The land a controversial chemical plant in New Plymouth is built on should be thoroughly tested for dioxin contamination before ever being used again, an expert in occupational and environmental medicine says.
Corteva Agriscience recently announced it was closing its Paritutu manufacturing facility.
Under previous management it produced the herbicide 2,4,5T which has been linked to health concerns.
From the 1960s through to 1987, Ivon Watkins (later Ivon Watkins-Dow) made 2,4,5-T at Paritutu.
The herbicide was a key component of the military defoliant Agent Orange used in the Vietnam War from 1962, which contained the toxic dioxin TCDD.
Associate Professor Dave McBride of Otago University has studied dioxin levels in the blood of Dow workers and contamination at the former the Fruitgrowers' Chemical Company in Mapua.
Prof McBride said Dow ran a cleaner operation than at Mapua, but it was not perfect.
"In the '80s a disc broke in a valve so they did have a release of Trichlorophenol (TCP), which was the precursor to one of the pesticides and we've shown that the staff there had high levels of dioxin in their blood. There were also some emissions from an incinerator on the site."
A 2002 Institute of Environmental Science and Research report noted that in the mid '80s elevated levels of TCDD were found in the soil on the Paritutu site's boundaries with reserve land and a residential street.
In 1998, similar levels were found on Mt Moturoa some distance away.
Dioxins are insoluble and break down very slowly in soil, if at all.
Prof McBride warned repurposing the 16 hectare coastal site would not be straightforward.
"There should be some soil testing done and any future building on that site should be closely regulated I think, because problems often turn up years afterwards.
"And I think if this land was going to be sold off and it's going to be used for residential, I would worry about that.
"If you look at Mapua as I believe it that remediated land has not been built upon."
The land at Mapua has been turned into a park at a cost of millions of dollars to the Tasman district council.
About $US60 million was spent containing dioxin contamination from a plant that made Agent Orange in Newark, New Jersey, while in Homebush, Sydney more than $A20m was required to remediate a Union Carbide site, and fishing was banned indefinitely in Homebush Bay.
Prof McBride believed the Paritutu site could remain in industrial use.
But Andrew Gibbs, a researcher for Dioxin Investigation Network and CEPRA (Chemically Exposed Paritutu Residents Association) with regard to the historic exposures of primarily historic residents, said there were still information gaps about the site that needed filling.
"We need some transparency about the soil [contamination] levels on the site.
"The historic dioxin levels in the 1980s were quite elevated where there used to be piles of drums stacked on the back of the site, and particularly under the plant where they made 2,4,5T and TCP when they had the high levels in the '60s. There's not any published data on those levels [under the plant] currently."
Gibbs said covering over contaminated areas could be one remediation solution.
"Site's like that have been remediated and used for industrial, but it would probably need some significant capping in some areas.
"If you disturb the dioxin, which binds to soil, it can release it and generally it's a lot better to cap it and contain it especially when you're in the middle of a residential area.
"But in saying that, the site probably has pockets of contamination rather than the whole area."
Taranaki Regional Council director of environment quality Gary Bedford said a 2001 council investigation of five soil and groundwater sites at Paritutu found negligible dioxin contamination.
A 2002 Pattle Delamore report reinforced that finding, and concluded dioxin levels were so low "the Ministry for the Environment and the Ministry of Health see the risk for current and future residents to be so low as to be negligible".
Bedford confirmed, however, there were no records for soil contamination under the building where 2,4,5T was made.
The Ministry for the Environment said there was no requirement for the Paritutu site to be remediated prior to any sale.
It said, if remediation was required, any purchaser would need to be notified and undertake that work.
Corteva, which took over Dow last year, said no decision had been made on the site's future.
It said it would work with authorities to ensure the site was managed appropriately whatever use was decided upon.