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Posted 6 September 2008
An experimental technique developed by a joint Australian-Canadian research effort offers the potential for new insight into long-term hydrogeological processes through the virtual acceleration of time.
Led by Dr Wendy Timms of the UNSW Water Research Laboratory (WRL) and Professor Jim Hendry of the University of Saskatchewan in Canada, the research team has succeeded in simulating a time period spanning tens of thousands of years using centrifugation to exert forces of more than 300 times that of normal gravity, speeding up processes that would otherwise take place naturally over millenia.
Detailed understanding of how water moves through the earth is essential to hydrogeologists studying groundwater. Different materials such as rock, sand and clay all vary in permeability, which limits the rate at which water can be conducted through them as it moves between their constituent particles.
While clay is capable of conducting water, it does so at very slow rates. Because of this low conductivity, clay often plays an important role in the natural containment of groundwater within aquifers (confined aquifers). Where drinking water supplies are obtained from aquifers, clay layers can also provide a barrier to external contaminants such as industrial or agricultural chemicals, sewage and seawater that would diminish the water quality.
While useful, this property also limits the measurement of water conductivity in clay when currently available equipment and techniques are applied, because of the relatively long time-spans required for testing. Speeding up the water conduction process would provide a major advantage to scientists in their efforts to develop our understanding of groundwater movement.
The researchers set out to see if they could speed up time virtually by exploiting centrifugal force to accelerate gravity.
The concept of centrifugal force refers to the resultant forces apparent on a rotating mass (depending on the frame of reference of the observer). The magnitude of the forces involved increases with the rate of rotation, applying stress to a spinning object that is measured in multiples of that which would normally be applied by the earth's gravity (multiples of G).
This effect is often employed in industry and science, where centrifugal force is used to inject material into moulds, separate mixtures of substances with different densities, test the tolerance of human pilots to the stresses imposed during high-speed flight and even in domestic washing machines to squeeze water from wet clothes through the side of the tub during the spin cycle.
The clay used in the experiment was obtained from a layer of fine-grained till - a sediment formed by glacial activity - at the King Site south of Saskatoon in Saskatchewan, Canada. This material has been a focus for hydrogeological research because it naturally confines a deep aquifer.
Samples of this clay, measuring just over 5cm thick, were mounted in a centrifuge capable of rotating at 1840 revolutions per minute (rpm). The rotation subjected the spinning samples to the equivalent of 330 times the earth's gravity (330G). This effective accelerated gravity increased the pressure of the water within the samples, causing it to move through them at a much faster rate than normal.
The experiment was able to simulate hydraulic movement through clay 17m in thickness over a period of approximately 24,000 years, after running for only 90 days.
Analysis of the chemistry of the water that passed through the samples confirmed the technique could simulate processes occurring over long-time periods in relatively short periods of time using comparatively tiny samples.
This technique has the potential to be developed for further applications that measure the movement of water underground. This could provide better understanding of important processes such as the rate at which aquifers are replenished by water moving through different kinds of soils and sediments (recharge), and the rate at which clay yields groundwater in response groundwater pumping (dewatering).
Accelerated gravity modelling techniques could also be used to test the suitability of clay as a barrier for containing stored carbon dioxide (sequestration) and to predict how contaminants emanating from human waste landfills might affect the environment hundreds and thousands of years in the future.
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