Most river pollution comes from streams that don't need fencing, research shows
Most of New Zealand's river pollution comes from streams that would be exempt from waterway fencing rules, new research shows.
It calls into question the effectiveness of current efforts, which would become law under legislation proposed by the National Government.
A paper published by the American Journal of Environmental Quality found that 77 per cent of contaminants in New Zealand's rivers came from smaller waterways exempt from proposed fencing rules.
Both voluntary and regulatory fencing efforts by rural and community groups have sought to protect larger waterways, sometimes described as wider than a stride and deeper than a redband gumboot.
In policy terms, it refers to streams more than 1m wide and 30cm deep.
The paper, by Ag Research scientist Dr Richard McDowell as part of the Our Land and Water National Science Challenge, found that most pollution does not come from those waterways.
It modelled the contaminant load for over half a million stream segments around the country, which were divided into those that would need to be fenced and those that would not.
It determined 77 per cent of the pollution load nationwide came from streams that would not require fencing.
The contribution of individual pollutants from smaller streams ranged from 73 per cent of total nitrogen to 84 per cent of dissolved reactive phosphorus.
It was particularly evident in agriculturally-productive regions such as Canterbury, Southland, Nelson and Hawkes Bay, which "exhibited large contaminant loads from exempt catchments", the research said.
"These regions have significant downstream rivers that are used for recreation and tourism," it said.
"Our data suggest that not requiring fencing may significantly delay or reduce the ability to mitigate water quality impairment unless other measures are taken."
Because most of the waterways in the country were smaller, it was no surprise they were the major contributor of pollution, McDowell said.
It showed fencing off waterways could not be relied on in isolation to improve water quality.
"Because there are so many of those [small] streams, it's a bit of a no-brainer, but most of the contaminant load comes from those streams," he said.
"The take-home message is not to put all your eggs in one basket and just rely on fencing to decrease your water quality impact."
For farmers, it would mean looking at other ways to reduce water quality impacts.
Fencing all streams in the country, including smaller ones, would cost $1.4b, according to a regulatory impact statement by the Ministry for Primary Industries.
It would be uneconomic to do so, but there were other measures to reduce the impact on water quality, McDowell said.
"Don't expect fencing to do everything. Fencing costs a lot, but we do have a large array of other management practices we can do to improve water quality and not impact on the bottom line."
Fish & Game chief executive Bryce Johnson said it was an "extremely important piece of research" that showed an urgent need to rethink the national riparian fencing strategy.
"We now have the science to show what we have long suspected – small waterways are crucially important to the environment and need to be properly protected from contamination," he said.
"They flow into the bigger streams and rivers and [the] research shows that by the time they join up with bigger streams, much of the pollution has already occurred."