Council staff collecting samples of the river that has been contaminated by an orange substance. Photo: Supplied / Waikato Regional Council
Elevated levels of arsenic have been detected in the Ohinemuri River, after it turned bright orange last week, the Waikato Regional Council says.
The council has been testing the water in the Karangahake Gorge since it changed colour on Friday.
An old mining shaft was confirmed as the cause of the changed colour.
Ohinemuri River Photo: The Valley Profile/Davidda Hikatangata
The arsenic content was well within drinking water and ecological protection guidelines for fresh water, the council said in a statement.
High levels of iron in the sediment accounted for the bright orange colour.
Environmental chemistry senior scientist Jonathan Caldwell said the minerals found posed a less immediate risk to fish in the water, but could still be a longer term risk in downstream areas where sediment built up.
"So far, the preliminary sampling results have not detected the presence of cyanide which was often used to help separate out the gold and silver from the mined materials," he said.
"The results also show that the water was not very acidic, which also reduces the concern around short-term effects on fish."
Council staff carry out tests at the site of the old mine. Photo: Supplied / Waikato Regional Council
Regional compliance manager Patrick Lynch said staff were working with the Department of Conservation (DOC) on the issue.
"It's quite obvious that a lot of people have been coming here and entering the mine," he said.
"There is still more sediment coming out, and if we get more rain then we can expect to see more orange plumes in the river."
Lynch said there was no evidence to suggest the buildup of sediment was due to a manmade structure.
He said it appeared the water leaving the mine cut its own natural channel, which was much deeper on Monday than it had been on Saturday.