Arsenic levels drop from previous day’s sampling results

Waikato Regional Council now has results from testing for all 30 heavy metals, plus cyanide, following a bright orange sediment discharge into the Ōhinemuri River on Friday.

Regional Compliance Manager Patrick Lynch says the latest results from follow-up sampling on Saturday show no mercury has 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,” says Mr Lynch.

“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 (Environmental Chemistry) Jonathan Caldwell says the greatest health risk regarding the elevated arsenic levels in the sediment is from ingestion (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.

“Arsenic can make its way into the food chain via uptake by fish and shellfish, but it typically converts into a less toxic form,” says Dr Caldwell.

“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 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. High iron in the sediment accounts for the bright orange colour.

The results will be shared with the Department of Conservation and Hauraki District Council.

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.

There is still sediment built up at the mine entrance, and a lot of sediment has accumulated in the stream directly below the entrance and which flows directly into the Ōhinemuri River. This means more orange plumes – but unlikely to be of the scale of Friday’s event – can be expected during rain events.

The council will update information as it becomes available.