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Ponds’ removal improves Manukau water quality

22 June 2005

Oxidation ponds’ removal improves Manukau water quality

Removing the Manukau harbour oxidation ponds has substantially improved the quality of the local marine environment, research by AUT masters in applied science student Sarah MacCulloch shows.

Sarah, who recently completed her thesis, says removing the 40-year-old ponds in 2001 provided a unique opportunity to monitor the response of physical, chemical and biological biodiversity in the reef and surrounding environment.

“There was a huge build-up of fine clay and silt sediment around the oxidation ponds and nearby Nga Kuia e Toru reef over several decades.

“The removal of the ponds resulted in greater exposure to tidal flows – in turn this led to a substantial decrease in the amount of fine clay and silt sediment around the reef.”

Sarah’s project, which took two years to research and write up, revealed that levels of heavy metals such as copper, zinc and lead, which are often found in clay and silt sediment, decreased because of the reduction in sediment grain size.

“Since the breach of the oxidation ponds, Nga Kuia e Toru reef has become a dynamic environment, with the removal of fine sediments and therefore heavy metals.”

She also measured benthic biota (organisms living in the sediment) levels and found species diversity increased during the study, although organism numbers remained relatively constant.

Sarah says higher standards for the quality of effluent discharged into the harbour have contributed to better water quality and this has helped the reef recover.

The area measured is safe for shellfish gathering and boating but she says swimming could be dangerous because of pollution, especially after heavy rainfall, which could lead to increased land run-off and overflows from the local wastewater treatment plant.

“This part of the Manukau harbour has in the past been one of the most polluted marine environments in New Zealand. While the diverse undersea communities that make their homes on or near the reef appear to be recovering, any future degradation of the environment could easily destroy them,” says Sarah.

Thesis supervisor and renowned marine conservationist and AUT Senior Research Fellow Dr Steve O’Shea says “congratulations” are in order for those who have helped improve water quality in the area.

“For decades the Manukau harbour has been abused. It is truly remarkable that the environment has been able to recover to the extent reported by Sarah.”


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