Water Resources Research

Tide-induced head fluctuations in a confined aquifer with sediment covering its outlet at the sea floor



[1] A confined coastal aquifer usually extends under the sea for some distance with its submarine outlet covered by a thin layer of sediment with properties dissimilar from the aquifer. Previous theoretical studies neglected this outlet capping. In this paper an analytical solution is derived for a confined aquifer subject to tidal fluctuations with a leaky boundary condition at the outlet capping. For the cases that offshore length of the aquifer and/or the leakance of the outlet capping are either zero or infinity, existing solutions in literature are obtained. It is shown that our analytical solution can also be used to describe the tidal wave propagation in a confined aquifer extending infinitely under a tidal river. The full solution agreed well with the observations in a piezometer in the Jahe River Basin in China, 200 m inland from the coastline where the tide-induced head fluctuations without delay were observed but cannot be explained using previous analytical and numerical solutions ignoring the effect of the outlet capping. The sensitivity analysis showed that the combined actions of the tidal loading and outlet capping lead to complex dependency of the head fluctuation on the outlet capping's leakance. When the offshore confined aquifer is short, the inland head fluctuation increases with the outlet capping's leakance. In this case significant negative phase shift occurs if the leakance is small. For offshore aquifer length greater than a threshold value the inland head fluctuations are independent of this length and the outlet capping's leakance.