The Nature of Longwall Mining

Photo: Coal Leader
"Longwall mining" has been allowed to take place underneath the Illawarra Plateau - close to, and in some places under, the rivers which supply Sydney's water.

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Longwall mining is a form of underground coal mining which uses a massive machine called a longwall. The longwall equipment consists of a number of hydraulic jacks, called chocks, roof
supports or shields, which are placed in a long line up to 400m in length in order to support the roof.
Diagram of Longwall Mining
BHP.
Rivers SOS website An individual chock can extend to a maximum cutting height of up to 5m. The coal is cut by a rotating drum with bits called a shearer that moves along the length of the face in front of the chocks, disintegrating the coal. The coal is then removed from the coal face by a moving chain or AFC (articulated face conveyor) (also called armoured face conveyor) to the main gate. Here it is loaded onto a conveyor belt and transported to the surface.
As the shearer removes the coal, the chocks move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the cavity behind the longwall, known as the goaf, increases. This goaf collapses under the weight of the overlying strata."
Text from Wikipedia

Sometimes longwall mining is called destructive or environmentally unsafe because it causes the land above the mined-out panel to sink. This can damage underground water tables, structures at the surface, and can cause erosion of the soil.wisegeek.com

As the coal is removed, and the hydraulic props are removed, the stone strata above the cavity collapse down to fill the cavity. This collapsing produces subsidence all the way up to the surface layers, resulting in cracked rocks.


Cracked rocks in river bank

Contamination of Cataract River
Photos - from Rivers SOS

When the rock strata are cracked, it can allow toxic gases to escape through the rocks, and allow water from creeks above to flow down into the mined areas, (drying up the rivers, of course) and allowing that water to become contaminated with minerals. That contaminated water may well escape back to the surface, bringing the dissolved mineral contaminants into the river systems.

Such contamination is now occurring in the lower sections of the rivers in the Sydney Catchment system.

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Contamination of Cataract River
Photo 2 - from Rivers SOS

Unanswered questions:

What notice is the SCA taking of the environment of the rivers?

What is the point of trying to transfer water from the Shoalhaven, if it just gets wasted down cracks into BHP Billiton's mines? Surely that is wasteful.

Why should the Kangaloon Aquifer's pure water be allowed to be wasted and/or contaminated, once it flows downstream?

How does any of this fit with the NSW Government's Advertising campaign on water running on TV at present?