Long ago, the Ohinemuri River gauged out the Karangakahe Gorge, between Waihi and Paeroa, on its way to the Waihou, which flows into the Firth of Thames. Last month, the Ohinemuri became “the orange river”.
It was also notorious during the area’s gold mining heyday of the late 1800s, when it was officially a sludge channel. That meant it could be used to dump tailings – the toxic material left over after gold is extracted from rock.
But tailings didn’t cause the orange river, says Waikato Regional Council senior scientist and environmental chemist Jonathan Caldwell. “The evidence we’ve got is that it’s not tailings, it’s mine waste [from the Comstock mine in the gorge]. It’s the material that’s left after they hewed out the tunnel.” The ore would have been processed at the nearby Crown Battery then discharged to the river.
Tailings, he says, would have shown up as elevated lead, mercury and other heavy metals in the council’s sediment and water testing, but these were only at background levels. The most concerning finding was high levels of arsenic, a metal often found alongside gold in rocks. “When people are mining for gold, there will be lots of arsenic around as well,” says Caldwell.
Fortunately, arsenic binds to iron, making it less bioavailable. It was oxidised (rusted) iron sediment that stained the river orange. This appears to have happened when sediment build-up at the mine outlet was released abruptly. Only the sediment showed high arsenic levels. “We saw very little arsenic in the dissolved phase of the water. If you have a high dissolved level, that’s where you start seeing real risk to aquatic organisms.”
The sediment will settle, mostly far downriver where the flow slows. It will join sediment rich in arsenic and lead left by historical mining. There’s potential for these metals to be released from their protective union with iron if water conditions change, says Caldwell, such as a drastic change in pH or dissolved oxygen levels.
The sediment may harm creatures living within the riverbed, and fish could eat them. But that’s not necessarily concerning, except for the benthic creatures themselves. “One of the mitigating factors with arsenic is that it tends to convert to a less toxic organic form within fish,” says Caldwell. “In humans, it’s excreted reasonably rapidly. It’s not something that tends to accumulate in the body. The bigger risk is getting a lot of doses over a long period of time.”
To Caldwell’s relief, the water wasn’t overly acidic. Gold-containing rock is often rich in sulfide minerals, which form sulfuric acid when exposed to water and air. The acid dissolves heavy metals from rock. “Our first concern was that if it was mine drainage and tailings, it would be very acidic, and we would expect significant fish kill.” But there’s no evidence of that. Caldwell suspects any sulfuric acid leaching is either very slow or occurred long ago.
Overall, he is relieved. “It’s been lucky. This could have been a lot worse.” A few kilometres away lies a substantial riverside impoundment of toxic mining remnants.
All mines produce tailings that are contaminated in perpetuity. What protects us and the environment? Modern tailings storage is engineered far more safely than in the past, and a global industry standard on tailings management was created after some catastrophic tailings collapses overseas.
But there’s no legal obligation to follow the standard, says Barry Barton, a professor of law at the University of Waikato. “We need better provisions to deal with large, long-term projects, and to modify the details of the project,” he says.
“The Resource Management Act pays attention to whether a resource consent should be granted, with conditions, and then it’s all over. It becomes a matter of compliance, monitoring and enforcement by regional councils.”
That poses a considerable burden on those councils, he says. If there’s a tailings failure after a mine is closed, there will probably be no bond in place to finance the clean-up.
