Slowbut sure for recharging aquifer
A leaky pond in Mid Canterbury could restore a depleted aquifer. Tony Benny reports.
The aquifers beneath Canterbury Plains provide what once seemed an unlimited source of clean water for domestic supplies and agricultural use but the system is now under serious pressure.
Near Ashburton wells and spring-fed drains have dried up and there’s been health warnings about the level of nitrates in some house water.
The aquifers are naturally recharged by rainfall and by water seeping down from rivers and in Mid Canterbury historical inefficient border dyke irrigation used to contribute ‘‘accidental’’ recharge as well, with excess water seeping downward from paddocks.
But as border dykes have been replaced by centre pivots which use perhaps half as much water, this source has gone. Adding to the pressure are thousands of pumping bores, plus two years of drought.
‘‘We got the perfect storm probably,’’ says Hinds Drains working party chairman Peter Lowe. ‘‘Pivots are a lot more efficient, the border dykes have gone, the open races have gone and we’ve had two years of no recharge thanks to drought. April 2014 would be the last rainfall month with enough to recharge aquifers.’’
When the aquifer under this part of Mid Canterbury is full it feeds springs further out towards the coast, keeping a network of springfed waterways called the Hinds Drains flowing, keeping alive fish and insect life, allowing the gathering of kai (by iwi) and providing water for irrigators.
In the past there was sometimes too much water in the system and farmers in the Hinds Drains area would have flooded paddocks, attributed to border dyke irrigation inland seeping down and when coupled with high-rainfall events, overwhelming the aquifers. But today that’s just a memory and many drains have dried up.
Experience overseas has shown aquifers can be recharged, simply by pouring water into ‘‘infiltration basins’’ to percolate down into the aquifer to refill it.
The Ashburton Zone Committee, responsible under the Canterbury Water Management Strategy for formulating local water management policy, established the Hinds Drains working party to investigate managed aquifer recharge here.
Principal hydrologist Bob Bower (Golder Associates), who has extensive managed aquifer recharge experience in the United States, lead the investigation and last month a trial project opened at a 1.8ha site at Lagmhor, about 15 minutes inland from Ashburton, using water from a disused stock water consent.
Up to 500 litres a second of water from the Rangitata Diversion Race, a huge irrigation canal built in the late 1930s, is delivered via Valetta irrigation scheme’s underground pipes and an old open race into an excavated ‘‘leaky pond’’ from which the water percolates down into the aquifer.
‘‘Aquifer recharge does a really good job of slowly putting water into an aquifer – you can’t shove it in. It’s low-tech, it’s cheap, you can maintain the basin operations easily but unlike a storage dam which can grab great chunks of storm flow water and store it, it’s sort of a more steady background thing,’’ Bower says.
‘‘If you think about that as a storage system and you use those spring-fed drains as your spillways and you understand your main outcome is to provide enough in that system where you’re not flooding people but you’re providing those consistent baseflows by placing artificial recharge in the right locations at the right time of year when water’s available, you’re learning to manipulate and understand your catchment’s natural ability to capture and store water so we’re not always at the whim of nature.’’
Part of Bower’s job was convincing sceptics that managed aquifer recharge could work but as he showed them overseas examples, of which there are about 1200 catalogued, support among the working party grew.
‘‘My whole career has been in this social hydrology space. This group is really remarkable and engaged with iwi and got on with it. My experience with these kind of groups is they won’t stick around if all they’re going to do is meet and talk. They need to do something, the process demands ‘action’.’’
As part of the project, an extensive network of scientific testing equipment has been installed to monitor where the underground water goes and other organisations are also taking the opportunity to run their own tests. ‘‘This is consistent with the wishes of the Ashburton Zone Committee, who encouraged us to foster developing new solutions to the water management problems in the catchment.’’
One of them is Lincoln Agritech which is doing real time tracking of nitrogen in groundwater, taking readings every 15 minutes to get a picture of what happens below the soil in paddocks and better understand the movement of nitrates into and through Canterbury groundwater. This will help to build upon predictions made by modelling software such as Overseer.
It’s expected the influx of clean water will have the effect of diluting nitrates that have contaminated some domestic wells and the Canterbury Health Board will test wells before and after the trial to see if this really is the case. Bower warns though that this isn’t really a practical way to fix this problem.
’’First and foremost, it must be done at paddock level, reducing the amount of nitrogen leaching. A hydrologist would never suggest you can dilute your way out of a contamination problem, you’ve got to control the source, otherwise it just never stops.’’
Two years ago Bower travelled to California, Nevada and Arizona with IrrigationNZ chief executive Andrew Curtis and other stakeholders, visiting managed aquifer recharge sites and finding out about the politics of water there.
‘‘In California we saw some really bad examples of arguing and then some good, innovative examples. If you just yell at each other across the room, you’re dead, nobody wins, and we saw the worst and the best of that.’’
He believes we are well placed to learn from California. ‘‘They’re not building big dams there, it’s not going to happen politically, environmentally or cost-wise. They’re building small off-channel capture dams to grab those peak flows when they can, irrigating when needed through highly efficient irrigation but then storing the extra water into the ground, which is used during peak demands.
‘‘In Canterbury, the way the groundwater allocation system is set up, the fact that we have metering to understand what we are using and when, we’re in a really nice space that makes real common sense to do it that way too.’’
The success of the Hinds managed aquifer recharge trial will be judged by whether it can raise groundwater levels to provide a base flow to the drains systems and whether nitrate levels in the groundwater in Tinwald, south of Ashburton, decrease.