Waimakariri groundwater investigation suggests long-term increase in nitrate

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A scientific investigation into potential groundwater movements under the Waimakariri River predicts a long-term increase in the level of nitrate in deep groundwater in Belfast and north Christchurch. 

Key points

  • We know from the modelling studies that increasing nitrate levels under Christchurch is a risk
  • Nitrate levels are currently very low, and we will be working very hard to keep it that way
  • The Waimakariri Water Zone Committee's ZIPA recommends a 3.8 mg/litre of nitrate which is based on national guidelines for protecting aquatic species
  • While the limit is challenging, it is achievable, and is well below the national drinking water standards, which allow for a maximum of 11.3 mg/L
  • The Zone Committee recommendations go further than current farming rules by requiring farmers to change their practices and reduce nitrate losses
  • There will be a rigorous public hearing process to decide these rules. Anyone will be able to make a submission on the proposed rules and to speak and present evidence at the public hearings, which will be run by a panel of expert independent commissioners.

 

 

Dr Tim Davie about the state of Waimakariri's groundwater

In July 2017, Environment Canterbury announced it would extend the Waimakariri sub-regional planning process to allow more time to reach agreement on the nature of deep groundwater movements under the Waimakariri north of Christchurch.

“The key issue was whether nitrate, from farming and other land use on the north side of the Waimakariri River, could enter the groundwater system and flow south under the river into the Christchurch groundwater system,” says Dr Tim Davie, Environment Canterbury’s Chief Scientist.

Over the past few months the scientific investigation involved the drilling of nine new wells, water quality and geochemistry testing, a piezometric[1] survey, as well as a sophisticated programme of groundwater modelling.

“It’s important to note there is no measured evidence of any increase in nitrate in the aquifers under the main part of Christchurch city.

“The investigation predicts, however, it is likely nitrate levels in deep groundwater in the area around Belfast and north Christchurch will increase if current land uses persist.

“We are talking a timeframe of 50 to 100 years for the full effect, and even then, any increase in nitrate would be below the Maximum Acceptable Value to ensure safe drinking water,” said Dr Davie.

The source of the nitrate is thought to be farming and intensive land use north of the Waimakariri River.

“It’s possible, over the next 50 to 100 years, that nitrate in aquifers below the main part of Christchurch will also increase, but there is less certainty over whether this will happen.  Nitrate levels are again predicted to remain below the Maximum Acceptable Value.”

The results of the science investigation have been discussed with the Waimakariri water management zone committee at a briefing. During 2018 the committee will use the information from the investigation to help develop a set of recommendations and solutions for land and water management in the Waimakariri water management zone.

These recommendations will be used by Environment Canterbury to set both water quality and water allocation limits for the Waimakariri water management zone, through a plan change to the Canterbury Land & Water Regional Plan.

Under the National Policy Statement, the plan is required to maintain or improve water quality.  Members of the public will be encouraged to have a say through the Plan submission and hearing process, which will be well publicised.

Background to the Waimakariri hydrogeological investigation

Over the past 20 years, scientists working on the Waimakariri groundwater system have developed a good understanding of how water from the Waimakariri River enters the shallow groundwater system south of the river and flows towards Christchurch, where it replenishes the city’s aquifers.

There was, however, some uncertainty over what happens at deeper levels (100 metres plus), and over longer periods (for instance greater than 50 years).  In July 2017 Environment Canterbury started an investigation which included: the drilling of nine new wells alongside the Waimakariri River to measure groundwater levels, water quality sampling; a piezometric survey across the zone to better understand groundwater flows; and sophisticated modelling.

The project has drawn on the expertise of scientists from three research institutes, Environment Canterbury, as well as independent freshwater scientists.

The Maximum Acceptable Value of nitrate in drinking water

What is a safe nitrate level in drinking water?

Drinking Water Standards for New Zealand set a Maximum Acceptable Level (MAV) of 50mg/L for nitrate, which is equivalent to 11.3mg/l nitrate-nitrogen. Visit Community and Public Health for information on health risks of nitrate in drinking water.