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This includes spent cell liner (SCL) waste that contains cyanide and toxic fluoride, and is banned from being buried untreated in both the US and Australia.
The warning last October from the compliance section of the regional council Environment Southland is in documents released to RNZ under the OIA.
"Former staff of the smelter report burying of spent cell linings or contaminated material in various parts of the Tiwai site," said the report into "key matters" for cleaning up the huge site next to conservation land once the smelter shuts in 2024.
Waste burial reportedly went on "particularly prior" to the Resource Management Act's enactment in 1991, by which stage tens of thousands of tonnes of hazardous waste had already been produced.
The report followed this with a warning: "It is likely that a number of unmapped or unconsented contaminated sites exist as a result of these uncontrolled activities."
The government has said it is "blind" on contamination at Tiwai, and abruptly called off talks with Rio Tinto last month, until the company gives it more information.
The Rio Tinto-controlled New Zealand Aluminium Smelters (NZAS) company is also reportedly not sure what it will find underground, though its pollutant discharges have been consented, at least since the 1990s.
NZAS and the government have parallel investigations going on to uncover what is there.
The smelter company denied any material had been buried.
"We are unaware of any material being buried at NZAS, in unmapped or unconsented sites," chief executive and general manager Stew Hamilton said in a statement.
RNZ asked Environment Southland what it is doing to ascertain if the reports of buried, unmapped waste are true, but it hasn't responded.
The documents also show that engineers recently discovered that sheds containing masses of toxic SCL waste are "structurally weakened".
This has put a halt "for a few months" to monitoring groundwater around the sheds for contaminants, a December 2020 council report said.
The floor of the first shed built in 1992 cracked almost from the word go. Contaminants have from then till now leaked into barrier membranes in the special foundations, as RNZ reported from previous OIA releases. Four sheds hold in total 75,000 tonnes of the waste, the company has said.
The council's October report noted the area around the SCL storage pad "has a legacy of poor management and may have residual groundwater contamination of fluoride from the failure of the storage system in the 1990s".
The regional council has consistently praised the smelter's cooperation on environmental issues over the years.
But now its report lists two issues of "high concern": The SCL storage pad that it notes is "susceptible to coastal erosion"; and the leachate from the landfill getting into groundwater and Foveaux Strait.
"Some groundwater monitoring bores on the south side of the landfill show some high levels of contamination for a number of contaminants including several nitrogen species and fluoride, and minor levels of heavy metals and polycyclic aromatic hydrocarbons," it said.
The smelter company said it operated in compliance with all standards set under its resource consents as monitored by Environment Southland and "conducts a range of monitoring, often in excess of those required". It said it provided reports to Environment Southland every quarter.
Hamilton said the smelter had operated at Tiwai Point for 50 years and during that time environmental monitoring regimes had evolved.
The company acted immediately to remedy any past non-compliances and will close the smelter in a responsible manner, including removing all spent cell lining from the site, Hamilton said.
"We want to make sure the community is confident in the way we deal with any contamination on the site, how waste will be managed and disposed of and address any environmental concerns about an operation that has been part of the community for 50 years," he said.
"We recognise that over time, community and regulatory expectations evolve, and we will ensure our closure processes and outcomes are transparent and meet the needs of today."
It has stockpiled 180,000 tonnes of much more toxic spent cell liner waste, and has exported 58,000 tonnes of it, it said; however, production figures suggest a further 20,000 tonnes of SCL may have been created than accounted for in these figures, and RNZ has queried this with NZAS.
The smelter was allowed to store up to 250,000 tonnes of SCL waste, the city council told RNZ.
The European Union describes SCL waste as "carbon-based linings and refractories from metallurgical processes containing dangerous substances".
Government looking at stronger laws
The long-term risk is from sea-level rise, with engineers advising the government the sea will breach the smelter landfill within a century, releasing toxic material that will damage the environment and even kill people. The smelter rejects this advice.
A powerless-looking government is now looking at law changes to force polluters to clean up.
"I have asked officials for advice on including a clear hierarchy to attribute liability for contamination in legislation or regulation, either through the current RM [Resource Management] reform process or through separate stand-alone legislation," Environment Minister David Parker said in a statement.
In contrast, Australian states have laws to enforce clean-ups, recently deployed in Victoria to ensure Alcoa pays for what is turning into a decade-long clean-up at the defunct Port Henry smelter.
Clean-up order legislation of this type "does not currently exist in New Zealand", the Environment Ministry told RNZ.
The government provided indemnities in 2003 and 2004 over any remediation NZAS may owe for cleaning up relatively minor amounts of dross.
In Southland, the regional council is leading the investigation though activists have accused it of taking a light hand with the smelter for many years.
Neither Environment Southland nor Invercargill City Council has a register of the amount or types of hazardous substances stored.
As part of the investigations, the company has this year been drilling a huge number of new bores - about 240.
It relies on self-monitoring just eight existing bores to know what is happening under the sprawling landfill.
Though the water flows south/southeast towards Foveaux Strait, five of those bores are on the upstream north or west side, and only three on the downstream side to the south, by the beach.
Annual reports of about 100 pages each to the regional council about environmental monitoring, give "typical" levels across a range of analytes in groundwater, including 1-10 grams per cubic metre (gm3) for fluoride, 1-6gm3 for "total nitrogen ", and 15-40gm3 for sulphate.
Fluoride - 57gm3 (bore A6) and 77 (bore A24)
Total nitrogen - 20 and 200
Sulphate - 780 and 90
Nitrogen in bore A24 was trending down but sulphates up; nitrogen and fluoride in bore A6 were largely steady, but sulphates dipped, then doubled by 2019.
By comparison, water in New Zealand is fluoridated to 0.7-1.0gm3, and the US has standards for naturally occurring fluoride in drinking water of 2-to-4gm3.
High concentrations of fluoride can harm human health.
Typical sulphate levels in fresh water are 20gm3; seawater has 2700gm3.
An aerial photo of the landfill; this shows carbon and Haysom’s dross waste, which contains toxics material, is close to the beachAn aerial photo of the landfill; this shows carbon and Haysom’s dross waste, which contains toxics material, is close to the beach Photo: SUPPLIED/ Environment Southland
NZAS' reports on its bores contain commentary that quite often contradicts the measurements in key ways.
For example, the 2019 report said fluoride at one southern bore had stabilised at 50gm3 "for a few years", when in fact it averaged 77.
In 2016, the measurements for two separate bores were exactly the same, as if copied over.
In 2017, fluoride was noted to have had a "slight increase" at bore A24, when in fact it rose almost 40 percent.
By contrast to the landfill bore data, the reports contain no measurements for the bores under the SCL pad or storage sheds.
Instead, each annual report said virtually the same thing - that monitoring showed "similar levels" to the year before, without saying what those levels were.
Other reports RNZ has obtained show that concerns existed in 1990 about SCL leachate getting into water-extraction wells. Since 1995, this leachate has been collected and treated, then discharged into the sea, where the cyanide in it disperses.
The permitted level of cyanide discharge into the sea at Tiwai (20gm3) is four times higher than India's limit on cyanide in smelter leachate.
Reports also showed NZAS had used patchy ways of measuring or recording such things as saltwater intrusion.
Long, complicated clean-up
The regional council documents show the smelter's plans at the point in late 2020 when it expected to shut the smelter in August 2021 (a date pushed back to December 2024 under a deal with power supplier Meridian).
It had been planning for a "make-safe" phase of five months.
Only after this were "detailed plans for the future of the site" to be drawn up.
These included "final site investigations as well as marketing the site for sale (approximately a year)".
"Once the final land use and the state of the site's environmental legacy is determined, a plan for decommission will be made. At this stage there is no final closure plan," the report said.
The Port Henry smelter clean-up in Victoria shows what could be ahead at Tiwai: It is long and highly complicated - but because of much tougher laws there than here, there is a lot more public information that must be made available.
At Port Henry in December 2014 the groundwater was found to be so polluted it was "detrimental to any beneficial use".
Victoria is toughening its clean-up powers still further, this July, under what its Environment Protection Authority calls the "biggest change in 50 years to Victoria's environment protection laws".
Rio Tinto co-founded the aluminium stewardship initiative (ASI) in 2012 and was the first to be ASI certified, including for its Tiwai Pt smelter.
The certification said it "provides proof of responsible production, responsible sourcing and material stewardship".
About half of SCL waste worldwide is estimated to be landfilled.
However, international guidelines say this is the least preferable disposal method, and that it should be treated first. The US has banned its untreated burial since 1998, and it only goes into landfills that are double-lined and have two levels of leachate collection, plus leak detection.
Even treated, it is still classified as a hazardous waste due to its caustic nature.