Exact and approximate analytical solutions of groundwater response to tidal fluctuations in a theoretical inhomogeneous coastal confined aquifer

The influence of hydraulic conductivity heterogeneity on tide-induced head fluctuations is presented for a theoretical coastal confined aquifer. The conceptual model assumes that the hydraulic conductivity increases linearly with the distance from the coastline. This type of heterogeneity has been observed in many alluvial coastal aquifers. An exact analytical solution that predicts induced head fluctuations is obtained in terms of a Hankel function. The exact solution can be approximated by a simple mathematical expression, valid for small rates of increase of hydraulic conductivity. Both exact and approximate solutions show significant differences from the classical solution obtained for a homogeneous aquifer. Near the coastline the amplitude of the induced head fluctuation is damped but it is enhanced as the distance to the coast increases. The time-lag between sea tide and induced head fluctuation in the aquifer is not linear; it behaves as a square-root type function leading to a faster transmission of the tidal fluctuation. Hypothetical examples show that the influence of hydraulic conductivity heterogeneity can be significant and should be considered for a correct description of the groundwater response.