Waikato Regional Council has confirmed it now has results from testing for all 30 heavy metals, plus cyanide, following a bright orange sediment discharge into the Ōhinemuri River on Friday.
Council regional compliance manager Patrick Lynch said the latest results from follow-up sampling on Saturday showed no mercury had been detected in sediment or water samples.
The arsenic levels, while still elevated, had dropped significantly compared to the levels detected in samples taken on Friday.
“In layman’s terms, what this means is that if you see orange sediment or an orange water plume, which is associated with sediment, then the caution we issued on Friday as soon as we were made aware of the event still stands: treat orange water and sediment as contaminated and do not touch,” Lynch said.
“Clear water is good; the monitoring shows the levels of arsenic in the clear running water is well within drinking water and ecological protection guidelines for fresh water.
“We have more sample results to come but expect them to be consistent with these first findings, if there are any changes to the results, we will update our communications.”
Senior scientist Jonathan Caldwell said the greatest health risk regarding the elevated arsenic levels in the sediment was from ingestion (eating or drinking).
Due to the high iron content in the sediment, the arsenic would be well bound up and was more likely to be excreted than taken up by the body, he said.
“Arsenic can make its way into the food chain via uptake by fish and shellfish, but it typically converts into a less toxic form.
“Also, arsenic is not absorbed very well through the skin, and this route of exposure to arsenic would increase if you had broken skin or an open wound.”
The results also showed no cyanide was detected in the sediment or water, although there had been some minor detections of antimony and thallium, which were typically associated with gold mining, in the stream samples.
Copper and zinc were also identified as being elevated in the stream samples but at levels similar to what could be seen in urban stormwater.
High iron in the sediment accounted for the bright orange colour.
The council said it would share the results with the Department of Conservation and Hauraki District Council.
The sediment came from a mine entrance associated with the Comstock Low Level Drive, an exploratory mine started in 1896 and which never proved profitable, mining in the 700-metre-long drive stopped in the early 1900s, the council said.
There was still sediment built up at the mine entrance, and a lot of sediment had accumulated in the stream directly below the entrance which flowed directly into the river.
This means more orange plumes, but unlikely to be of the scale of Friday’s event, could be expected during rain events.