The Marlborough District Council and an agency working for the Conservation Department have been dumping 1080 poison baits at Blenheim's Taylor Pass tip since 1998, records show.
Blenheim resident Arthur Pillans and hunter Lloyd Hanson are among those who say they are concerned about the quantities they believe had been buried at the landfill and are worried the disposal was not safe.
But the district council and the department disagree.
Poisoning programmes by the Animal Health Board - working through local councils - and the department use about 80 per cent of the world supply of sodium fluoroacetate, the United States-manufactured poison which is the active ingredient in 1080.
The department uses the poison against possums destroying native forest and preying on nesting native birds.
The Animal Health Board uses it to control possums as carriers of bovine tuberculosis.
Both organisations have increasingly come under attack from community groups opposed to the use of 1080 - particularly recreational hunters critical of aerial drops which they say kill other pests such as deer and wild pigs.
Other critics include dog owners whose pets are vulnerable when scavenging poisoned possum carcasses, and some rural residents who worry 1080 could contaminate drinking water.
Marlborough council's operations and maintenance engineer, Stephen Rooney, said that since February 2000 about 3 tonnes of 1080 packaging and pellets past their use-by date had been dumped.
Mr Rooney said material was not accepted if there were any doubts over its suitability for the landfill, and materials prohibited within the council's consent were not accepted for disposal.
The council had in the past turned down an application to dump a load of 1080 because of its high toxicity and the fact that it was still usable.
The material was being sent back to the manufacturer for reuse.
Mr Rooney said the combined operation of the landfill and sewage treatment system assisted the breakdown of waste.
The 1080 was not disposed of in stage one of the landfill, an area where the leachate drains had been compromised during construction.
It was discovered in 1997 that leachate from that part of the Taylor Pass landfill was seeping out of the area meant to contain it.
All the 1080 had been disposed of in stages two and three of the landfill, where at least 3m of refuse existed between the disposal area and the leachate collection drains beneath the landfill.
"All the leachate is collected and discharged to the Blenheim sewer network," Mr Rooney said.
He said 1080 was disposed of by spreading the pellets across the working face of an approved landfill, increasing its exposure to air, moisture, soil and sunlight - all elements which helped break it down.
The working face was then covered before the end of the working day.
He said access by the general public was controlled.
- NZPA
nzherald.co.nz/environment
Tonnes of 1080 at Marlborough District Council tip
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