A Turnbull government plan to dump 2600 Olympic swimming pools of rubble in Alpine lakes risks "catastrophic" damage to Australia's most important river system if it goes awry, experts and environmentalists have warned.
The plan forms part of the "supercharged" Snowy 2.0 proposal announced by Prime Minister Malcolm Turnbull last year. It would expand the Snowy Hydro project to secure electricity supplies to the east coast market, acting as a giant battery and increasing generation capacity by 2000 megawatts.
The project involves two large underground caverns and a 27-kilometre tunnel linking the Tantangara and Talbingo reservoirs, which would be bigger than the Sydney Harbour crossing and the Melbourne Metro project.
A feasibility study released last month said the project is expected to extract 6.5 million cubic metres of compressed dirt and rubble, known as spoil.
The study proposed disposing of the spoil in the Talbingo and Tantangara reservoirs - lakes on rivers that feed the Murray Darling Basin, on which millions of Australians depend.
It conceded the plan came with "risks ... which will require further investigation and management" such as "liberating constituents from spoil into the reservoirs".
The study said "reactive" spoil containing sulphur was a potential source of acid rock drainage - a process where sulphides in rocks are exposed to air and water, producing sulphuric acid.
University of Canberra Associate Professor Ben Kefford, an expert in the effect of contaminants in freshwater systems, said such a process can be "catastrophic".
"It's very expensive to fix and has huge environmental impacts which last decades if not centuries," Dr Kefford said.
"If the water was to go acidic in the dam then any water you release downstream would be acidic.
"It would have huge impacts on the flora and fauna living along these rivers. It would affect the use of the water in these rivers ... the water can be unfit for human consumption and irrigation."
Dr Kefford said such a scenario was unlikely, however "you want to be very careful and do a lot of testing and consider worst case scenarios," including conditions centuries into the future.
He said acid rock drainage could be prevented if the spoil was confined to the bottom of the reservoirs where oxygen levels were low or non-existent. But he warned such conditions were rarely permanent and depended on weather and other factors.
Dr Kefford said depositing spoil in the lakes could also mean some "drifts out and smothers areas on the edges of the reservoir or ... puts sediments downstream".
Nature Conservation Council chief executive Kate Smolski questioned why the spoil would be dumped in reservoirs designed to hold water.
"It would also pollute the waters released from the dam by increasing sedimentation and turbidity, which is very harmful for freshwater native fish, their food sources and ecosystems downstream," she said.
Snowy Hydro has refused to release its preliminary environmental risk assessment, but says an environmental impact statement will be made public as part of the planning approvals process.
A Snowy Hydro spokeswoman said "all possible measures will be taken to avoid and mitigate the environmental impacts" of the project, and it was seeking to reuse spoil as road base or in construction materials.
"Snowy Hydro is commissioning a series of technical studies which will look at geology, material characterisation, hydrodynamics, biogeochemistry and ecotoxicology," she said, in response to pollution concerns.
"Snowy Hydro have a long track record of being responsible environmental managers in the Kosciuszko National Park."
The spokeswoman said displacing a small percentage of the water with spoil would not impact Snowy Hydro's ability to capture, store or release water.
If Snowy 2.0 was not built, alternative electricity generation methods would be required which "have their own carbon footprint, greenhouse gas and other emissions and have impacts on the environment," she said.
Environmentalists have questioned why the government wants to build the project in Kosciuszko National Park, when research has shown Australia has more than 22,000 sites potentially suitable for pumped hydro storage.