Waikato Regional Council is expecting preliminary results in the next 24 hours from sampling of sediment that turned the Ohinemuri River in the Karangahake Gorge bright orange on Friday afternoon.

The council’s incident response team were on site on Friday afternoon and Saturday, taking water and solid samples upstream and downstream of where a load of bright orange sediment was discharged from a historic mine shaft and entered the Ohinemuri River.

Regional Compliance Manager Patrick Lynch says it is appropriate to urge people to treat the discoloration as contaminated until sample results had been obtained.

“We only know what could be lingering inside a mine and that could include residual arsenic and heavy metals, so people need to be cautious and not touch any of the orange sediment that still may be lingering in the environment.”

Waikato Regional Council incident response team member taking water and solid samples. Photo: Waikato Regional Council.

The Department of Conservation has put up signs near the site, warning track users of potential contamination.

On Saturday morning council staff confirmed the discharge point of the sediment as Comstock Mine, an old mining asset above a stream opposite the Karangahake Gorge car park.

Patrick Lynch says the sediment appears to have been shifted from the Comstock Mine entrance in the hills above the Karangahake Gorge car park, and the flow of water coming from the shaft was running relatively clear by Saturday.

There was still sediment that had settled in the river in quiet locations or around rocks on Saturday, 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," says Patrick.

“Getting results from our sampling takes time, and it will be days before we know.” 

The entrance to the Comstock Mine, where the discharge of water came from that flowe down into the Ohinemuri River on Friday afternoon. Photo: Waikato Regional Council.

Sample results can take up to 10 days.

Council staff took water samples upstream and downstream on Saturday, as well as samples of the settled sediment.

On Saturday, Patrick said they only know what might be in sediment in an old mine shaft and it could be some days until they get the results back so they 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.”

Waikato Regional Council incident response team member taking water and solid samples. Photo: Waikato Regional Council.

Patrick says a plane was used on Saturday 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, August 23, at about 3pm.

Council investigators and scientists returned to the mine entrance again today.

Patrick says there is heavy rain forecast this week and a lot of water does come down the hills into the Ohinemuri River, so that may trigger more flushing of the settled sediment down the stream, into the river. 

"People can expect to see more discolouration until all the sediment has been flushed out of the stream."

Patrick says council staff will now complete a thorough scene examination, including how the event occurred and what a “normal” discharge from the mine should look like.

The council will update information as it becomes available.