A scientific paper spells out in stark terms the unintended consequences of efficient irrigation in one corner of Canterbury. David Williams reports
When former Aucklander Cole Groves, his wife Ginny and her parents bought a dairy farm in Hinds, Mid Canterbury, four years ago it triggered an audit by the farm’s irrigation company.
MHV Ltd wanted to ensure the farm would be managed at least as well as it had been by the previous owner, as the Hinds/Hekeao Plains had a known problem with nitrates (oxidised nitrogen) in water, and there was pressure to reduce nitrogen leaching.
A regional council plan change, known as variation two, which mandated reductions in nitrogen losses, was in the wings.
The 120-hectare farm, Coldstream Pastures, took out inefficient border-dyke irrigation and installed fixed-grid irrigation (sprinklers on posts, controlled by smart phone). The farm’s fertiliser was reduced, and cows dropped from about 440 to 410.
“We’ve made significant changes in four years and reduced [nitrogen losses] by 20 percent,” Groves says. The farm’s audit grade has moved from B to A, and he wants it to be A-plus next year – when cow numbers will be cut further.
There’s pressure to do more. “To do another 20’s not going to be easy at all. But there might be some technological gains or stuff that we can do to get another 10 in four or five years.”
Farming practices have changed rapidly between the Hinds and Rangitata Rivers, a 580-square-kilometre plain between Christchurch and Ashburton.
In 2006, dairying was only 35 percent of farming but a decade later that increased to 62 percent. Over the past two decades, irrigation practices have flipped from being almost entirely border-dyke irrigation (flooding paddocks using temporary dams) to sprinkler-type irrigation, mainly by hulking centre-pivots.
But as recently published research points out, more efficient water-use for farming has reduced groundwater levels and increased groundwater contamination, backing international findings. The paper has provoked strong comments from environmental groups, who are adamant the Canterbury Plains are the wrong place for such intensive farming.
The regional council, ECan, says it’s known about the issue for a long time and despite the “drawbacks”, the total nitrate lost to the catchment’s groundwater will reduce.
MHV Ltd – which has just had a controversial discharge consent renewed – says its groundwater programme was founded on the study’s data and nitrate trends are improving.
High levels of nitrate-nitrogen in groundwater can be toxic to fish and invertebrates, and can cause excessive weed growth and algal blooms, as well as human health effects.
Drinking water limits have been set to avoid potential health threats to bottle-fed babies. However, a recent study in Denmark has linked colorectal cancer to people with prolonged exposure to much lower concentrations of nitrate-nitrogen.
Add those concerns to the climate and biodiversity crises, and there’s a potent environmental cocktail.
“Environmentalists in Canterbury and around New Zealand are extremely alarmed at the amount of pollution going into the ground and into the rivers in Canterbury,” says freshwater conservation advocate Annabeth Cohen, of lobby group Forest & Bird. “It is ground zero for New Zealand’s agriculture problems. It’s serious, and something needs to be done urgently.”
The efficiency paradox
The paper on the Hinds-Rangitata Plain, published in the journal Agricultural Water Management in February, was written by William Dench and Leanne Morgan, then of University of Canterbury’s Waterways Centre for Freshwater Management. It tracked groundwater quantity and quality between 2005/2006 and 2016/2017.
Groundwater levels fell up to 17m in some areas, the most significant of which were inland from State Highway 1, about 13km from the coast. The volumetric decrease over that period was estimated to be 262.6 gigalitres, at an annual rate of 23.9 gigalitres. (The annual loss is equivalent to about two-thirds of Auckland’s biggest water supply dam, Mangatangi.)
Modelling showed a substantial spike in nitrate-nitrogen (nitrate-N) concentrations, especially in shallow wells. Two factors were likely to blame: reduced dilution of nitrates by groundwater, and the rise of dairy farming.
Between 2005 and 2012, shallow groundwater wells showed increases in average annual nitrate-N of roughly 7mg/L to 11mg/L. (This country’s maximum allowable value for human health is 11.3mg/L.) Meanwhile, concentrations at surface water drains increased from 6mg/L to 10mg/L.
“Nitrate-N concentration in groundwater within the study area has increased substantially over the past decade and poses a serious risk to drinking water sources.”
The study notes a managed aquifer recharge scheme – a multi-million-dollar project pumping extra water from the Rangitata River into the ground – attempted to address these “unintended consequences”. (ECan’s long-term plan earmarks more than $5 million for the scheme over the next three financial years. The regional council wants to hike rates by between 18 and 24.5 percent next year.)
Greenpeace’s Aotearoa executive director Russel Norman says irrigation efficiency was sold as a way to avoid some environmental consequences. “What this study shows is that’s completely untrue.”
Norman accuses authorities of effectively overseeing an unregulated expansion of intensive dairying, leading to the toxification of the country’s freshwater. There’s no technical fix for groundwater contamination, he argues. “The answer is fewer cows and less synthetic nitrogen fertiliser.”
Freshwater ecologist Mike Joy says efficiency rarely leads to positive change.
“Heat pumps are more efficient, right? But then everybody gets a bloody heat pump and then they all turn them on all the time, and the net amount of energy use actually goes up. Cars have got more efficient over time but there are more cars than there ever were.”
The Canterbury Plains tilt about 50km from the Southern Alps foothills – about 400m above sea level – to the coast. The drought-prone soils are generally shallow, with a low capacity to hold water, meaning water passes through to gravels which have groundwater flowing through them.
Joy says: “They shouldn’t even be having cows in Canterbury.”
“The answer is fewer cows and less synthetic nitrogen fertiliser.” – Russel Norman
A study for the Ministry for the Environment, published last year, estimated Canterbury’s irrigation footprint at 546,000ha – an area larger than Kahurangi National Park – or 61 percent of the country’s total.
A preliminary study completed last year found exposure to nitrate in drinking water “is likely to be a significant risk factor for colorectal cancer in New Zealand”, potentially affecting hundreds of thousands of people. (High nitrates can also lead to a higher risk of thyroid cancer and neural tube defects in babies.)
Study author Dr Tim Chambers, a senior research fellow in University of Otago, Wellington’s, Health, Environment and Infection Research Unit, says 11 epidemiological studies have now considered a link between colorectal cancer and nitrates, with the Danish study, “by far the best one”. It suggests a link at levels in drinking water from as low as 0.87mg/L.
“Ingested nitrate under conditions that result in endogenous nitrosation is probably carcinogenic to humans,” he says. “We want to make sure that we limit any exposure that we can from any potential carcinogen.”
(A Ministry of Health working group tasked with considering the issue has been disbanded.)
The International Agency For Research On Cancer hasn’t thoroughly reviewed its recommendations on nitrate levels in drinking water for 15 years.
It’s not clear what level of exposure could lead to potential health effects, Chambers says.
Canterbury’s dairy herd has doubled to 970,000 cows in a decade, with Ashburton having 364,000 cows, while its northern neighbour, Selwyn, has 173,000.
Across the country, the use of nitrogen and phosphorus-based fertilisers – applied to improve soil fertility, but often lost to groundwater and waterways – increased 630 percent to 452,000 tonnes, between 1991 and 2019.
Cohen, of Forest & Bird, says if the country’s serious about protecting waterways and improving health, agriculture needs a transition plan – starting in Canterbury. Central Government should offer financial help to farming communities, she says.
“We need to be looking at what’s the right thing for us to be farming in New Zealand given the limitations of our environment today and the limitations of the climate tomorrow.”
Environment Minister David Parker couldn’t be reached for comment.
Working with communities
ECan monitors and protects water quality in the region’s aquifers, rivers and lakes. It can’t knowingly allow declines.
Yet in its latest groundwater quality survey of 234 wells, 47 percent showed nitrate-nitrogen concentrations were likely or very likely increasing. It’s fair to ask, too, what happened on the Hinds-Rangitata Plain during Dench’s study, which showed substantial increases. (Our story on MHV’s consent application noted the company’s own expert said modelled nitrogen concentrations flowing into “soil drainage waters” increased 40 percent over the 2009-2013 baseline period.)
Dr Carl Hanson, ECan’s groundwater science manager, defends the council’s record. “Over the past 10 years, we’ve been working with communities to set limits on nitrogen inputs to waterways and to minimise nitrogen losses from farming and other land uses.”
(Cohen, of Forest & Bird, thinks there’s an irony the regional council is raising rates, partially to tackle water pollution, while MHV Ltd’s discharge consent is rolled over.)
A move to more efficient irrigation will, on its own, lead to lower groundwater levels and higher nitrate concentrations, Hanson says. “But the total amount of nitrate lost to groundwater across the catchment will also reduce, along with the total nitrogen load to coastal waters. That is why we’ve encouraged more efficient irrigation in spite of its drawbacks.”
Managed aquifer recharge will focus on cleaning up legacy groundwater contamination, he says, while improved on-farm practices will reduce new nutrients entering the system.
“Long-term reductions in groundwater nitrate concentrations will only come from improvements in farm management.”
MHV sponsored Dench’s thesis, and he was employed by the company to start its initial groundwater programme. Chief executive Melanie Brooks says the company’s knowledge has “moved on significantly” since 2017.
“We have had an improving nitrate trend since September 2019 in both shallow and deep bores, and we continue to refine our geological and hydrological understanding of our catchment.”
Sample sizes for well testing were small to start, she says. In many cases the same bores weren’t visited more than once, which could account for volatility. “As we have increased our sample size, across the whole catchment we are seeing a smoother trend emerging.”
Shallow groundwater bores have a slightly higher average of 8.19mg/L.
MHV monitors its 140 bores quarterly, or monthly at some sites. The company has just employed another person in its groundwater and surface water team.
“We remain concerned about water quality, so even though we are seeing improvements in water quality we continue to make on-farm improvements” Brooks says. “We know there is further to go.”
Groves, the Coldstream Pastures farmer who gets water from MHV, is buoyed by farmers’ drive to improve. Audits will pick up laggards quickly, he says.
“The scheme holds our licence to operate. If everyone within the scheme doesn’t operate at a high level and is striving for even higher, then we won’t be able to operate in 10 years’ time.”
Anything in life has an impact, he says, but he understands why people are pushing for a quicker response to farm-related water pollution. Farmers want change to come more quickly, too, he says.
“But what I do today, I’m not going to see the benefit tomorrow – I’m going to probably see it in 10 years’ time. That’s the hard thing, probably, for the public to understand it, and it’s not because we’re doing it slowly it’s just the way the environment works.”
Groves isn’t a fan of chlorinated drinking water in Christchurch, where his sister lives. He’d rather drink his own, he says, which he realises is influenced by farming practices upstream – just as his farm influences downstream drinking bores.
What’s the nitrate level in the drinking water well 100m from his house? “We’re sitting around 7.4mg/L, I think.”
How does he feel about that level, considering studies have linked people with prolonged exposure to nitrates at lower levels to colorectal cancer? “It could always be better, can’t it?”
However, Norman, Greenpeace’s boss, is shocked. “Holy ferlutti! And he drinks it?”
Chambers, of University of Otago, Wellington, says 7.4mg/L is “very high”. In the short-term, if the family is worried, they can eat lots of fruit and vegetables, which inhibit the process, and avoid red meat.
“Long-term, I would be looking for an alternative water supply.”
Groundwater movements snaking underneath the Hinds-Rangitata Plains might be complex, but understanding water contamination might be as simple as testing a glass of water and weighing up the risks of drinking it.