The two infiltration basins of the Managed Aquifer Recharge pilot at Hinds, in Canterbury.
The two infiltration basins of the Managed Aquifer Recharge pilot at Hinds, in Canterbury.
We know the phrase "saving for a rainy day". But in the case of a pilot managed aquifer recharge (MAR) scheme by the Hekeao reach of Hinds River in south Canterbury, it's more about saving water when we don't have enough rainy days.
MAR is a tool used widely in parts of the USA and Australia and it may help us in some locations in New Zealand.
The people behind the trial project in Hinds deliberately picked a part of the catchment that had the highest groundwater nitrogen concentrations and the most depleted stream-flow conditions. They wanted it to be a true 'test' of the technique - and initial results are promising.
In his report on New Zealand's fresh waters, the Prime Minister's chief science advisor Professor Sir Peter Gluckman said solutions to our water quality issues will work best if there is a partnership approach across sectors of society.
The Hinds MAR pilot ticks that box: It's being driven by a governance group that includes ECan Ashburton Zone Committee members, residents and farmers, iwi and representatives from DoC, Fish & Game and Forest & Bird.
Considerable effort has been invested in getting an understanding of the geology of the MARS site at Hinds. These are some of the sonic drilling cores. Photo / Golder Associates
They've harnessed the technical skills of Golder Associates, including hydrologist and 20-year veteran of MAR initiatives in the USA, Robert Bower - a speaker at the Federated Farmers National Council meeting in Wellington earlier this year.
There is very complex science behind effective managed recharge but the concept can be boiled down to thinking of an aquifer as a giant, underground very slow moving river/ reservoir. It can be topped up with water when it is surplus, such as during the agriculture off-season and the big winter river flows, and then tapped again when demand is high.
The whole system has progressively got out of balance. We're looking to replenish groundwater in terms of quantity and quality.
At the pilot site two infiltration basins have been constructed. Fed at 100 litres of water per second (but consented up to 500 l/s), the first is designed to retain natural sediments to lessen clogging. The water then flows into the second and larger infiltration basin and percolates into the underlying shallow aquifer.
More than 2.4 million cubic metres of high quality water had been recharged in the first 274 days of operation.
There are problems with low aquifer levels and nutrient concentrations because over time 'accidental' recharge of the aquifers has declined due to leaky races being piped and more efficient irrigation becoming commonplace. Intensification of agriculture, as well as town and septic tank impacts, has seen a rise in nitrates and other contaminants.
"The whole system has progressively got out of balance," Mr Bower says. "We're looking to replenish groundwater in terms of quantity and quality."
Mr Bower says when he was first fronting early sessions to explain the pilot to locals, people would be yelling at him, "where's the water going to come from" and "you're crazy".
But Ashburton District Council came up with 500 L/s of un-utilised stock water from the Rangitata River and there are other consents for less reliable water sitting in trust and other stock water rights.
"The reality is the peak of the peak events, such as with the recent cyclone, that's not the kind of water we want to put into the basins because it would just clog them. But two days before, and a couple of weeks after, once that initial slug of rainwater has come through...
"I think the water is there," Mr Bower says. "Some of it's free and sits in consents because it's shoulder water, there's other water initially used for hydro. It's a political and adaptive process we have to go through."
There are costs, but this needs to be weighed up against the benefits MAR can deliver over time. "How much is it worth not having to build herd homes and feed pads by using MAR quality water, and avoiding the expense of re-drilling wells."
It's only a year into what is expected to be a five-year trial, and full analysis of year one results won't be completed until next month, but the governance committee is heartened by preliminary indications.
The quality of groundwater within a 3km radius of the MAR basins is on the upswing. At one bore where nitrogen concentrations were 13.2 milligrams per litre of water (significantly above the national drinking water standard of 11.3), readings have been as low as 1.2. In deeper parts of the aquifer, some 30m below ground, readings have dropped from 4.0 mg/L to near zero.
Groundwater levels in the catchment are also showing improvement. According to hydrological data in Environment Canterbury's 'State of the Water' report for the year ending December 2016, a well connected to the Hinds MAR project was among the three per cent of those which showed a "very high" improvement in water levels across Canterbury, and the only site in Mid Canterbury.
It all bodes well for the potential to roll out MAR techniques across the Canterbury Plains and other parts of New Zealand where water shortages are an issue.
Mr Bower says the end goal is managing groundwater on a catchment scale. It's a technique to use alongside ongoing improvements to on-farm nutrient management practices and irrigation.
When what Mr Bower calls a "sustainable yield scenario" is achieved, it would ensure that reliable groundwater resources are maintained for irrigation and drinking water as well as more consistent and improved quality base flows to the spring-fed water bodies.
From there, schemes could push for an "additional yield position", which could help protect both water users and ecological habitats from erratic weather patterns driven by climate change, such as prolonged droughts.
Another MAR trial scheme is right now getting underway in Poverty Bay. It's a deeper aquifer system there so the recharge will be by injection rather than infiltration.
reliable groundwater resources are maintained for irrigation and drinking water as well as more consistent and improved quality base flows to the spring-fed water bodies. From there, schemes could push for an "additional yield position", which could help protect both water users and ecological habitats from erratic weather patterns driven by climate change, such as prolonged droughts.
