The Nepean River is one of Macarthur's most beautiful natural resources however a University of Western Sydney professor is concerned that mine waste is damaging the river's ecosystem.
Professor Ian Wright has spent a decade researching the effects of waste water from the Tahmoor Coal mine on the Nepean and Bargo river system.
He said the level of pollutants in some sections of the river was "unnaturally high".
"The Environmental Impact Statement (EIS) for the mine shows that waste from the mine makes up two thirds of the flow of the Bargo River - which is really high," he said.
"How I distill it down is that for the past 10 years we have been trying to find out how this waste affects the ecology of river life and it shows that the growth of algae has choked the river.
"The Nepean River is actually in really good shape but mine waste changes the river in a negative way where it meets the Bargo River."
The plans included a reduced number of longwall mines and the extension of mining in the region for 10 years.
Documents about the plan also stated that "the mine intends to install the new advanced water treatment plant as soon as possible to further improve the quality of the water released from the mine".
A SIMEC spokesman said the company took its responsibility to the environment and community very seriously.
"As technology has improved, so has our water treatment and testing processes," he said.
"Water assets like the Bargo River are carefully monitored throughout the life of the mine.
"The quality of water discharged from the site is mandated by our environment protection licence (EPL) issued and monitored by the NSW Environment Protection Authority (EPA).
"Typically, the water monitoring results are far better than those limits allowed by the licence.
"Our proposed new advanced water treatment plant will further improve the water discharged from site."
However Professor Wright said he had reached out to government officials to seek more information about the environmental effects.
"I am not against the expansion of the mine however if you are producing waste you should be managing and treating it appropriately," he said.
"I have written to the Premier (Gladys Berejiklian) and the EPA about this issue - which is something I have never had to do in my career.
"We need more details about the environmental impacts."
Professor Wright said various gasses and metals from mine waste were affecting the health of the waterway.
"The waste is really salty and also contains a cocktail of metals like barium, lithium, nickel and zinc," he said.
"The waste contains the sewage from the mines - basically a small town of workers.
"There is also a high level of nitrogen and phosphorous in the water which leads to increased levels of algae and river weeds - something the EPA has been successfully campaigning against for years."
Professor Wright said the river had always been a popular swimming location for locals due to its beauty.
"I think people are unwittlingly putting themselves or their children at risk," he said.
"I don't think people should be made to stop swimming there but I think more testing needs to be done - especially when it comes to bacteria.
"I think it is a potential health issue that needs to be investigated. It should be monitored and people should be warned if it is unsafe.
"The river is a really important place and is spiritual for a lot of people so it needs to be protected."
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