Selwyn council to spend $5m on hunt for nitrate-free drinking water

Tuesday, 7 May 2024

The Selwyn District Council has acknowledged five of its bores have nitrate at levels associated with pre-term birth and other reproductive risks.

The Selwyn District Council has allocated $5 million to investigate new wells after high nitrate levels were found in drinking water supplies in several towns including Rolleston and Darfield.

A long-term solution to move away from nitrate-contaminated water supplies could cost more than $400m.

Selwyn District Council is on the hunt for water sources with lower nitrate levels as concentrations of the contaminant have been found to be creeping up in several water supplies in the area - including in towns.

Nitrate in drinking water comes primarily from farmers using synthetic nitrogen fertilisers to add nutrients to the soil, some of which are ingested by cows and excreted onto the land.

Traditionally the highest nitrate levels in drinking water have been relegated to private bores, which district and regional authorities say they have no obligation to monitor, but rising levels in council controlled supplies have led the Selwyn council to acknowledge that five of its water sources exceed half the maximum amount of nitrate levels allowed, and a spokesperson said a further three were approaching similar levels.

Campaigners argue the official New Zealand nitrate limit of 11.3mg/L, which is based on 1958 World Health Organisation guidelines, is too high for human and waterway health.

Nitrate in drinking water comes mainly from farmers using synthetic nitrogen fertilisers on the soil. (File photo of a Selwyn farm)

Some scientific evidence suggests levels over 5mg/L pose an increased risk of pre-term birth and other reproductive risks. The New Zealand College of Midwives advises women to consider an alternative water source over 5mg/L.

In its long-term plan (LTP) consultation document, the Selwyn District Council notes five water supplies have nitrate levels that have reached or topped 5.65mg/L.

Depending on the size of the water supply, any result over 5.65mg/L triggers closer monitoring requirements, as well as reporting to Taumata Arowai and the regional Public Health Service.

The council’s test results for April showed water supplies in Kirwee (5.99mg/L), Darfield (6.3mg/L) and Rolleston’s Illinois Drive bore (7.11 mg/L) all above half the maximum allowable value, but historic testing also shows other supplies, including Edendale and Overbury bores breaching the 5.65mg/L mark. Some of the affected bores have come close to the maximum allowable value of 11.3mg/L in the past.

A Selwyn District Council spokesperson said the bore with the highest levels, Illinois Drive, is one of four bores that supply Rolleston and is the least used.

Testing shows water supplies in Kirwee (5.99mg/L), Darfield (6.3mg/L) and Rolleston’s Illinois Drive (7.11mg/L) are all above half the maximum allowable value.

The spokesperson said a further three bores were approaching the 5.65mg/L limit, but did not elaborate on where they were.

Head of asset management Murray England said the council was proposing “to investigate whether we can centralise some of our water treatment to come from one low nitrate source”.

The LTP consultation document says the council has allocated $5.3 million over the next decade to “investigate the feasibility and do early design”, while a longer term strategy document puts a $405m price tag on carrying the plan out.

Recent Greenpeace testing found similar levels in supplies from Canterbury towns, as well as several very high readings from private wells.

The Selwyn council’s 30-year infrastructure strategy notes an “extreme” risk of waste-water or drinking water not being treated to an acceptable standard, the potential for significant public health and environmental impacts, and the possibility of legal action and cultural offence. It specifies the “growing awareness of the potential impacts of nitrates on drinking water and the high cost for treatment/removal”.

The document lists several assumptions, including: that centralised water takes for high growth areas of plains will be able to be consented including the transfer of existing consents; that growth occurs at projected rates (despite Selwyn regularly confounding growth predictions); that low nitrate water remains available at the rate council will need; and that groundwater nitrate concentrations remain high across the Canterbury Plains.

Environment Canterbury, which is responsible for addressing nitrate contamination in waterways and groundwater, said on its website nitrate concentrations in Canterbury groundwater were generally increasing. It said due to the time taken for nitrate to travel through groundwater, levels were likely to get worse before any improvement from “today’s strict farming rules” become apparent.