Massive project to ‘pull the plug’

MINING the Hope Downs iron ore deposit in the Pilbara will require dropping the level of the surrounding water table more than 180 metres below its current level, and raising it back up when the mine is eventually decommissioned.

“It is going to be like emptying a bathtub eight times the size of Mundaring Weir and filling it up again,” said one source contacted by Business News.

Details of the mind-boggling engineering feat are revealed in the project’s recently released Public Environmental Report (PER).

The Hope Downs project is a joint venture between Hancock Prospecting Pty Ltd and the South African group, Iscor Limited. The partners plan to spend $450 million to develop a mine based on a deposit of marra mamba ore about 75 kilometres north-west of Newman in the Pilbara.

The PER sets out how they propose to do it. According to the 192-page document, the partners want to mine two identified ore bodies Hope North and Hope South using conventional open pit mining techniques.

The ore bodies, however, lie mainly below the surrounding water table which is critical to the well-being of a well-vegetated, groundwater-fed surface water feature called the Weeli Wolli Creek system located about seven kilometres downstream of the Hope North deposit.

To get to the ore the partners are going to have to “pull the plug” on groundwater levels in a process engineers refer to as dewatering. At the same time, in order to meet their state and federal environmental obligations, they will have to maintain a supply of water to the surrounding vegetation.

And when mining finishes they are going to have to put the plug back in and refill the aquifer, a process the PER says will take 20 years.

“(The partners) consider that the key environmental issue relating to the project is the potential impact of the development on the Weeli Wolli aquifer and spring,” the PER says.

“Considerable effort has been spent on refining the existing hydrogeological model of the catchment using the latest available data.

“This work demonstrates how mining will avoid a serious impact on the aquifer, spring and associated vegetation.

“Water from dewatering activities will be used to maintain Weeli Wolli Spring flow, maintain vegetation, supply mine water requirements and inject water into an aquifer storage area for later recovery.

“Following mining the pit voids will be backfilled to a level that eliminates any standing water and water will be recovered from the aquifer storage area near the spring to increase the rate of groundwater recovery.

“It is expected that this will result in a self-sustaining system in relation to Weeli Wolli Spring flow and groundwater dependent vegetation.”

Mundaring Weir has a 64 gigalitre capacity. According to the PER the Hope Downs partners will de-water and re-inject the groundwater supply at a rate averaging about 40,000 kilolitres a day over the lifetime of the project and for 20 years after decommissioning.

A company spokesperson said this would be the equivalent of moving 70,000 tonnes of water a day but this was on a similar scale to dewatering operations at other well-known mines, including the Pilbara’s Mt Whaleback.

“Many large ore bodies around the world go below the water table,” the spokesman said.

“Mt Whaleback is an example of a large orebody that is being mined below the water table and it has been for about 15 years.”