Ōhinemuri River incident

The sediment comes from a mine entrance associated with the Comstock Low Level Drive, an exploratory mine started in 1896 and which never proved profitable. Mining in this 701-metre long drive stopped in the early 1900s. Investigations show that this incident was not caused by any current mining operations in the Karangahake area.

Downstream drinking water suppliers were notified and should be taking extra precautions. Results have detected elevated arsenic in the sediment, while the arsenic content present in the water is well within drinking water and ecological protection guidelines for fresh water.

We responded to this incident immediately upon notification. However, sediment cannot be treated like an oil spill. Oil and hydrocarbons float on water. That is why booms can be effective in containing an oil spill. Sediment like this is throughout the vertical water column (the depth of water) and cannot be boomed, and it then settles.

We have not received any reports of fish kill. Monitoring results show that the water was not very acidic, which reduced the concern around immediate risk to fish.

However, the arsenic in the sediment could still pose longer term risk to fish in areas where it accumulates.

Aquaculture industry in the Firth of Thames was notified on Friday and will be taking their own precautions.

High iron in the sediment accounts for the bright orange colour. There was a lot of sediment discharged very quickly, so when it mixed with the river it turned the water bright orange. Now you’ll see the water is running clear even where the sediment has settled in the river.

There are 30 separate metals and cyanide that we’re testing for in each sample. To give a full picture of environmental effect, there are many samples taken to show what is happening upstream, downstream and at the point of discharge. The first results came in the Monday after the Friday afternoon event.

Sediment usually moves with gravity and water. The heavier the rainfall, and the steeper the country, the more sediment is likely to move. When it is in water it can ‘stay together’ for some time and travel as a mass which is referred to as a plume. Over time, the heavier material will settle to the bottom and along banks but can be moved again during high rainfall events when rivers run fast and high.   

We are planning a meeting with all stakeholders this week (6 September) and potential for remediation will be part of that discussion.

However, removing the sediment from the mine mouth and downstream would certainly be challenging, not to mention that the sediment still in the mine may move over time. Some of the sediment that has moved from the mine shaft is suspended beneath the mine mouth and some has dropped about 8-10 metres into a stream that connects with the Ōhinemuri River. The stream is rocky, and the area is steep and covered in native bush. The sediment already sitting in the Ōhinemuri River is now quite widespread and would also be difficult to remove.

There is still a lot of sediment in the stream that enters the Ōhinemuri River. If we get more rain then we can expect to see more orange plumes (unlikely to be of the scale of Friday's event) in the river.

Local iwi Māori have been involved from the outset in the Ōhinemuri River incident. Tangata whenua conducted an assessment onsite, considering a rāhui, but deciding not to initiate one at that stage. They will reassess if circumstances change. The council values the involvement of local iwi Māori and sees cultural monitoring as an essential service provided by tangata whenua.

We do not advise consuming watercress growing among the contaminated sediments, as arsenic can accumulate in plants to toxic levels. As a result, it can enter the food chain and pose health risk to humans.

A tideline inside the drive shows a sediment depth – prior to the discharge event – of about 96 centimetres. There was no evidence of any manmade structures in front of the mine entrance that would have caused the buildup of the sediment inside. The mine entrance is in the face of a steep bank that drops away to the Nohopane Stream below. It would appear a natural dam of branches and foliage has given way or partially given way. It would appear that gravity and the weight of the sediment inside overcame the strength of the natural barrier, causing the discharge of sediment to occur.

The landowner is responsible for what happens on their property.

Waikato Regional Council will continue to provide ongoing support through environmental monitoring of water and sediment in the Ōhinemuri River, the Waihou River and the Firth of Thames, which will help provide information about the impacts of the event. There is information about this ongoing monitoring on this webpage.

We know there is more sediment to come down into the river and will be for some time. We advise against any contact with discoloured water or orange coloured sediment.