Impacts of stormwater focus of harbour seabed survey

How healthy is Wellington Harbour for the sediment-dwelling animals that live there?

Greater Wellington and Wellington City Council’s second survey of contaminants in Wellington Harbour sediments aims to find out.

The survey, which runs during November, follows a base-line assessment in 2006 and is designed to assess how urban stormwater is affecting the quality of the sediment on the harbour seabed and the health of the benthic (bottom-dwelling) invertebrates living there.

Surface sediment and benthic invertebrates will be collected from 16 sites near Lambton Harbour, Aotea Quay, Evans Bay and the Petone foreshore. The sediment samples will be analysed for a range of contaminants, including heavy metals, polycyclic aromatic hydrocarbons (PAHs, byproducts of fuel burning), and pesticides.

The baseline assessment from 2006 showed that heavy metals, PAHs and pesticides have accumulated in the sediments of Wellington Harbour with the highest concentrations found in inner Lambton Basin and Evans Bay where there is less flushing of contaminants.

Investigations show that urban stormwater, which can contain toxic chemicals such as paint and oil, copper from vehicle brake pad wear, and zinc from unpainted galvanized roofs, is a major contributor to the contamination. While some contamination is historical in origin, such as lead from leaded petrol, the contaminants are persistent and toxic and continue to enter our harbour via streams and stormwater outfalls.

The 2006 study reported a moderately healthy 101 species of invertebrates living in the harbours sediments, but also found that abundance and diversity of these species was higher in areas with less contamination. The present survey will sample additional sites closer to shore to further clarify this impact on invertebrate communities.

The survey is being coordinated by Greater Wellington’s Environmental Monitoring and Investigations Department and is co-funded by Wellington City Council (WCC) and Hutt City Council. WCC is required to monitor sediment quality at five-yearly intervals at a number of the sites as part of a new consent granted earlier this year that covers all of Wellington city’s stormwater discharges into the harbour.

A more formalised and integrated approach to stormwater and wastewater management is an outcome of the consent and will see WCC working with a variety of stakeholders in determining future strategies and community driven outcomes that lead to protecting our coastal waters.

We’re aiming to see steady improvements in the quality of stormwater discharges into the harbour over the 10-year term of WCC’s consent because water quality is crucial to the survival of marine life in the harbour. We cannot afford to compromise here,” Greater Wellington regional councillor Chris Laidlaw, Chair of Te Upoko Taiao – Natural Resource Management Committee says.

“This second survey will enable us to see whether there have been any changes in the health of the harbour seabed since 2006 and whether these are affecting bottom-dwelling invertebrates.”