A project to dredge the Woolston Cut has been delayed until December because asbestos was found in the silt.
City council land drainage manager Keith Davison said as part of the investigative and pricing study, the results of testing found asbestos in some samples.
“A specialist company was then engaged to carry out more extensive and detailed testing, which confirmed low levels of asbestos in some locations and at different depths within the sediment, suggesting historic contamination,” he said.
It was originally hoped the dredging would begin this month.
Mr Davison said due to the contamination, it would now begin in December, and should be complete by March.
It was also expected to be slightly more costly than the original $2.3 million budget, he said.
“The removal and disposal of contaminated silt is more
expensive, but as the majority is now believed to be uncontaminated the increase is much less significant than originally understood.”
The cut was built in the 1980s to prevent flooding in the lower Heathcote River area, and for other catchment-wide improvements, such as the upper Heathcote detention basins.
Water from the Heathcote flows into the cut, which can then be opened during floods using the tidal barrage.
The city council decided to dredge the cut so it continued to provide effective flood remediation. It was currently about 50 per cent full of silt and liquefaction.
Mr Davison said contamination mitigation strategies had been developed to ensure contaminated silt would be removed and disposed of safely and at the right facility.
“The majority of the silt to be removed is not contaminated and will be disposed of normally,” he said.
He said while the source of asbestos was not clear, it was not thought to be recent.
“The localised nature of the contaminated sites might suggest historic dumping of materials into the cut.”