The Dunedin City Council this month declined to release two reports into the poisonous leftovers from Dunedin’s former gasworks underground at 45 Hillside Rd.
The council also declined to release more than 2000 emails since 2021 related to the matter — due to the amount of work it would take to collate the material.
Three years ago, the council said it expected to spend more than $2 million last financial year to clean up the hazardous mess, but to date there is little evidence of any progress towards the cleanup.
In June, the Otago Daily Times informed the council the newspaper was seeking details about the progress the council had made towards understanding the scale of the problem and steps towards a solution.
The response came this week — the bulk of the request was declined.
One of the withheld reports contained "factual inaccuracies", the council said.
The other was in progress and yet to be formally considered — but it might be released at a later date, the council said.
The council did produce four high-level reports it had commissioned since 2021.
The most substantive of those reports was an update on the interim site management plan prepared by Tonkin + Taylor in May this year.
The council was investigating options for the remediation of the site, the report said.
In the interim there were measures to ensure the risk to human health and the environment was "minimised", the report said.
"The tar well contains a stratified mixture of heavy end and volatile tar and water phases.
"Tar sits at the base and contains heavy end petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAH) phenols, monoaromatic hydrocarbons such as benzene and ethyl benzene along with naphthalene.
"Ammonia and cyanide may also be present."
It said the underground well in Hillside Rd was believed to be filled with about 800cu m of tar and toxic chemicals.
Despite being covered in chip seal in 2005, the 22m timber tank lid was known to leak, the report said.
Council water-level monitoring indicated the water level in the well sat above ground level, the report said.
This was taken as evidence the tar well was not leaking.
It was also taken to show the water accumulating in the well was entering through the lid, or through the thinner well walls that extended above ground level.
The monitoring data further suggested about 60% of the rain that fell directly on the tar well was entering the tank, the report said.
"This appears consistent with entry of rainfall through the timber plank roof structure cover or edges rather than deeper inflows."
While levels of cyanide and ammonia were found in the groundwater at higher levels than some of the other surrounding wells in the monitoring network, "an absence of floating or submerged free product suggests the tar well structure is containing its tar".
Soils near the tar well might contain ammonia, cyanide, arsenic, volatile organic compounds (VOCs) and PAHs, the report said.
A well installed next to the tar well in 2014 — and now sampled annually — showed "a typical gasworks-related" profile which included high levels of ammonia and cyanide in the groundwater.
No personnel access on the tar well lid was permitted under any circumstances.
Council staff were to inspect the site every six months.
But, also, if there was a 100-year rainfall event, or flooding event, or something such as a car crash or earthquake that might damage the well, an inspection would be required.
The Dunedin City Gasworks made gas from 1863 to 1987, the report said.
The bulk of the land from the former site was sold by the council, but the council kept the ownership — and therefore the liability — of the tar well.
Remedial works were completed in 2005 to "seal and provide additional security for the structure".
In September 2021 the council received $200,000 from the Ministry for the Environment’s contaminated sites remediation fund to conduct a feasibility assessment and create a remedial action plan for the site.
In 2017, the council received about $20,000 from from the same fund for costs associated with a feasibility study for the site’s cleanup.
