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. Preliminary results have detected elevated arsenic in the discharged sediment, while the arsenic content present in the water was 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 poses less immediate risk to fish but could still pose longer term risk in downstream areas where sediments accumulate.

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 33 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. These take time and are being analysed with urgency.  

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.   

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.

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

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.