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A new minefield
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Hydrogeologist, John Polglase, examines some consequences of mining and drilling activities on the world beneath our feet. The Great Artesian Basin (GAB) is not only vast in area, it is 'vast' in its variety of sedimentary contexts over geological time. During more than a hundred million years, various sediments were deposited and 'lithified' in various basins and lowlands, which subsequently experienced various tectonic events and vertical rock movements. The result is that the sub-basins, troughs, embayments of today's GAB, are only loosely connected and related. The GAB is far from homogeneous, which is why understanding the local 3D geology beneath our feet is important.
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The sedimentary basins of different geological periods may also overlap each other. |
| For example, in Eastern Australia, the coal, groundwater and gas mass extractions from the Surat Basin (a major sub-basin of the GAB) may also sit above these resource types from the Permian Sydney-Gunnedah-Bowen Basin. Again, over 'vast' periods of time, this older basin system initially underwent extension or rifting, and then such tension or compression from the East that resulted in a major fault system (the Hunter-Mooki Fault System), leaving the Eastern sedimentary strata strained, folded and anything but horizontal and competent.
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Above: Sandstone Liesegang rings - bands coloured by variable amounts of iron oxide in sandstone. These are inside a cave near Bundeena (image by Ann Young).
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With respect to groundwater resources in the sedimentary strata of the GAB, there is typically no chemical (eg isotopic) investigation of the water horizons encountered by bores drilled for agricultural or mining purposes, that could indicate whether the waters were from the GAB or other deeper and distal basins (such as the Sydney-Gunnedah-Bowen Basin or the Clarence-Moreton Basin) and highs. Further, the notion that rainfall in the East makes its way at depth to the centre of the Australian continent, is simplistic and incorrect. For instance, the GAB sub-basins beneath northern NSW are separated by basement high areas such as the Tibooburra and Eulo Ridges, resulting in a series of loosely connected basins that act as groundwater reservoirs.
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Below ground, water and gas are constantly moving through fractures and fissures in the sedimentary rocks, at various rates under varying pressures. The rocks are also moving - continuously or abruptly - as Nature balances and rebalances the stresses and strains according to the laws of physics. |
| Alongside this physical balancing act, there is also chemical balancing between the solids in solution, precipitants from solution, and changes in acidity, salinity, temperature, oxygenation. Any extraction and redistribution of mass: solid (coal, ore, inter- over-burden), water or gas, creates voids which in turn cause Nature to attempt a new equilibrium by re-balancing the physical and chemical states of the fluids and rocks. Like Space, even the 'voids' are not really 'empty'. They contain reactive gases flowing in from above and below.
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Above: Displacement out of cutting. The crack here formed due to mine subsidence. It is about 5 m below ground level and is exposed in a cutting on the unfinished Maldon-Dombarton railway line (image by Ann Young).
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One consequence of mass extraction may be seismic activity due to rock movement. Similar movements may also occur when mass (solid, liquid or gaseous) is injected back into the pre-stressed basin strata. Some examples include re-injection of formation waters; injection of fluid and other wastes (such as carbon dioxide); and fraccing.
Denver, Colorado, provides the classic case study of this. The area had a stable seismic history until the US Army began injecting waste at depth into strata just beyond the first front of The Rockies. This resulted in an increasing series of earthquakes over ten years, something that took authorities some time to work out.
Today, there are many similar case studies:
there have been mining-induced earthquakes up to 4.6 Richter scale in Poland and Germany;
there is an earthquake a day in some South African gold mining operations;
earthquakes in Texas are linked to the extraction of water and oil.
A German geophysicist, Christian Klose, believes that the mass extraction of groundwater and coal from the lower Hunter Valley was responsible for the Newcastle earthquake.
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Below: Minor earthquake cluster proximate to longwall coal mining panels in the lower Hunter Valley (image from John Polglase). |
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There are, of course, no 'Terms and Conditions' requiring mining companies to monitor long-term seismic activity or groundwater contamination within and without their lease areas. Mine blasting and the transport and storage of coal and rock should also be taken into consideration for their impact upon the strains and pressures of the fractured and fissured basin strata below.
What is inevitable, is that some form of subterranean physical and chemical rebalancing will take place in the short- to long-term, to compensate for subterranean mass redistribution.
For example, there have already been cases of economically viable groundwater bores being contaminated within a few months of exploratory drilling a few kilometres distant.
Any contaminant introduced into any fluid flow does not remain localised as it becomes part of that flow. For example, antimony and arsenic, unwanted leftovers from derelict New England mining operations, are still moving seaward in the Macleay River catchment materials.
It is quite simplistic to believe that mining has no knock-on effects. Mass loading and unloading perturbs the physical and chemical balances beneath our feet, such that there will inevitably be 'unintended' physical and chemical consequences.
Indeed, it is clear that mining has not only altered the surface of the planet, it has also altered the atmosphere. Monitoring regulations for mining operations need to include local and regional long term monitoring of the physical and chemical expressions of the rocks and fluids within the sedimentary basins below.
We are in the midst of a new minefield.
John Polglase was interviewed for A Question of Balance by Ruby Vincent. Summary text by Victor Barry, December 2011
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