Title Analytical studies of groundwater-head fluctuation in a coastal confined aquifer overlain by a semi-permeable layer with storage

Analytical studies are carried out to investigate groundwater head changes in a coastal aquifer system in response to tidal fluctuations. The system consists of an unconfined aquifer, a semi-confined aquifer and a semi-permeable confining unit between them. An exact analytical solution is derived to investigate the influences of both leakage and storage of the semi-permeable layer on the tide-induced groundwater head fluctuation in the semi-confined aquifer. This solution is a generalization of the solution obtained by (Jiao JJ and Tang Z. Wat Resour Res 1999;35:747-751) which ignored the storage of the semi-confining unit. The analytical solution indicates that both storage and leakage of the semi-permeable layer play an important role in the groundwater head fluctuation in the confined aquifer. While leakage is generally more important than storage, the impact of storage on groundwater head fluctuations changes with leakage. With the increase of leakage the fluctuation of groundwater head in the confined aquifer will be controlled mainly by leakage. The study also demonstrates that the influence of storativity of the semi-permeable layer on groundwater head fluctuation is negligible only when the storativity of the semi-permeable layer is comparable to or smaller than that of the confined aquifer. However, for aquifer systems with semi-permeable layer composed of thick, soft sedimentary materials, the storativity of the semi-permeable layer is usually much greater than that of the aquifer and its influence should be considered. * Corresponding author. Address: Department of Earth Sciences, The University of Hong Kong, Hong Kong, P. R. China. Tel: +852 2857 8246; Fax: +852 2517 6912; e-mail: [email protected]