Abstract
Two simple frequency decomposition techniques were used as part of a tidal response method to derive the hydraulic diffusivities of a freshwater-lens aquifer. Digital high-pass filtering can separate the tidal components of diurnal and shorter periods from longer-period components. Discrete Fourier transform can be used to isolate a specific tidal component. These techniques are easy to practice using the built-in functions of spreadsheet software. The applied techniques were each optimized for the frequencies of known major tidal components. Isolation of the specific tidal signals helps to reduce the errors of a basic tidal response method that uses in its calculations the amplitude attenuation and phase lag of a simple sinusoidal wave of groundwater fluctuations. Another advantage of the present tidal method is the utilization of two groundwater time series collected from near-shore and relatively inland sites affected by the same ocean tide. The method does not use surface-water observation, thus avoiding errors derived from generally possible surface-water/groundwater boundary effects.The tidal response method with simple decomposition techniques was used to investigate the aquifer properties of an uplifted limestone island located in a subtropical region of Japan. A freshwater lens is the principal water resource for this island and its sustainable development is desired. Significant hydraulic layering has not been reported in the limestone aquifer. Pairs of groundwater-level time-series data collected by simultaneous observations at near-shore and inland sites were analysed by the tidal response method. The results demonstrated heterogeneous aquifer diffusivity on the island, typically with larger values in the southeastern coastal part than in the northwestern coastal part, which is consistent with the planar distribution of the entire freshwater lens and the position of its maximum thickness that are slightly biased toward the northwestern side.
Highlights
In many small islands in the oceans and seas composed of hydraulically permeable geologies, freshwater lenses are the only source of abundant freshwater supply for the residents and others, as well as for agriculture that is often an important industry on these islands
Isolation of the specific tidal signals helps to reduce the errors of a basic tidal response method that uses in its calculations the amplitude attenuation and phase lag of a simple sinusoidal wave of groundwater fluctuations
Another advantage of the present tidal method is the utilization of two groundwater time series collected from near-shore and relatively inland sites affected by the same ocean tide
Summary
In many small islands in the oceans and seas composed of hydraulically permeable geologies, freshwater lenses are the only source of abundant freshwater supply for the residents and others, as well as for agriculture that is often an important industry on these islands. Such groundwater resources are known or at least considered to be susceptible to natural and anthropogenic threats such as long-term unprecedented sea-level rise, extended drought, and over-abstraction (e.g., TERRY & CHUI, 2012; ISHIDA et al, 2015; BARKEY & BAILEY, 2017). Analysis of the natural tidal response of groundwater fluctuations gives a general idea of the aquifer hydraulic parameters (PERRIQUET et al, 2014) and would be an appropriate and convenient approach for studying a freshwater-lens aquifer
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