Wednesday, 13 November 2019 08:55

Freshwater plan a killer blow

Written by  Nigel Malthus
ECan chief scientist Dr Tim Davie. ECan chief scientist Dr Tim Davie.

A national limit on dissolved nitrogen would “essentially eliminate” intensive agriculture in the Selwyn Waihora catchment, says Environment Canterbury chief scientist Dr Tim Davie.

The limits are contained in the proposed national policy statement on freshwater.

“Rough modelling” done on the catchment suggested that 15-20% of it could be intensive but the rest would have to be in dryland sheep farming or forestry, he said.

Davie was speaking at a New Zealand Institute of Primary Industry Management (NZIPIM) forum on the practical steps farmers can take to improve water quality.

The forum, at Lincoln, occurred on the day submissions closed on the proposed policy statement, which among other things suggests a national limit of 1mg/litre of dissolved inorganic nitrogen (DIN).

“To get our spring-fed systems to a value of 1 would require, under current management practices and the best that we can do, would require a land use change,” said Davie.

Explaining some of the underlying science, Davie said there was more to stream health than just nitrogen and nutrients, which is why the policy statement on freshwater was bringing in other measures.

He said the nitrate ion NO3 is toxic to aquatic species, impairing breeding and other functions above about 2mg/litre. 

But it was important to distinguish between toxicity and eutrophication -- the growth of “stuff” in waterways including algae and weeds. The dissolved inorganic nitrogen (DIN) measure is being brought in because it includes ammonia and nitrite as well as nitrate, and all are drivers of eutrophication.

Davie also explained that the region’s alpine-fed rivers generally had low DIN concentrations, most of them well under 1mg/litre, but a lot more of the spring- and groundwater-fed streams of the lower reaches of the plains were well above 1mg.

“The proposed DIN limits are a significant challenge for our spring-fed streams,” he said.

DairyNZ water quality specialist Justin Kitto said the proposed policy statement put a huge weight on managing a complex interactive system on two measures – a DIN of 1 and a DRP (dissolved reactive phosphorus) number of about .02.

But there was a lot of controversy around that and concern about how those numbers have been come up with, he said.

DairyNZ’s recommendation was that the policy put the onus on regional councils to manage the ecosystem health of their rivers, said Kitto.

“As opposed to saying DIN has to be no more than 1, look at what other factors are driving ecosystem health in a given catchment, possibly more strongly than dissolved inorganic nitrogen.”

He said the current national bottom line of 6.9mg/litre was based on an 80th percentile target aimed at protecting 80% of species from the effects of nitrate toxicity. DairyNZ was suggesting a 90th percentile target, which would move the national bottom line for nitrate toxicity to 3.8.

Kitto said that would be much more stringent than the current bottom line for, say, the Hinds catchment “but it’s much better off than 1 which is currently proposed”.

“The Government’s not going to accept 6.9. But we can’t accept 1. So we’ve got to come up with a compromise.”

Other speakers at the event included DairyNZ’s Dr Ina Pinxterhuis and Virginia Serra, and a panel of Canterbury farmers.

Serra told the forum that a stocktake of farmers revealed that 90% were planning changes to irrigation, 80% to reduce N fertiliser, and 66% were planning changes to effluent management. 60% were looking to try catch crops, 46% plantain and 45% low-N feeds.

More like this

Editorial: Sense at last

OPINION: For the first time in many years, a commonsense approach is emerging to balance environmental issues with the need for the nation's primary producers to be able to operate effectively.

Dark ages

OPINION: Before we all let The Green Party have at it with their 'bold' emissions reduction plan, the Hound thought it wise to run the numbers through the old Casio.

Featured

Brendan Attrill scoops national award for sustainable farming

Brendan Attrill of Caiseal Trust in Taranaki has been announced as the 2025 National Ambassador for Sustainable Farming and Growing and recipient of the Gordon Stephenson Trophy at the National Sustainability Showcase at in Wellington this evening.

National

Machinery & Products

Farming smarter with technology

The National Fieldays is an annual fixture in the farming calendar: it draws in thousands of farmers, contractors, and industry…

RainWave set to cause a splash

Traditional spreading via tankers or umbilical systems have typically discharged effluent onto splash-plates, resulting in small droplet sizes, which in…

» Latest Print Issues Online

Milking It

Misguided campaign

OPINION: Last week, Greenpeace lit up Fonterra's Auckland headquarters with 'messages from the common people' - that the sector is…

Fieldays goes urban

OPINION: Once upon a time the Fieldays were for real farmers, salt of the earth people who thrived on hard…

» Connect with Dairy News

» eNewsletter

Subscribe to our weekly newsletter