In 2015 a report by Environment Canterbury - the regional council - showed the effect irrigation could have on nitrates. Photo: RNZ/Vinay Ranchhod
A change in irrigation practices a decade ago has likely contributed to nitrate spikes in Waimate's drinking water, but it is ratepayers who could foot the bill in the long-term.
A do-not-drink notice was lifted on Thursday for about 650 households, in place since 2 December.
In 2015 a report by Environment Canterbury - the regional council - showed the effect irrigation could have on nitrates.
Switching from border dyke irrigation, where channels are cut and the water runs down into the pasture, to spray irrigation using large, often mobile sprinklers, would risk a breach.
"Assessment results indicate that border dyke conversion could cause nitrate concentrations to exceed the MAV (maximum acceptable value) here," the report said, finding a jump from 33 percent of bores at risk of exceeding the limit, up to 59 percent.
According to the Waimate District Council's website, significant rainfall over the past two months was likely partly to blame, flushing nitrates held in the soil into the groundwater.
"However, nitrates within the groundwater have increased over the last 20 years and are almost certainly related to changing land use (intensive farming) and changing irrigation practices (the replacement of border dyke irrigation with more efficient and sustainable pivot irrigation)," it said.
Jennifer Rochford, regional council compliance manager, said new rules had introduced stricter regulations on how much nitrogen, which was usually used as fertilizer, could be used on farm land.
"Despite stricter rules, the concentration of nitrate has been increasing within the catchment and we are gathering more information on understanding why this is occurring," Rochford said.
"Our team is checking that landowners are following the rules, as well as looking for other nitrogen sources, such as silage pits and effluent discharge in the catchment."
One solution could be a denitrification plant - but that would fall under the remit of the district council, rather than Environment Canterbury.
Waimate District Mayor Craig Rowley. Photo: RNZ / Logan Church
Waimate District Mayor Craig Rowley told RNZ in the days after the breach the council had considered it, but it would not be a working solution anytime soon.
Marnie Prickett, a research fellow in the Department of Public Health at University of Otago, said it was an unfair burden to place on the district council, when the regional council could reduce the problem at the source.
The Havelock North Drinking Water Inquiry, published in 2017, found: "Protection of the source of drinking water provides the first, and most significant, barrier against drinking water contamination and illness."
Prickett said regional councils had a responsibility to their communities to protect their source water.
"When the regional councils fail to protect that source water, the cost of treatment starts to go up, and district councils can't control the pollution of their source water, but they have to pay for, and find the money for from their ratepayers, to treat that water."
That cost also fell on those who owned private bores, as well.
"It's an unfair system," Prickett said.
Waimate District Council said in a statement: "We have always said that elevated nitrates in the shallow groundwater are beyond the control of Waimate District Council.
"There is also the question of fairness as to why the potential costs for denitrification fall with ratepayers. The capex and opex costs are significant."
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