Tidal Response of Groundwater in an Anisotropic Leaky Aquifer

Groundwater tidal response analysis is a valuable tool for monitoring leakage in groundwater systems, yet the interpretation of this response has often been incomplete. Notably, the impact of anisotropic aquifer permeability on tidal response has not been addressed in existing models. This study presents an analytical model to examine the effect of anisotropy on the tidal response of an aquifer overlain by a semi-confined aquitard with finite storage. After verifying our model against previous models and numerical simulations, we fund: (a) At high vertical aquifer conductivity and aquitard leakage, the amplitude ratio of the tidal response is small, and the phase shift is positive, making our solution closely align with the existing leaky aquifer model. (b) As the vertical aquifer conductivity decreases, the amplitude ratio increases and the phase shift decreases and becomes negative at relatively low leakage, similar to that of a confined aquifer. (c) When the vertical aquifer conductivity is smaller relative to the horizontal one, the existing leaky aquifer model tends to underestimate the amplitude and overestimate the phase shift. (d) The aquitard storage has a significant effect on the tidal response of the aquifer when the aquitard leakage is large, but a negligible impact when the vertical aquifer conductivity is small. Applying our model to field data from four monitoring wells in the North China Plain, we find that when the shale content in the aquifer reaches 40.09%, our anisotropic model more effectively fits the observed phase shift compared to the existing leaky aquifer model.