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Report assesses nitrate discharge into Canterbury groundwater

Published: 19/05/2009 4:42 p.m. 

A report has been released assessing nitrate discharge into groundwater on the Canterbury Plains from rural land uses.

The report, commissioned by the Canterbury Water Management Strategy Steering Group, arose from public concern about the effects of agricultural land use on groundwater quality.

The report looks at nitrate discharge from rural land use and its implications for water quality in two situations: shallow groundwater quality on the Canterbury Plains and deeper groundwater quality for Central Canterbury.

Environment Canterbury Director Investigations and Monitoring, Ken Taylor, says continued access to clean drinking water always comes through in public consultation as one of the key issues for water management and the report provides the Steering Group with some predictions about the impact of nitrate discharge on groundwater quality.

“This is a subject that will require much study. This report was compiled over six years and provides a good starting point for understanding this important matter,” he says.

“The report provides evidence that nitrate discharge from agricultural land use on the Canterbury Plains has a negative impact on the quality of water at shallow groundwater levels, and in some situations, exceeds drinking water nitrate standards,” he says.

“It also shows groundwater quality generally improves with depths below the groundwater surface because of mixing with high quality groundwater from river recharge,” he says.

“Most large drinking water supplies, including Christchurch, take water from deep groundwater. In Christchurch there are two other factors that help protect water quality from land use effects. The first is the confining layers above the aquifers where Christchurch city takes its water. The second is that we are in the process of implementing land use controls over the area of land where nitrate could get into the aquifer,” he says.

Mr Taylor says the report shows contamination in shallow ground water is directly related to land use and there are areas of compromised groundwater drinking supplies in rural Canterbury. Careful land use management is therefore a reality and further land use intensification in the Canterbury Plains will be dependent on reducing current levels of nitrate leaching.

“There is potential to improve practice and the rural industries have stated a commitment to working together to promote improved practice that will lead to a reduction in nitrates.

“The report compares the effects of improving practices on existing agricultural land to the effects of changing land use. Improving practices on existing agricultural land will have the greatest effect on the availability of quality drinking water from shallow groundwater,” he says.

“This is a very helpful report and provides new data and insights on this important issue. I would commend those with an interest in water management to read it.”

The report will be used by the Canterbury Water Management Strategy, run by the Mayoral Forum, to help guide them and others when looking at the best way to sustainably manage Canterbury’s future water resources.

A Water Management Strategy public consultation programme is currently underway. For a copy of the nitrate report and more information on the public consultation please visit www.canterburywater.org.nz/

Background:
Nitrate is leached by water draining from the soil into the groundwater below, referred to as nitrate discharge. The amount of nitrate transported into the groundwater surface depends on the type of land use and the associated management practices. This soil-water drainage is one of the major sources of recharge to groundwater, called land surface recharge.

The other major source of groundwater recharge is leakage from rivers, and this river recharge is a significant feature of the groundwater resource of the alluvial aquifers of the Canterbury Plains.

All groundwater eventually discharges into surface waters such as springs, wetlands, streams, lakes or directly to the sea, and supplies most of the freshwater flow to these water bodies in the lowland and plains area of Canterbury.

There is a national threshold for safe nitrate levels in drinking water. High nitrate levels may lead to nitrate poisoning in animals and bottle-fed babies less than six months old.

For more information: Ken Taylor, Environment Canterbury, Director Investigations and Monitoring, 03 371 7109, 027 295 4359 or Vince Bidwell, Lincoln Ventures, 03 325 3704.

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