Two-dimensional flow response to tidal fluctuation in a heterogeneous aquifer-aquitard system

In alluvial coastal aquifers, finer sediments are preferentially deposited along the downstream direction, so the hydraulic conductivity is generally heterogeneous and changes with distance from the coastline. To investigate the influence of aquifer heterogeneity on seawater-groundwater interaction, a new two-dimensional model characterising groundwater flow in an aquifer-aquitard system was developed assuming that the hydraulic conductivity of the aquifer linearly increases with the distance from the coastline along the inland direction. A closed-form analytical solution was derived using the separation-of-variables method. Comparing the new solution with the numerical solution by comsol Multiphysics (Sweden) based on the finite-element method, one can see that the new solution agreed with the numerical solution very well except at the early time. We found that both aquitard leakance and the heterogeneity factor (b) could result in the propagation bias. The propagation bias represents the inconsistency between the theoretical calculation and the observed strong attenuation and small time lag between the head and tide fluctuations. The attenuation decreased with perpendicular distance from the coastline (x-axis), whereas the time lag increased with distance along the x-axis. The relationship between the time lag and the distance along the x-axis seemed to be linear when b was 0.001m(-1), whereas it obeyed a power function when b was greater than 0.01m(-1). Copyright (c) 2014 John Wiley & Sons, Ltd.