The Waimate District Council issued a public notice yesterday to inform residents to stop drinking the water.
The council’s online nitrate probe of the Lower Waihao rural water scheme returned a result of 50mg/l, right on the maximum allowed value.
A combination of land use within the catchment and heavy rain contributed to the increase, council asset group manager Dan Mitchell said.
"The increased nitrate levels are almost certainly linked to land use within the catchment. Whenever heavy rainfall occurs, nitrates that are typically held within the soils are leached into the shallow groundwater and increase the concentration."
It was difficult to know how long the water would be undrinkable, he said.
"Various factors, such as additional rainfall, can have an adverse effect. From experience in 2022 a reduction could take weeks."
This was the first time since late 2022 that the nitrate levels had reached the maximum allowed value.
The council has continued to provide water tanks of fresh drinking water in Glenavy, Morven and Waimate. The council has been providing those since 2022 when dangerously high nitrate levels were recorded and the council restricted drinking from the tap for about a month.
Mr Mitchell said they were continuing to investigate options for the "reduction in nitrate concentration in the source water".
But the situation is not new to Glenavy residents. The known quantity of the issue began in November 2021, when Greenpeace provided residents with free water testing. The results showed 86% cent of the drinking water samples brought in by local people had levels over 1mg of nitrate per litre, which research shows starts to increase the risk of bowel cancer.
To solve the issue, the council originally planned to commission a denitrification plant midway through 2023. It then moved the date to the end of that year.
In April this year residents told the Otago Daily Times they refused to drink the tap water provided because of the nitrate threat, yet the council further delayed its planned denitrification plant.
Mr Mitchell said dealing with excess nitrate in drinking water was complicated.
"These options are both complex and require resource consents to discharge into the receiving environments."
The office of the prime minister’s chief science adviser highlights that human activities such as farming have increased the level nitrates in the environment. On its website it states there has been a reported 627% increase in nitrogen fertiliser use in New Zealand between 1990 and 2019.
Nitrates are soluble and leach into groundwater.
In October, Mr Mitchell said progressing the denitrification plant was complex "given the potential effects".
There is still no denitrification plant in Glenavy.
