Keith Faulkner, of Ok Tedi Mining Ltd., confirmed that company executives saw the effects of acid rock drainage along the Middle Fly River in the western province last year.

Acid rock drainage usually exposes sulfide-bearing tailings to air and water, forming sulfuric and other acids. These acids then dissolve metals such as lead, copper, arsenic and mercury into the river, poisoning ground and drinking water.

The mine has been dumping up to 80 million tonnes of tailings and waste rock every year for the past 20 years into the Ok Tedi and Fly River systems, with flooding causing forest dieback over 1,500 square kilometres.

But Faulkner said acid formation had now killed off large patches of vegetation.

"The dredged sands that have reduced the die-back, are presenting long-term management problems with acid rock drainage from sulphur content in the sands and the potential to release copper and other metals into the river, particularly under certain climatic conditions," he told AFP this week.

The OK Tedi mine was previously majority owned by Australian mining giant BHP Billiton, but the company sold its 52 percent stake to largely Papua New Guinea entities because of concerns over its environmental impact.

The largest private listed stake holder in the mine is now Canada's Inmet Mining Corporation which holds 18 percent.

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