The nitrate limit in Aotearoa New Zealand’s drinking water standards is set at 11.3 mg/L to prevent blue baby syndrome. But in the past decade, international studies have observed exposure to nitrate levels as low as 1 mg/L increasing the risk of colorectal cancer. The risk of premature births increases from 5 mg/L.
WSP Technical Principal - Water and Wastewater Bridget O’Brien and University of Otago Senior Research Fellow in Public Health Tim Chambers recently analysed the emerging body of evidence and applied it to predicted future land use scenarios in Christchurch and Waimakariri.
They found an increase in nitrate of 2.3 mg/L in the aquifers below Christchurch and Waimakariri could cause an estimated 33 additional annual cases of colorectal cancer and 10 more premature births per year. An increase of 6.9 mg/L could result in 72 more cancer cases and 24 more premature births per year. The health and treatment costs would range between $21 million and $48 million per year.
The takeaway is clear. If the level of nitrate in drinking water is lower, Christchurch and Waimakariri will reduce its potential risk of colorectal cancer and premature births.
Higher concentrations of nitrate from intensive land-use are finding their way into Canterbury’s aquifers and groundwater supplies. Bridget says elevated levels of nitrate in groundwater is a long-term problem that needs a long-term solution – especially in Canterbury where stony soils make it that much easier for nitrates from fertiliser, cow urine and wastewater discharges to soak into groundwater.
“The youngest groundwater used for drinking water in Christchurch is around 20 years old. This means the increase we’re seeing now in groundwater nitrate reflects land use from decades ago. The nitrate from subsequent land use intensification is yet to make its way into the city’s groundwater.”
Bridget and Tim also looked at the cost of treating Christchurch’s water to remove nitrate before the water gets to people’s taps. The cost would run into the hundreds of millions of dollars upfront, with millions more a year in operating costs.
The pair recommend tightening farm management practices to better protect the region’s groundwater sources from nitrate contamination. To safeguard people’s health, a much lower nitrate limit should also be considered for New Zealand’s drinking water standards, they say.
In a nod to the relevance and importance of the issue, Bridget and Tim’s research was last month awarded Paper of the Year at Water New Zealand’s annual conference.
