Environmental Science and Research principal scientist Murray Close said its proposed denitrification barrier near Christchurch would be the first time such a wall had been inserted into a gravel groundwater system.
A denitrification wall, also known as a denitrifying permeable reactive barrier, worked like a groundwater filter.
The organic matter fuelled microbial activity that stripped nitrate from groundwater passing through the wall.
Similar walls had worked well in Waikato sand aquifers but water flowed through Canterbury gravel up to 100 times faster, Close said.
The barrier at Siilverstream Reserve close to Kaiapoi and the Waimakariri River was likely to be 60 metres wide and 10 metres deep – dug two or three metres below the water table.
It needed to be long enough, wide enough and deep enough to handle the volume of water flowing through a network of underground channels.
The twisting, two or three metre wide “open channels” along braided rivers like the Waimakariri had no natural clay or silt barriers to slow the movement of water and nitrate.
ESR was working with geophysicists to find the channels, using salt to track the directional flow.
“On a broad scale you can get a fairly accurate measure of where water’s going because it goes downhill.
“But on a local scale the angle (of the channels) could shift about 30-40 degrees from the average direction.”
Close said they would dig a trench, screen gravel, mix it with compost at a 1:1 ratio and backfill it with woodchips.
Fine and very large gravel would be removed from the groundwater flow and microbial reaction.
ESR had been working on the “fate and transport” of nitrate in NZ groundwater systems for the past decade.
The Crown agency found the country’s alluvial gravel aquifers had “negligible ability to naturally attenuate nitrate and, in many locations, groundwater systems have reached the limit of mitigating nitrate pollution through natural dilution”.
ESR was applying to Waimakariri District Council for consent to use Silverstream Reserve and if successful it would apply to Environment Canterbury for resource consent.
ESR had consulted Ngai Tahu about the project, including any cultural or technical concerns.
In an unrelated project, ECan was working with Geological and Nuclear Science on groundwater modelling for the Waimakariri to determine whether water channels on the north side of the river were connected to Christchurch’s aquifers.
