Denitrificaton wall could be answer to nitrate problems
Farmers trying to lift the water quality of rivers might get help from a nitrate wall proposed to go into Silverstream Reserve near Kaiapoi to lower the steam's high nitrate levels.
The Institute of Environmental and Scientific Research (ESR) was keen to assess whether the wall, which had been successfully used with sandy aquifer systems at Waikato and the United States, could have the same results for gravel aquifer systems such as those in Silverstream and elsewhere in Canterbury.
The Silverstream waterway had nitrate levels "well beyond" being environmentally sound, according to research by Environment Canterbury.
If nitrate pollution can be reduced using a denitrification wall, also known as a woodchip permeable reactive barrier (PRB) it could improve the safety of freshwater and groundwater resources and potentially provided means for farming within water quality limits.
The study was proposed by ESR senior scientist Lee Burbery who has done a lot of work in nitrate pollution and has been instrumental in planning the pilot project at Silverstream Reserve.
Silverstream was identified as an excellent field study location for several reasons, including its high nitrate levels which were above the national bottom line for lowland streams.
Gravel aquifers contained much of New Zealand's usable groundwater and were particularly vulnerable to nitrogen contamination, Burbery said.
"Much of our groundwater research is targeted directly at answering a question or providing some scientific solution to an active groundwater pollution issue, such as nitrate contamination from intensified farming or microbial contamination from land-based effluent disposal practices," he said.
The denitrification wall operates as a groundwater filter. A wall made of woodchips, mixed with gravel aggregate, strips nitrate from groundwater passing through it. Carbon within the wall provides a food source for bacteria to convert nitrate in groundwater to nitrogen gas (80 per cent of the air we breath).
ESR principle scientist Murray Close said the pilot project would be a world first as denitrification walls had not been tested in gravel aquifer systems.
"We are interested to see if the same method can work in these fast flowing gravel aquifers. With nitrate levels being a concern in the groundwater feeding Silverstream, residents and the Waimakariri Water Zone Committee want to know if a denitrification wall could be a viable solution to the problem."
Close said ESR was running a salt tracing experiment alongside the Silverstream Reserve to measure the speed of the groundwater.
"This will enable us to design the thickness of the denitrification wall. Once we have this data, we will design a 60 metre pilot wall which will give us an opportunity to measure the effectiveness of the wall."
Once denitrification walls are installed, they require little maintenance and can remain active for up to 30 years. However, there were socio-political and cultural issues to consider along with the risk of pollution swapping in the initial stages of operation when the woodchip is fresh.
"So far leaching tests in the lab have not shown any significant risks however we can evaluate this further with the pilot project. The pilot wall will be located far enough away from any potable supplies and surface waterway to ensure it doesn't have any adverse impact on them," Close said.
Although the intended result is improving water quality and altering the chemical state of the water, there are conflicts with the Canterbury Land and Water Regional Plan, so ESR is in the process of applying for a resource consent from Environment Canterbury to carry out the pilot project.
An important part of the resource consent application is the management plan which sets out all the monitoring that will take place during the project.
If consent was granted Close hoped the project could begin this summer.
"We are looking forward to carrying out the pilot study. If the denitrification wall can remove nitrate from groundwater before it reaches Silverstream and improves its water quality, it will be a brilliant result for the local community."