By David Hill, Local Democracy Reporter
Deputy mayor Neville Atkinson questions the urgency of asking the council to fund an assessment of all private and individual water supplies as part of the 2023/24 annual plan. Photo: North Canterbury News / David Hill via LDR
Private water supplies could come under greater scrutiny, amid concerns about nitrates in drinking water.
The incoming Waimakariri District Council will be asked to consider funding an assessment of all private and individual water supplies as part of next year's 2023/24 annual plan.
The recommendation comes from the council's land and water committee in response to recent reports about water quality trends across the Canterbury region.
Council staff considered two reports from Environment Canterbury (ECan) considering long term trends and a paper from University of Otago scientist Dr Tim Chambers.
Deputy mayor Neville Atkinson questioned the urgency.
"Not all rivers have increasing nitrate levels, so I want to see some more facts.
"At Silverstream the nitrate levels are still the same as they were 10 years ago.
"For me this needs a hell of a lot of work to get this to a decision-making point."
But he supported providing staff with more resourcing to monitor the situation.
Three waters manager Kalley Simpson said while nitrate-nitrogen concentrations in drinking water supplies were relatively low at present, it could take decades for groundwater to flow downstream and into aquifers.
The Waimakariri District Council's water supplies, which served around 54,000 people, relied on groundwater.
Regular testing found the average nitrate-nitrogen concentration was 1.9 milligrams per litre (mg/L), well below the maximum acceptable value of 11mg/L.
There were around 2750 private wells in the Waimakariri district used by an estimated 6900 people and ECan's regular testing of a small number of wells indicated an average nitrate-nitrogen concentration of 3.5mg/L.
A 2021 ECan report found that 10 percent of the 300 wells it regularly monitored throughout the Canterbury region had nitrate levels above the maximum acceptable value.
A nitrate risk map produced at last month's Waimakariri Zone Committee found that private wells in the coastal parts of the zone were low risk, but the bulk of the district was considered "moderate risk".
In his paper, Dr Chambers said increased levels of nitrates in drinking water could lead to an increase risk of colorectal (bowel) cancer and preterm births.
Studies had identified other possible risk factors such as E coli in drinking water, smoking and even eating meat.
High levels of nitrates could also impact on aquatic life.
It was not just private water supplies at risk in the long term.
A recent ECan case study found there was an increased risk over time of groundwater containing elevated nitrate-nitrogen concentrations, from land intensification, passing under the Waimakariri River into deep aquifers beneath Christchurch.
The costs of treating water downstream to remove nitrates was expensive, so it was better to treat it at the source.
"If water had to be treated to remove nitrates, ion exchange is the most likely treatment method as this is well-proven
and more cost-effective than other methods," Dr Chambers said.
Because of the large number of pump stations in Christchurch, he estimated this could cost about $610 million to construct and $24m per year to operate.
"This equates to 19 years of planned capital expenditure by Christchurch City Council on water supply and would result in a 75 percent increase in operational costs," Dr Chambers said.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air