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Testing Tailings

The long-term environmental effects of mine wastes in Otago will be manageable, with the most common arsenic and mercury minerals decomposing only slowly, according to a study by Otago University researchers. The study has focused on arsenic and mercury in old and active gold mining areas of Otago.

"The combination of Otago's dry climate and underlying schist rock ensures that ground-water remains near-neutral, and acidification of mine wastes is likely to be minimal," programme leader Dr Dave Craw says.

Otago's environment is unique.

"It's therefore starkly different from many overseas sites where environmental problems have been encountered."

The research provides a useful scientific background on which to base decisions about long-term storage of mine tailings from gold mines in Otago. This study looked at a recent mining site so that results could be compared with old mine tailings.

"This one was a large pile of arsenic-bearing mine tailings constructed by Macraes Mining Co Ltd in 1990, but abandoned in 1993. This provided an excellent and unique opportunity to examine mine tailings four years after deposition," he says.

Most such sites around the world are quickly cleaned up and are lost for scientific study. Research found that theoretical and laboratory results were different from what the ground was revealing.

"Arsenic is therefore not as significant a pollutant as laboratory data would suggest," Craw says.

Nor is mercury. Heavy metals are concentrated in the fine-grained portions of soils on iron oxides and clay minerals, and arsenic forms the secondary mineral scorodite.

"These sites weaken arsenic strongly, and help to keep arsenic contents of polluted water down to four parts per million. This is well above acceptable water quality."

He says that high arsenic contents are localised. Dilution in main streams ensures that water quality can be kept to acceptable standards.

"This research provides a useful scientific background on which to base decisions about long-term storage of mine tailings from gold mines in Otago. Results are being used for that purpose."