Loading effect of water table variation and density effect on tidal head fluctuations in a coastal aquifer system

Water table variation will result in considerable variation of the weight of the pore water, which has loading effects on the groundwater flow in the underlying confined aquifer. Here we present an analytical solution on such a loading effect in a coastal aquifer system consisting of an unconfined aquifer, a confined aquifer, and an impermeable layer between them, with the two lower layers extending under the sea. The solution generalizes several existing ones in the literature. The water table variation's loading effects tend to enhance the amplitude and to reduce the phase shift of the tide-induced head fluctuation. These effects become considerable when the tidal loading coefficient is large, the aquifer's offshore extending length is long, and the unconfined aquifer has large values of hydraulic conductivity and specific yield. Numerical examination indicated that the assumption of ignoring the density variation causes an error not greater than 2.5% of the tidal amplitude.