Cost to rid red zone of coal tar skyrockets to $100m

Monday, 29 July 2024

The cost to remove coal tar from Christchurch’s river red zone has sky-rocketed to around $100 million - seven times more than previous estimates.

Testing has revealed most of the disused roads in the Ōtākaro Avon River Corridor (ŌARC) were built using coal tar, which if disturbed, is harmful to humans and aquatic eco-systems.

Coal tar, a black, sticky by-product of coal gasification, was used as a waterproof binder in roads up until the 1980s.

The council estimates there is 335,000 tonnes of coal tar-contaminated material that has to be removed from the red zone.

“So a big problem, a massive problem for us in the ŌARC,” council residential red zone manager David Little said at a recent council briefing.

Most of the roads in the red zone will need to come out for various reasons. Some areas will be used for stormwater treatment and others might end up being floodplains.

“If there’s contamination in there, you’re going to have contamination long term so you really don’t want to be leaving these roads in place.”

Coal tar contains “polyaromatic hydrocarbons”, some of which cause cancer in humans.

The council was last year warned by staff that contaminants buried under Christchurch’s red zone would cost more than the $12.1m budgeted for the clean-up, but no figure was provided at the time.

The $100m cost was associated with dumping the contaminated material at Kate Valley landfill in North Canterbury. This would involve 4700 truck trips, covering a total of 700,000 kilometres - twice to the moon and back.

Those trips would also generate 750,000 tonnes of carbon emissions - equivalent to the annual emissions of 50,000 people.

However, in a bid to limit costs and emissions, the council has teamed up with Canterbury University to find another more sustainable solution.

Little said the council was looking at a bioremediation process, which involved using microbes and organic material like compost to eat the coal tar and rid it of contaminants.

Soil scientists at the university have been carrying out experiments in their labs since February and the results were so far promising, Little said.

The next phase was to create a pilot project with larger amounts of coal tar-contaminated aggregate.

Little said he hoped to start the pilot this summer, but the timing relied on a number of things falling in place.

“Firstly, we need to replicate our results and iron out some inconsistencies in our method that are reducing the consistency of our results, then we need to confirm our on-site methodology, and finally obtain resource consent for the works.”

The council wanted to set up the pilot in Bexley to reduce the carbon footprint but a consent, which was needed from Environment Canterbury, might require an off-site solution.

The project would not require a physical plant to be built, and would not involve large piles of compost being stored, Little said.

The compost would be mixed with a low flat pile of the coal tar-contaminated aggregate.

If successful, Little said it could take less than a year to reduce coal tar contamination levels to below acceptable levels.

He was not sure how much the bioremediation process was expected to cost since they were still working on the methodology but it would be significantly cheaper than the $100m cost of dumping the coal tar.

“We should be able to provide more certainty around this over the coming months.’

If successful, the method could be used on coal tar found in other areas of the city.

Coal tar was adding to the cost of a number of council roading projects. A recent upgrade of Ilam Rd found 2500 cubic metres of coal tar-contaminated material that cost $1.3m to dispose of.