Impact of Depth-Dependent Heterogeneity in Aquifers on the Response of Unsaturated-Saturated Flow to Earth Tides

Heterogeneity is a natural characteristic of aquifers, which has a profound influence on groundwater flow and solute transport. However, the heterogeneity in an aquifer is difficult to characterize and its impact on groundwater response to Earth tides remains insufficiently explored. A common heterogeneity in thick aquifers is the decrease in porosity and hydraulic conductivity with depth. In the present study, we present an analytical model that includes this particular heterogeneity in both the unsaturated and saturated zones to explore its impact on the tidal response. Our results reveal that, as the hydraulic conductivity decays with depth, the amplitude ratio of the tidal response increases and its phase shift decreases, and the impact of the capillary zone on tidal responses is primarily restricted to the shallow region of the aquifer. Neglecting the decay of conductivity in the previous models would result in underestimation of the amplitude ratio and overestimation of the phase shift. The solutions are applied to the field data to effectively estimate the decay in conductivity with depth. This study highlights the importance of considering aquifer heterogeneity in the analysis of the response of saturated and unsaturated flow to Earth tides.