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Council confirms source of orange sediment

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Published: 24/08/2024

Waikato Regional Council has confirmed the discharge point of the sediment that turned the Ōhinemuri River bright orange yesterday.

Regional Compliance Manager Patrick Lynch says the sediment appears to have been shifted from an old mining asset in the hills above the Karangahake Gorge car park, and the flow of water coming from the shaft was now running relatively clear.

“There is still sediment that has settled in the river in quiet locations or around rocks, but most of it has been flushed through the system.

“The settled sediment may be present and visible for some time, and we still advise caution as we do not know yet what it contains.

“Getting results from our sampling takes time, and it will be days before we know.” [Sample results can take up to 10 days.] 

Council staff have again taken water samples upstream and downstream, and taken more samples of the settled sediment.

“We only know what might be in sediment in an old mine shaft and it will be some days until we get the results back so we know what is actually in this sediment.

“The colour indicates it will be iron-related, but it is likely to be highly acidic so again we advise people to treat it as contaminated and not touch it.”

Substances that may be found in old mine assets could include residual arsenic and heavy metals such as cadmium, chromium and antimony, and they could cause risk. So avoiding skin contact is the main thing.

“We are testing for these heavy metals, and also mercury, both totals and dissolved.

“We really want to understand how this event happened and that investigation will take time. It’s also one of those situations where we are relying on mother nature – a rain event – to wash the contaminant away. The more water it’s exposed to, the more diluted it will get.”

Lynch said a plane was being used to see if a sediment plume was apparent in the Firth of Thames, or whether it had dissipated.

The incident was reported to the council on Friday, 23 August, at about 3pm.

The council will update information as it becomes available.