Impacts of longwall coal mining on our water supply areas
The intensive longwall mining method used by the collieries under the water supply catchment is causing damage. Longwall mining extracts long blocks (‘panels’) of a coal seam from deep underground. A panel may be three or four metres high, up to 350 metres wide, and several kilometres long! The typical longwall panel can take a year to mine. The longwall panels are located in parallel, separated by coal pillars.
When the coal panel is being mechanically cut, the strata above the advancing machine-cutter are propped up by very powerful roof supports, protecting the mine workers. But as the supports are moved forward with the advance of mining operation, the immensely heavy overlying rock collapses into the cavern created by the removal of the coal seam.
Fractured rock may extend to a height above the seam of 25 to 35 times the thickness of the mined coal panel. Above this level, the surface rocks settle and may crack – perhaps in a creek or under a swamp. Cracking and subsequent water loss can result in permanent changes to water catchments and groundwater aquifers.
In addition to cracking and falling groundwater levels, surface subsidence can cause hill slopes to collapse, escarpments to topple, increased erosion and eco-toxic stream pollution. In hilly country, the surface damage may occur as far as 1.5 kilometres from the mined area.
Several places in the water supply catchments south of Sydney have suffered serious damage due to longwall coal mining. At the Metropolitan Colliery, Peabody Energy has mined directly under the Waratah Rivulet that provides about 30% of water to Woronora Dam. The Rivulet has been badly cracked by longwall mining: much of its water is draining away; and Flat Rock Swamp, an important headwater source, has all but completely eroded away. For much of the Rivulet’s length, it only flows after heavy rain, and three attempts at remediation to restore flows have failed.
In the case of the BHP Elouera Mine, two creeks in the water supply catchment have been damaged. The longwalls are 185m wide at a depth of 340m, the damage to the creeks includes extensive and intense cracking of their rock beds and draining of all rock pools (small and large) in mined areas. Under normal unmined circumstances the affected streams would be flow during drought times (and this is the case with unmined creeks in the vicinity). Upland swamps above this mine have dried out, and then were scoured away in the following heavy storms.
There is a persuasive argument that water flow is being lost underground. In 2001 the Elouera Mine reported that water inflows had increased to 225megalitres a month (see graph of increasing water losses). In late 2007 and early 2008, there have been two events when the adjoining Dendrobium Mine suffered an inrush of water during heavy rain, including 50 megalitres of water flooding the mine over just four days. So there is a risk of water supplies draining into the mines.
Strong and effective protection measures are needed to protect our pristine catchments that are critical to Sydney and Wollongong and that will otherwise drain away and become polluted. The catchment is managed by the Sydney Catchment Authority, which was created in 1998 after the water contamination incidents. The Catchment Authority has a legislative duty to preserve the ecological integrity of the area, but does not have any power to prevent the impacts from coal mining.
The intensity of longwall coal mining has increased over the years. With underground longwall coal mining, the amount of surface movement and damage is greatly influenced by the width of the longwall mining panels and the depth of the mine. In recent longwall mining in the Metropolitan water supply catchments and nearby areas, widespread surface damages occur when wide panels of coal are extracted.