Predictive models of groundwater flow largely depend upon reliable estimates of hydraulic properties. Limitations of traditional hydraulic tests for aquifer property estimation include low signal-to-noise ratios, limited applicability, and high costs, as well as the need to dispose of potentially contaminated water. Sinusoidal aquifer testing provides an efficient, cost-effective alternative means of estimating hydraulic properties, particularly in settings where traditional methods do not produce a measurable response. The results of sinusoidal testing are typically frequency-dependent, which also provides a potential means of assessing aquifer heterogeneity.
We designed and fabricated a digitally controlled winch to perform sinusoidal aquifer testing. Stepper motor programming was used to generate cyclic variations in the elevation of a two-metre-long solid PVC slug. Testing of the winch was undertaken at a field site in Aldinga, South Australia. This site featured two multi-level piezometer nests located 12 metres apart and completed in a non-karstic Tertiary limestone aquifer. Three single frequency tests of 60 minutes duration were performed, each of which featured a unique forcing signal. The same three signals were then combined simultaneously to perform a separate multi-frequency test.
Decimetre-scale fluctuations in water column elevations were achieved in the tested production well piezometer. Responses measured at the same elevation in an observation well piezometer were attenuated by two orders of magnitude and featured time delays in the order of minutes. Signal attenuation and time delay were also observed in piezometers screened at other elevations within the tested aquifer. Aquifer hydraulic parameters (i.e. transmissivity and storativity) were estimated through the inversion of analytical forward models. Transmissivity values were found to be positively correlated with the frequency of the forcing signal applied.
Field testing demonstrated that sinusoidal slug testing is a cost-efficient and robust means of estimating aquifer hydraulic properties, which is especially well-suited to relatively more complex hydrogeological settings.