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Stormwater continues to affect harbour sediment

Survey reveals stormwater continues to affect harbour sediment quality

The results of a 2011 Wellington Harbour sediment survey show Wellington’s stormwater continues to be an ongoing source of heavy metal contamination for the harbour.

The survey which was carried out by Greater Wellington Regional Council and part funded by Wellington City Council, was the second of its kind. It investigated sediment quality and the health of the sediment-dwelling animals, at 16 subtidal sites in the Wellington Harbour.

Results show that concentrations of copper, lead, zinc and mercury exceeded nationally recognised guidelines at several sites with the highest concentrations found at sites closer to the city. Sources of these contaminants include vehicle brake and tyre wear, galvanised roofs, road dust and soil that has in the past been contaminated with leaded petrol and lead based paints.

Hydrocarbon concentrations (such as from vehicle exhausts and tyre wear) also exceeded guidelines at inner harbour and Evans bay sites but it is thought that much of this contamination is historic from industries such as the gasworks at Miramar. Despite being banned in the late 1980s, the pesticide DDT is also apparent throughout the harbour with higher concentrations closer to the city.

Examination of creatures living on the harbour floor identified 124 species, among them tiny crustaceans, worms, urchins, bivalves and brittle stars. These invertebrate communities have shown little change in the last five years and remain reasonably diverse. However, invasive species and those with a high degree of pollution tolerance appear to dominate at sites with higher amounts of mud and contaminants.

Regional Council coastal scientist, Dr Megan Oliver says that while there were some statistically significant changes in contamination levels between this survey and the 2006 study, it is still too early to determine if the changes are environmentally meaningful. “The accumulation of contaminants is a relatively slow process and at least another three surveys will be needed before we can say if the situation is improving or worsening.” Dr Oliver says. “Overall there are contamination hotspots but the animal communities living there appear reasonably healthy and diverse and are similar to harbour communities found elsewhere in New Zealand”.

The contaminated sediment poses little risk to people but could, if the accumulation of contaminant continues, have an adverse effect on marine life as sediment-dwelling organisms provide vital habitat and food for numerous fish and other species.

To confirm assumptions about contamination source, tests were also carried out on sediment collected from roadside stormwater catchpits. These also contained substantial amounts of copper, lead and zinc indicating that urban stormwater run-off is an ongoing source of heavy metal contamination for the harbour.

Wellington Regional Council’s Environmental Management spokesperson, Councillor Chris Laidlaw says the study reinforced the need for us to increase our awareness of how our actions on land impact on the quality of water.

“Wellington Harbour is one of our most important coastal environments. We have built our communities around it and because of this everything we do has an impact on it. The study is a timely reminder that we need to reduce the amount of contaminants we allow to flow into the stormwater system and ultimately the harbour. It is up to us to protect the marine ecosystem and we do that by caring for our waterways both, natural and manmade,” the councillor says.

Councillor Laidlaw points out that a catchment committee is soon to be established in the Hutt/ Wellington area and this committee will focus on land and water management ultimately influencing the rules and policies in the Regional Plan.


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