Abstract
The tidal response method of analyzing responses of groundwater levels to the tide for estimating aquifer hydraulic parameters was applied to an unconfined coastal aquifer on Tarama Island, Okinawa Prefecture, in order to investigate aquifers beneath wide areas of the island. The method was combined with isolation of specified tidal oscillations by digital filtering and discrete Fourier transform. The length of the used groundwater-level observation data was set based on an appropriate length for the discrete Fourier transform to isolate predominant tidal components, 3,279 hours (about 137 days), and additionally required 10 days for employing prior digital filtering. Estimated hydraulic diffusivities for aquifers beneath several investigation lines in coastal areas of Tarama Island were 4 to 15 m2·s-1. Hydraulic conductivities estimated therefrom were in the order of magnitude of 10-2 m·s-1, the same as the result of a pumping test previously reported. Several coastal parts of the aquifer vary three- to four-fold in estimated diffusivities and conductivities, with smaller values for the northern part and the smallest for the north-northwestern part of the aquifer. This also is consistent with the freshwater-lens distribution biased toward similar directionality.
