Oral Presentation Australasian Groundwater Conference 2017

Using atmospheric and Earth tides as a natural tracer to hydraulically characterise groundwater systems (212)

Gabriel C Rau 1 2 3 , Landon J S Halloran 1 2 3 , Wendy Timms 1 4 , Martin S Andersen 1 2 3 , Ian R Acworth 1 2 3
  1. Connected Waters Initiative Research Centre, UNSW Sydney, Sydney, NSW, Australia
  2. Water Research Laboratory, UNSW Sydney, Sydney, NSW, Australia
  3. School of Civil and Environmental Engineering, UNSW Sydney, Sydney, NSW, Australia
  4. School of Mining Engineering, UNSW Sydney, Sydney, NSW, Australia

Determining groundwater state of confinement and calculating aquifer compressible storage properties through geological sequences at high vertical resolution is difficult and time-consuming. We demonstrate that atmospheric and Earth tides can be used as a natural stressor to hydraulically characterise the subsurface over depth instead of aquifer testing and subsequent inversion of heads. Atmospheric and Earth tides induce oscillations in hydraulic heads that can be identified in the frequency domain. We analyse hydraulic heads measured in 9 piezometers screened at different vertical depths in a sequence of unconsolidated clays between 5-55 m in conjunction with atmospheric pressure and synthetic Earth tide records. The amplitude and phase response to atmospheric tides at 2 cpd frequency is accurately quantified by removing the influence of the Earth tide. This permits precise calculation of barometric efficiency (BE) values over depth. In conjunction with porosity estimates, this leads to vertically resolved values of specific storage and compressibility. We verify our results by using the hydraulic head response to rainfall loading at the surface. Further, when the system is confined there is a phase difference of 180° between atmospheric pressure and hydraulic head at 2 cpd, as is stipulated by the stress balance. We illustrate that this phase difference can be used to determine state of confinement over depth, and that this confinement changes over time depending on moisture storage within the overlying soil. Our new approach only requires a continuous record of water level and atmospheric pressure that is longer than ~16 days. The method simplifies hydraulic characterisation of multilayered groundwater systems and can therefore significantly improve conceptual models with less effort compared to traditional approaches.

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