Constructed wetlands could reduce nitrate levels in Waimakariri

Green Cay Nature Centre and Wetland in Boyton Beach, Florida is a 100-acre integrated constructed wetland created on former farmland in 1999. Photo / Supplied

A recent presentation has inspired the Waimakariri Water Zone Committee to consider a pilot project to reduce nitrate, phosphorous, sedimentation and E. coli levels in local waterways and improve biodiversity in the district.

The presentation, by wetland scientist Dr Michelle McKeown of science and engineering consultancy Wallbridge Gilbert Aztec (WGA), was on the benefits of integrated constructed wetlands.

An integrated constructed wetland is an engineered water treatment system that uses vegetation and microbes in the soil to treat water from farms and other sources while also integrating the wetland structure into the surrounding landscape fabric.

These wetlands, which have been used in Ireland, the USA, and the UK since around 2007, act as a biofilter to remove suspended solids, pathogens, and nutrients from waterways.

Waimakariri Zone Committee Chair Michael Blackwell said he was excited by the potential of integrated constructed wetlands to improve rivers and streams.

The wetlands also had the potential to be incorporated with the vision of Te Mana o te Wai as a vehicle for the community "to move forward together on improving the water quality of our rivers and streams," while also providing valuable recreational, cultural, and educational opportunities, Blackwell said.

"Wetlands are the kidneys of our ecosystem, and we've already lost over 90 per cent of our wetlands across New Zealand. It's important to think outside the box to reduce contaminants and increase biodiversity values.

"We can also learn from current projects underway elsewhere in Canterbury including the Whakaora Te Ahuriri project near Lake Ellesmere which will create a constructed wetland at Ahuriri Lagoon to improve mahinga kai, water quality and biodiversity values."

McKeown described the concept as a "reanimation" of the landscape using an approach that not only integrated ecologists, soil scientists, hydrologists, and engineers but also the community.

People could have an active input into what they would like the space to be used for recreationally, while also delivering key environmental benefits to society, McKeown said.

Constructed wetlands do not provide all the benefits of a natural wetland but in areas where wetlands have been completely lost, they can provide an alternative to traditional wastewater treatment plants.

"We facilitate natural processes through an engineered ecosystem-based approach, which act as landscape kidneys to filter nutrients from water, which is carried out by vegetation and microorganisms living in the soil," McKeown said.

These systems could also act to sequester carbon through capturing and storing atmospheric carbon dioxide in their highly organic soils, McKeown said.

"The carbon is essentially locked into the system. Their carbon sequestration potential works in a similar way to forests and there is a real potential that these integrated constructed wetlands could qualify for carbon credits in the same way forests do."

People would lead the design in terms of how they wanted to use the space, McKeown said.

"Instead of the traditional model of separating people from water treatment facilities, this approach includes recreational, cultural, and educational aspects such as walking tracks, biota nodes for school groups to manage, and a place to share cultural history and practices."

Spending an adequate amount of time on planning and research was essential when it came to designing a successful integrated constructed wetland, McKeown said.

Wetland scientist Michelle McKeown hopes to create a pilot project in Waimakariri to test the benefits of integrated constructed wetlands. Photo / Supplied

Factors, such as the size of the wetland and land type, were key during this process, along with consideration for consent requirements, to ensure that any development didn't have adverse impacts, she said.

"The planning phase is absolutely vital. You need to spend a good amount of time consulting with the community, experts and iwi and get everyone to decide on the purpose and use of the wetland area."

The size of each wetland was determined by the level of contaminants entering the land area, McKeown said.

"You need the right amount of land to purify the contaminated water. Through a series of interconnected wetland cells, that mimic marsh wetlands, you can achieve a reduction in contaminant levels."

For them to operate effectively, the balance between the functional benefits of natural wetlands (vegetation diversity and the appropriate substrate) and the engineered design (ensuring water is flowing correcting through the system) needed to be right, McKeown said.

"The wetland cells should ideally be on flat land to ensure the water does not short-circuit and takes time to move slowly through the vegetation and soil. This slow movement of water in this zone is where the contaminants are removed. Thus, the flat land of Canterbury is an excellent place to implement this concept".

Features such as islands (for bird habitat) and walkways could be incorporated into the design, however, the location of such features needed to fit within the entire ecosystem, rather than being viewed purely from an aesthetical perspective, McKeown said.

"Having an island can be attractive and provide habitat for birds. However, if it is placed too close to the end of the system you can have issues with faecal contamination from birds, so you need to consider the entire design of the system before you begin construction and how each aspect interacts with other areas of the wetland."

McKeown was currently working on developing a pilot project concept with interested parties in Waimakariri and hoped her vision could become a reality.

"It would be so exciting to see this happen as there are many benefits for the land, water and the local community."