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Waimakariri Zone Committee chairman Dave Ashby said he was confident the work of Canterbury's zone committees would stop this worst-case scenario - presented by Canterbury medical officer of health Dr Alistair Humphrey - from happening.
Dr Humphrey appeared in an episode of a publicly funded episode of Frank Film, a current affairs series exploring news issues in the South.
In the episode, Dairy farming and the water we drink, he warned of the growing risk of nitrate contamination in Christchurch's water.
But Mr Ashby took a different view.
''We are going to beat water quality. It's clear we can beat water quality because the science is good,'' he said.
''We recognise we have an issue and we are moving as fast as we can, but we also recognise there has been a significant investment in agriculture and we've got to manage any reductions accordingly.''
Mr Ashby said the Waimakariri Zone Implementation Plan Addendum (ZIPA), signed off in December, set a framework for reducing nitrate levels, requiring dairy farmers in priority areas to make a further 15% reduction in nitrate levels beyond ''good management practice'' by 2030.
Federated Farmers environment spokesman Chris Allen said most Canterbury farmers were heading in the right direction.
''We've got to remember that without some form of nitrates in the soil we're not going to grow anything, but we all drink from the same water, from the same place.
''If anyone is concerned about the water they are drinking, they can get it tested or talk to ECan (Environment Canterbury) or the Ministry of Health.
''Knowledge is king and that's the advice I give to farmers.''
While nitrate levels remained low, ECan revealed in December 2017 that scientific investigations into potential groundwater movements under the Waimakariri River found low levels of ''nitrate-nitrogen'' in aquifers and wells in Kaiapoi, Belfast and northern Christchurch.
An ECan spokesman said a public hearing process was expected to take place next month to finalise rules for the Waimakariri zone, when a proposed plan change is notified following last year's ZIPA.
-By David Hill