Saturday 31 December 2011

Acid spill from gold mine adds to Edith River's woes.

Heavy rainfall associated with Cyclone Grant (now downgraded to Tropical Storm Grant) has caused serious problems along the Edith River in Northern Territory, Australia. There has been widespread flooding, road and rail links have been cut, and a train derailment has lead to a spill of toxic copper concentrate. Now news has emerged that the rainfall has caused a retaining pond at the Mount Todd Gold Mine to fill up and overspill into the river.

The Stuart Highway near the Edith River.

There are typically two types of ponds at gold mines, settling tanks in which ore is deliberately mixed with chemicals that help extract the gold (typically cyanide and mercury compounds) and retaining ponds holding water drained from the mine. The pond affected appears to have been of the latter type, which would be less of a problem, but is still potentially nasty. A mine in the (high precipitation) Northern Territory is likely to produce a lot of water, which will have been contaminated by passing through metal rich deposits. The mine operators, Vista Gold, have conceded that the water is acidic, but have declined to release the results of tests on the water, saying this would 'confuse' the public.

The pond has a history of releasing sulphuric acid (H₂SO₄) and heavy metals into the river, leading to fish kills, and there is no reason to suspect that this spill will be very different. Sulphuric acid is common in wastewater from mines, formed when water reacts with sulphur compounds in the metal-bearing rocks. Retaining ponds containing sulphuric acid are commonly treated with calcium carbonate (CaCO₃; usually powdered limestone) in order to neutralize the acid. The nature of any metals in the water is less clear; iron and copper compounds are common in gold mine runoff, and aluminum, mercury, lead and cadmium are known at other mines.

On Thursday (28 December) the rate of water running out of the pond was estimated at 1700 m² per hour (1 700 000 liters per hour) and a spokesman from the company stated the water had a pH of 3-3.5 (roughly the equivalent of lemon juice or vinegar), though it was unclear if this figure related to the pool before the rains, the pool after the rains, or the river.

Toxicologist Barry Noller of the Centre for Mined Land Rehabilitation at the University of Queensland has observed that there is a danger of the released water reacting with copper concentrate spilt by a train derailment at the Edith River Crossing. The owners of the spilt material, OZ Minerals, have undertaken to clean up any material remaining at the sight, but admit that there is little they can do about any dissolved material entering the river system. The spill apparently contained copper metal, which is insoluble, copper sulphate, which is soluble, and iron sulphate, which is also soluble; all of these are toxic. Any copper and iron sulphate is likely to have been washed away already, copper could react with the dissolved sulphuric acid to form more copper sulphate, which would then be washed away.

There has been little official response from the authorities in the Northern Territory, which has been criticized by Opposition Minister for Natural Resources, Peter Chandler (Country Liberal), who has accused the Northern Territories Government of being unaccountable and helping the company to keep information from the public.