Nitrate rates rise in Selwyn
Selwyn District Council is on the hunt for water sources with lower nitrate levels, as concentrations of the contaminant creep up in several of the council’s water supplies.
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 on to 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. A spokesperson said a further three were approaching similar levels.
Campaigners argue that the official New Zealand nitrate limit of 11.3mg/L, based on 1958 World Health Organisation guidelines, is too high for human and waterway health. Some scientific evidence suggests that levels over 5mg/L pose an increased risk of preterm birth and other reproductive risks.
In its Long Term Plan (LTP) consultation document, the Selwyn council notes that five water supplies have nitrate levels that have reached or topped 5.65mg/L, half the maximum amount allowed.
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 council spokesperson said the bore with the highest levels, Illinois Drive, was one of four bores that supplied Rolleston, and was the least used.
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 $5.3m 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.
In the council’s 30-year infrastructure strategy, the council notes the risk of wastewater 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 as “extreme”, and 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 the 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 that council will need; and that groundwater nitrate concentrations remain high across the plains.
On its website, Environment Canterbury, which is responsible for addressing nitrate contamination in waterways and groundwater, says nitrate concentrations in Canterbury groundwater are generally increasing, and that, due to the time taken for nitrate to travel through groundwater, levels are likely to get worse before any improvement from “today’s strict farming rules” becomes apparent.