Water quality trial 'helping nature along'
- Dana Dopleach
- Feb 27, 2015
- 3 min read
A low-tech approach to replenish and improve water quality in Mid Canterbury's over-allocated and polluted aquifers will be trialled soon.
Managed aquifer recharge (MAR) has succeeded in restoring the health of aquifers in the US and now one of the scientists involved in that work, hydrologist Bob Bower, is leading a pilot scheme in the Hinds Plains area to see how well the technique will work in here.
"Historically, we put bores in the ground and we take water out and sometimes we take way too much out and the only way to fix it is to take the bores back or turn them off so the idea behind MAR is just helping nature along," Bower says.
Simply, the idea is pour clean water from the Rangitata River, via the Rangitata diversion race and the Valetta irrigation scheme, into an "infiltration basin" and let it percolate down into the aquifer to refill it. Up to 500 litres per second is available from an unused stock water consent held by the Ashburton District Council.
A 1.8ha site at Lagmhor, about 15 minutes inland from Ashburton, has been selected for the trial and half the required funding of $350,000 has been secured. The pilot has the support of Ashburton zone committee, responsible for formulating policy under the Canterbury water management strategy, and Environment Canterbury commissioners.
"We're going to dig a leaky pond, it's kind of the worst pond ever," says Bower. "We go in and strip the top soil to get rid of the pesticide history and we might dig some trenches and fill them with clean gravels so what you're trying to do is really open up that aquifer to get the percolation rate really high."
The zone committee want more water in the aquifer to reinvigorate streams and wells in the area, but more importantly wants water quality improvements. Groundwater under the Ashburton and Tinwald area has some of the highest concentrations of nitrates in New Zealand.
Ironically, pressure on the aquifer has increased as irrigation has become more efficient, with spray irrigation replacing border dyke and piped, pressurised supply replacing leaky open races. Excess irrigation water used to sink into the ground and recharge the aquifers by default but less and less is finding its way underground.
"We know it's worked in the past unmanaged so let's try to manage it. We'll bring some specific tools and we'll track the results."
Sensors placed in existing and possibly specially drilled bores will measure water temperature and the level of the aquifer and a tracer study will reveal just where the extra water is going.
The pilot should reveal the sweet spot, where there's enough water in the aquifer to feed springs, streams and wells but so much that it causes flooding in low lying areas closer to the coast.
Bower says it's not intended to fix groundwater nitrate levels simply by dilution and that it is important to continue with efforts to reduce nitrate leaching from farms, as proposed under the Hinds Plains sub-regional water plan.
"The majority of water in Canterbury is out of groundwater, it represents a massive underground resource of storage and if we can manage that better and use it and complement surface storage and irrigation efficiency and use it as a whole package, that's what the goal is," Bower says.
Canterbury's not the only New Zealand region where MAR is being trialled and Bower is also involved in a pilot study in Gisborne, in partnership with Leader Brands, iwi and the district council. The aquifer there is more confined than those in Canterbury and acts more like a tank but over the past 30 years of extraction it is in decline.
Instead of an infiltration basin an injection bore will be used to feed clean water from a reservoir into the aquifer to try to restore it to a sustainable state.
Bower says he hopes the pilot projects will lead to improved water quality and quantity, but almost as important, he says, is educating people about aquifers.
"It is critical as you develop a MAR pilot that you work with the community to get their ideas and knowledge of an area to ensure it is a success," he says.
"When people think ground water, they think, 'I put a bore in, there's water, what are you talking about?'.
"People don't get ground water, so I see these pilots and the signs and the communications as essential to getting people to think about ground water - it is a resource and how do we manage it better? A healthy aquifer's good for everybody."
TONY BENNY 05:00, Feb 27 2015
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