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The Department of Conservation says it has succeeded with a $2.6 million clean-up of the second most arsenic-contaminated site in the world, at Waiuta.
Geologists had assessed the old Prohibition Mine as the most toxic mine site in New Zealand, with arsenic levels among the highest recorded in the world and second only to a site in California that is also being remediated.
Doc said the site posed a high risk to people as contact with soil or water could have caused acute or chronic poisoning, as well as adverse effects on aquatic ecosystems from water runoff.
The highest ground residue arsenic concentrations ranged between 20% and 40%.
The site was reopened to the public in December 2016 and senior ranger Gavin Collis said there had been a steady decline in arsenic levels.
Just two sites showed a spike in arsenic levels. During the clean-up phase contaminated water from the site was collected downhill from the site.
Doc Western South Island director Mark Davies said the department was also assessing other sites, such as the Snowy River battery, for possible remediation work, but as yet no decisions had been made on available funding and timing.
West Coast Conservation Board chairman Mike Legge said the clean-up had gone "amazingly well".
"Looking at the figures they have produced, it's (arsenic) down to practically nothing."
- By Laura Mills