Water Quality Impacts of a Point-Source Discharge of Tikitere Geothermal Water in Lake Rotoiti

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti

NIWA Client Report: HAM2005-149 January 2006

NIWA Project: RDC06203

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti

Scott Stephens Sue Clearwater

Prepared for

Rotorua District Council

NIWA Client Report: HAM2005-149 January 2006

NIWA Project: RDC06203

National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone +64-7-856 7026, Fax +64-7-856 0151 www.niwa.co.nz

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Contents

Executive Summary iv

1. Introduction 1

2. Methods 3 2.1 Water-quality data from chemical analyses 3 2.2 ANZECC water quality guidelines 3 2.3 CORMIX 5 2.3.1 Bathymetry 6 2.3.2 Ambient conditions 8 2.3.3 Discharge conditions 9

3. Results 12 3.1 Modelling of contaminant dilution 12

4. Discussion 16

5. References 20

6. Glossary 21

7. Appendix – CORMIX flow description table 23

8. Appendix – aquatic toxicity information from ANZECC (2000) 24 8.1 Ammonia 24 8.2 Arsenic 25 8.3 Boron 26 8.4 Chromium 27 8.5 Copper 28 8.6 Mercury 29 8.7 Zinc 30

Reviewed by: Approved for release by:

John Oldman Kit Rutherford

Formatting checked

Executive Summary

Recent investigations showed that a diversion wall could be built in Lake Rotoiti to direct the majority of the Ohau Channel flow into the Okere Arm and directly down the Kaituna River, thereby short- circuiting the main basins of Lake Rotoiti (Stephens 2004; Stephens 2005) and improving the trophic status of the lake (Hamilton & Uraoka 2004). In 2005 an environmental consent was obtained by Environment Bay of Plenty to construct the diversion wall. The Tikitere geothermal springs enter the eastern side of Lake Rotorua via the Waiohewa Stream, and have a high nitrogen load. This nitrogen load could be removed from Lake Rotorua (and Lake Rotoiti) by piping it to a discharge point in the Okere Arm on the Kaituna River side of the diversion wall. This may improve the trophic status of Lake Rotorua, but the effects of such action on the Kaituna River must also be considered. Thus, Rotorua District Council commissioned NIWA to investigate the dispersal characteristics and potential impacts on flora and fauna of a direct discharge of Tikitere geothermal water into the Okere Arm, following construction of the diversion wall in Lake Rotoiti. The dispersion of the Tikitere geothermal water within Lake Rotoiti was predicted using the CORMIX model. Fully-mixed contaminant concentrations were also calculated at the Okere Falls, the entrance point to the Kaituna River. These contaminant concentrations were then put into context using the Australian and New Zealand guidelines for fresh and marine water quality (ANZECC 2000), using trigger values for “slightly to moderately disturbed” systems.

After initial mixing with the receiving water, concentrations approached or exceeded the trigger values for environmental concern for the contaminants arsenic, boron, chromium, copper, mercury and zinc. Of these, mercury contents were particularly high. No trigger values are available for antimony, beryllium, caesium, lithium or thallium. All measured nutrients exceeded the trigger values. Of these ammonia-N is particularly high and is of particular concern due to the possibility of ammonia toxicity. After becoming fully-mixed with the water entering the Kaituna River, high concentrations of mercury, total ammonia-N and total nitrogen were still predicted to exist, but would fall below the trigger values for environmental concern. CORMIX simulations indicated that discharges became fully vertically mixed within 40 m from a lakebed diffuser, and within 150 m from a surface diffuser.

The high contaminant levels predicted indicate that adverse aquatic toxicity levels are likely to occur near the diffuser and considerable caution should be exercised before making a direct discharge of Tikitere geothermal water into the Okere Arm. Our recommendation is that environment-specific aquatic toxicity testing should be conducted before proceeding with any such engineering works. Alternatively, stripping of the contaminants (particularly mercury and ammonia) would have to be undertaken to reduce contaminant concentrations before discharge.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti iv

1. Introduction

Water quality in Lake Rotoiti has been declining over many years (e.g. Hamilton & Uraoka 2004), resulting primarily from increased nitrogen and phosphorus loads to the lake. These nutrients promote algal growth leading to increased turbidity near the lake surface; subsequent sedimentation and decomposition of algae leads to oxygen depletion near the lakebed, which promotes further nutrient release and thus perpetuates this undesirable situation. Public interest has been sharpened by the prevalence of blue-green algal blooms, which are toxic to human consumption and have caused the closure of the lake to contact recreation in summers, particularly since 2000.

Lake Rotoiti receives nutrients from geothermal sources and catchment runoff, but approximately 75% of nutrients are thought to enter the lake from Lake Rotorua via the Ohau Channel. The Ohau channel provides about ¾ of the total flow into Lake Rotoiti, and enters the lake a relatively short distance from its main sink, the Kaituna River. Therefore there is considerable potential to redirect some or all of the flow from the Ohau Channel directly to the Kaituna River. Doing so would remove a large nutrient source from Lake Rotoiti that could lead to considerable improvements in lake health almost immediately.

The Tikitere geothermal springs enter the eastern side of Lake Rotorua via the Waiohewa Stream, and have a high nitrogen load. Rotorua District Council (RDC) and EBOP have recognised that this nitrogen load could be removed from Lake Rotorua (and Lake Rotoiti) by piping it to a discharge point in the Okere Arm on the Kaituna River side of the diversion wall. This would reduce the nitrogen load to Lake Rotorua, contributing to improvement in its trophic status. However, the effects of such action on the Okere Arm of Lake Rotoiti and on the Kaituna River must also be considered.

RDC commissioned NIWA to investigate the dispersal characteristics and potential impacts on flora and fauna of a direct discharge of Tikitere geothermal water into the Okere Arm, following construction of the diversion wall in Lake Rotoiti.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 1

The dispersion of the Tikitere geothermal water within Lake Rotoiti is predicted using the CORMIX (Cornell Mixing Zone Expert System) model. The modelling concentrates on predicting geothermal discharge concentration in the near-field region (NFR , see glossary), in the relatively uniform bathymetry and ambient flow conditions that would occur in the lee of the diversion wall. Since the plume will become more dilute beyond the NFR, the NFR concentrations can be used as a conservative estimate to assess the implications of the discharge on water quality behind the diversion wall in the Okere Arm. Fully-mixed contaminant concentrations are also calculated at the Okere Falls, the entrance point to the Kaituna River.

These contaminant concentrations are then put into context using the Australian and New Zealand guidelines for fresh and marine water quality (ANZECC 2000), using trigger values for “slightly to moderately disturbed” systems.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 2

2. Methods

2.1 Water-quality data from chemical analyses

A number of water sample analyses have been undertaken for geothermal water samples collected from the Tikitere hot springs. These samples were collected from the top field and the upper, middle and lower culverts on 21 occasions between July and November 2004, encompassing a variety of flow rates. Rotorua District Council (RDC) supplied results of the analyses. EBOP supplied results of chemical analyses undertaken on water samples collected from the Ohau Channel and Okere Arm and these, along with data presented in Stephens et al. 2004, provide the available background concentrations. As a conservative measure, we have used the maximum measured concentrations in our assessment to give the “worst-case” concentration estimates in the lake. In other words, the predicted levels of contamination in the lake should be biased toward the high side. The data we employed are summarised in Table 1. The table incorporates all the supplied metal contaminant concentrations, plus the supplied nutrient concentrations that are commonly used in water-quality analyses and which had associated trigger values in the ANZECC guidelines (ANZECC 2000). Table 1 reports total concentrations.

We expect that the contaminants within the Tikitere geothermal fluid will be conservative while travelling through the pipe to the proposed discharge location, i.e. the chemical makeup of the fluid will remain unchanged.

2.2 ANZECC water quality guidelines

Much of this section has been taken directly from the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC 2000). The guidelines provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality are designed to help users assess whether the water quality of a water resource is good enough to allow it to be used for humans, food production or aquatic ecosystems (these uses are termed environmental values ). If the water quality does not meet the water quality guidelines, the waters may not be safe for those environmental values and management action could be triggered to either more accurately determine whether the water is safe for that use or to remedy the problem.

In most situations, a decision should be made before the ecosystem becomes adversely affected so that management actions can be implemented in time to prevent the ecosystem becoming damaged. In other words, a ‘threshold value’ of the indicator

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 3

must be selected that is smaller than that which indicates that the ecosystem has been impaired. How much smaller this value needs to be depends on the nature of the impact, the level of our understanding of the relationship between changes in the indicator and ecological impact, and the lead-time necessary to implement management actions.

For the metal contaminants we have referred to Table 3.4.1 (ANZECC 2000) to obtain trigger values for toxicants applying to slightly–moderately disturbed systems . These guideline trigger values represent the best currently-available estimates of what are thought to be ecologically low-risk levels of these indicators for chronic (sustained) exposures . For most contaminants the trigger values used were at the 95% protection level, i.e., expected to protect 95% of all species.

For the nutrient contaminants we have referred to Table 3.3.10 and Table 8.3.7 in ANZECC (2000), and adopted the trigger values for slightly modified lowland rivers.

Some metals/metalloids form different species, with specific trigger values for each species that are highly relevant to their environmental impact. Thus, Table 8 lists trigger values for the different species, where available, e.g., arsenic and selenium (ANZECC 2000).

It is important to realize that the guidelines are not sufficient in themselves to protect ecosystem integrity; they must be used in the context of local environmental conditions and other important environmental factors, for example, habitat, flow and recruitment. Thus the guidelines do not provide mandatory standards because there is significant uncertainty associated with the derivation and application of water quality guidelines. For example, data on biological effects are not available for all local species; there is uncertainty over the behaviour of contaminants in the field; there is uncertainty in water quality measurements. However, the guidelines provide a framework for recognising and protecting water quality for the full range of existing environmental values.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 4

Table 1: Maximum contaminant (total) concentrations measured in the Tikitere geothermal field (source: RDC), background contaminant concentrations in the Ohau Channel and at Okere Falls (source: EBOP).

Contaminant Symbol Maximum Contaminant Contaminant Tikitere concentration concentration concentration in Ohau at Okere Falls (µµµg l -1) Channel (µµµg l -1) (µµµg l -1) Antimony Sb 5.4 – – Arsenic As 27 7 6 Beryllium Be 0.3 – – Boron B 3570 310 280 Cadmium Cd 0.07 0.025 0.025 Calcium Ca 9050 3100 2200 Caesium Cs 8.6 – – Chromium Cr 2 0.25 0.25 Copper Cu 5.9 0.6 0.6 Lead Pb 5.9 0.05 0.05 Lithium Li 63.3 134 119 Mercury Hg 57.5 0.05 0.05 Nickel Ni 1.2 0.05 0.05 Selenium Se <1 – – Silver Ag <0.1 – – Thallium Tl 0.34 – – Zinc Zn 20 5 0.5 Dissolved reactive phosphorus DRP 52 46 – Total ammonia-N NH4-N 58601 295 – Total phosphorus TP 486 77 – Total nitrogen TN 57186 648 –

2.3 CORMIX

The dispersion of the Tikitere geothermal water within Lake Rotoiti was predicted using the CORMIX (Cornell Mixing Zone Expert System) model, version 4.3. This is a software system for the analysis, prediction, and design of aqueous toxic or conventional pollutant discharges into diverse water bodies. CORMIX has been used here to design a diffuser pipe for the discharge, and to assess the behaviour of the discharging plume under a range of ambient and discharge conditions.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 5

The term “diffuser” refers to the configuration of the outlet pipe at the point of discharge; for example the number and diameter of exit holes and the physical orientation of the diffuser. The diffuser was modelled as an unmodified pipe end discharging into the lake. Multi-port diffusers generally give higher contaminant dispersion than unmodified pipes, but the extra complexity was not warranted given the enclosed results, which showed very high mercury levels. This issue is discussed further below.

2.3.1 Bathymetry

A representative bathymetry profile was obtained for the CORMIX modelling using bathymetry soundings (Figure 1) collected by the University of Waikato in 2004 (Hamilton & Uraoka 2004). This data was interpolated onto a 5 m square grid using Kriging software, from which the representative profile (red line in Figure 1) was selected (Figure 2). The selected profile lies about 100 m northeast of the boat ramp. Using this profile, dispersion was modelled for the case where the diffuser discharged at the water surface at the lake edge, and for the case where the diffuser was located underwater on the lakebed (Figure 2). The surveyed depths and the resulting bathymetry profile used in CORMIX have a zero datum coinciding with 179.15 m above mean sea level relative to the Moturiki datum. The maximum variation in lake level since year 2000 is <0.3 m, which is small enough to treat the water depths as constant in the CORMIX simulations.

For the lakebed discharge simulations CORMIX represents the bathymetry as a rectangular box (red dashed line in Figure 2). The box was set to have the same cross- sectional area as the bathymetry profile (heavy blue line in Figure 2), giving a mean depth of 2.26 m and width of 45.76 m. The channel is assumed to be uniform in the downstream direction, which is a reasonable approximation in the lee of the diversion wall (e.g., Figure 1). For the surface discharge the depth profile was represented by a sloping bathymetry (green dashed line in Figure 2) with a slope of 2.5 °.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 6

Diversion wall

Profile Boat Ramp

Figure 1: Map showing the locations of depth survey positions (black lines), the proposed diversion wall (green) and the bathymetry profile (red) adopted for the CORMIX simulations (Figure 2).

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 7

Lakebed diffuser

Figure 2: Bathymetric cross-section in the vicinity of the lakebed diffuser, true-left bank at left and diversion wall at right. Blue = actual cross-section; Red = schematised rectangular bathymetry used in CORMIX for near-bed discharge. Note distorted scale.

2.3.2 Ambient conditions

Average, maximum and minimum ambient discharges from the Ohau Channel of 15.84 m3 s-1 (Hamilton & Uraoka 2004), 23.3 m3 s-1 and 6.3 m3 s-1 (Environment Bay of Plenty (EBOP) records from 1992–1993 and 2001–2002) respectively, were used in the simulations. The mean flows in the water balance for Lake Rotoiti for the period 1 January 1999 to 28 February 2004 are: Ohau Channel 15.84 m 3 s -1, Kaituna outflow 21.09 m 3 s -1, rainfall 1.30 m 3 s -1, evaporation 0.96 m 3 s -1 and the residual term 4.93 m 3 s-1 (Hamilton & Uraoka 2004). EBOP records show temperatures ranging from 7– 23 °C between winter and late summer, so these two extremes were used in the simulations. Lake Rotoiti is temperature stratified during much of the year, with strong stratification occurring in late summer – early autumn. However, the water depth in the Okere Arm adjacent to the diversion wall (< 5 m) will be shallower than the typical pycnocline depth in the lake (~10 m). Thus we expect the lake to be reasonably well-mixed (unstratified) behind the wall. For completion, we trialled a strongly stratified case in CORMIX, but found the results to be relatively insensitive to stratification. This was because of the shallow water and the high temperature of the

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 8

geothermal discharge that negated the effects of even strong stratification. Thus we present results from unstratified simulations only.

2.3.3 Discharge conditions

The geothermal water would be supplied to the lake via a 160 mm outer diameter and 144 mm internal diameter polyethylene pipe (Peter Dine, email 7 Dec 2005), at a discharge rate of 0.017 m3 s-1 and temperature of 30 °C (Peter Dine, email 25 Nov 2005).

Firstly, a lakebed diffuser was modelled, whereby the geothermal water was assumed to exit a diffuser pipe located on the lakebed in the channel formed behind the diversion wall. The schematised bathymetry for these simulations is shown in red- dashed box on Figure 2, and the non-varying parameters used in the lakebed discharge simulations are shown in Table 2. It was assumed that the diffuser pipe points away from the west bank at 90 ° to the ambient flow in the channel behind the wall; changes in this alignment were tested but caused only minor changes in the dispersal characteristics of the geothermal water. Likewise, simulations were relatively insensitive to the height of the diffuser above the lakebed within several diffuser widths, so results are presented only for 0.072 m above the lakebed, corresponding to the ½-width of the diffuser. Dispersal from the lakebed was most sensitive to the steady ambient flow rate past the diffuser and the vertical angle of the diffuser. Two extremes of Ohau Channel temperatures were also modelled. The values assigned to these variable parameters are presented in Table 3.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 9

Table 2: Non-varying parameters as used in the simulations of a lakebed diffuser.

Water body depth (m) 2.26 Depth at discharge (m) 2.2 Bounded water body width (m) 45.76 Distance to nearest bank (m) 15 Port diameter (m) 0.144 Nearest bank on the Right Port Height (m) 0.072 Water body is Fresh Geothermal discharge flow rate (m 3 s -1) 0.017 Stratification is Uniform Geothermal discharge temperature °C 30 Wind speed 0 Horizontal angle w.r.t. ambient flow ( °) 90 Manning’s n 0.025

Table 3: Variables used in the simulations of a lakebed diffuser.

Steady ambient flow rate (m 3 s-1) 6.3, 15.84, 23.3 (min., av., max. Ohau flows) Vertical angle of diffuser w.r.t. lakebed, positive upward ( °) 0, 15, 30 Receiving water temperature ( °C) 7, 23

Secondly, a surface discharge was modelled, where the diffuser pipe was assumed to enter the lake at the land-edge as shown in Figure 3. The non-varying and variable parameters used for the surface discharge simulations are given in Table 4 and Table 5 respectively. Dispersal from the left bank water surface was most sensitive to the steady ambient flow rate past the diffuser and the horizontal angle of the diffuser.

Table 4: Non-varying parameters as used in the simulations of a surface diffuser.

Average water body depth (m) 1.48 Depth discharge outlet (m) 0.16 Bottom slope at discharge ( °) 2.5 Distance to nearest bank (m) 0 Bounded water body width (m) 65 Nearest bank on the Left Port diameter (m) 0.144 Water body is Fresh Geothermal discharge flow rate (m 3 s -1) 0.017 Stratification is Uniform Geothermal discharge temperature °C 30 Wind speed 0 Horizontal angle w.r.t. ambient flow ( °) Manning’s n 0.15

Table 5: Variables used in the simulations of a surface diffuser.

Steady ambient flow rate (m 3 s-1) 6.3, 15.84, 23.3 Horizontal angle of diffuser w.r.t. true-left bank ( °) 45, 90 Receiving water temperature ( °C) 7, 23

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 10

Figure 3: Bathymetric cross-section in the vicinity of the surface lake-edge diffuser, true-left bank at left and diversion wall at right. Blue = actual cross-section; Red = schematised sloping bathymetry used in CORMIX for surface discharge.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 11

3. Results

3.1 Modelling of contaminant dilution

A simple calculation of the fully-mixed contaminant concentrations can be made if the contaminants are assumed to be conservative in the receiving water. This is probably a reasonable assumption in the relatively small water body that would exist behind the diversion wall from the diffuser to the Okere Falls. The fully-mixed concentrations entering the Kaituna River can then be calculated from the ratio of the average flow of 21.1 m3 s-1 at the Okere Falls and the 0.017 m3 s-1 diffuser discharge. These values are presented in column 2 of Table 8.

In CORMIX, the metal and nutrient contaminants were also treated as conservative pollutants i.e., they did not undergo any decay/growth processes. This is a reasonable assumption over the <100 m distance to the edge of the near-field mixing region. The discharge concentration of the contaminant is defined as the excess concentration above any ambient background concentration of that same material. Initially, generic simulations were run assuming a contamination level of 100% above background, i.e., assuming no background contamination levels. This enabled the effects of the physical variables to be assessed without specific reference to individual contaminants. The results of these assessments are presented in Table 6 and Table 7. For, example, the lowest dilution (highest NFR concentration) occurred at low ambient discharge with a high vertically-angled diffuser (simulation 4, Table 6), which might be considered “worst-case”.

Having identified the “worst-case” physical conditions for both lakebed and surface diffusers, simulations were run for these worst-case scenarios using the measured contaminant and ambient values presented in Table 1. Ambient contaminant concentrations were subtracted before running dispersal simulations, then re-added to the predicted concentrations. The results of these assessments are presented in Table 8, including comparisons with available trigger values from the ANZECC Water Quality Guidelines.

It is important to remember that hydrodynamic modelling by any known technique is not an exact science. Extensive comparison with field and laboratory data has shown that the CORMIX predictions on dilutions and concentrations (with associated plume geometries) are reliable for the majority of cases and are accurate to within about ±50%. As a further safeguard, CORMIX will not give predictions whenever it judges the design configuration as highly complex and uncertain for prediction.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 12

Table 6: Results of CORMIX simulations using a lakebed diffuser and assuming an input contaminant concentration of 100% relative to background (i.e., no background contamination). Ambient refers to the receiving water, NFR = near-field region. See Table 9 in Appendix for explanation of flow classification. The simulation giving highest predicted concentration is highlighted as a “worst-case” scenario.

Simulation Ambient Ambient Vertical NFR effluent Downstream Cross- Plume Plume Downstream Downstream Flow number flow rate temperature diffuser concentration distance to shore ½-width thickness distance to dist. to bank classification (m 3 s -1) (°°°) angle ( °°°) (%) NFR edge distance (m) (m) fully-mix (m) contact (m) to NFR edge 1 6.3 7 15 3.3 14.2 -8.9 5.7 0.6 41.2 14.2 H4-90 2 6.3 23 0 2.9 8 -7.2 2.2 2.2 22.3 74 A3 3 6.3 23 15 5.5 7.1 -6.9 2.4 0.6 31.4 76.2 H4-90 4 6.3 23 30 8.2 5.4 -4.7 2.1 0.4 34.3 130 H4-90 5 15.84 7 15 3.1 5.3 -3.2 1.3 1.3 14.7 195 H2 6 15.84 23 15 2.6 7.5 -3.5 1.5 1.5 14.7 188 H2 7 23.3 7 0 1.4 16.3 -2.8 1.6 1.6 22.4 219 A5 8 23.3 7 15 2.4 8.4 -2.9 1.3 1.3 16.3 207 H2 9 23.3 7 30 4 4.5 -2.4 1 1 13.2 221 H2 10 23.3 23 15 2.1 12.3 -3.2 1.4 1.4 19.9 202 H2

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Table 7: Results of CORMIX simulations using a surface diffuser at the western channel bank, and assuming an input contaminant concentration of 100% relative to background (i.e., no background contamination). Ambient refers to the receiving water, NFR = near-field region. See Table 9 in Appendix for explanation of flow classification. The simulation giving highest predicted concentration is highlighted as a “worst-case” scenario.

Simulation Ambient Ambient Horizontal NFR effluent Downstream Cross- Plume Plume Downstream Downstream Flow flow rate temperature diffuser concentration distance to shore ½-width thickness distance to dist. to bank classification (m 3 s -1) (°°°) angle ( °°°) (%) NFR edge distance (m) (m) fully-mix (m) contact (m) to NFR edge 11 6.3 7 90 21.7 102 -24 9.52 0.05 134 267 SA2 12 6.3 23 45 21 72 0 11 0.06 104 0 SA2 13 6.3 23 90 25.2 102 -12.3 8.1 0.06 122 113 SA2 14 15.84 7 90 25 14 -1.9 1.9 0.11 63 0 SA1 15 15.84 23 90 22.9 12.6 -1.4 1.4 0.16 32 0 SA1 16 23.3 7 45 26.6 19.9 0 2 0.09 39 0 SA1 17 23.3 7 90 23.4 9.1 -1 1 0.14 28.9 0 SA1 18 23.3 23 90 17.2 8.9 -0.8 0.84 0.2 18.7 0 SA1

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 14

Table 8: Estimated contaminant concentration in Lake Rotoiti after mixing. Columns indicate: 1) contaminant substance; 2) ANZECC trigger contaminant concentration (“ID” signifies that insufficient data exists to develop a guideline for the contaminant, “-“ signifies no data); 3) the increase in contaminant concentration assuming the geothermal flow of 0.017 m3 s-1 becomes fully-mixed with the average flow over the Okere Falls of 21.1 m3 s-1; 4) the fully-mixed contaminant concentration expressed as a percentage of ambient concentration measured at the Okere Falls (or in Ohau Channel where unavailable e.g., nutrients); 5) the fully-mixed contaminant concentration expressed as % of the ANZECC trigger values for slightly-moderately disturbed systems ; 6) Predicted concentration at the edge of the near-field mixing region, from a lakebed diffuser using the “worst-case” parameters from Table 6; 7) col. 6 values expressed as % of ANZECC trigger values; 8) as for col. 6 but using “worst-case” surface diffuser results; 9) col. 8 values expressed as % of trigger values.

1 2 ( µµµg l -1) 3 ( µµµg l -1) 4 5 6 ( µµµg l -1) 7 8 ( µµµg l -1) 9 Sb ID 0.0044 – – 0.4 – 1.4 – 0.2, As 13, 24 1 0.022 0.4% 0.1% 1 8.6 66, 36% 1 12.3 95, 51% 1 Be ID 0.0002 – – 0 – 0.1 – B 370 2.88 1% 1% 578 156% 1177 318% Cd 0.2 0.0001 0.2% 0.0% 0.025 13% 0.025 13% Ca – 7.29 0.3% – 3589 – 4682 – Cs – 0.0069 – – 0.7 – 2.3 – Cr ID, 1 2 0.0016 1% 0.2% 2 0.35 –, 35% 2 0.75 –, 75% 2 Cu 1.4 0.0048 1% 0.3% 1 71% 2 143% Pb 3.4 0.0048 10% 0.1% 0.55 16% 0.21 6% Li – 0.051 0.0% – 139 – 150.8 – 25583, – Hg 0.06,ID 3 0.046 93% 77% 3 4.75 7917, –%3 15.35 %3 Ni 11 0.001 2% 0.0% 0.1 1% 0.3 3% Se 5, ID 4 0.0008 – 0.0% 0.1 2, –%4 0.3 6, –%4 Ag 0.05 0.0001 – 0.2% 0 0% 0 0% Tl ID 0.0003 – – 0 – 0.1 – Zn 8 0.016 3% 0.2% 6.2 78% 9 113% DRP 10 5 0.042 0.2% 0.4% 5 23.8 238%5 24.9 249%5 NH4- N 2180 6 47.2 41% 5% 6 4904 225%6 15615 716%6 TP 33 0.39 1% 1% 95.6 290% 171 517% TN 614 46.1 8% 8% 5235 853% 15622 2544% 1Using trigger values for As(III) and As(V) respectively. 2Using trigger values for Cr(III) and Cr(V) respectively . 3Using trigger values for inorganic Hg and methyl Hg respectively. 4Using trigger values for total Se and Se (IV) respectively. 5Using trigger value for lowland river from Table 3.3.10 (ANZECC 2000) 6Trigger value derived for pH 7.0

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 15

4. Discussion

The Ohau Channel links Lakes Rotorua and Rotoiti and is a highly regarded trout fishery, particularly in the winter months when large numbers of spawning trout move through. The proposed diversion wall will effectively extend the length of the Ohau Channel, into which the Tikitere geothermal water would be directly discharged. This study shows that a zone will occur where some contaminants considerably exceed the ecologically low-risk levels for chronic (sustained) exposures.

Close to the diffuser, at the edge of the near-field region, concentrations approached (≥ 50%) or exceeded the trigger values for environmental concern for the contaminants arsenic, boron, chromium, copper, mercury, and zinc (Columns 7 and 9 of Table 8). Of these, mercury contents were particularly high. All nutrients (dissolved reactive phosphorus, total ammonia nitrogen, total phosphorus and total nitrogen)

exceeded the trigger values after initial mixing from the diffuser. Of these NH 4-N is particularly high and is of particular concern due to the possibility of ammonia toxicity. Further information pertaining to the possible toxic effects of these contaminants is presented in sections 8.1–8.7.

After becoming fully-mixed with the water entering the Kaituna River, high concentrations of mercury, total ammonia-N and total nitrogen were still predicted to exist, with the geothermal discharge predicted to nearly double the amount of Mercury entering the river (Column 4 of Table 8). However, once fully-mixed with the water in the Okere Arm, all concentrations would fall below the trigger values for environmental concern, including mercury. CORMIX simulations indicated that discharges became fully vertically mixed within 40 m from a lakebed diffuser, and within 150 m from a surface diffuser.

The fact that the ambient receiving water already contains diluted Tikitere geothermal water has not been considered thus far. Hoare (1985) estimated a geothermal inflow of ~0.6 m3 s-1 to Lake Rotorua, of which the Tikitere geothermal water makes up 1/35 th (2–3%) at 0.017 m3 s-1. Likewise, the Waiohewa Stream that delivers the Tikitere geothermal water to Lake Rotorua is ~2% of the total Lake discharge through the Ohau Channel (Hoare 1985). Assuming that other geothermal waters also contain high contaminant contents (there will be some variability between sources) then we would expect much higher ambient concentrations in the Ohau Channel and at Okere Falls if the contaminants were conservative through the lake systems. But the substances are not conservative through these systems, there is loss to sediments and uptake by flora and fauna, for example. In bypassing the lake systems the Tikitere diversion therefore represents a concentrated injection of the geothermal water to the Okere Basin and

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 16

Kaituna River system that considerably elevates contaminant concentrations near the discharge point, relative to current ambient levels. In addition the mass loading (i.e., long-term inputs) of contaminants such as mercury (that can biomagnify), and arsenic and selenium (that can bioaccumulate in aquatic foodwebs) should be considered.

The high contaminant levels predicted indicate that adverse environmental effects are likely to occur and considerable caution should be exercised before making a direct discharge of Tikitere geothermal water into the Okere Arm. Our recommendation is that environment-specific aquatic toxicity testing should be conducted before proceeding with any such engineering works. Alternatively, stripping of the contaminants (particularly mercury and ammonia) could be undertaken to reduce contaminant concentrations before discharge.

There are no trigger values for some of the contaminants/contaminant-species identified in the Tikitere geothermal water, because there was insufficient data to form guidelines. However, further environment-specific aquatic toxicity investigation is not warranted for these contaminants at this stage, given the very high mercury and ammonia levels already present in the geothermal fluid that must first be managed.

In this investigation we have simulated discharges from simple diffusers, but multi- port diffusers can give greater dispersion. However, a multi-port diffuser would not reduce mercury levels to below the relevant ANZECC guidelines. Further simulations with multi-port diffusers could be made pending further investigations for management of the geothermal contaminants.

The appendix (8.1–8.7) includes information about the likely aquatic toxicity of those toxic contaminants (metals and ammonia) that were predicted to approach or exceed the ecologically low-risk levels for chronic (sustained) exposures. Much of this information is quoted directly from ANZECC (2000) and references quoted therein. To assist the reader, the relationship between the predicted contaminant levels from section 3 and the ANZECC guideline information in sections 8.1–8.7 are synthesised in the following paragraphs:

Ammonia – At the edge of the near-field region NH 4-N values of 4904 and 15615 µg l-1 were calculated for the lakebed and surface diffusers respectively. Thus we would expect to see harmful effects upon freshwater fish, crustaceans and possibly insects near the diffuser. We would not expect harmful effects at the concentrations predicted when the plume becomes fully-mixed with the Kaituna River (47 µg l -1).

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 17

Arsenic – In the near-field region total arsenic values of 8.6 and 12.3 µg l -1 were predicted for the lakebed and surface diffusers respectively. The maximum predicted value is about half of the trigger value. Based on the LC50 and NOEC values quoted above, we would not expect harmful effects from arsenic for most species at the edge of the near-field region. However, minor harmful effects may be observed on algae.

Boron – In the near-field region total boron values of 578 and 1177 µg l -1 were predicted for the lakebed and surface diffusers respectively, both of which exceed the recommended trigger value. Based on the LC50 and NOEC values quoted above, we would expect harmful effects upon algae and plants at the edge of the near-field region, but no harmful effects on fish or crustaceans. No harmful effects are expected at the concentrations predicted when the geothermal water becomes fully-mixed with the Kaituna River (2.9 µg l -1).

Chromium – In the near-field region total chromium values of 0.35 and 0.75 µg l -1 were predicted for the lakebed and surface diffusers respectively, which approach or exceed the recommended trigger value of 1 µg l -1. Based on the LC50 and NOEC values quoted above, we would not expect harmful effects at the edge of the near-field region for any of the listed species. However, the reliability of the trigger value is low and species-specific aquatic toxicity tests may be necessary to determine the response of local species to these levels of chromium contamination. No harmful effects are expected at the concentrations predicted once the geothermal water becomes fully-mixed with the Kaituna River (0.002 µg l -1).

Copper – In the near-field region total copper values of 1 and 2 µg l -1 were predicted for the lakebed and surface diffusers respectively, which approach or exceed the recommended trigger value. Based on the LC50 and NOEC values quoted above, we could expect harmful effects at the edge of the near-field region on freshwater fish, crustaceans, insects and molluscs. Species-specific aquatic toxicity tests are recommended to determine the response of local species to these levels of copper contamination. No harmful effects are expected at the concentrations predicted once the geothermal water becomes fully-mixed with the Kaituna River (0.005 µg l -1).

Mercury – In the near-field region total mercury values of 4.75 and 15.35 µg l -1 were predicted for the lakebed and surface diffusers respectively, which dramatically exceed the recommended trigger value of 0.06. Based on the LC50 and NOEC values quoted above, we could expect harmful effects at the edge of the near-

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 18

field region on freshwater fish, crustaceans, and molluscs, but not plants. Again, species-specific aquatic toxicity tests are recommended to determine the response of local species to these levels of mercury contamination. Harmful effects are still possible at the concentrations predicted, when the geothermal water becomes fully-mixed and diluted with the Kaituna River (0.05 µg l -1). Of further concern is the potential for mercury to biomagnify up the food chain. Therefore it is highly recommended that some form of treatment be undertaken to strip mercury from the geothermal discharge.

Zinc – In the near-field region total zinc values of 6.2 and 9.0 µg l -1 were predicted for the lakebed and surface diffusers respectively, which approach or exceed the recommended trigger value of 8 µg l -1. Based on the LC50 and NOEC values quoted above, we could expect harmful effects at the edge of the near-field region on freshwater crustaceans and insects. Species-specific aquatic toxicity tests are recommended to determine the response of local species to these levels of zinc contamination. No harmful effects are expected at the concentrations predicted when the geothermal water becomes fully-mixed with the Kaituna River (0.02 µg l -1).

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 19

5. References

Australian and New Zealand Environment and Conservation Council; Agriculture and Resource Management Council of Australia and New Zealand. (2000). Australian and New Zealand guidelines for fresh and marine water quality. Volumes 1 and 2 of 4.

Hamilton, D.P.; Uraoka, T. (2004). Lake Rotoiti fieldwork and modelling to support considerations of Ohau Channel diversion from Lake Rotoiti. Centre for Biodiversity and Ecology Research, The University of Waikato, Consulting report to Environment Bay of Plenty, 93 p.

Hoare, R.A. (1985). Inferred geothermal inflows to Lake Rotorua. New Zealand Journal of Marine and Freshwater Research 19(2) : 151-156.

Jirka, G.H.; Doneker, R.L.; Hinton, S.W. (1996). User's manual for CORMIX: a hydrodynamic mixing zone model and decision support system for pollutant discharges into surface waters. U.S. Environmental Protection Agency, CX824847- 01-0, 164 p.

Stephens, S.A.; Gibbs, M.; Hawes, I.; Bowman, E.; Oldman, J.W. (2004). Ohau Channel Groynes. NIWA consulting report to Environment Bay of Plenty, HAM2004-047, 89 p.

Stephens, S. (2004). Modelling diversion walls for diverting the Ohau Channel inflow from Lake Rotoiti. NIWA consulting report to Environment Bay of Plenty, HAM2004-164, 48 p.

Stephens, S. (2005). Supplement to Lake Rotoiti diversion wall modelling - refined wall designs and response to peer review. NIWA consulting report to Environment Bay of Plenty, HAM2005-032, 21 p.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 20

6. Glossary

Ambient conditions – the geometric and dynamic characteristics of a receiving water body that impact mixing zone processes. These include plan shape, vertical cross sections, bathymetry, ambient velocity and density distribution.

Ambient discharge / ambient flow – the volumetric flow rate of the receiving water body.

Background contaminant concentration – the ambient background concentration of a contaminant in the receiving Lake Rotoiti/Ohau Channel water, before addition of Tikitere geothermal water via a diffuser.

Contaminant – defined here as a substance contained in the Tikitere geothermal water that has been measured during laboratory analyses and presented in Table 1.

LC50 – Median lethal concentration. The concentration of material in water that is

estimated to be lethal to 50% of the test organisms. The LC 50 is usually expressed as a

time-dependent value, e.g., 24-hour or 96-hour LC 50 , the concentration estimated to be lethal to 50% of the test organisms after 24 or 96 hours of exposure.

LOEC – Lowest observed effect concentration. The lowest concentration of a material used in a toxicity test that has a statistically significant adverse effect on the exposed population of test organisms as compared with the controls.

MATC – Maximum acceptable toxicant concentration.

NOEC – No observed effect concentration. The highest concentration of a toxicant at which no statistically significant effect is observable, compared to the controls; the statistical significance is measured at the 95% confidence level.

Far-field – the region of a receiving water where buoyant spreading motions and passive diffusion control the trajectory and dilution of the effluent discharge plume.

Near-field – the region of a receiving water where the initial jet characteristic of momentum flux, buoyancy flux and outfall geometry influence the jet trajectory and mixing of an effluent discharge.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 21

Near field region (NFR) – a term used to describe the zone of strong initial mixing where the so called near-field processes occur. It is the region of the receiving water where outfall design conditions are most likely to have an impact on in-stream concentrations.

Pycnocline – a horizontal layer in the receiving water where a rapid density change occurs.

Sorption – Process whereby contaminants in soils adhere to the inorganic and organic soil particles.

Stratification – occurs when less dense (lighter) water overlies more dense (heavier) water, creating buoyancy forces that make the water column resistant to mixing in the vertical direction. In freshwater this occurs due to temperature changes, whereby warm water overlies denser cold water.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 22

7. Appendix – CORMIX flow description table Table 9: Description of flow classification for the dispersing geothermal plume in the receiving lake water, from CORMIX (Jirka et al. 1996). Flow Description classification A3 Controlled primarily by the geometry of the discharge, the near-field of this flow configuration is dominated by Coanda attachment. The port orientation is more or less horizontal and/or the height of the discharge port above the bottom is too small. This leads to rapid dynamic attachment (Coanda attachment) of the discharge flow to the bottom and the formation of a wall jet. At some distance the discharge flow will lift off from the bottom due to its strong buoyancy. A5 Controlled primarily by the geometry of the discharge, the near-field of this flow configuration is dominated by Coanda attachment. The port orientation is more or less horizontal and/or the height of the discharge port above the bottom is too small. This leads to rapid dynamic attachment (Coanda attachment) of the discharge flow to the bottom and the formation of a wall jet. The discharge flow will remain attached to the bottom due to its weak or negative buoyancy. H2 A submerged buoyant effluent issues horizontally or near- horizontally from the discharge point. The discharge configuration is hydrodynamically "stable", that is the discharge strength (measured by its momentum flux) is weak in relation to the layer depth. The discharge buoyancy plays a minor role in this case. H4-90 A submerged buoyant effluent issues horizontally or near- horizontally from the discharge port. The discharge is at, or approximately at, a right angle with the ambient current. The discharge configuration is hydrodynamically "stable", that is the discharge strength (measured by its momentum flux) is weak in relation to the layer depth and in relation to the stabilizing effect of the discharge buoyancy (measured by its buoyancy flux). The buoyancy effect is very strong in the present case. This discharge configuration is very susceptible to attachment of the jet/plume to the bottom of the receiving water. SA1 This flow is dynamically attached to the downstream bank. Along the bank is a zone of recirculating effluent that will reduce the dilution. The penetration into the crossflow is reduced due to this dynamic attachment. For this case, the flow does not interact with the bottom in the near field. SA2 This flow is dynamically attached to the downstream bank. Along the bank is a zone of recirculating effluent exists which reduces the dilution. The penetration into the crossflow is reduced due to this dynamic attachment. Since the discharge depth is equal or nearly equal to the depth of the receiving water at the discharge point, the flow becomes attached to the bottom. This attachment to the bottom could effectively block off the ambient current and be the cause of the attachment to the downstream shoreline.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 23

8. Appendix – aquatic toxicity information from ANZECC (2000)

8.1 Ammonia

Ammonia is a non-persistent and non-cumulative toxicant to aquatic life. In solution, + ammonia consists of ionised ammonium (NH 4 ) and un-ionised ammonia (NH 3). The toxicity of ammonia can depend on pH, temperature and ionic composition of

exposure water. The toxicity of ammonia is primarily attributed to the un-ionised NH 3, but the ammonium ion can also contribute significantly to ammonia toxicity under certain conditions. There have been many reviews of ammonia toxicity with ANZECC (2000) quoting one study that found ammonia was acutely toxic to freshwater organisms at concentrations (uncorrected for pH and temperature) ranging from 5 to 23 mg l-1 for nineteen invertebrate species and from 0.88 to 4.6 mg l-1 for 29 fish species. Values of 4.9 mg l-1 and 15.6 mg l-1 were predicted at the near-field mixing region for the Tikitere discharge (Table 8). Invertebrates are generally more tolerant to ammonia than fish, and phytoplankton and aquatic vascular plants are more tolerant again. Salmonid fish appear to be particularly sensitive to ammonia (e.g., trout). Thus we expect that a direct discharge of Tikitere geothermal water would provide a serious health hazard to trout in the vicinity of the diffuser, and may form a substantial barrier to their passage through the Ohau Channel.

The values given below are geometric means of species data taken from all screened data that concurrently measured pH and temperature. Figures were adjusted to a standard pH of 8.0 and calculated in terms of total ammonia-N.

Freshwater fish: 15 spp, 24−96 h LC50, 3944−169 873 µg l-1. Chronic NOEC and EC20 for 9 spp (28−6 d, growth and survival) of 1350−19 720 µg l-1.

Freshwater crustaceans: 10 spp, 24−96 h LC50, 7754−108 500 µg l-1. The cladoceran Simocephalus vetulus was the most sensitive (24-h EC and LC50 values around 1580 µg l-1) and the amphipod Crangonyx pseudogracilis was least sensitive. Chronic NOEC and EC20 for 4 spp (7 d−10 weeks, reproduction) of 1450−19 770 µg l-1.

Freshwater insects: 8 spp, 24−96 h LC50, 15091−282 400 µg l-1. Chronic NOEC for 2 spp (29 d, reproduction) of 1790−4400 µg l-1

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 24

Freshwater molluscs: 7 spp, 12 558−74 623 µg l-1. Chronic NOEC and EC20 for 2 spp (42−60 d, reproduction and survival) of 540−2620 µg l-1. The most sensitive species under chronic exposure was the New Zealand species Sphaerium novaezelandiae with NOEC (60 d mortality and reproduction) of 540 µg l-1 total ammonia-N.

Freshwater annelid: 2 spp, 24−96 h LC50, 20 071−79 788µg l-1

Freshwater rotifer: Brachionus rubens , 24-h LC50 of 1300 µg l-1

Freshwater Platyhelminthes: Polycelus tenuis , 24−96 h LC50 of 37 634 µg l-1

Guideline trigger values were calculated by converting all acceptable chronic NOEC data, reported at different pH values, to total ammonia at a common pH value of 8 before applying the statistical distribution derivation method. No temperature conversions were used in the procedures. Water managers need to refer to table 8.3.7 (ANZECC 2000) in the section on ammonia (see 8.3.7.2) every time that ammonia toxicity is being considered. It is important to determine the pH and temperature whenever ammonia concentrations are measured. When ammonia concentration is expressed as that of un-ionised ammonia instead of total ammonia, table 8.3.6 can be used to derive total ammonia. Table 8.3.6 reports the percentage of un-ionised to total ammonia at different pH and temperatures.

A freshwater high reliability trigger value of 900 µg l-1 TOTAL ammonia-N was calculated at pH 8.0 using the statistical distribution method with 95% protection. This translates to about 35 µg/L un-ionised ammonia-N at 20 oC. Table 8.3.7 (ANZECC 2000) indicates how the guideline figure changes at different pH values.

8.2 Arsenic

Phytoplankton are among the most sensitive organisms to both forms of arsenic. Higher trophic levels are less sensitive to arsenic because they generally accumulate the element from food rather than the water column. Acute toxicity of arsenic (III) to freshwater invertebrates occurred at concentrations as low as 812 g l-1 Adult freshwater fish are generally less sensitive to arsenic. Concentrations of arsenic (III) causing an acute toxic response in fish ranged upwards from 13 300 g l-1, and the lowest acute toxic concentration of arsenic (V) for freshwater fish (rainbow trout) was 10 800 g l-1.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 25

The chronic toxicities of arsenic (III) and arsenic (V) to freshwater organisms are detailed below. Arsenic (V) seems to be more toxic to plants than arsenic (III).

Screened chronic toxicity data for arsenic (III) comprised 7 taxonomic groups, to give the following figures (pH range 6.9–8.03):

-1 Fish: 7 spp, chronic LC 50 between 540 and 67 300 g l , converting to NOEC range of 108–13 460 g l-1 plus a measured NOEC figure of 961 g l-1

-1 Amphibian: Ambystoma opacum , chronic LC50 of 4450 g l to give NOEC of 890 g l-1

Crustaceans: 2 spp, NOEC of 88–961 g l-1 (the geometric mean was 290 g l-1 for the more sensitive Gammarus sp.)

Insect: 1 sp, NOEC of 961 g l-1

Mollusc: 2 spp, NOEC of 961 g l-1

-1 Macrophyte: 2 spp EC50 (growth) of 3600–4100 to give NOEC of 720–820 g l

-1 Algae: 2 spp, EC50 (population growth) of 79–31 200 g l , to give NOEC of 16– 6240 g l-1.

A high reliability freshwater trigger value of 24 g l-1 was derived for arsenic (III) using the statistical distribution method with 95% protection.

8.3 Boron

Boron is a trace element of igneous rocks and is common in sedimentary rocks derived from marine waters. Groundwater in contact with volcanic rocks can contain high concentrations of boron. The main species present in freshwaters, depending on pH,

are borates e.g., B(OH) 4 - and boric acid B(OH) 3, a weak acid, and the main removal mechanism is adsorption onto suspended clays or sediments, particularly on contact with seawater. Boron is an essential element required by aquatic plants. Concentrations of boron in New Zealand rivers, with low or no geothermal influence ranged from <0.5 to 410 µg l-1 with a geometric means of 16 µg l-1. Boron is an important buffer for maintaining the pH of seawater. The current analytical practical quantitation limit (PQL) for boron is 0.5 µg l-1 in fresh water.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 26

Screened chronic freshwater data (around 30 points) for boron were available for 5 taxonomic groups as follows. Toxicities are expressed as NOEC equivalents other chronic end-points have been adjusted to NOECs.

Fish: 4 spp, 40 µg l-1 ( O. mykiss from 32-d LOEC, mortality) to 27 600 µg l-1 ( O. mykiss from 32-d LC50. Other chronic O. mykiss data were orders of magnitude higher than 40 µg l-1, including those from the same paper (2100 µg l-1 for 87-d NOEC and 27 600 µg l-1 for 32-d LC50). All other geometric means were >4000 µg l-1.

Crustaceans: 2 spp, 4665 µg l-1 ( D. magna ; from 21-d MATC, growth) to 53 200 µg l-1 (D. magna from 21-d LC50). A measured NOEC, reproduction of 6000 µg l-1 was also reported.

Macrophytes: 2 spp, 1000 µg l-1 ( Elodea canadaensis ; from 21-d LC50) to 34 200 µg l-1 ( Myriophyllum spicatum ; from 32-d EC50)

Algae: 2 spp, 400 µg l-1 ( Chlorella pyrenoidosa ; 14-d NOEC, population growth) to 5200 µg l-1 ( C. vulgaris ; NOEC, population growth)

A freshwater high reliability trigger value for boron of 370 µg l-1 was calculated using the statistical distribution method at 95% protection.

8.4 Chromium

Cr (VI) is usually more toxic than Cr (III). Freshwater organisms are generally much more sensitive to chromium than marine organisms. Freshwater algae and invertebrates are more sensitive to chromium (VI) than fish, with Cr (VI) being the most toxic species. Crustaceans are particularly sensitive to Cr (VI). Chronic data on chromium (III) were screened for hardness and other standard factors to give 7 data points, Hardness has a significant influence on the toxicity, with chromium (III) being more toxic in soft water. The pH range was 7.2–8.0. These figures were corrected to a -1 common hardness value of 30 mg l as CaCO 3 to give the following figures:

Fish: 3 spp, 7–28 d LC50, 66 ( O. mykiss ) to 442 µg l-1 ( Micropterus salmoides )

Amphibians: 1 sp. Ambystoma opacum , 8-d LC50, 795 µg l-1

Crustaceans: 1 sp, D. magna, 21-d EC50, reproduction, 430 µg l-1

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 27

Algae: 1 sp, Selenastrum capricornutum , 4-d EC50, population growth, 397 µg l-1.

The screening process reduced acceptable chronic data to 6 species from 3 taxonomic groups, and acute data for species from 2 taxonomic groups. Hence only a low reliability trigger value could be calculated.

A low reliability freshwater trigger value for chromium (III) of 3.3 µg l-1 was derived. -1 This applies to low hardness water at 30 mg l as CaCO 3. This figure should only be used as an indicative interim working level.

8.5 Copper

For freshwater guideline derivation, only the chronic data that were linked to pH and hardness measurements were considered and further screened. This reduced the dataset to around 130 data points covering 4 taxonomic groups, and these were -1 adjusted to a common hardness of 30 mg l as CaCO 3, as follows (data are reported as geometric means of NOEC after adjustment from other chronic end-points (pH range was 6.96−8.61):

Fish: 10 spp, 2.6 µg l-1 ( Ptylocheilus oregonensis , from 7-d LC50) to 131 µg l-1 (Pimephales promelas , 7-d LC50); 7 species had geometric means <25 µg l-1

Crustaceans: 5 spp, 1.7 µg l-1 ( D. pulex and G. pulex , NOEC, reproduction & mortality) to 12.1 µg l-1 ( Hyalella azteca , from 10−14 d LC50)

Insects: 3 spp, 2.2 µg l-1 ( Tanytarsus dissimilis , from 10-d LC50) to 11µg l-1 (Chironomus tentans , 10−20 d LC50)

Molluscs: 3 spp, 1.64 µg l-1 ( Flumicola virens , from 14-d LC50) to 56.2 µg l-1 (Corbicula manilensis , from 7−42 d LC50). The latter figure was not included in calculations as it was outside the pH range.

A freshwater high reliability trigger value for copper of 1.4 µg l-1 was derived using the statistical distribution method with 95% protection. This applies to waters of -1 hardness of 30 mg l as CaCO 3.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 28

8.6 Mercury

Mercury in the aquatic environment exists mainly as complexes of mercury (II) and as organomercurials. Of particular concern to the aquatic environment is the fact that inorganic forms of mercury (of relatively low toxicity and availability to bioconcentrate) may be converted by bacteria in situ into organomercury complexes (particularly methylmercury), which are more toxic and tend to bioaccumulate.

Sorption onto suspended matter or bottom sediments is the most important process controlling the concentration of mercury in natural waters. Only a small proportion of total mercury is found in the dissolved phase; dissolved mercury concentrations rarely exceed 12 ng l-1 in freshwaters. This is one reason that Mercury is non-conservative in its transport from its existing geothermal sources entering Lake Rotorua to the headwaters of the Kaituna River. Comparison of the concentrations predicted in Table 8 to the figures quoted below (from ANZECC 2000), show that the direct discharge of the Tikitere geothermal water into the Ohau Channel extension will raise Mercury levels to chronic toxicity levels in the vicinity of the diffuser.

Chronic freshwater data for mercury were screened to 4 taxonomic groups, as follows (pH range 7–8.7):

Fish: 7 spp, 7−91 d LC/EC50, 0.7 µg l-1 ( Carassius auratus ) to 6355 µg l-1, which converted to NOEC values of 0.14−1271 µg l-1 Crustacean: 1 sp, Hyalella azteca , 42−70 d NOEC, 1.12 µg l-1

Mollusc: 1 sp, 7-d LC50, 60−95 µg l-1, converting to NOEC of 12−19 µg l-1

Algae: 3 spp, 14-d NOEC, growth, 33−85 µg l-1

Blue-green algae: NOEC, 253 µg l-1

Macrophyte: 1 sp, Myriophyllum spicatum , 32-d EC50, growth, 1200−3200, converting to NOEC of 240−640 µg l-1

A freshwater high reliability trigger value of 0.6 µg l-1 was calculated for inorganic mercury using the statistical distribution method with 95% protection. This has not specifically considered bioaccumulation. The 99% protection level is 0.06 µg l-1, and is the figure recommended for slightly-moderately disturbed systems for three reasons: a) as a precaution for bioaccumulation; b) the 95% figure is close to the chronic LC50 figure for Carassius auratus ; and c) the 95% figure is only 3−4 fold lower than the

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 29

lowest acute LC50 for D. magna . There were insufficient data for ANZECC to derive a trigger value for methyl mercury.

8.7 Zinc

For freshwater guideline derivation, only the chronic data that were linked to pH and hardness measurements were considered and further screened for quality and other factors. This reduced the dataset to around 85 data points. These were adjusted for -1 uniform lower hardness (30 mg l as CaCO 3) and other end-points adjusted to NOECs. The NOEC values from 6 taxonomic groups were as follows (pH range 6.75– 8.39):

Fish: 11 spp, 24 µg l-1 ( O. tshawytscha ; from LC50) to 1316 µg l-1 ( Ptylocheilus oregonensis ; from LC50); 7 species had geometric means <250 µg l-1 and a measured NOEC of 38 µg l-1 was reported for P. promelas

Amphibians: 1 sp, Ambystoma opacum , 180 µg l-1 (from LOEC)

Crustaceans: 3 spp, 5.5 µg l-1 ( C. dubia ; from LC50) to 25.3 µg l-1 ( C. dubia ), plus a figure of 18 480 µg l-1 for the crayfish Orconectes virillis )

Insect: 1 sp, Tanytarsus dissimilis , 5 µg l-1 (NOEC)

Molluscs: 3 spp, 54µg l-1 ( Dreissena polymorpha ) to 11 200 µg l-1 ( Velesunio ambigua ), a NOEC of 487 µg l-1 was measured for Physa gyrina

Annelid: 1 sp, Limnodrilus hoffmeisteri , 560 µg l-1 (from LC50)

The geometric means for zinc were distinctly bimodal with two values at least 9.4 times the next highest. However, all the data fitted the model and they were not excluded. The trigger value is above the lowest measured NOEC for an insect and the recalculated NOEC for C. dubia (from chronic LC50 of 27.5 µg l-1). However, given the essential nature of zinc and the fact that the chronic end-points are NOECs, the risk is low and the 95% protection level is considered acceptable for slightly- moderately disturbed systems.

A freshwater high reliability trigger value of 8 µg l-1 was calculated for zinc using the statistical distribution method with 95% protection. This applies at hardness of 30 -1 mg l of CaCO 3.

Water quality impacts of a point-source discharge of Tikitere geothermal water in Lake Rotoiti 30