by George Houen
Deregulation of the Dewatering of Mines
To clear the way for the extraction process in open cut and longwall underground coal mining (and to a lesser extent other minerals), miners pump accumulated groundwater out of the pit, they also pump from bores to dewater the ground in advance.
|Image sourced [here]|
The pit becomes a sump which drains all aquifers overlying the target coal seam, and the coal seam itself. The resulting drawdown extends outward in a radius determined by permeability of the ground and interconnection between aquifers. One expert report for the Galilee Basin predicted the radius of permanent drawdown to extend 30km – even with lesser distances, the scope for damage to landholder’s bores is obvious.
That this dewatering has major implications for the environment and for owners of water bores is authoritatively shown by Land Court’s Alpha Coal judgement1, in which the primary recommendation was to reject the mining application. Despite extensive expert study and evidence on groundwater, the Court found that evidence unsatisfactory as to the impact of the mine’s dewatering on other groundwater users and on the ecology. The Court’s fallback recommendation made any approval for the mine conditional upon:
- the proponent first obtaining water licenses on conditions which resolve all concerns pursuant to the precautionary principle, and
- additional groundwater monitoring points on each of the objectors’ properties, and
proponent must enter into make good agreements with each of the three
objectors either within 12 months of grant of the mining lease, or before
mining activities commence, whichever is the sooner.
In contrast to the impacts of mine dewatering, dewatering for CSG (which is unregulated) targets the specific gas-yielding coal seam. It is usually deeper than the water supply aquifers and usually with poorer quality water. It is dewatered to liberate the gas from fissures in the coal. Except where private bores actually draw from that same seam, the impact of CSG dewatering on water bores is indirect. In response to the alteration in relative water pressures, and depending on permeability and interconnections, water from other aquifers may flow to the dewatered coal seam. Because the coal itself is not disturbed, presumably it is possible in the long term that the original saturation and pressure balance would be reinstated.
The Land Court’s Alpha Coal decision shows, as is common sense, that mine dewatering impacts are difficult to predict but involve major, direct and permanent impacts on the groundwater relied upon by landholders and townsfolk, as well as significant environmental impacts. Accordingly mine dewatering should be made an integral part of the approvals process. But as explained below, CSG dewatering is very different in its process and its impacts. The proposition that mine dewatering needs to be deregulated to put miners on an even footing with CSG operators is contrived and invalid.
|Skid mounted pump installed on a barge in a coal mining operation|
Photo sourced [here]
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