Nitrates to climb over time

SHARE

By DAVID HILL

A scientific investigation into potential groundwater movements under the Waimakariri River has found low levels of “nitrate-nitrogen” in aquifers and wells in Kaiapoi, Belfast and northern Christchurch.

This is expected to increase over the next 50 to 100 years if land-use changes are not made.

Environment Canterbury (ECan) chief scientist Dr Tim Davie says about 2-3 micrograms per litre of “nitrate-nitrogen” was found in urban aquifers and wells.

Modelling suggests it could increase by a further 5mg/l over the coming decades, well below the national drinking water standard of 11.3mg/l.

He says there is no evidence of nitrates from the Waimakariri catchment reaching water supplies in central and southern Christchurch, but modelling suggests this could happen in the future.

“This is saying, ‘this could happen if we continue the way we are going’, and this is decades away. We might see changes in a 20 to 50-year time period.

“This is low levels, but we realise people want to see Christchurch’s drinking water remain in a pristine state.”

Dr Davie says the studies focused on nitrates, rather than bacteria or E. coli.

The source of the nitrates was primarily from agriculture north of the Waimakariri River.

While there are natural ways of treating nitrates in water, including the use of plantain, once it gets into aquifers and wells it is difficult to treat as nitrates dissolve in water.

Dr Davie says using chemicals to treat it is expensive, unpopular and potentially dangerous.

He says it has long been known that nitrates from land-use south of the Waimakariri River could infiltrate Christchurch’s drinking water, hence only low intensive land-use has been permitted in the West Melton area.

A hypothesis was first put forward to ECan in 2002 that groundwater containing nitrates from north of the Waimakariri River could move under the river and get into Christchurch’s drinking water, but this was rejected based on the modelling at the time, Dr Davie says.

ECan drilled nine new wells along the Waimakariri River and in Christchurch to test water quality, geochemistry and to complete a piesometric survey, the measurement of groundwater levels across a large area at the same time.

Dr Davie says the findings have been examined by a panel of scientists and water quality experts from Canterbury and around New Zealand.

The evidence will be passed to the Waimakariri Zone Committee to consider as part of its sub-regional plan process.

Dr Davie says while nitrates from the Waimakariri catchment can get into Kaiapoi’s drinking water, Rangiora is supplied by the Ashley River.