By ELEANOR BLACK and PHILIPPA STEVENSON
An environmentalist estimates it would take $1 billion and 10 years to bring farming up to a standard required so that it does not threaten the nation's waterways and soil quality.
But farmers say the figure is nearer $30 billion and there should be greater understanding of the issue before any money is spent.
Ecologic Foundation executive director Guy Salmon said increased nitrogen in Lake Taupo and the threat it posed to the clarity of the lake's water was just the most visible example of a nationwide problem.
Nitrogen saturation also threatened Lake Rerewhakaaitu near Rotorua, the Waikato River, Lake Hayes near Queenstown and lowland streams in Marlborough, Canterbury and Otago.
He said it would cost the Government $1 billion to help farmers convert to more ecologically friendly methods of farming.
In the Waikato alone, it would cost $300 a hectare to fence off the margins of the Waikato River to stop "gross pollution" by stock.
For Lake Taupo, Mr Salmon favoured an artificial wetland downstream from the five dairy farms at the centre of a nitrogen runoff controversy to improve the water before it entered the lake.
But Federated Farmers policy adviser Graham Pinnell said fencing the estimated 4 million km of streams and major waterways running through pasture nationwide would cost $30 billion for a conventional fence or $10 billion for an "el cheapo two-wire electric fence."
Mr Pinnell said it might be possible to capture nitrogen in an artificial wetland in some areas, but in Taupo's pumice soils a lot of nitrogen leached through the subsoil.
"Really, there is only one place you can do any [capture] work, and that's as it leaves the cow's tail."
He said that 30 years ago willows were promoted to farmers as "the godsend" for stream bank protection, but today they posed huge problems.
They exacerbated the streambank erosion they were meant to control by taking large chunks of bank with them when they fall, causing flooding by blocking streams.
Taupo farmers last week asked Environment Waikato to produce more proof that farming was behind the lake's deteriorating water quality.
Sheep and beef farmer Bob Cottrell said farm values had dived 30 per cent in the past seven months since the regional authority made the issue public.
While farmers were committed to preserving the lake, more research was needed.
Environment Waikato scientist Bill Vant said: "Environment Waikato and farmers need to agree on a strategy before that strategy can be effective." He said that although the evidence was strong, more water samples would be taken in future, from three additional sites.
Water samples were regularly taken from nine streams feeding the lake and three of those showed a marked increase in nitrogen levels since testing began in the 1970s.
Mr Vant said the increases were unlikely to be due to dairy farming, which had come to the lake's shores in 1996.
But runoff from intensive sheep and cattle farming operations was responsible for an estimated 30 per cent of the nitrogen in the lake.
The rest came from rainfall, decaying vegetation and pine forest runoff, none of which could be controlled.
He said dairy farming had become a hot issue in the region because it was known to have contributed to the pollution of Waikato waterways and the effects of that should not be allowed to spread along the lakefront.
"One hectare of national park loses 2kg of nitrogen per year. Intensive dairying land loses 40kg per year."
Federated Farmers policy executive director Catherine Petrey said farmers were not being cavalier "but you can't expect a farm not to have any impact whatsoever on the environment.
"You mitigate those effects. There is no evidence of dying streams of trout floating belly up."
There had been significant other growth in Taupo, including of houses and septic tanks, she said.
"Only 30 per cent [of Lake Taupo's] turbidity is estimated to come from the farms, but they have decided the farms should change, not anything else."
She said the initial environmental standards were so tough that not even sheep and beef farmers could have met them, and pastoral farming could have been halted in the Taupo catchment.
"That really has a significant economic impact on the community, and they do need to think it through."
Environment Waikato is expected to decide between four options - ranging from doing nothing to converting pastoral land to forestry and other crops.
The Ministry for the Environment is producing a draft guideline on riparian management.
Cleaning murky waters likely to cost billions
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