About 780,000 litres of viscous tar phase material are planned to be removed from the tar well in Hillside Rd, treated and stabilised on site and disposed of at the Smooth Hill landfill in the hills above Brighton.
City councillors will consider an update report on tar well remediation at tomorrow’s meeting.
The well collected tar-contaminated water used to remove impurities during the gas-making process at the former Dunedin City Gasworks, which operated from 1863 to 1987.
The viscous tar phase material contains known contaminants such as volatile organic compounds, cyanide and ammonia.
Property services group manager and report co-author Anna Nilsen told the Otago Daily Times the timing of intended remediation would align with the planned opening of the Smooth Hill landfill in 2029/30.
A working group established in 2021 — comprising Dunedin City Council, Otago Regional Council and Ministry for the Environment staff and a remediation contractor — fully considered other disposal options, Mrs Nilsen said.
The group "found [Smooth Hill] to be the most suitable — it's a modern facility that's capable of taking waste", she said.
The preferred remediation method involved removing all liquid and tar from the well, but retaining the below-ground structure, which would later be filled and sealed.
In 2024, early cost estimates for the physical work sat between $7.2m and $8.5m, although Mrs Nilsen said construction costs did increase over time.
Three years of detailed planning were expected before physical remediation, expected to take six months, began.
Operational funding of $1m was included in the council’s 2033/34 property services budget — onsite work was expected to begin by 2035/36.
Three Waters and transition general manager David Ward said while the council had a base methodology, largely the cost would be "establishment and the particular equipment used".
"That may change, and it may change to the positive."
Contamination was present in the city block around the tar well, although this was from the former gasworks site — Mrs Nilsen said there was no suggestion the tar well was leaking.
"The site's really well managed as it is currently and we will continue to monitor for any changes over the time in the interim," she said.
"Whether that gets reviewed in the next [10-year plan] or not, we will actively and responsibly look after it.
"[The tar well’s] been there a long time so far, so it’s really prudent that we do consider remediation and I’m really glad we’ve set out on this work and got support from everybody that’s involved, but right now nothing’s showing it’s drastically changing."
In the past, there were concerns it was not possible to remediate the site effectively or safely, she said.
"I can tell [councillors] now we can fix this, we can remediate it. It’s just a matter of timing, but it’s totally possible."
About 2200 tonnes of contaminated material, or up to 4400 tonnes of stabilised contaminated material, would be disposed to Smooth Hill.
The material would be treated to meet Smooth Hill’s discharge conditions of "treated hazardous waste", the report said.
"We know how we’re going to do it, we know we can do it, but getting into the minutiae detail we’ll work through next," Mrs Nilsen said.
Another 400 tonnes of contaminated water, which entered the well though its leaky lid, would be trucked to Christchurch’s mixed liquid tar disposal facility — water was removed in this fashion annually.
