Tahmoor Coal has been fined $15,000 by the Environment Protection Authority after a creek pollution incident noticed by researchers who happened to be on the scene.
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Environmentalists say this reveals gaps in the regulatory regime for mines.
The incident, in April, was identified by researchers from Western Sydney University (WSU).
The EPA said fine particles of clay passed through the mine's water treatment system to the licence discharge point, and into Tea Tree Hollow Creek and the Bargo River downstream to the popular swimming spot Mermaids Pool, making them turbid and discoloured.
It was obvious that something very wrong was occurring
- Dr Ian Wright
Dr Ian Wright, a senior lecturer in the WSU school of science, was one of the team there who spotted the problem.
"I have not seen the cloudy, turbid water (a strange grey colour) before from this or any other mine," he said.
"I have studied mine waste water impacts for more than 20 years. We could see as soon as we got to the site ... that the mine waste was causing pollution.
"It was obvious that something very wrong was occurring. With our instruments onsite we instantly knew that the wastes were outside of the EPA licence conditions with excessive turbidity, and we let the EPA know that day (as we have a duty of disclosure)."
Dr Wright said the laws under which the EPA regulates water pollution were "very strong", and this case was a reminder to the community to be the "eyes and ears" of the EPA.
But he questioned how well it works.
"In other ways I am not satisfied with the regulatory regime, as in my opinion the mine releases poorly treated wastes continuously, every day and has for years," he said.
"The EPA licence fails to impose meaningful discharge conditions, which I think cause pollution to the Bargo and even Nepean River. This is through very high permitted concentrations of metals (nickel, zinc and arsenic), salt and even nitrogen."
Asked about this, the EPA encouraged people to report pollution they see.
"Tahmoor Coal has now installed monitoring equipment to detect turbidity and provide an early warning of problems in the water treatment system," an EPA spokeswoman said.
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