"Catastrophic" consequences of "extensive, irreversible" environmental damage and even loss of life is being forecast from sea level rise breaching a giant landfill including toxic waste at Tiwai Point aluminium smelter in Southland.
The smelter company rejects the findings, but engineers' reports to Treasury say it is "almost certain" the sea will swamp a landfill that contains 250 Olympic-size pools of waste.
That report is looking a century ahead but Rio Tinto plans to leave the smelter in less than four years, and talks on how to clean up the site foundered this month, with the government protesting lack of transparency by the company.
In the reports, a risk assessment on coastal inundation said there were no "existing controls" to deal with it.
"Sea level rise of 1m combined with erosion generating storm surge events will cause an inland movement of the coastline by 100 metres within 100 years, eroding landfill
'berms' and releasing contaminants, causing extensive irreversible environmental damage and health impacts, with the potential to cause fatalities," the report by consultants Aurecon said.
Before that happened, seawater would intrude underground with similar impacts including "potential health impacts to humans with the potential to cause loss of life".
Environment Minister David Parker told RNZ they were serious issues.
But he remained "blind" on the extent of the contamination, despite the 170 pages of reports released to RNZ under the OIA; these went to Parker and Treasury last September-October, and a second tranche of as-yet unreleased advice went to them last November.
"I really don't know how contaminated the site is. I do need to know," Parker said.
The company pushed back, saying it had not see the Treasury reports until RNZ sent them through on Tuesday.
New Zealand Aluminium Smelters chief executive Stew Hamilton said in a statement: "We are very concerned by what we believe are inaccurate assertions and conclusions made by this desktop study".
Their preliminary modelling showed "no impact" from sea level rise on the landfill "within the next 100-200 years", he said.
Parker said the reports did not say "we've got immediate sea level rise around the corner, it says that we've got a future problem".
The edge of the 19ha landfill southwest of the main smelter is within 100m of the beach, with Bluff just across the channel.
The regional council, Environment Southland, said it took sea level rise into account when it issued a 20-year landfill consent in 2003.
The landfill is the second of two major threats at Tiwai Point, in addition to the tens of thousands of tonnes of spent cell liner (SCL) waste that is too nasty to be buried and is piled on a concrete pad also within 100m of the beach with the regulating councils' knowledge, as well as being stored in four sheds.
The company is trying to find a buyer for the 181,000 tonnes of SCL.
It confirmed Aurecon's estimate that about 620,000 cubic metres of waste had been buried at the landfill since the smelter started in 1971.
But there were data gaps - the Treasury reports said the company had not provided landfill records prior to 1992, or for 2003-17.
A Treasury report from 2012 said the company was dealing with the waste by covering it over and revegetating the land - but these latest reports suggest the sea would wash that out.
Hamilton rejected that.
"NZAS would not allow conditions to exist which could result in the landfill being breached, and will take all necessary preventative action to ensure that such a scenario could not occur, including management of the landfill and comprehensive monitoring of coastal erosion," he said in a statement.
NZAS had not been consulted and Aurecon had not visited the site, but it and the government team working on the issue was welcome to, he said.
The smelter's landfill management plan runs to many scores of pages.
The landfill has taken 20,000 tonnes of waste or more in some years, of all kinds; not all toxic but dominated by huge amounts of used refractory material, including heavy metals, toxic fluorides and asbestos.
'We've made no decisions' - Parker
Last month, Parker said he was almost "completely blind" about the residual contamination, and soon after the government suspended talks that began last September with mining giant Rio Tinto.
The reports, released under the Official Information Act, list risk mitigation options for breaches by sea level rise across two pages, but they were redacted.
RNZ has appealed to the Ombudsman to have that reversed.
Parker at first said he could not recall what the options were, then said with regard to remediation options that there was "a large range of cost around them, and we don't want to disclose what that range is to Tiwai Point smelter owners in case we get it wrong".
He later said that in general, the landfill options were to "either dig stuff up and take it away, or you cap it" or contain it in cells.
"We've made no decisions as to what is necessary because we don't know the state of the site."
Aurecon did not know either, its reports were based on "estimates", he added.
Parker read out from Aurecon's report a line that was not in the redacted copy RNZ had, that said "the key activity that we consider is likely to be occurring and does not appear to have been authorised is the discharge of contaminants into land and groundwater".
"I still don't know the extent of that," Parker said.
The government is relying on a $300,000 investigation by Environment Southland to shed more light on the issue.
Parker said he trusted the council to do that, though groundwater monitoring that was overseen by the council did not seem to have been "done to the highest of standards".
"I saw in a report a reference to the fact that there has not been groundwater monitoring on the site, which disturbs me, which is one of the reasons why I have asked the regional council to do that [investigation]," Parker said.
The council said the landfill existed before the Resource Management Act was introduced.
Environment Southland chief executive Rob Phillips said: "The current landfill consent is due for renewal in 2023 and will be subject to landfill rules under our proposed Southland Water and Land Plan and the RMA".
"Addressing the climate change impacts is part of the investigation process."
Under the NZ Coastal Policy Statement 2010, planners must look at least 100 years ahead.
The council refused to release information to RNZ about what clean up maybe required or interventions needed, saying that would likely prejudice the maintenance of law (including the prevention, investigation and detection of offences).
Past problems at the landfill include dross leachate getting into groundwater when it shouldn't have, according to a 2006 council-smelter report.
The Treasury reports showed the amount of waste being buried rose in 2018 and 2019.
The smelter must also now deal with 22,000 tonnes of dross from Mataura and the elsewhere which is being returned to Tiwai over the next four years.
The Treasury reports also showed that the government knew in October the smelter had stockpiled tens of thousands of tonnes of spent cell liner waste it also has to deal with, and that this had previously leaked cyanide and fluoride into the groundwater and sea for years, probably for three decades from the 1970s.
RNZ revealed that publicly last month.
Aurecon estimated there might be 330,000 tonnes of SCL all up.
But the company told RNZ yesterday that it had exported 58,000 tonnes and the total in storage was 181,000 tonnes.
The Ministry for the Environment has said it did not know if it could hold the smelter company legally liable to clean up.
The Environment Protection Authority told RNZ it had no involvement in contamination issues at Tiwai or any clean up, though it got new RMA enforcement powers last year.