Could turning aquifers into managed reservoirs prevent water shortages and seawater contamination?

Wednesday, 30 January 2019

Managed aquifer recharge is being trialled as a solution to water problems in Gisborne and in Hinds, Mid Canterbury (pictured).

It seems the neatest of solutions: take the winter water that rages, unneeded, to the sea, put it in a great underground tank, and drag it out again as the summer dry threatens to brown the grass and suck the life out of parched apples, lettuces, peas.

Managed aquifer recharge is the new buzz phrase in the search for answers to New Zealand's twin problems of increasingly scarce water and weed-choked rivers toxic enough to kill fish.

Proponents tout it as a potential solution to everything from aquifers being sucked dry by irrigation to nitrogen pollution to seawater contamination of drinking water supplies. But critics say it could actually worsen New Zealand's water pollution problems.

The concept is straightforward – you remove water from rivers during winter, when it's not needed to water fields and crops, and when flows are high from plentiful rainfall. Then you put it into the aquifer below and hope it stays there ready for irrigators and thirsty households to pump out when temperatures soar.

The Hinds/Hekeao managed aquifer recharge (MAR) site.

* Water's local solutions: Are Canterbury farmers finding fixes to water quality?

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It's an apparent win win – no eyesore dams and storage schemes, with the added bonus of restoring the summer flow of springs and rivers, which can reduce to a trickle as irrigation pillages the aquifer that feeds them.

But, like many things to do with water, it's not as simple as it sounds.

Mid-Canterbury managed aquifer recharge project technical lead Bob Bower and Hinds Drains working party chairman Peter Lowe at the trial site.

Managed aquifer recharge, or MAR as it's commonly known, has been widely used around the world. One of the oldest schemes is in California's Orange County, where in 1933 irrigation dragged so much water from the underground aquifer that seawater seeped into the space, threatening the freshwater supply. Now, the district injects excess river water and treated wastewater into coastal wells to keep seawater out, and inland artificial ponds allow water to seep down to recharge the aquifers.

Managed aquifer recharge is being investigated for the Hawke's Bay's parched Ruataniwha Basin, where the Waipawa River dried out in parts in 2018.

Los Angeles pumps stormwater into its underground aquifer, for its town drinking supply. Perth recycles wastewater, letting it filter through purifying soils into the aquifer, then pumps it out in times of high demand.

Bob Bower, a consultant hydrologist and key New Zealand MAR advocate, came across the concept in North America, while trying to restore salmon rivers. He estimates New Zealand is 50 years behind the United States – as over-allocation of water is only now biting here.

MAR provides an opportunity to be smarter about what's coming in – and out – of aquifers, Bower says. Climate change will make that increasingly important, with longer and more severe droughts.

'It isn't a silver bullet. It's really part of a conversation about living within our means. Because there's only so much water in a catchment – you can't argue your way out of that.'

In New Zealand, trials are already underway for MAR schemes in Poverty Bay, Gisborne, and the Hinds catchment, on the Canterbury Plains. MAR is also being considered for Hawke's Bay's Ruataniwha aquifer and Wairarapa's Ruamahanga valley. The details vary with the environment – in Poverty Bay, water is injected through wells to reach the deep aquifer below. In Canterbury, the underground reservoir sits closer to the surface, so the Rangitata River water is diverted into filtration ponds, where it's left to percolate naturally into the aquifer.

Early results from both schemes show the method works, in terms of raising the water level in the underground storage aquifers. But there are risks. In Poverty Bay, the injection water introduced e-coli bacteria. And all water is not created equal. The river water being injected is oxygen-rich, while the aquifer water is not. While that might sound insignificant, the trial report notes the mixing of the two 'could in some cases cause pyrite oxidation and the release of arsenic in the aquifer'.

As GNS head of hydrogeology Stewart Cameron puts it, mixing waters of different chemical composition can cause 'odd reactions'. Cameron says MAR could work in New Zealand, but one of the biggest barriers is that we don't know enough about what's in our aquifers, and how they work.

'The volumes you have to put in to have a meaningful recharge are quite large, so the cost effectiveness of it is sometimes questionable. It's definitely got applications as long as the setting is well researched and understood.'

In fact, New Zealand has had its own MAR scheme. Established in Hawke's Bay's Heretaunga aquifer in 1988, it drew water from the Ngaruroro River into four infiltration basins. It was abandoned as ineffective in 2008, due to problems with clogging and the discovery that water leaked back into the river.

One of the touted benefits of the Hinds MAR scheme is that it could help dilute nitrogen pollution from farms, which in some places has reached levels toxic to both fish and humans. The trial showed groundwater nitrate-nitrogen levels fell from 4g/m³ to less than 1 beneath the recharge site and from 14 to 4, further away.

As surface dams have become less culturally and environmentally acceptable, turning aquifers into underground storage reservoirs is being trialled as an alternative.

While that sounds like a good thing, it could have perverse consequences, acting as a licence for farmers to continue polluting, instead of reducing nitrogen use and runoff. That's the fear of Waikato University professor of freshwater science Troy Baisden. On the one hand, he says, MAR could dilute contaminants and improve water reliability for higher value, lower-polluting agriculture, such as crops or grape vines.

'On the other hand, it creates pretty extreme opportunities to make water quality worse, by putting more agriculture in places that are already in danger of exceeding ecological and possibly health levels of contaminants like nitrate. And that's a big issue.

Bob Bower says MAR is not a silver bullet for diluting pollution, and farmers will still have to reduce nitrogen runoff from their farms.

New Zealand has plenty of water, but not always in the right place at the right time to meet our needs.

'So there's a danger with managed aquifer recharge, that it's just the latest fad that's helping create a pathway towards more irrigation and more agricultural intensity.'

Forest & Bird hydrologist Annabeth Cohen calls it rearranging the deckchairs on the Titanic.

'Building a dam or augmentation or managed aquifer recharge might buy us a little bit of time, but we're really kidding ourselves, in the sense that we think we can engineer our way out of this one. We've got to be more resilient in the face of climate change. And that means a type of farming that accepts the limits that exist now. We're in denial of the limits that exist now, and we haven't even acknowledged the fact those limits are going to change in the future.'

Bower says pollution dilution is not the Hinds scheme's main purpose and the nitrogen pollution is too severe to dilute your way out of: 'There have to be reductions.'

However, a 2015 conference paper he co-wrote, entitled 'Is dilution the solution for water pollution?', prices MAR as an alternative to advanced farm mitigation measures.

'The value of water-quality improvement was assessed by considering the forgone cost of farm level nutrient mitigation practices that alternatively farmers would have to adopt to improve the water quality.'

A business case is due in 2019 to investigate the economics of the Hinds scheme, and where the water should come from. Early cost-benefit assessments found the scheme would generate $164 for every $1 spent, but that's based on getting the source water for free. That could fall to just 1:4 if society decided irrigators should pay for water.

Plant and Food soil scientist Brent Clothier reckons the jury is still out on MAR: 'Nature is bloody leaky.'