Determination of Hydraulic Diffusivity of a Limestone Aquifer Using Spectral Analysis Techniques

The estimation of aquifer flow properties is commonly based on the application of a mechanical stress on the aquifer that causes a response in the flow field. By observing and measuring the response in the aquifer, hydrogeologic properties like transmissivity, hydraulic conductivity, and storativity can be inferred. The application of an anthropogenic aquifer stress (e.g., pumping or injection wells) is a widely-employed method of aquifer testing since the stress can be controlled and easily measured. Aquifer response to non-anthropogenic aquifer stresses, such as oceanic and earth tides, can also be used in the determination of aquifer properties, with the additional complication that the stress must first be well understood and quantified.

In a tidally influenced aquifer system, hydraulic diffusivity of the aquifer is proportional to the speed and amplitude at which a finite pressure pulse of the tide propagates through the aquifer system. This pressure pulse results in a spectral signature in observed groundwater wells. Frequency domain analysis of the tidally impacted groundwater wells and tides provides a meaningful and non-invasive approach to determination of hydraulic diffusivity of an aquifer.

Hydraulic diffusivity was estimated for a prolific aquifer in South Florida based on identification of tidally influenced spectral signature, lag time, and amplitude in groundwater wells and tides. An analytical solution of sea-aquifer model with a leaky semi-confining layer separating an overly unconfined aquifer from an underlying semiconfined aquifer was applied to estimate hydraulic diffusivity; based on the parameter values obtained from the spectral analysis technique. The hydraulic diffusivity values for the aquifer obtained from the spectral analysis techniques was within the same order of magnitude as that obtained from pumping tests.