An investigation into the discharge of orange sediment into the Ōhinemuri River in Waikato has found “no clear evidence” of culpability – other than it being the result of historic mining activity.
The river turned orange on the afternoon of August 23 when the sediment flowed into the river.
At the time, council officials feared the discoloured water was due to an old mine shaft collapse, which could send heavy metals and contaminants into the river.
After an investigation into how the sediment entered the river, the Waikato Regional Council said there was “no clear evidence of culpability other than it being the result from mining activities more than 100 years ago”.
Regional compliance manager Patrick Lynch said there was no evidence of any manmade structures in front of the mine entrance that could have caused the buildup.
Lynch said the sediment was discharged from a mine associated with the Comstock Low-Level Drive. The mine was used as an exploratory mine between 1896 and the early 1900s but was never profitable.
“There’s a tideline inside the drive that shows the sediment had built up to a depth of about 96cm,” Lynch said.
“The mine entrance is in the face of a steep bank that drops away to the Nohopane Stream and this entrance has, over the years, become overgrown and covered with branches and foliage.
“This vegetation created a natural dam effect, enabling the buildup of sediment inside. It appears that gravity and the weight of the sediment has overcome the strength of the natural barrier, causing the discharge into the stream and then into the Ōhinemuri River.”
The council tested for 30 heavy metals and cyanide. A streambank and mine outlet water sample showed “very high arsenic levels that would pose a human health risk if ingested”.
“This sediment is still in the Nohopane Stream and has settled along parts of the Ōhinemuri River,” Lynch said.
“We continue to urge people to avoid contact with this sediment, which is bright orange in colour due to high iron.”
Initial results from water monitoring also showed elevated arsenic in the river caused by suspended sediments.
“However, all levels were back to normal by the Monday following the incident.”
Plumes of sediment can be seen moving through the Ohinemuri River, turning it bright orange. (Source: Pine J Brown) (Source: Supplied)
Lynch said regional council staff met with stakeholders on Friday last week to discuss the investigation’s results. This group included representatives of Hauraki iwi, staff from Hauraki District Council, the Department of Conservation and Ministry for the Environment, Waikato regional and Hauraki district councillors, and the Coromandel Watchdog of Hauraki.
He said the purpose of the meeting was to start talking about what to do next.
The Waikato Regional Council said it was committed to providing “ongoing support through environmental monitoring to help provide information about the impacts of the event”.
