Tikitere Diversion
Introduction
The Tikitere geothermal field is located on SH 30 about 4km east of the junction with SH 33. The field actually straddles the catchment boundary between Lakes Rotorua and Rotoiti. This report deals with the surface flows from the southern (Rotorua) side of the field. There are two main areas of surface discharges, both of which have been developed as a Tourist Attraction (Hells Gate). Fluids from the lower of the two fields have been collected and used to develop a spa complex.
Discharges from both fields are high in nitrogen mainly in the ammonium form. The field naturally drains to the Waiohewa Stream which discharges to LakeRotorua. The proposal here is to collect these nitrogen rich discharges and by-pass the lakes (Rotorua and Rotoiti) by piping them directly to the KaitunaRiver. This is just one component of a series of initiatives aimed at reducing the nutrient load on the RotoruaLakes.
Tikitere Nutrient Load
Nutrient loads from the two separate fields have been measured over the period from April 2004 until present. Monitoring is continuing. Mass loads are summarised as follows:
Average Flow / Nitrogen (t/annum) / Phosphorus (t/annum)Top Field / 13.6 l/s / 18.8 / 0.04
Bottom Field / 3.3 l/s / 5.7 / 0.04
Note that the flow in the bottom field is the actual complex discharge calculated from the difference in measured flows upstream and downstream. Flows from the site are not defined in an outlet(s). Continued monitoring is aimed at better defining long term loads which may vary seasonably and isolating those flows as much as possible.
Other Constituents
Other elements have been measured in the discharge. Most are at low levels and present no issues. Boron levels are however reasonably high resulting in a mass annual load of 1.5 tonnes. Suspended solids can also be high and are mainly sourced from the upper field, which has periodic overflows from mud pools. Suspended sediment loads have been measured as high as 567mg/l directly from the top field. Corresponding levels downstream have reduced by a factor of 5 but significant sediment quantities in the discharge do have an impact on the design of any diversion pipeline.
The silica content averages around 100 mg/l in the form of silica dioxide. This will potentially form scale deposits on the inside of the pipeline increasing roughness and hence increase the head loss. Similarly calcium content is around 8 mg/l and could cause a similar problem. This issue will require detailed evaluation as it could effect the long term viability of the piping options.
There are four potential options/sub-options for diversion of this nutrient rich in flow to LakeRotorua.
a)Diversion Pipeline along SH 30 and 33 to the Okere arm of LakeRotoiti where it would be discharged down the KaitunaRiver.
b)Diversion Pipeline along SH 30 to the Rotorua lakeshore and then along the lakeshore to the Ohau Channel. This avoids the hill just prior to Mourea.
c)Diversion of the Waiohewa Stream itself along the shoreline of LakeRotorua to the Ohau Channel.
d)Absorption of the nutrient, which is principally in the form of ammonium by passing through a zeolite filter.
Option 1 – Roadside Diversion
For this option discharges from the Tikitere field would be collected and piped along the road edge to the boat-launching ramp just north of the Ohau Channel. Flows from the fields would need to be isolated as much as possible to minimise flow rate, and hence reduce both cost of capital and any future ongoing costs. Flows from the top field are well defined can be collected relatively easily, however the discharge from the bottom field is not well defined being part of the thermal spa complex with no single discharge point. Considerable benefit for the project could be gained by isolating the flows from the spa complex which could involve re-plumbing and re-building part of the facility. This has not been investigated in any detail but is supported by the Hell’s Gate management provided any impact on the business is catered for. The alternative is to pipe the total flow which includes discharge from the Rotokawau catchment above which is virtually nutrient free.
From the intake works a pipeline of approximately 250mm diameter would run down the northern side of SH 30 to SH 33 and then along the eastern side of SH 33 to discharge in the Okere Arm of LakeRotoiti at the boat ramp just north of the Ohau Channel. The total length is 9000metres. There is about 40m of fall over the route of 9km but along SH 33 just before Mourea, there is a hill of about 30m elevation. The route will require pipe thrusting over a length of approximately 650 metres. This would entail a 200mm ID pipe thrust. The simplest option is a straight thrust, which will entail going under private land. The land is in forestry held under Maori Trust title.
The pipe thrust required is at or near the practical limit that current technology will permit and there are some risks that it would not be successful, i.e. settlement of drilling rods, too higher friction. It would be prudent to undertake the thrust prior to the remainder of the pipe laying.
Other issues with the pipeline include, scale deposits of silica and the high sediment load. The sediment will likely require occasional flushing and a sluicing system could be incorporated into the design through the low point along the start of SH 33, and prior to the Ohau Channel crossing. Deposits of silica within the pipeline will be an ongoing issue and specialist advice will be required.
An initial very brief assessment from Contact Energy suggests that the levels recorded should not affect long term viability.
Option 2 – Diversion along lakeshore
This option involves piping along SH 30 and out to the lakeshore, and then along the lakeshore to the Ohau Channel. The difference means the difficult pipe thrusting through the Mourea hill can be avoided, and construction of the pipeline will be a lesser problem, but there will be greater short term construction impacts and the final discharge will be to the Ohau Channel rather than the Okere arm of LakeRotoiti. This option is only viable if the Ohau Channel diversion is implemented.
Option 3 – Diversion of Waiohewa Stream
This option involves the diversion of the Waiohewa Stream itself by containing it in a long shore channel and discharging direct to the Ohau Channel. There are a number of options for construction of the channel, but the simplest would be to use a pre-cast concrete panel laid 4 to 5 metres out in the lake. This has the added advantage of diverting additional nutrient inflows that occur downstream of Tikitere. Currently this is an extra amount of approximately 10 tonnes per annum or a total of about 39 tonnes per annum. The land based inputs are also expected to increase with time.
The Waiohewa Stream also currently has a total phosphorus load of about 1.5 tonnes/annum which will be an additional benefit not available with option 1 or 2.
Clearly there will be added visual impacts with such an option, and it is only viable should the Ohau Channel diversion be implemented.
Option 4 – Zeolite Filter Bed
Zeolite has the property of absorbing cations. The principal constituent of the nutrient discharge in ammonium in the form of NH4+. This is 98% of the nitrogen in the discharge from the Tikitere field. Nitrogen can be recovered from the zeolite filter and used as fertiliser. There is some potential therefore for a commercial operation though the zeolite will also absorb other minerals which may inhibit the commercial viability. The University of Waikato are undertaking some trials on the viability of this technique.
Estimated costs
Costs at this stage are preliminary only with detailed hydraulic design and site investigations yet to be undertaken.
Option 1
Preliminary and General$ 50,000.00
Intake Works$ 50,000.00
Pipe Supply and Lay$1,170,000.00
Air and Scour Valves$ 80,000.00
Pipe Supply – Thrust Section$ 88,000.00
Pipe Thrust $ 550,000.00
Contingency$ 220,000.00
TOTAL$2,188,000.00
Option 2
Preliminary and General$ 50,000.00
Intake Works$ 50,000.00
Pipe Supply and Lay$1,080,000.00
Lake Shore Works$ 250,000.00
Air and Scour Valves$ 80,000.00
Contingency$ 150,000.00
TOTAL$1,588,000.00
Option 3
Preliminary and General$ 50,000.00
Supply Concrete Panels$2,400,000.00
Site Laying$2,400,000.00
Contingency$ 480,000.00
TOTAL$5,330,000.00
Option 4
Not costed.
Summary
It is technically feasible to collect and divert the high nutrient flows from the Tikitere geothermal field, which discharges to the Rotorua Catchment. This will prevent some 25 tonnes per annum of nitrogen from entering Lake Rotorua, meeting some 10% of the target for proposed reduction of nitrogen inflows.
The various options now need to be investigated in some detail, which will initially involve hydraulic and geotechnical design. There are some risks and technical challenges with all options, which need careful evaluation. Options 2 and 3 will have a much greater environmental and visual impact and are likely to attract a higher level of opposition, but particularly option 3 has the added benefit of preventing a total of some 39 tonnes of nitrogen and 1.5 tonnes of phosphorus per annum, entering Lake Rotorua.
The option of mining the nitrogen on site is being pursued by the University of Waikato who have had some discussions with a commercial operation. At this stage option 4 is best left to the University but depending on the outcome may become a viable alternative that requires detailed engineering evaluation.
Future Strategy
1)That the first three options now need to proceed to detailed engineering design and evaluation.
2)That in conjunction with this an AEE is required to be prepared.
3)This is best undertaken as a single professional services contract.
A professional services contract for this work is estimated to cost $100,000.00 which includes about $30,000.00 worth of geotechnical site works and testing which will be necessary to design the pipe thrust.
Design Requirements
This will involve detailed hydraulic design inclusive of pipe lives and the issues surrounding silica scale and sediment flushing. Management options for control of these effects do exist and detailed analysis of those options and financial implications will need to be undertaken. There will be a need to undertake some significant geotechnical, work particularly drill logs and soil strength testing on the possible pipe thrust and lakeshore options. This investigative work will require consents itself. Each of the options will require design and detailed costings and feed into the AEE. The separate option of which the feasibility work is being undertaken by the University of Waikato may then be available also for engineering evaluation and inclusion in the AEE.
Assessment of Environmental Effects
This study will have to consider various aspects of each option. In summary these will be:
Option 1
The effect of the discharge on the receiving environment that includes both the Okere arm of Lake Rotoiti and the Kaituna River.
Some assessment will also be required on the impact on the Waiohewa Stream itself in the removal of these discharges. There is in this case the likelihood of water quality improvement with the removal of the geothermal discharges.
There will be short term construction effects but these are likely to be minor and easily manageable.
Option 2
As in option 1 above but with the added impact of the effect on the Ohau Channel.
Construction impacts will also be significant with the effect on 4kms of Lake Rotorua Shoreline. While this is short term the high sediment movements along the shoreline will mean that recovery will be rapid.
Option 3
The discharge to the Ohau Channel will be modified from (2) above being the total flow of the Waiohewa Stream. This will require an additional monitoring of the Stream flow itself.
Construction impacts will also be significant as in (2) above but longer term there is the aesthetic issue of the permanent inshore Channel. There are various issues to resolve inclusive of flood flows and sedimentation both within the inshore Channel and on the Lake side of the wall.
Attachments
1)Plan of Monitoring Sites at Tikitere.
2)Plan of Options 1, 2 and 3.
3)Preliminary Details of Pipe Trust through hill at Mourea (Option 1).
4)Preliminary Deisgn of Inshore Channel (Option 3).
5)Monitoring Data to Date.
Doc No: IT-12814 / 1 / File No: 41-01-131Doc No: IT-12814 / 1 / File No: 41-01-131
ROTORUA DISTRICT COUNCIL
ENVIRONMENTAL LABORATORY
ANALYSIS REPORT
Client:RDC Professional ServicesDate Received:21 April 2004
Address:Private Bag RO 3029Report No.:04/252
Rotorua
Contact:Peter DineOrder No.
Issue Date:24 May 2004
Client ID / Lab ID / Ag / As / Be / B / Cd / EC / Cr / CsTop field / 04/893/1 / <0.0001 / 0.027 / 0.0001 / 1.29 / 0.00007 / 708 / 0.002 / 0.0023
Upper Culvert / 04/893/2 / <0.0001 / <0.001 / <0.0001 / 0.019 / <0.00005 / 132 / <0.0005 / 0.0002
Middle Culvert / 04/893/3 / <0.0001 / 0.007 / <0.0001 / 2.78 / <0.00005 / 388 / 0.0006 / 0.0086
Bottom Culvert / 04/893/4 / <0.0001 / 0.01 / 0.0003 / 1.29 / <0.00005 / 1008 / 0.0007 / 0.0029
Client ID / Lab ID / Cu / DRP / Flow / Hg / Li / NH4-N / Ni / Pb
Top field / 04/893/1 / 0.0059 / 0.014 / 6.27 / 0.0575 / 0.0047 / 59 / 0.0012 / 0.0059
Upper Culvert / 04/893/2 / <0.0005 / 0.045 / 9 / < 0.00008 / 0.013 / 0.040 / <0.0005 / <0.0001
Middle Culvert / 04/893/3 / 0.0005 / 0.020 / 12 / 0.0005 / 0.0633 / 10 / <0.0005 / 0.0002
Bottom Culvert / 04/893/4 / 0.0017 / 0.009 / 11 / 0.0146 / 0.0226 / 45 / <0.0005 / 0.0017
Client ID / Lab ID / pH / Sb / Se / SS / Temp / TKN / Tl / TOXN
Top field / 04/893/1 / 6.3 / 0.0054 / <0.001 / 567 / 31.8 / 57 / 0.00034 / 0.026
Upper Culvert / 04/893/2 / 6.6 / <0.0002 / <0.001 / <1 / 13.6 / 0.16 / <0.00005 / 0.58
Middle Culvert / 04/893/3 / 4.2 / 0.0012 / <0.001 / 4 / 18.5 / 10 / <0.00005 / 0.45
Bottom Culvert / 04/893/4 / 3.1 / 0.0018 / <0.001 / 93 / 27.2 / 45 / 0.00008 / 0.027
Client ID / Lab ID / TP / Zn
Top field / 04/893/1 / 0.49 / 0.02
Upper Culvert / 04/893/2 / 0.057 / 0.002
Middle Culvert / 04/893/3 / 0.068 / 0.013
Bottom Culvert / 04/893/4 / 0.173 / 0.012
Doc No: IT-12814 / 1 / File No: 41-01-131
10-May-04
Client ID / Lab ID / EC / DRP / Flow / NH4-N / pH / SS / Temp / TKN / TOXN / TP
Top Field / 04/1042/1 / 979 / 0.009 / 8.86 / 53 / 3.1 / 139 / 33 / 56 / 0.013 / 0.107
Upper Culvert / 04/1042/2 / 136 / 0.044 / 11 / 0.43 / 7.0 / <1 / 15.4 / 0.23 / 0.68 / 0.065
Middle Culvert / 04/1042/3 / 553 / 0.049 / 16 / 12 / 3.4 / 3 / 19.4 / 11 / 0.52 / 0.082
Bottom Culvert / 04/1042/4 / 1309 / 0.009 / 13 / 44 / 2.8 / 10 / 27.8 / 40 / 0.041 / 0.049
17-May-04
Client ID / Lab ID / EC / DRP / Flow / NH4-N / pH / SS / Temp / TKN / TOXN / TP
Top Field / 04/1106/1 / 1095 / 0.007 / 12.3 / 50 / 3.0 / 44 / 30.3 / 48 / 0.026 / 0.104
Upper Culvert / 04/1106/2 / 135 / 0.045 / 14 / 0.027 / 6.8 / <1 / 12.6 / 0.19 / 0.74 / 0.066
Middle Culvert / 04/1106/3 / 667 / 0.046 / 16 / 12 / 3.2 / 2 / 17.8 / 12 / 0.58 / 0.070
Bottom Culvert / 04/1106/4 / 1335 / 0.010 / 17 / 39 / 2.7 / 9 / 25.2 / 41 / 0.078 / 0.052
24-May-04
Client ID / Lab ID / EC / DRP / Flow / NH4-N / pH / SS / Temp / TKN / TOXN / TP
Top Field 10.20am 24/5/04 / 04/1170/1 / 968 / <0.005 / 15.7 / 48 / 3.1 / 72 / 30.9 / 51 / 0.023 / 0.095
Upper Culvert 11am 24/05/04 / 04/1170/2 / 133 / 0.037 / 14 / 0.030 / 6.8 / 1 / 12.2 / 0.18 / 0.71 / 0.063
Middle Culvert 11.10am 24/5/04 / 04/1170/3 / 639 / 0.049 / 17 / 12 / 3.2 / 3 / 17.4 / 13 / 0.52 / 0.076
Bottom Culvert 11.15am 24/5/04 / 04/1170/4 / 1236 / 0.009 / 18 / 39 / 2.8 / 21 / 25.6 / 42 / 0.067 / 0.044
31-May-04
Client ID / Lab ID / EC / DRP / Flow / NH4-N / pH / SS / TKN / TOXN / TP
Top Field / 04/1227/1 / 972 / 0.006 / 16 / 42 / 3.0 / 60 / 42 / 0.022 / 0.094
Upper Culvert / 04/1227/2 / 119 / 0.040 / 16 / 0.024 / 6.5 / 2 / 0.24 / 0.74 / 0.064
Middle Culvert / 04/1227/3 / 644 / 0.045 / 22 / 11 / 3.1 / 6 / 11 / 0.58 / 0.066
Bottom Culvert / 04/1227/4 / 1190 / 0.006 / 23 / 34 / 2.8 / 27 / 37 / 0.13 / 0.057
8-Jun-04
Client ID / Lab ID / EC / DRP / Flow / NH4-N / pH / SS / Temp / TKN / TOXN / TP
Top Field / 04/1293/1 / 780 / <0.005 / 17 / 38 / 3.2 / 65 / 29.8 / 39 / 0.023 / 0.079
Upper Culvert / 04/1293/2 / 128 / 0.029 / 13 / 0.025 / 6.6 / <1 / 11.3 / 0.19 / 0.90 / 0.050
Middle Culvert / 04/1293/3 / 432 / 0.030 / 20 / 7.7 / 3.5 / 4 / 15.3 / 7.9 / 0.71 / 0.053
Bottom Culvert / 04/1293/4 / 1076 / <0.005 / 17 / 33 / 2.9 / 14 / 25.6 / 33 / 0.060 / 0.047
14-Jun-04
Client ID / Lab ID / As / B / Ca / EC / DRP / Flow / Hg / NH4-N / Pb
Top Field / 04/1342/1 / 0.003 / 1.2 / 8.0 / 653 / 0.005 / 15.9 / 0.0049 / 40 / 0.0005
Upper Culvert / 04/1342/2 / 7.29 / 132 / 0.040 / 14 / 0.024
Middle Culvert / 04/1342/3 / 0.015 / 3.57 / 9.05 / 507 / 0.037 / 18 / 0.0013 / 11 / 0.0003
Bottom Culvert / 04/1342/4 / 0.003 / 1.15 / 8.56 / 969 / 0.006 / 18 / 0.00063 / 34 / 0.0002
Client ID / Lab ID / pH / Sb / SiO2 / Si-Total / SS / Temp / TKN / TOXN / TP
Top Field / 04/1342/1 / 3.5 / 0.0005 / 102 / 50.2 / 124 / 31.3 / 40 / 0.020 / 0.081
Upper Culvert / 04/1342/2 / 6.6 / 77.6 / 36.7 / <1 / 0.22 / 0.88 / 0.049
Middle Culvert / 04/1342/3 / 3.5 / 0.0026 / 92.3 / 42.1 / 5 / 16 / 12 / 0.70 / 0.065
Bottom Culvert / 04/1342/4 / 3.0 / <0.0002 / 111 / 52.8 / 12 / 26.2 / 33 / 0.071 / 0.044
Doc No: IT-12814 / 1 / File No: 41-01-131