Thursday, 02 September 2021 12:55

Wetlands could combat nitrates

Written by  Staff Reporters
The size of each wetland is determined by the level of contaminants entering the land area. The size of each wetland is determined by the level of contaminants entering the land area.

The Waimakariri Water Zone Committee in Canterbury is considering a pilot wetlands project which would aim to reduce nitrate, phosphorus, sedimentation and E. coli levels in local waterways, while improving biodiversity in the district.

This follows a recent presentation on the benefits of integrated constructed wetlands by wetland scientist Dr Michelle McKweown of science and engineering consultancy Wallbridge Gilbert Aztec (WGA).

An integrated constructed wetland is an engineered water treatment system that uses vegetation and micrbes 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 says he is excited by the potential of integrated constructed wetlands to improve rivers and streams, and to be incorporated with the vision of Te Mana o te Wai as a vehicle for the community to work together on improving water quality.

"Wetlands are the kidneys of our ecosystem, and we've already lost over 90% 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."

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

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

"These systems can also act to sequester carbon through capturing and storing atmospheric carbon dioxide in their highly organic soils. 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."

Blackwell says spending an adequate amount of time on planning and research cannot be underestimated when it comes to designing a successful integrated constructed wetland. Factors such as the size of the wetland and land type are key, plus ensuring any development does not have adverse impacts.

"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 is determined by the level of contaminants entering the land area. 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.

"The wetland cells should ideally be on the land."

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