The river turned orange with contamination from a historic mine on Friday. Photo: The Valley Profile/Davidda Hikatangata
Arsenic continues to be the most problematic element in the Ohinemuri River.
The river turned orange with contamination from a historic mine on Friday.
Waikato Regional Council said it had tested for all 30 heavy metals, and results showed no cyanide or mercury contaminates. However, there were minor detections of antimony and thallium, and copper and zinc at elevated levels.
Regional compliance manager Patrick Lynch said the arsenic levels, while still elevated, had dropped significantly compared to the levels detected in samples taken on 23 August when the orange bloom appeared.
He said there was still sediment built up at the mine entrance, and a lot of sediment had accumulated in the stream below the entrance which flows directly into the Ohinemuri River.
This means more orange plumes can be expected during rain events, although it is unlikely the plumes will be of the scale seen last week.
Lynch said that if people see orange water and sediment, they should assume it is contaminated and not touch it.
"Clear water is good - the monitoring shows the levels of arsenic in the clear running water is well within drinking water and ecological protection guidelines for fresh water," he said.
Senior scientist (Environmental Chemistry) Jonathan Caldwell said the greatest health risk regarding the elevated arsenic levels in the sediment was from ingestion (eating or drinking it). However, due to the high iron content of the sediment, the arsenic will be well bound up and is more likely to be excreted than taken up by the body.
"Arsenic can make its way into the food chain via uptake by fish and shellfish, but it typically converts into a less toxic form," Caldwell said.
Arsenic is typically not absorbed very well through the skin.
"This route of exposure to arsenic would increase if you had broken skin or an open wound," he said.
Antimony and thallium are typically associated with gold mining. Copper and zinc were at levels similar to what could be seen in urban storm water.
The sediment involved comes from a mine entrance associated with the Comstock Low Level Drive, an exploratory mine started in 1896 and which never proved profitable. Mining at the drive site stopped in the early 1900s.
Waikato Regional Council compliance manager Patrick Lynch told Checkpoint people should treat orange water as contaminated and not touch it.
"We've got a bunch of sample results back now, they've just confirmed what our preliminary results were telling us, the sediment is high in arsenic, that's really the only thing that we've got any more concerns about there," he said.
Lynch said there are other levels of metals in the background that are elevated, but not to any level of concern.
"It's really about the arsenic. The warning that we have been giving over the weekend during the event is about staying out of any orange plumes, staying out of any orange sediment that that still stands," he said.
"There's still quite a lot of sediment up there. There's a large plug of sediment which is sitting immediately beneath the mine shaft opening and it's sort of a little bit suspended there so over time that's going to wash away the stream, it goes down quite a few metres down into a stream, that stream in places is quite heavily choked with that orange sediment."
He said for members of the public that are visiting the Karangahake Gorge, they will be able to see orange material coming out of the stream for some time.
Water running through the mine slowly brought the orange sediment out, he said.
"There's a tree down in front of the mine shaft and then what we think is that over time that's caught foliage, caught sediment, it's sort of built up somewhat like somewhat of a natural dam."
The sediment over time had slowly built up behind the dam, he said.
"For whatever reasons, this natural dam failed or at least partially failed last Friday and that's when the very large volume of material would have gone down the stream out into the river and turning it orange," he said.
He said there was evidence that people had been going into the mineshaft.
"It's open, people clearly know it's there, we understand that there's quite a bit of interest actually, like historical interest. We know people have been down and clambering around," he said.
Regarding whether any people had a part in the dam giving way, he said the council had opened a few avenues of inquiry they are following.
"We certainly haven't found anything to suggest that this has been a man-made structure that has somehow been let go intentionally by anyone."
He said as part of the inquiry, they would not go into the mine itself due to safety concerns.
"We could really only see the first, maybe 10 or 12 metres of it, it's a concrete structure with a very clear arched roof and you can see where the sediment has been. It's left a tide mark up the side of that wall," he said.
Expect to see more orange
When it rained next, and the heavier the rain was, the more sediment would be picked up and carried into the river, he said.
"That's why we're letting people know they can expect to see further orange clouds, pluming clouds coming out into the river. But we wouldn't think it would be at the scale that it was last Friday."
He said due to the chemicals in the water and the arsenic settling in the sediment, there was a risk that shellfish and finfish would take in arsenic.
"Our scientists tell us that arsenic tends to be less toxic when it is taken up in that form, and it also doesn't bioaccumulate, so it doesn't make its way up the food chain like mercury, where it sort of increases in concentrations as it makes its way up through the food chain."
He said they were telling people to stay away from the water.
"If it's orange, stay out of it. If the sediment clay type material they can see in that stream is deposited and placed down the river, we're saying to just stay away from it."
He said precautions had been issued to downstream users through to the aquaculture industry out in the Firth of Thames.
"All we can really do is inform people of this as it's happening and just to take precautions."
