A denitrification wall at Silverstream Reserve, North Canterbury, has already significantly reduced nitrate levels in less than two months. (Video first published in January 2019)

Nitrate levels have significantly reduced at a North Canterbury stream less than two months into a pilot project to improve its water quality.

The Institute of Environmental and Scientific Research (ESR)-led denitrification wall trial at Silverstream Reserve, near Kaiapoi, has resulted in nitrate levels in groundwater dropping from 7.1mg/l to 0.5 mg/l.

The 25 metre-long wall, installed in November, is a world first, having never been tested in a fast-flowing gravel aquifer system before.

EMMA DANGERFIELD/STUFF The project aims to filter nitrates out of groundwater using a permeable barrier made from woodchip and gravel.

ESR senior scientist Lee Burbery​ said he was delighted the wall was working as he anticipated.

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"The wall itself is effectively removing all of the nitrate in the groundwater that passes through it. We are seeing a plume of treated groundwater extend down-gradient from the wall."

EMMA DANGERFIELD/STUFF Silverstream has high nitrate levels as a result of land intensification.

The denitrification wall acts as a groundwater filter. Woodchip mixed with gravel removes nitrate from the groundwater that passes through the wall. Carbon from the woodchip provides a food source for bacteria that then convert the nitrate into the harmless di-nitrogen gas (N2), which makes up 80 per cent of the air we breathe.

Two mixes are being tested in the wall to determine which type of material is the most cost effective and efficient at removing nitrates.

The test site was selected because of its shallow water table and high nitrate levels.

SUPPLIED ESR lead scientist Murray Close and senior scientist Lee Burbery are working to reduce nitrates in Silverstream.

"We have 34 monitoring wells on the site, which will be increased to about 50. It's important to ensure that there won't be any adverse effects with pollution swapping from the woodchip, but we haven't seen any evidence of this so far," Burbery said.

The trial will run for two years, but the wall itself is expected have an operational lifespan of about 30 years.

ESR lead scientist Murray Close expected the site to function as a demonstration model to show landowners how the concept worked.

"We're trying to establish guidelines on how denitrification walls work, and this site will provide a practical example."

Close said denitrification walls could be part of the answer to reducing Silverstream's high nitrate levels.

"All of the work being carried out at Silverstream addresses different parts of the puzzle. Our piece of the puzzle is what we can do to address the nitrates that are already in the groundwater.

"I think it will take a combination of approaches to deliver the improvements that we want to see for Silverstream."