Toxic waste has been uncovered in at least 12 Nelson-Tasman sites, including some now used for housing and recreation, according to a secret 1996 report.
The chemical waste was dumped by the former Fruitgrowers Chemical Company (FCC) and includes truckloads of faulty chemicals, sump waste, packaged DDT and crushed drums.
The dumping took place from the early 1950s to when the Mapua chemical plant closed in 1988.
It occurred with the knowledge of the former Waimea County Council, Nelson City Council and Richmond Borough Council.
The contamination has been revealed to Tasman District councillors in an until-now confidential 1996 report compiled after 77 former workers, drivers and landowners were interviewed.
The report, obtained by The Press under the Official Information Act, identified toxins leaching into ground and surface water, buried chemicals and discarded drums.
Tasman District Council resource scientist Jenny Eastonsaid the report was not made public because the council had not completed investigations and it was council policy not to publicly disclose its contaminated-site register.
The dumps were not included in the FCC site clean-up as the mechano-chemical dehalogenation (MCD) reactor could process only pesticides, not herbicides, Easton said. Some of the sites had since been cleaned up, but most had been capped, contained and monitored as the most effective management of the environmental risks posed.
Ten dump sites were identified in the report. Two more contamination sites have since been found, one of which, a pallet of packaged DDT, was found buried in sand dunes next to a stream near the lagoon by the Mapua causeway last year. The former FCC site is the most contaminated pesticide site known in New Zealand.
It was used by the company from the early 1930s and produced tonnes of pesticides, such as DDT, DDD, aldrin, lindane and dieldrin, from the late 1940s until it closed in 1988.
The $8 million clean-up of the 5.5ha site, which started in August 2004, has uncovered hundreds of small bags of organochlorine pesticides and toxic soil.
The 10-page 1996 report noted leachate problems and contaminated ground and surface water at a Redwoods Valley farm where 10 to 12 truckloads of waste were dumped in pits.
Contaminated soil and seepage was found at FCC's old herbicide plant bordering the Waimea Estuary.
The report noted organochloride, organophosphate and herbicide leachate from two chemical waste pits at the Mariri transfer station.
A 30m by 10m gravel pit in Tahi Street, Grossi Point, next to the FCC site, was used to dispose chemical waste during the 1950s, the report said.
Easton said the Redwoods Valley site had been cleaned up by the current landowner and FCC's old herbicide plant had been capped and cleaned up to residential standard by the current landowner.
The Tahi Street area had been capped and contained, as had the hazardous-waste trenches at the Mariri transfer station.
The old Mariri tip, Beach Road transfer station and former Founders Park dump in Nelson had been capped and tested for residues, she said.
No contamination was found on Rabbit Island or at the old Ngatimoti tip, where excavation revealed rusted chemical drums.
Stream monitoring of run-off from the suspected Takaka Hill quarry dump showed healthy aquatic life and no contamination, Easton said.
Easton said all properties affected were on the council's contamination register and the sites were noted on individual land information memorandum reports. Those properties that had not been cleaned up had individual management plans. It was possible further chemical dump sites could be uncovered as FCC managers had burnt company records when the factory closed.