High hopes for Kaiapoi River infiltration trial

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By David Hill

An infiltration trial in Kaiapoi River’s upper reaches has delivered promising early results.

A three-year trial being carried out by Environment Canterbury (ECan) and Waimakariri Irrigation Limited (WIL) aims to reduce nitrate levels by introducing more water into the upstream section of the river known as Silverstream.

Silverstream has nitrate levels above 10 micrograms per litre, which exceed the maximum level of 6.9 mg/L set by national policy guidelines for fresh water.

The trial is operating on a WIL shareholder’s farm using a trench connected to an irrigation race which receives water from the Browns Rock intake of the Waimakariri River.

The trench, 7km east of Silverstream, is 1.5 metres deep and 150 metres long.

ECan senior hydrogeologist Zeb Etheridge says initial results from the first eight weeks of the trial are promising.

“We knew within the first week that it was a really good infiltration location as we’re getting at least 80 litres of water per second into the trench.

“We’ve also recorded a drop from 10mg/L to 4.5 to 5mg/L of nitrate at the piezometer located next to the trench, which is really positive.”

The trench is an old braid of the Waimakariri River which helps the water move towards Silverstream through porous gravel.

Mr Etheridge says adding this clean water to Silverstream could help reduce nitrate issues, while providing a cost-effective solution.

“Water quantity is the key.

“Silverstream flows at about 400 litres per second in its upper reaches, so if we could get 100 litres per second of additional low nitrate water flowing through there, then in the best-case scenario we could reduce nitrate levels to 7mg/l, which would put us well on the path to reaching the guidelines.

“Infiltration is one of the lowest-cost and most practical solutions, so if it does work then it would be a real win-win from an environmental and a cost point of view.”

WIL environmental manager Paul Reese says shareholders reacted positively to the idea from the start because they saw the trial as a chance to use their resources and infrastructure to reduce their impact on the environment.

“The landowner has been exceptional. He has given up a significant area of productive land for this project as he and all our shareholders recognise that Silverstream has significant issues and we all need to work together to solve them.”

The next stage of the trial will measure how much water is moving from the trench through to Silverstream.

If the trial is successful, WIL and ECan will consider replicating the trial at other sites.