Waikato Regional Council says it has now received test results for all 30 heavy metals following a bright orange sediment discharge into the Ohinemuri River last week.
Earlier this week, preliminary test results from sediment samples taken from below the entrance of the mine shaft which flows directly into the Ohinemuri River returned "very elevated" arsenic levels.
In an update this afternoon, Waikato Regional Council's compliance manager Patrick Lynch said the latest results from follow-up sampling on Saturday showed no mercury has been detected in sediment or water samples.
It also found that while still elevated, the arsenic level 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.
"The monitoring shows the levels of arsenic in the clear running water is well within drinking water and ecological protection guidelines for fresh water."
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 701m-long drive stopped in the early 1900s.
Lynch said there is still sediment built up at the mine entrance, and more orange plumes can be expected during rain events — but they are unlikely to be of the same scale as Friday's event.
Waikato District Councils's environmental chemistry scientist Jonathan Caldwell said the greatest health risk regarding the elevated arsenic levels in the sediment is from ingestion through eating or drinking it.
"However, due to the high iron content of the sediment, the arsenic will be well bound up and is more likely to be excreted than taken up by the body," he said.
He said arsenic can make its way into the food chain through fish and shellfish, but it typically converts into a less toxic form and is "not absorbed very well through the skin".
"The results also show no cyanide was detected in the sediment or water, although there have been some minor detections of antimony and thallium, which are typically associated with gold mining, in the stream samples. Copper and zinc were also identified as being elevated in the stream samples, but in levels similar to what could be seen in urban storm water."
The council said results would be shared with the Department of Conservation and Hauraki District Council.
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