No clear evidence of culpability in final stages of investigation into orange sediment discharge

Waikato Regional Council’s investigation into the discharge of orange sediment into the Ōhinemuri River is in its final stages, with no clear evidence of culpability other than it being the result from mining activities more than 100 years ago.    

Regional Compliance Manager Patrick Lynch says there was no evidence of any manmade structures in front of the mine entrance that would have caused the buildup of the sediment inside, and that discharged during one event on the afternoon of Friday, 23 August.

The sediment discharged from a mine entrance associated with the Comstock Low Level Drive, an exploratory mine that was mined between 1896 and the early 1900s and which never proved profitable.

“There’s a tideline inside the drive that shows the sediment had built up to a depth of about 96 centimetres,” says Mr Lynch.

“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 build-up 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.”

On Friday last week, Waikato Regional staff met with stakeholders to brief them on their investigation.

The meeting was attended by 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 Coromandel Watchdog of Hauraki.

Mr Lynch says the purpose of the briefing was to generate conversations about “where to from here”, with Waikato Regional Council committing to provide ongoing support through environmental monitoring to help provide information about the impacts of the event.

The council tested for 30 heavy metals and cyanide in all water and sediment samples taken.

A solid sediment sample taken from the streambank and from the mine outlet 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,” says Mr Lynch.

“We continue to urge people to avoid contact with this sediment, which is bright orange in colour due to high iron.”

The sediment could pose longer term risk to aquatic organisms where it has accumulated.

Initial results from water monitoring in the Ōhinemuri River also showed elevated arsenic levels from suspended sediments (unfiltered water samples), however, all levels were back to normal by the Monday following the incident.

For more information on the event, including details of the monitoring results and frequently asked questions, go to waikatoregion.govt.nz/Ohinemuri-river.