Groundwater table fluctuations in a coastal unconfined aquifer with depth-varying hydraulic properties

Abstract

While many analytical studies have been carried out to examine groundwater table fluctuations in tidally-influenced coastal unconfined aquifers, they generally focused on homogeneous aquifers and overlooked actual field situations with sound physical underpinnings. Taking the inelastic intergranular movement into account, this study presented a new governing equation under the Dupuit assumption, in which the compressibility factor was considered to incorporate the vertical variations of both effective porosity and hydraulic conductivity. An analytical solution was derived using Taylor expansion and the perturbation method. Time-varying groundwater table from the present model lies in between the homogeneous solutions predicted using grain parameters in the upper and bottom aquifer layers, which is almost overlapped with the homogeneous solution if applying the grain parameters in the middle layer. Effects of depth-varying hydraulic properties were further examined with respect to the total fluctuation range and phase shift of the groundwater table, which quantitatively present the differences between the present solution and Nielsen (1990) homogeneous solution. Such differences become significant for the large compressibility factor. In addition, it was mathematically proved that compressibility factor has no effect on the mean water surface as landward distance approaches infinity.