Big port clean-up under way
The clean-up of Port Nelson’s slipway, one of the most contaminated sites in the country, is moving ahead one bucket at a time.
The $8.9 million remediation project of the Calwell Slipway officially got under way this week.
The marine sediments around the slipway were contaminated by pollutants such as tributyltin and copper from ship repair work dating back to 1970.
The remediation project uses excavators and dredging equipment to dig up the contaminated sediment. It is processed to form ‘‘mudcrete’’, ensuring any pollutants are contained, before being returned to form a new reclamation at the port.
The project is being managed by Port Nelson, with Aucklandbased civil engineers Heron Construction tasked with digging up the area.
The slipway is number two on the Ministry for the Environment’s Contaminated Sites Remediation Fund priority list, and its clean-up has been on the cards since 2010.
Port Nelson chief executive Martin Byrne said the start of the project was a big step forward after seven years of preparation.
‘‘It’s good we’re able to finally do something – some of the environmental practices that went on in our industry and a lot of others 30-40 years ago would be unacceptable now,’’ he said.
Byrne said the construction of the reclamation was a ‘‘win-win’’ for the port in cleaning the slipway as well as creating extra land to reconfigure logging areas.
The total project cost was being split between Port Nelson and central government, with a $4.2m government contribution given to the remediation in 2016.
The dredge and reclamation construction programme is due to be completed by April.
Project manager Kelly Leonard, of Port Nelson, said the last few weeks had focused on commissioning the mixing plant and completing the pilot trial of mudcrete production.
Leonard said dredging was the Heron’s ‘‘bread and butter’’ and it had previously carried out similar work with Ports of Auckland.
"There was a lot of preparation to get people on-site and start and now it’s all ready it’s nice to be involved in one of the country’s biggest remediation projects.’’
Dredging is carried out with a long-reach excavator within a contained area. A silt curtain prevents contaminated sediment from spreading.
The excavated material is placed on a barge and taken to a mixing plant nearby, mixed with concrete and binded with activated carbon, before the new mudcrete product is returned to the site and placed on the seabed.
It will then be used to construct a new 5000m2 reclamation east of Kingsford Quay adjacent to the slipway.
The main dredge area covered the water immediately north and to the east of the slipway.
So far 3000 cubic metres of the estimated 30,000 cubic metres has been shifted.
Excavators would dig between three and eight metres deep to allow for the contaminated area as well as a area of liquefiable material that also needed to be dredged.
‘‘It’s material that is natural, but in an earthquake it’s been identified as a high risk for liquefaction, and if we’re building that concrete reclamation over top, then that needs to come out too,’’ Leonard said.
A computer screen inside the excavator enabled the operator to see how far they were digging. As the bucket dropped into the water, it can be tracked where it’s going through GPS equipment.
The screen turns blue when it hits material, green when it reached the dredge zone and red once it goes beneath the dredge zone.
Validation testing would be carried out to ensure that the mudcrete was in an appropriate state to be used.
A website has been set up to keep the public informed on remediation work, including timelapse footage of the progress.