April 9, 2015

Leaky aquitards

The Australian continent is huge, and the degree of detail and accuracy of various forms of mapping may be patchy in remote regions. Scanty geological maps are not confined to the far Outback. This is a recurring theme among hydrogeological expert witnesses in Land Court cases against large international mining companies proposing to excavate huge holes in the Galilee Basin in Central Queensland.

Maps are at the heart of resource exploration in the first place and, later, hydrogeological conceptualisation for the groundwater modelling required by Environmental Impact Statements. Geologists are still using geological maps by Vine et al (1969) and a hydrogeological map of the Great Artesian Basin by Habermehl and Lau (1997). State governments are even reluctant to fund the updating of topographical maps used by bushwalkers every day.

There are other methods available to those conceptualising the landscape – topographical observations on the ground, helicopter flyovers, remote imaging, drill hole data, seismic surveying, radiometrics – but assumptions abound in the absence of certainty, and consensus must be reached between experts who disagree and necessarily strive for the 'best representations' given the data available.

Today I was back in the Land Court for the second 'groundwater day'. There was a lot of discussion about a series of springs near the mine site. Whether or not they continue to flow once the mine is in operation depends on how they are sourced; the potential pathways along which groundwater can flow; whether those pathways include faults or fractures; whether or not essentially impermeable layers of rock 'leak' or not.

In these discussions many technical terms are used all the time, and members of the public gallery must grasp their meaning in order not to drown. So, an aquitard is a layer of rock that restricts groundwater flow between one aquifer and another as a result of components that have a low hydraulic conductivity, that is, the ease with which water flows through pore spaces or fractures. A flow of water is artesian if it is caused by a difference in pressure between one layer of rock and another. The potentiometric surface is an imaginary level, or plane, that a reservoir of fluid would find – that is, equalise out to – if allowed to flow unrestricted.

A few hours of groundwater conceptualisation is enough to give the amateur enthusiast a headache. I'll be back for more tomorrow, however. There are still three expert witnesses to go. There are several extremely important aspects to this case, but the potentially catastrophic impact of mining on groundwater resources in an arid region is, arguably, the most crucial.

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