the 1978 to 1982 period was 0.53 g


s-1 (Gibbs, 1987). If this decrease is assumed to have occurred for all other

natural geothermal chloride ion mass flows into the river from the Wairakei

part of the Field, then the pre-development mass flows of 98 g s-1 for all

inflows other than the Wairakei Stream probably decreased to about 2 to 3 g

s-1 after field development.

3.25 Thus, the total post-development natural chloride ion mass flow is probably

about 7 g s-1 for the Huka Falls to Aratiatia section and 52 g s-1 for the Taupo

Gates to Aratiatia section.

Present-day flow of separated geothermal water equivalent to the decline in

the natural flows from the Wairakei/Tauhara Field

3.26 Allowing for the uncertainty of the post-development natural chloride ion

mass flow of between 61 g s-1 and 156 g s-1, the decline in the natural

chloride ion mass flow is between about 432 and 527 g s-1.

3.27 This decline can be expressed as an equivalent discharge of present-day

Wairakei separated geothermal water. The flow-weighted chloride ion

concentration in the present-day separated water is 1930 g t-1. This gives an

equivalent discharge of SGW from the Wairakei bore field of between about

19,300 t d-1 (tonnes per day) and 23,600 t d-1.

3.28 The low end of this range assumes no decline in the unmeasured pre-

development chloride ion mass flow of 89 g s-1 into the river between Huka

Falls and Aratiatia; a very unlikely scenario. Accordingly, the most probable

equivalent present-day discharge of separated geothermal water is nearer

the upper end of this range, say, about 23,000 t d-1.

Pre- and post-development arsenic mass flows from the Wairakei/Tauhara

geothermal field

3.29 The ratios of chloride to arsenic in fluids from the Wairakei/Tauhara

Geothermal Field are plotted against time in Figure 1. The earliest results for

the period December 1952 to January 1959 were reported by Ritchie (1961).

Mahon and Glover (1965) reported the results for 1965 and the results for the

Wairakei drains in 2000 were reported by Ray et al. (2001)

3.30 There is one anomalous feature of the data in the figure and that is the

increasing ratio for the Wairakei bore fluids from about 430 to about 500 over

the period 1952 to 1959 (all these data are reported in Ritchie,1961). Ritchie


did not comment on this apparent trend but there is good evidence to show

that these measurements were incorrect.

3.31 The analytical methods Ritchie used to measure chloride and arsenic in the

1959 samples differed from those used for the earlier samples. The pre-

1959 method Ritchie (1961) used for arsenic was published by the American

Public Health Association, one of the definitive authorities on water quality

analyses. This method was, therefore, likely to have been reasonably

reliable.

3.32 The method used for the 1959 analyses was adapted by Ritchie from a

literature method and although he achieved almost complete recovery of

arsenic added to distilled water, this does not guarantee that he achieved

complete recovery from natural samples. In fact, Ritchie’s result for Lake

Taupo water using the 1959 method was 0.5 mg m-3 whereas the present

concentration is about 10 mg m-3. Geothermal fluid discharges into Lake

Taupo have almost certainly decreased since the 1950s (those from Tauhara

have) so the lake water concentration of arsenic in the late 1950’s would

have been at least 10 mg m-3 and possibly higher. It seems likely that the

higher chloride to arsenic ratios obtained for the 1959 samples were the

result of the 1959 method developed by Ritchie recovering only about 90% of

the arsenic from the geothermal samples.

3.33 Excluding these anomalous high results, the range for the pre-1959 ratios is

similar to the range for the post-1959 ratios. It can be concluded, therefore,

that there has been little change in the ratio over time and that the ratio for

fluids from the Wairakei part of the Field are similar to those obtained during

the 1950’s for fluids from the Tauhara part of the Field. There are no recent

data for Tauhara fluids.

3.34 Excluding the 1959 data, the mean chloride to arsenic ratio for the other 22

results is 485. The mass flow of chloride in the natural discharges of

geothermal fluids from the Wairakei/Tauhara Field was estimated to be about

588 g s-1 before development of the Field and between 61 g s-1 and 156 g s-1

now. Applying the above mean chloride to arsenic ratio gives estimates of the

natural mass flow of arsenic of about 1.2 g s-1 or 38 t year-1 before

development and between 0.13 g s-1 (4 t year-1) and 0.32 g s-1 (10 t year-1)

after development.

3.35 It is almost certain that the pre-development natural flows of geothermal

water into the Waikato River between Taupo Gates and Aratiatia have


decreased as explained previously for the chloride ion, and for this reason it

is likely that the present natural mass flow of arsenic is towards the lower end

of this range, i.e., closer to 4 t year-1 than to 10 t year-1. The natural mass

flow of arsenic has, therefore, reduced from about 38 t year-1 before

development began to about 4 t year-1, a reduction of 34 t year-1.

350

400

450

500

550

600

650

700

Jan-50 Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00 Jan-05

Ch

lori

de/

Ars

enic

con

cen

trat

ion

rati

o

Wairakei boresWairakei drainsGeyser Valley geysersTauhara geysersTauhara bores

Figure 1. The chloride to arsenic concentration ratio over time in fluids from the

Wairakei/Tauhara geothermal field.

4. THE EFFECTS OF ARSENIC IN THE WAIRAKEI POWER STATION

SEPARATED GEOTHERMAL WATER DISCHARGE ON WAIKATO RIVER

WATER AND SEDIMENT QUALITY.

Water Quality

4.1 The concentrations of arsenic in geothermal water are much higher than in

other surface and groundwaters and, consequently, the separated

geothermal water discharge from the Wairakei Power Station causes

elevated concentrations of arsenic in Waikato River water.

4.2 At all locations downstream of the Wairakei/Tauhara Geothermal Field,

arsenic concentrations in the river water exceed 10ppb which is the New

Zealand Maximum Acceptable Value for treated drinking water (Ministry of

Health, 2005). A ppb is a part per billion and is one thousandth of a ppm or

part per million.


4.3 This Maximum Acceptable Value would be exceeded most of the time and at

most locations on the river even if there were no geothermal water discharge

from the Wairakei/Tauhara Geothermal Field. This is because the arsenic

concentration is at 10ppb in Lake Taupo water, and natural flows of

geothermal water into the river downstream of the Taupo Control Gates

(TCG) add additional arsenic.

4.4 To enable the effects of changing the separated geothermal water discharge

from the Wairakei Geothermal Field on arsenic concentrations in the Waikato

River water to be estimated, I developed a mathematical model of arsenic

concentrations in the Waikato River water.

4.5 This model was developed from the known inputs of arsenic to the river

(Timperley and Huser, 1996) and is based on the assumption that arsenic is

not lost from the river water during its passage to the sea. This assumption

is not completely correct, it has been estimated that approximately 7% of the

arsenic entering the river is retained in the reservoir bed sediments (Aggett

and Aspell, 1980). Model estimates of arsenic concentrations in the river

water are, therefore, slightly greater than the actual values.

4.6 The concentrations calculated with the model assume that the flow at Taupo

Control Gates has persisted long enough for the flows at all points in the

Waikato River to have stabilised and that the ratio of the river flow at any

location to the flow at Taupo Control Gates is at its long-term average value.

For example, the long term average ratio of the flow at the Waipapa Dam to

the flow at Taupo Control Gates is 1.434 so the model assumes that when

the flow at Taupo Control Gates is 83 cumecs, the flow at the Waipapa Dam

is 119 cumecs.

4.7 For the river upstream of the Karapiro Dam, the concentrations estimated by

the model agree very closely with those measured by Environment Waikato

in its long-term river monitoring programme. Downstream of the Karapiro

Dam there are unmeasured flows of both arsenic into the river, possibly from

waste discharges and groundwater. The model was adjusted to fit the

measured concentrations in this lower section of the river by adding

additional inputs.

4.8 Figure 2 shows the concentrations of arsenic in the river water at various key

locations calculated using the model for different discharges of geothermal

water from the Wairakei/Tauhara Geothermal Field and for different river

water flows at Taupo Control Gates. The flows used were 130 cumecs for the


pre-development situation, 160 cumecs for the present-day situation with a

separated water discharge of 60,000 t d-1 and 50 cumecs for both situations.

130 cumecs was the approximate pre-development average river flow at

Taupo Control Gates and 160 cumecs is the approximate present day

average river flow at Taupo Control Gates. The concentrations shown in

Figure 2 for the two flows are, therefore, the approximate annual average

concentrations for the pre-development and present-day situations

respectively. A separated geothermal water discharge of 60,000 t d-1 is the

maximum permitted discharge in Resource Consent No 104711.

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350Distance from Taupo Control Gates km

Ars

enic

con

cen

trat

ion

pp

b

Tau

po

Co

ntr

olG

ates

Oh

aku

riD

am

Hu

ntl

y

Ham

ilto

n

Wai

pap

aD

am

Wh

akam

aru

Dam

Ara

tiat

iaD

am

Mer

cer

Predevelopment natural geothermal discharge. TCG flow 50 cumecs

Predevelopment natural geothermal discharge. TCG flow 130 cumecs

60,000 t/d SGW discharge. TCG flow 160 cumecs

60,000 t/d SGW discharge. TCG flow 50 cumecs

Oh

aaki

Ara

pu

niD

am

Mar

aeta

iDam

Figure 2. Calculated concentrations of total dissolved arsenic in Waikato River

water for various quantities of geothermal water discharged from the

Wairakei/Tauhara geothermal field at river flows at Taupo Control Gates of 130

cumecs for the pre-development situation, 160 cumecs for the present-day situation

with a separated water discharge of 60,000 t d-1, and 50 cumecs for both situations.

4.9 Prior to development of the Wairakei/Tauhara Geothermal Field, the annual

average arsenic concentrations in the river water at Hamilton would have

been about 21ppb and 45ppb for flows at Taupo Control Gates of 130

cumecs and 50 cumecs respectively. At Mercer where Watercare now

extracts water for treatment and transport to Auckland, the average arsenic

concentrations would have been approximately 19ppb and 42ppb

respectively for these two Taupo Control Gate flows.


4.10 At a separated geothermal water discharge of 60,000 t d-1, the approximate

present discharge, the average concentrations would be about 24ppb and

63ppb at Hamilton and about 21ppb and 55ppb at Mercer, for river flows at

Taupo Control Gates of 160 cumecs and 50 cumecs respectively.

4.11 Thus, the annual average arsenic concentrations in the river water at

Hamilton and Mercer prior to development of the Wairakei/Tauhara

Geothermal Field would have been 85 to 90% of present-day concentrations.

4.12 Previous studies (McLaren and Kim, 1995, Webster-Brown and Lane, 2005)

found considerable variation over a year in the river water dissolved arsenic

concentrations, with summer concentrations up to three times higher than

winter concentrations. For example, at Hamilton summer concentrations of

about 60ppb and winter concentrations of about 20ppb have been recorded.

It is possible, therefore, that for short periods during summer river water

concentrations could be 50% higher than the annual average concentrations

calculated with the model.

4.13 Monitoring by Contact between November 2005 and the end of May 2006 did

not, however, detect these high summer concentrations. The reasons for this

are unknown.

Sediments

4.14 Table 2 shows the concentrations of arsenic in the bed sediments of Lake

Taupo and of the reservoirs of the Waikato River measured prior to the start

of reinjection of Wairakei separated geothermal water (Hickey et al, 1995).

All sediments contained arsenic at concentrations exceeding one or both of

the ANZECC 2000 Interim Sediment Quality Guidelines.

4.15 These guidelines are an indication of the risk of sediment-bound arsenic to

aquatic life. In laboratory tests about 10% of sediment samples with arsenic

concentrations equal to the ANZECC ISQG-Low showed adverse effects on

the test animals. For the ISQG-High, about 50% of samples showed adverse

effects.

4.16 A small proportion of the arsenic in these sediments originates from the

weathering of minerals in non-geothermal environments, but the major

proportion originates from geothermal fluids.

4.17 The discharge of separated geothermal water from the Wairakei Geothermal

Field to the Waikato River has reduced since reinjection commenced. As a


consequence, the arsenic concentrations in the surface layers of reservoir

bed sediments would have begun to decrease as the old sediment has been

mixed with new sediment containing less arsenic. Provided that the quantity

of separated geothermal water does not increase in future above about

60,000 t d-1, then this decrease in the sediment arsenic concentration will

continue for sometime into the future.

4.18 The eventual extent of this decrease in sediment arsenic concentrations is

predictable. If the discharge of separate geothermal water remains at about

60,000 t d-1 then the sediment arsenic concentrations will eventually stabilise

at about 60% of the concentrations that existed prior to the start of

reinjection, assuming that the chemical conditions that influence the

accumulation of arsenic into the reservoir sediments do not change. The

present discharge is about 60% of the discharge before reinjection started.

Table 2. Arsenic concentrations in bed sediments from Lake Taupo and the Waikato

River. Numbers in italics exceed the ANZECC 2000 ISQG-Low guideline. Numbers

in bold italics exceed the ISQG-High.

Site Arsenic concentration (ppm)

Taupo 7.9

Aratiatia 69.4

Ohakuri 111.0; >200; 103 – 1340

Maraetai 101 – 233

Waipapa 859 –1520

Karapiro 222

Hamilton 60.1

ANZECC guidelines ISQG-Low 20

ANZECC guidelines ISQG-high 70

5. THE EFFECTS OF HYDROGEN SULPHIDE IN THE WAIRAKEI POWER

STATION COOLING WATER DISCHARGE ON THE WAIKATO RIVER

WATER QUALITY

5.1 As I have described previously, geothermal fluids add hydrogen sulphide, a

water soluble gas, and mercury to the Waikato River. However, with the

exception of flows of fluids into the river through the river bed and the cooling

water from the Wairakei Power Station that contain condensed geothermal

steam, the amounts of hydrogen sulphide and mercury added to the river by

geothermal fluids are likely to be small. This is because most of these fluid

flows are able to lose gases, including hydrogen sulphide and mercury, to the


atmosphere before they reach the river. For flows through the river bed,

however, the hydrogen sulphide and mercury could dissolve into the river

water rather than escape to the atmosphere.

5.2 Early investigations by NIWA scientists (Ray et al, 2001) identified hydrogen

sulphide arising from the cooling water discharge as a potential threat to

aquatic life in the Waikato River. A comprehensive programme was

undertaken by NIWA and Contact staff to determine the range of

concentrations and the level of effects of hydrogen sulphide in the Waikato

River and to assist in developing appropriate strategies for avoiding,

remedying or mitigating those effects. The proposed strategies for avoiding

the discharge of hydrogen sulphide to the river are based on replicating

natural oxidation processes which I will discuss in a moment.

5.3 Hydrogen sulphide is one of three chemical species containing sulphur that

together constitute total sulphide in aqueous systems. The other two species

are bisulphide and sulphide. All three species are soluble in water and all are

present if total sulphide is present. The proportions of the species change as

the water pH changes. As the pH decreases, that is, the water becomes

more acidic, the proportion of hydrogen sulphide increases. Hydrogen

sulphide is the toxic form of total sulphide.

5.4 Hydrogen sulphide is difficult to measure. Surveys of the type undertaken for

this study measure the concentrations of total sulphide and from these

results, and the measured water pH, temperature and concentrations of other

dissolved substances, the concentrations of hydrogen sulphide are obtained

by calculation.

5.5 Total sulphide was measured on numerous occasions by NIWA and Contact

staff during the period November 1999 to July 2000 in the cooling water and

at various sites in the river downstream of the cooling water outfall. The

results are shown in Figure 3. The source of steam supplied to the Wairakei

Power Station has not changed appreciably since that time so the total

sulphide concentrations in the river will not have altered to any extent either.


0

1

10

100

1000

10000

0 2000 4000 6000 8000 10000 12000

Distance downstream from Huka Falls (m)

To

tals

ulp

hid

eco

nce

ntr

atio

n(p

pb

) coo

ling

wat

er

Ara

tiat

iad

am

Rap

ids

Jet

jett

y

Fu

lljam

esra

pid

s

Figure 3. Total sulphide concentrations in the cooling water and in Waikato River

water at various distances downstream from Huka Falls. Huka Falls is position 0m

on the figure.

5.6 The concentration of total sulphide in the cooling water was approximately

1000ppb. The total sulphide concentration in the river water was about

20ppb at the Aratiatia Dam, a reduction of about 50-fold from the

concentration in the cooling water, and 2ppb at the Fulljames Rapids, that is

an overall reduction of about 500 fold relative to the cooling water. The wide

range of concentrations at the first sampling site downstream of the outfall

was due to the incomplete and variable mixing of cooling water into the river

water.

5.7 To assist with assessing how the concentrations of hydrogen sulphide in the

river water would change under varying cooling water discharges and river

flow regimes, I developed a mathematical model for hydrogen sulphide

concentrations in the river water based on the measured concentrations of

total sulphide shown in Figure 3.

5.8 The mathematical model developed for hydrogen sulphide firstly calculates

the concentration of total sulphide in the river water due solely to dilution at


any selected distance downstream of the cooling water outfall and at any

selected river water flow.

5.9 The difference between the measured total sulphide concentration and that

calculated from dilution is the total sulphide lost. This loss can be attributed to

oxidation and volatilisation. These processes are not differentiated in the

model but their combined effect, that is, the loss of total sulphide, at any

particular location in the river is explained almost entirely by the residence

time of total sulphide in the river between the outfall and the location.

5.10 A mathematical equation relating the loss of total sulphide to the residence

time was derived. Figure 4 compares the model estimates with the measured

values for the total sulphide lost.

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 200

TS loss m easured m g m -3

TS

loss

pre

dic

ted

mg

m-3

Figure 4. Comparison of the estimated and measured losses of total sulphide (TS) in

the Waikato River water downstream of the Wairakei Station cooling water outfall.

5.11 The model calculates the river water pH due to the carbon dioxide in the

cooling water taking into account the composition of the river water upstream

of the cooling water outfall and the river water flow and temperature. This pH

together with the calculated effects of other chemicals in the river water on


total sulphide are then applied to the calculated river water concentration of

total sulphide to produce an estimated concentration of hydrogen sulphide.

5.12 This model has been used to estimate the effects of reducing the cooling

water concentration of total sulphide on the hydrogen sulphide concentration

in the river downstream of the cooling water outfall.

5.13 Figure 5 shows the concentrations of hydrogen sulphide calculated using the

model in Waikato River water downstream of the Wairakei Power Station

cooling water outfall. A maximum cooling water discharge of 17.2 cumecs

with a total hydrogen sulphide concentration of 80ppb and the existing

concentration of carbon dioxide of 43ppm together with a river water

temperature of 15oC were assumed for these calculations.

5.14 A total hydrogen sulphide concentration of 80ppb is the quarterly average

maximum concentration in the cooling water proposed in the Statement of

Agreed Matters between Technical Experts on behalf of the Appellant

(Contact Energy Limited) and respondent (Waikato Regional Council).

5.15 The existing concentration of carbon dioxide was used because, firstly, it is

not known at this time by how much the proposed treatment system for total

sulphide will reduce the carbon dioxide concentration and, secondly, using

the existing concentration produces a worst case model result for river water

hydrogen sulphide concentration.


0

2

4

6

8

10

12

14

16

18

20

0 2000 4000 6000 8000 10000 12000 14000 16000

Distance downstream of Huka Falls (m)

Hyd

roge

nsu

lphi

deco

ncen

trat

ion

(ppb

)

Fu

lljam

esR

apid

s

Ara

tiat

iaD

am Present discharge

Treateddischarge

50

Riverwaterflows

160

250

250160

50

Co

olin

gw

ater

ou

tfal

l

0

2

4

6

8

10

12

14

16

18

20

0 2000 4000 6000 8000 10000 12000 14000 16000

Distance downstream of Huka Falls (m)

Hyd

roge

nsu

lphi

deco

ncen

trat

ion

(ppb

)

Fu

lljam

esR

apid

s

Ara

tiat

iaD

am Present discharge

Treateddischarge

50

Riverwaterflows

160

250

250160

50

Co

olin

gw

ater

ou

tfal

l

Figure 5. Concentrations of hydrogen sulphide calculated using the model in river

water downstream of the cooling water outfall.

6. THE EFFECTS OF MERCURY IN THE WAIRAKEI POWER STATION

DISCHARGES ON THE WATER, SEDIMENTS, AQUATIC ANIMALS AND

PLANTS OF THE WAIKATO RIVER.

Water quality

6.1 In the early 1970s the separated geothermal water from the Wairakei

Geothermal Field and cooling water from the Wairakei Power Station were

found to carry about 5 and 50kg year-1 respectively of mercury into the

Waikato River (Weissberg and Zobel, 1973). During the period 1988 to 1992

the average cooling water discharge of total mercury was 46.5kg year-1

(Timperley, 1997). Of this total, elemental mercury which is the form of

mercury present in the original geothermal fluid before separation of the

water and steam, constituted an average of 19.3 kg year-1. The chemical

form of the other 27.2kg year-1 in the cooling water discharge is unknown.

6.2 Reinjecting 40000 t d-1 of separated water (approximately the present

reinjection rate) has reduced the annual load of mercury discharged to the

river in the separated geothermal water by about 5kg to about 3kg.


6.3 Incorporating the 1988 to 1992 data for the cooling water (Timperley, 1993)

with the 1970’s result for the separated geothermal water (Weissberg and

Zobel, 1973) adjusted for reinjection and the total mercury concentration of

0.0005ppb in Lake Taupo water (Kim, 1995), gives an average concentration

of total mercury in the river water of about 0.010ppb immediately after full

mixing of the cooling water at a flow of 160 cumecs at Taupo Control Gates.

At a Taupo Control Gates flow of 50 cumecs (the minimum flow under the

Mighty River Power consents), the concentration would be about 0.031ppb.

Both these concentrations are below the ANZECC 2000 trigger value for 99%

protection of aquatic species of 0.06ppb, and below the New Zealand

drinking water Maximum Acceptable Value for total mercury of 2ppb

(Ministry of Health, 2005).

6.4 GNS Wairakei have estimated on the basis of my data as set out in the AEE,

an assumed cooling water flow of 17.2 cumecs, and an assumed separated

geothermal water discharge of 60,000 tonnes per day, that there will be a

combined annual discharge from Wairakei of 37 kg +/-_ 9 kg per annum of

mercury. The apparent high variability in this estimate reflects the variance in

the mercury concentration data which in turn reflects difficulties associated

with sampling and analysing mercury at low concentrations. This value

provided the basis for one of the mercury limits discussed by Mr Venus in his

evidence.

Sediments

6.5 Dissolved forms of mercury entering surface waters, for example, from the

atmosphere and from catchment sources, are incorporated into, or adsorbed

onto, suspended particulate matter. Where water velocities and turbulence

are sufficiently low, these particles settle carrying the mercury into the bed

sediment. This is a natural process occurring in all surface waters. The

scavenging of dissolved mercury by particulate matter is extremely efficient

and results in very low natural concentrations of dissolved mercury in surface

waters. The concentration of total mercury of 0.0005ppb in Lake Taupo

water (Kim, 1995) is an example.

6.6 Mercury in the Wairakei Power Station cooling water discharge undergoes

the same process in the Waikato River with the result that much of the

mercury discharged from the Station has accumulated in the bed sediments

of the Waikato River reservoirs.


6.7 Table 3 gives the total mercury concentrations in the surface layers of bed

sediments from Lake Taupo and the Waikato River reservoirs (Hickey et al,

1995). Except that from Lake Taupo, sediment at all sites had mercury

concentrations exceeding the ANZECC 2000 ISQG-Low guideline.

Table 3. Mercury concentrations in bed sediments of Lake Taupo and the Waikato

River. Numbers in italics exceed the ANZECC 2000 ISQG-Low.

Site Mercury concentration (ppm)

Taupo 0.025

Aratiatia 0.47 – 0.87

Ohakuri 0.12 – 0.74

Maraetai 0.36 – 0.83

Waipapa 0.30 – 0.38

Karapiro 0.43

Hamilton 0.31

ANZECC guideline ISQG-Low 0.15

ANZECC guideline ISQG-high 1

6.8 The quantities of mercury in the bed sediments of Lakes Ohakuri,

Whakamaru, Maraetai and Arapuni have been measured (Timperley, 1993).

These data can be extended with a high level of confidence to the other four

lakes to enable the total amount of mercury contained in the lake sediments

to be estimated. This estimate indicates that approximately half of the

mercury discharged to the Waikato River from the Wairakei Power Station

during its operational lifetime remains in the reservoir bed sediments. It can

be assumed that the other half has been lost to the atmosphere, the

sediments of the lower Waikato River and its flood plain, and the Tasman

Sea.

6.9 The highest concentrations of mercury in the reservoir bed sediments occur

in the upper part of Lake Ohakuri. This is the first section of river

downstream from the cooling water outfall with sufficiently low water

velocities to allow fine suspended particulate matter to settle. The bed

sediments in this part of Lake Ohakuri contain about one quarter of the

quantity of mercury discharged from the Wairakei Power Station over its

lifetime.


Aquatic life

6.10 The principal environmental concern with mercury is the accumulation of a

highly toxic organic form of mercury, methylmercury, in edible fish.

Methylmercury is produced from inorganic mercury, the predominant natural

form, mostly by bacteria living in fine-grained muddy bed sediments. These

types of sediments are common in lakes. Methylmercury diffuses from the

sediment into the overlying water where it is absorbed into algae. Small

animals ingest the algae and the methylmercury. These animals with their

methylmercury are, in turn, eaten by larger animals. Because animals

excrete methylmercury very slowly, it accumulates in their tissues. The

highest concentrations of mercury occur in the oldest predator animals in the

ecosystem, for example, trout and eels in the Waikato River. This process of

mercury “bioaccumulation” occurs in all water bodies where conditions are

suitable for the production of methylmercury and where there are animals

feeding on other animals.

6.11 Because of the health risks associated with ingesting methylmercury,

guidelines have been developed for the consumption of fish, the principal

source of this compound in human diet. New Zealand has adopted a

Maximum Level for mercury in fish. For trout and eels this Maximum Level is

0.5 mg kg-1 (Australia New Zealand Food Standards Code, 2002).

6.12 Table 4 compares the concentrations of mercury in trout from the Waikato

River with the concentrations in trout from a selection of other lakes in the

central North Island.

Table 4. Concentrations of mercury in mg kg-1 wet weight in the flesh of trout from

various lakes in North Island. Data on different lines are from different studies.

Concentrations above the New Zealand Maximum Level in fish are shown in bold.1Concentrations are for methylmercury (approximately equal to total mercury).2Median concentrations.


Water body Number of

fish

analysed

Mean or

median

concentration

Concentration

range

Lake Taupo 7

7

21

76

0.065

0.12

0.19

0.102

0.03 – 0.11

0.02 – 0.24

0.03 – 0.53

0.01 – 0.87

Lake Aratiatia 49 0.112 0.23 – 0.80

Lake Ohakuri 23 0.28 0.12 – 0.55

Lake Atiamuri 2

25

0.32

0.29

no range

given

0.11 – 0.62

Lake Whakamaru 5

34

0.32

0.20

0.28 – 0.37

0.06 – 0.54

Lake Maraetai 7

18

0.14

0.17

0.09 – 0.21

0.07 – 0.55

Lake Waipapa 5 0.082 0.06 – 0.13

Lake Arapuni 21 0.185 0.07 – 0.27

Lake Karapiro 11 0.211 0.06 – 0.34

The Waikato at

Ngaruawahia

17 0.122 0.04 – 0.35

The Waikato at

Huntly

7 0.166 0.08 – 0.27



fish

analysed

Mean or

median

concentration

Concentration

range

Lake Rotorua 26

20

11

4

0.5311

0.46

0.85

1.68

0.146 – 1.991

0.23 – 0.93

0.06 – 2.1

1.1 – 2.57

Lake Okareka 19

13

5

0.2211

0.24

0.35

0.117 –

0.5011

0.10 – 0.63

0.33 – 0.37

Lake Okaro 25 0.6501 0.420 – 1.151

Lake Tarawera 25

20

0.1821

0.09

0.069 –

0.6121

0.054 – 0.1

Lake Tikitapu 5 0.3 0.15 – 0.56

Lake Rotokakahi 6 0.25 0.10 – 0.39

Lake Rotomahana 30

15

5

1.841

1.24

2.1

0.71 – 4.131

0.98 – 1.56

1.52 – 2.92

Lake Rotoiti 23

6

1.11

1.0

no range

given

0.47 – 3.0

Lake Rotoehu 17

9

0.11

0.22

0.06 – 0.19

0.12 – 0.63



fish

analysed

Mean or

median

concentration

Concentration

range

Lake Okataina 20

4

0.06

0.38

0.03 –0.11

0.06 – 0.62

Lake Rotoma 13 0.24 0.08 – 0.54

6.13 In Lake Taupo, where water concentrations of mercury are very low, mercury

is bioaccumulated into trout but the concentrations are, on average, quite

low, about 0.1 mg kg-1. Thus, about 500g of Lake Taupo trout could be eaten

per week before reaching the USEPA reference dose for methymercury.

6.14 Trout in Lake Aratiatia contain concentrations of mercury similar to the low

concentrations in Lake Taupo trout, despite the discharge of Wairakei Power

Station cooling water into the upper reaches of Lake Aratiatia. The low

concentrations of mercury in Lake Aratiatia trout could be due to trout

avoiding the warm cooling water containing the mercury and/or to the lack of

suitable sediment conditions in the lake for the production of methylmercury.

6.15 The most extensive survey of trout mercury concentrations in the Waikato

River, conducted by Environment Waikato in 1985/1986 (Mills, 1995),

analysed 161 trout collected from 9 locations. Only 6 (4%) of the 161 trout

contained mercury more than 0.5 mg kg-1. These trout were generally, but

not exclusively, among the oldest caught in the survey and all were from

Lakes Ohakuri, Atiamuri, Whakamaru and Maraetai. Trout from Lake Ohakuri

had the highest median concentration of mercury.

6.16 The data in Table 4 show that even in Lake Taupo some trout contained

mercury at concentrations exceeding 0.5 mg kg-1 and further, that the highest

concentration recorded in Lake Taupo trout was greater than the highest

concentration recorded in trout from the Waikato River. The average

concentrations in trout from the Waikato River reservoirs were well below the

average concentrations in trout from Lakes Rotorua and Rotoiti, two of the

most popular trout fishing lakes. In these latter two lakes, more than half of

the trout caught had mercury concentrations above 0.5 mg kg-1 ppb.


6.17 Although a trout with mercury concentrations above 0.5 mg kg-1 can be

caught occasionally from the Waikato River, (and also from Lake Taupo),

almost all of the trout caught from the River had mercury concentrations well

below 0.5 mg kg-1.

6.18 In the Waikato River downstream from Lake Maraetai, the average

concentrations in trout were about 0.2 mg kg-1 or less.

6.19 The same Environment Waikato study referred to earlier found that eels in

Lakes Arapuni and Karapiro contained mercury at concentrations below 0.5

mg kg-1.

6.20 Mercury concentrations in freshwater mussels were higher in Lake Aratiatia

than they were in most of the other lakes, although in all lakes that have been

studied the concentrations exceeded 0.5 mg kg-1. Concentrations in Lake

Aratiatia mussels were over 100 times higher than were the concentrations in

Lake Taupo mussels. There were few mussels in Lake Aratiatia but they

were abundant in some other parts of the river. They are, however, difficult

to obtain and are not normally taken for human consumption.

6.21 There do not appear to be any reported results for mercury concentrations in

aquatic plants from the Waikato River. Based on the low concentrations of

mercury I have measured in aquatic plants growing in some of the Rotorua

lakes receiving geothermal fluids, I would not expect mercury to accumulate

to high concentrations in the aquatic plants of the Waikato River.

6.22 Lake Ohakuri is one of several Waikato River reservoirs where elvers are

being released to establish eel populations in new habitats. These eels will

accumulate mercury and future old eels could contain high concentrations of

mercury. Mercury is unlikely to be an issue for a commercial eel fishery if one

were to be established on the lakes where elvers are now being released,

because young eels with relatively low mercury concentrations would

dominate the commercial catch.

6.23 The concentrations of mercury in the surface bed sediments of the Waikato

River reservoirs and the effects of this mercury on river animals including

bioaccumulation into trout and eels, will remain essentially unchanged for at

least as long as the discharges of mercury to the river continue at the levels

of the last decade.

6.24 If the discharges of mercury to the river were to be substantially reduced I

would expect the mercury concentrations in the surface few centimetres of


the bed sediments of the river’s reservoirs to slowly decrease as the existing

sediments become buried with new sediment containing lower concentrations

of mercury. It would probably be several years, however, before the resulting

lower concentrations in bed sediments and in trout and eels would be

detectable.

7. SUMMARY

7.1 The best estimate of the natural mass flow of chloride ion from the

Wairakei/Tauhara Geothermal Field before development is 588 g s-1. The

present-day natural mass flow is probably between about 61 g s-1 and 156 g

s-1. The difference of between 432 g s-1 and 527 g s-1 is the estimated decline

in the natural chloride ion mass flow due to the development of the

Wairakei/Tauhara field.

7.2 The pre-development flow of “naturally separated geothermal water” from the

Wairakei/Tauhara Field was about 24,000 t d-1, comprising about 18,000 t d-1

from the Wairakei part of the Field and about 6000 t d-1 from the Tauhara part

of the Field.

7.3 The decrease in the natural chloride ion mass flow is equivalent to a present-

day discharge of separated geothermal water from the Wairakei bore field of

between about 19,300 t d-1 and 23,600 t d-1. The most probable equivalent

present-day discharge of separated geothermal water is nearer the upper

end of this range, say, about 23,000 t d-1.

7.4 The natural mass flow of arsenic in the geothermal fluids from the

Wairakei/Tauhara field was about 1.2 g s-1 or 38 t year-1 before development

and between 0.13 g s-1 (4 t year-1) and 0.32 g s-1 (10 t year-1) after

development.

7.5 It is likely that the present natural mass flow of arsenic is towards the lower

end of this range, i.e., closer to 4 t year-1 than to 10 t year-1. The natural mass

flow of As has, therefore, reduced from about 38 t year-1 before development

began to about 4 t year-1, a reduction of 34 t year-1.

7.6 Arsenic concentrations in Lake Taupo water are at the Maximum Acceptable

Value of 10ppb for treated New Zealand drinking water. Natural flows of

geothermal water into the Waikato River below Taupo Control Gates add

further arsenic to the river water. Prior to development of the

Wairakei/Tauhara Geothermal Field the annual average river water


concentration at Hamilton would have been about 21ppb. With development

of the Field and with the present level of separated water reinjection, the

annual average arsenic concentration at Hamilton is about 24ppb.

7.7 Arsenic has accumulated in the bed sediments of the Waikato River

reservoirs to concentrations that exceed the ANZECC 2000 guidelines.

7.8 Approximately 80-90% of the total sulphide presently discharged from the

Wairakei Power Station is lost from the river water between the cooling water

outfall and the Aratiatia Dam primarily because of bacterial oxidation. The

total sulphide remaining in the river water raises hydrogen sulphide

concentrations above the site-specific water quality guideline of 2ppb for

between 8 and 10km downstream, depending on river water flow, from the

cooling water outfall.

7.9 Reducing the cooling water concentration of total sulphide to 80ppb would

reduce the distance downstream of the cooling water outfall over which the

site-specific water quality guideline of 2ppb would be exceeded to less than

2km at low river flow of 50 cumecs, 2km at mean river flow of 160 cumecs

and 3km at a river flow of 250 cumecs.

7.10 Mercury, mostly from the Wairakei Power Station cooling water discharge,

has accumulated in the bed sediments of the Waikato River reservoirs where

some of this mercury is converted by bacteria into methylmercury. This form

of mercury bioaccumulates in trout and eels. In the mid-1980’s 4% of trout

caught from the river had mercury concentrations above 0.5 mg kg-1. Mostly

these trout were old and all were from Lakes Ohakuri, Atiamuri, Whakamaru

and Maraetai. Elsewhere in the river average mercury concentrations in trout

were below 0.5 mg kg-1.

7.11 The average concentrations of mercury in trout from the Waikato River are

less than half the average concentrations in trout from Lakes Rotorua and

Rotoiti, two of the most popular trout fishing lakes in North Island.

7.12 In future old eels in Lake Ohakuri could contain high concentrations of

mercury but the concentrations in younger eels caught for commercial sale

are likely to be below this level.


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Timperley, M.H. (1983). Phosphorus in spring waters of the Taupo Volcanic Zone,

North Island, New Zealand. Chemical Geology 38: 287-306.

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Timperley, M.H.; Hill, L.F. (1997). Discharge of mercury from the Wairakei

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