Modelled discharges of up to 50 megalitres of saline treated water per day from just two of the mines, Ulan and Moolarben, “may be inconsistent” with the scheme, the Australian Government-established Independent Expert Scientific Committee said in a report published in January in response to a Yancoal Moolarben Coal plan to increase mining from 13 to 16 million tonnes per year.

The Environment Protection Authority credits the salinity trading scheme with “restoring the waters of the Hunter River to an unprecedented level of freshness”.

“Water salinity is more stable and lower and the river is now as fresh as many bottled mineral waters,” the EPA said.

The scheme aims to keep Hunter River salinity levels below 600EC at Denman and 900EC at Glennies Creek and Singleton. A NSW Government WaterInfo website notes “a large proportion of fresh water entering the Hunter River is expected to originate from the Goulburn River”.

But Ulan and Moolarben mines, between Denman and Mudgee, are licensed to discharge treated mine water at up to 900EC, with the scientific committee warning of a “heightened risk of uncontrolled water discharges” from Moolarben under certain scenarios.

“This could lead to changes to hydrology, water quality and aquatic ecology of the Goulburn River,” the committee said.

It said Moolarben’s environmental assessment ignored the potential impact on the Hunter River of mine water discharges of up to 50 megalitres per day into the Goulburn River from Ulan and Moolarben mines under dry conditions when discharges from mines into the Hunter River are not permitted.

“Water quality assessment has to consider cumulative impacts of other mines in the area, such as in elevating salinity in the upper Goulburn River, and how the altered water quality and volumes of discharged water influence the aims of the Hunter River Salinity Trading Scheme,” the committee said.

It advised the NSW and federal environment departments that a detailed assessment of the cumulative impacts of Moolarben, Ulan and Wilpinjong mines, and potentially a Bylong mine, on the Goulburn River is needed.

The impacts of the Moolarben expansion “cannot be considered adequately without the context of water discharges and water quality impacts from other mines”, the committee said.

The mine is approved to run four open cut and three underground mining areas until 2038. The mine has applied to lift its allowable daily discharges into Goulburn River from 10 megalitres of treated mine water per day to 20 megalitres. This is despite the mine only discharging mine water into the river on one occasion since 2010.   

The scientific committee report backs calls by community and environment groups for a full independent study of ground and surface water impacts of the three huge underground and open cut coal mine complexes between Denman and Mudgee, including the Peabody Wilpinjong mine that produces coal for domestic energy generation at Bayswater power station.

The scientific committee raised concerns that under certain modelled conditions there would be discharges from Moolarben to the river on most days between 2024-2026. It was also concerned that discharges of 20 megalitres per day from Moolarben could impact aquatic plants in the neighbouring Ulan Coal Mine Goulburn River Diversion, where four kilometres of the Goulburn River was diverted to the mine boundaries in the early 1980s to allow open cut mining.

Any impact on the aquatic plants raised the risk of disturbing sediments potentially containing high concentrations of heavy metals “given the history of mine water discharge” into the diversion channel, the committee warned.       

The scientific committee said Moolarben’s environmental assessment did not adequately assess the potential downstream impacts of large volumes of uncontrolled discharges into the Goulburn River under certain modelled scenarios. The committee has previously told the NSW and federal environment departments that uncontrolled discharges from the proposed Bylong mine and the Wilpinjong mine extension had been “inadequately assessed”.

In a submission to the Department of Planning, the Environment Protection Authority rejected a Moolarben proposal to store up to 2.5 megalitres per day of brine – a waste product of mine water treatment – in underground mine voids.

The Department of Primary Industries and Crown Lands and Water questioned why Moolarben needed to lift its potential Goulburn River discharge from 10 megalitres per day to 20 megalitres per day when its groundwater assessment said the proposed expansion would result in a negligible increase in groundwater entering the mine.

Moolarben said the proposed annual yield increase from 13 to 16 million tonnes of coal per year would improve productivity and “security of the continued employment of the existing workforce” and return an additional $80 million in royalties to the state.

It said a water treatment facility would control water quality discharges to the Goulburn River. It said modelling predicted “no adverse impacts to downstream water quality” and “negligible impacts to downstream aquatic ecology”.