Water testing confirms constructed wetland delivering impressive results

Analysis of water quality monitoring at a constructed wetland in Ashley Clinton has confirmed what many had hoped: constructed wetlands can make a significant difference to water quality while delivering wider environmental benefits.

Funded by Fonterra and Hawke’s Bay Regional Council (HBRC) in partnership with the National Institute of Water and Atmospheric Research (NIWA) and the Tukipo Catchment Care Group (TCCG), the 1.6 hectare White Wetland aims to improve water quality and biodiversity in the catchment while also providing valuable data on how effective constructed wetlands are in removing nitrogen and other contaminants from farm runoff.

Recognising the importance of measuring outcomes, Tukituki Land Care (TLC) funded 24 months of independent water quality monitoring. WaterSense collected monthly samples from both the wetland inflow and outflow, with the results analysed by Dr Andy Hicks, Principal Ecologist at Stradegy Ltd.

The findings are impressive. On average, dissolved inorganic nitrogen (DIN) concentrations were reduced by 73 percent as water passed through the wetland. Dissolved reactive phosphorus concentrations were reduced by an average of 82 percent, while E. coli levels also showed generally strong reductions.

"One of the standout findings from the White Wetland is that water entering the wetland exceeded nitrogen targets, while water leaving the wetland was meeting those targets," said Dr Hicks. "Results like this show the potential for constructed wetlands to become an important part of the solution to improving water quality, while also delivering biodiversity and flood management benefits that complement productive farming."

Dr Hicks also stressed the importance of strategic placement to achieving the best results. "Constructed wetlands are most effective where they intercept water with elevated nutrient concentrations and where water can be retained long enough for treatment processes to occur,” said Dr Hicks.

The White Wetland was built on a former marginal paddock donated by the White family and now includes 24,000 native plants alongside a seven-hectare remnant bush. Using NIWA's modular design, it treats runoff from around 180 hectares through a series of treatment cells that naturally remove contaminants before water returns to the catchment.

Annabel Beattie, Catchment Management Lead, Biodiversity at Hawke's Bay Regional Council, says wetlands also provide benefits well beyond water quality. "Wetlands provide habitat for some of New Zealand's most threatened species, absorb nutrients, filter sediment, slow floodwater and have important cultural and recreational values," she said. "Despite this, only around four percent of Hawke's Bay's historic wetlands remain."

The White Wetland demonstrates how one well-designed project can improve water quality, enhance biodiversity and support resilient farming systems. The full report and monthly water testing results are available on the Tukituki Land Care website.