In partnership with the Ministry for the Environment, Port Nelson has just successfully completed a significant environmental project involving the remediation of the port’s Calwell Slipway Basin.
With site-works kicking off in October 2017 and taking 7 months to complete, the equivalent of 13 Olympic sized swimming pools of contaminated sediment from the Calwell Slipway Basin has been removed, a total of 32,516 cubic metres, preventing its migration to other areas of Nelson Haven.
Removal of the contaminated sediment, built up over a 30-year period, has restored navigability of the Basin, ensuring the future operability of businesses reliant on access to the basin in Nelson’s Marine Engineering Hub with further benefits to the wider marine contracting sector.
Cleverly, the contaminated sediment has been recycled as ‘mudcrete’ and stabilised within 5000sqm of reclaimed land inside the Slipway and will be used for logging storage.
Two slipways operate within Calwell Slipway Basin itself. The larger of the two is owned and operated by the Port of Nelson and is the third largest slipway in New Zealand.
Supporting the largest fishing port in Australasia the primary users of the Calwell Slipway are fishing vessels from all over New Zealand. A smaller slipway is leased by Nelson Slipway from the Port of Nelson and used by smaller vessels.
The project was initiated in 2009 when leasees of the Slipway Basin approached the Port to do some maintenance dredging to improve navigability.
Kelly Leonard, the Ports Environmental Officer and the remediation project’s Project Manager says that when the Port undertook some pre-dredge monitoring sediment was found contaminated with tributylin (Tbt) and copper, historically used in antifouling paints.
As the contaminant levels were higher than the resource consent permitted, the Port was unable to carry out dredging of the slipway and had to find a solution.
Kelly explains that the contamination occurred pre-Resource Management Act when it was common for people to pull up beside the jetty or haul their vessel up out of the water, scrape the hull and wash it all back into the tide.
“Tbt in the marine environment is highly toxic toward non-target organisms. It’s one of the worst contaminants ever introduced into the marine environment with a worldwide ban put in place since the early 2000’s.”
Both slipways now operate with appropriate containment for the discharges so they don’t go
back into the tide.
In 2010 the Ministry for the Environment entered into partnership with Port Nelson in regard to the remediation project and investigation into the scale and nature of the contamination was launched, along with identifying options for remediation.
“The local Iwi didn’t want the contamination going out of the region and becoming another region’s burden and there’s no landfill here that will take the volume and concentrations of contaminates that we had. Bio-remediation with the concentrations we had didn’t really exist and it wasn’t feasible send it offshore for incineration given the volume we had.”
After a comprehensive viability assessment and consultation process, agreement was reached that remediation would involve dredging the contaminated sediment and stabilizing it using cement and activated carbon to make mudcrete for the construction of a reclamation adjacent to the dredging site.
With over 45 years experience in dredging projects, Auckland based Heron Construction Co. Ltd was engaged to undertake the project as lead contractor for both the dredging and reclamation site work. “Reclamation construction using mudcrete has been around since the late 90’s, says Kelly.
“We think this is the first application of it to stabilise these contaminants in NZ. “Mudcrete is a mix of the sediment along with the right ratios of cement and activated carbon. The mixing plant was designed by Heron Construction and built in NZ specifically for this job.”
Over 32,000 cubic metres of contaminated sediment was dredged from Port Nelson’s Calwell Slipway Basin.
Taking the time at the back end of the project to properly plan the project and do the research to ensure that the project would achieve what it set out to do before pressing go on the site works was a key success factor, along with making sure that the right people were on the project team.
“We spent nine years building up to the site works. Given the complex nature of the job and what could have gone wrong, the site works went extremely well.
“To come out the other side successfully and achieve the remediation targets for a complex project like this without significant involvement of health and safety, environmental or financial incidents is a pretty significant achievement.”
A notoriously complex project with many challenges along the way, Kelly says it was good to get to the other side and deliver what the project promised to deliver and meet the environmental goals.
“It’s also important to acknowledge the support of the New Zealand Government and Nelson Ports project partner, the Ministry for the Environment, which funded 50% of the site works $8.3m cost, through its contaminated site remediation fund.”
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Port project meets environmental goals
Final reclamation dredge bund and rock armouring, part of the remediation of Port Nelson’s Calwell Slipway Basin.
