Makauri aquifer trial Q&A
Some frequently asked questions about the Makauri aquifer Managed Aquifer Recharge (MAR) trial project.
What is the Makauri aquifer?
The Makauri aquifer is the largest aquifer underneath the Poverty Bay Flats extending from Kaiteratahi down to Makaraka. It spans both sides of the Waipaoa River but most of the aquifer is on the eastern side.
The aquifer slopes from Kaiteratahi where it is relatively shallow (about 20m deep) down to the toe of the aquifer by Makaraka where it is about 80m deep.
The aquifer is not an “underground lake” – instead it's an area of gravel and sand under the flats which is saturated with water.
The Makauri aquifer sits below other aquifers on the Poverty Bay Flats. Its natural way of recharging is for water to percolate from the river and higher aquifers down through the gravel into it. This happens slowly over decades, and most of the recharge currently occurs around the Kaiteratahi area.
How has the Makauri aquifer water level declined?
In the 1980s Poverty Bay had a significant boom in horticulture and water takes for irrigation of crops has remained at a constant level since then.
The aquifer has very little natural recharge from rainwater and river water, because of changes to the land like flood control, vegetation clearance, rainfall and climate changes as well as water taken for use by farmers and growers.
It's a semi-confined aquifer and once full it can't take any more water. The more that's pumped out, the greater the recharge as the aquifer refills naturally.
The amount of water recharging the aquifer is less than the amount of water abstracted so the overall effect is that the aquifer water level declines by up to 0.1m per year.
Why not reduce water takes and let natural processes recharge the aquifer?
The Makauri aquifer supplies nearly a third of all irrigation water on the Poverty Bay Flats. Actual water takes will need to be cut by about 2/3 of current use to equal natural recharge.
It’s highly likely water restrictions will be introduced on water takes from the Makauri aquifer by 2020. If irrigation water was restricted there will be significant economic impacts to the region’s producers.
If the trial is successful a MAR would be used in conjunction with other water management measures.
We’re supporting projects looking at natural infiltration from the surface into shallow aquifers.
The Freshwater Plan is now in place and requires water users to improve efficiency of water use, use of surface storage and better match the amount of water needed for their crop requirement, soil type and climate. Irrigation management plans will be required and enforceable when water users renew their permits.
What is a managed aquifer recharge (MAR)?
A managed aquifer recharge is one of the proposed solutions to managing water levels in the aquifer.
MAR is a tool that has been used around the world to increase water levels in aquifers, there are also different methods of Managed Aquifer Recharge.
We’re proposing to trial an injection method which ‘injects’ water from another source into the aquifer through a bore.
The trial is see whether an injection MAR will work, without any adverse effects.
Council will apply for 4 resources consents to take water from another source and inject it into the aquifer. The consent applications will be going through the Bay of Plenty Regional Council to make sure the consenting process is transparent and independent.
Why does the Makauri aquifer suit a recharge injection?
International scientists and engineers have assessed the feasibility of a MAR in the Makauri aquifer.
The Makauri aquifer is semi confined. This means it doesn't ‘leak’ so much into the sea or into other aquifers. This is due to its near impermeable soil layers of silts and clays that have been deposited by the river over time. As long as the water quality's not compromised it may be able to be used to store water without any evaporation or leakage losses.
The trial will tell us if it’s possible to increase the amount of water in the aquifer with minimal impact on the water quality and environment.
How are we deciding the source of water for the trial?
Originally it was proposed to use the spill over from the Mangapoike dams as the water source for the trial.
This water travels down the pipeline and joins water from the Te Arai River intake before being treated at the Waingake Water Treatment Plant. The trial could only proceed using this water if the Mangapoike dams were overflowing. Forecasted El Nino weather conditions mean the potential to use water from Waingake is more limited than first thought.
So we’ve investigated using Waipaoa River water further. One option is to use water from the Bushmere treatment plant that is also drinking water quality - but more costly to run for the small amount of water needed in the trial.
A third option is considering sourcing river water from the irrigation system used at Kaiaponi Farms Ltd.
Golder International consultants were asked to provide a recommendation after investigating these options in the last 6 months.
They’ve completed a range of water testing, including when the river is high with a lot of sediment and run-off. The findings show the water quality is still very good, and well suited for the MAR trial. The recommendation is that the Kaiaponi / Waipaoa River source should become the preferred water source.
What are the cost differences of the source water options?
Golder consultants have provided a decision matrix and this led to a recommendation that the Kaiaponi source be the preferred option. The main differences in costs is treating the water to an appropriate quality.
Because the Mangapoike dams water is treated, we had the notional cost of providing the treated water for the injection well. This water has an opportunity cost of $1.21 per cubic metre and although this was not a cash cost it still needs to be considered.
The maximum cost to treat the same quantity of water from the Waipaoa via the Kaiaponi infiltration chamber is assessed at $65,000 and is more likely to be around $22,000. This additional cash cost and the additional analysis costs have been taken into account and compared with the use of city supply water.
Will water quality deteriorate as a result of the MAR injection?
Existing water in the aquifer is a poor quality, its minerals and salts have dissolved and it’s also high in iron and manganese.
The injection trial will be using very high quality water. If there is a surplus of extra water from the Waingake treatment plant we’ll use it - if not we’re investigating the potential for water from the Waipaoa river. The small amount of injected water isn’t likely to alter the overall quality, but if high volumes were injected and it did alter quality it would more than likely be an improvement.
When the water's mixed with air or oxygen, chemical reactions can turn the iron into ‘rust’ like material that'll block irrigation sprinklers and stain anything that's washed in the water.
What are the risks and how will they be managed?
The water in the aquifer has little to no oxygen. It also has minerals such as iron that if mixed with oxygen will perform chemical reactions and the character of the water can change.
This will be managed very carefully. We’ll inject small volumes of water in increments and gauge any changes through monitoring bores close by.
The injection can be reversed and the injected water can be pumped back out if necessary.
Arsenic leaching and creation of chlorine derivatives are very unlikely to occur.
There is very low natural arsenic in the rocks of the Poverty Bay Flats which makes arsenic leaching particularly unlikely.
Any arsenic that might be present has to come from the rocks that the well drilling may disturb.
Cuttings from the well drilling will be collected and analysed.
The injection of oxygenated water from the river into the aquifer also creates chemical conditions that further reduce the likelihood of arsenic mobilisation.
Dissolved organic carbon
The groundwater is in a very low oxygen environment. When combined with the overall
chemistry of the aquifer and source water, carcinogens or disinfection by-products are very
unlikely, even if chlorine treated water is used.
‘Clogging’ can also happen if fine sediment enters the aquifer. If surface water from the river is used it will need to be treated to ensure that this does not occur.
The risk of disinfection by-products from chlorine is also very low due to the natural conditions within the aquifer and the river water. The right mix of chemical precursors and environment conditions need to be present for reactions to occur. Testing for these precursors have found:
1. Organic matter quantities in the aquifer are so low that they cannot be identified in the
2. Nitrite levels in the aquifer are low and no other inorganic precursors such as bromides
or iodides were found present.
3. Chlorine is the likely disinfection that will be used for treating injection water. If we use
chlorine we can break down residual chlorine with sunlight before injection, by passing
water though a transparent tank.
How will Council apply for consents to do the trial?
We will apply for 3 resource consents to take water from another source and inject it into
the aquifer. The consent applications will be applied and processed through the Bay of Plenty Regional Council to make sure the consenting process is independent, transparent and assessed fairly.
Anyone will be able to make a submission to support or oppose the trial when the consents are notified. Submissions would be assessed by commissioners or the Environment Court to decide if and how the trial would proceed.
How will we know if the injection trial has worked?
If the chemical reactions to the oxygen can be restricted to a small ‘bubble’ within the aquifer it will be able to be managed. The smaller the ‘bubble‘ the better, and nature will be able to handle the changes over a short period of time. The bacteria in the water will be more active but then should de-oxygenate quickly. Monitoring will let us know how well we've restricted the bubble of water. Ongoing water quality testing will be continued to ensure that the natural state is retained.
The other measure of success is the hydraulic response. The water level will rise some distance away even though the injected water will not move far from the injection location. A small chemical effect and a large hydraulic effect is the aim.
We’ll assess the results of the trial and consider if we should proceed with a business plan and economic assessment for a full aquifer recharge programme.
What happens after the trial?
Council has only committed to the trial to see if Managed Aquifer Recharge will work.
To actually stabilise and recharge the aquifer, multiple sites and injection bores would be needed.
Currently about 1 million cubic metres of water is taken out of the aquifer each year. Modelling shows that 360,000m3 per year is required to go back in to stabilise the aquifer - that could be up to 5 more injections bores. Even more water would need to go in on top of that to enable any expansion of irrigation.
If the trial shows that MAR is a good option for the Makauri aquifer, then an economic case would need to be made to go ahead with a full recharge.
We’re currently looking at how other irrigation schemes work in the rest of New Zealand.
How is the trial funded?
Eastland Community Trust has agreed to fund $200,000 with matching funds from The Ministry of Primary Industries Irrigation Acceleration Fund of $250,000. No other funding is necessary other than Council providing the water, staff time and in-kind contributions from local irrigators.
The funding covers the comprehensive modelling and research into the aquifer and source water, as well as the capital and operational costs of developing the injection well infrastructure and completing the trial.
We’ve been working with irrigators in the catchment on how a full MAR programme might be funded in future if the trial is successful. A detailed business case would be completed before any further support is committed by Council.