Abstract
Abstract Most studies about the tidal response of leaky aquifers have treated the layered groundwater system as a classical unconfined aquifer without unsaturated flow. However, a recent study has shown that the conventional hypothesis of free drainage of groundwater to the watertable may be defective and the unsaturated flow may strongly affect their tidal response. Hence, it is critical to examine if unsaturated flow may also affect the tidal response of a layered groundwater system. In this study, we apply two-dimensional multilayered numerical simulations to examine the tidal response of unsaturated flow in a leaky aquifer. The results show that unsaturated flow on the watertable may significantly affect the tidal response of deeply buried aquifers, and the thicker the unsaturated zone is, the greater influence on the groundwater response to earth tide would be. Besides, a dimensionless quality ω∗ is introduced to estimate the effect of the unsaturated flow. When ω∗>10−0.5, the effect of the unsaturated flow on the tidal response of the water level is evidently; otherwise, the effect can be neglected. We then apply the numerical model to interpret the tidal response of a well installed in Lijiang, Yunnan province, China. It perfectly explains that the phase shift and amplitude ratio, respectively, decrease and increase exponentially when the watertable is below the ground surface. This study emphasizes the necessity of considering unsaturated flow in the multilayered model to improve the accuracy of predicting the permeability of the leaky aquifer.
Highlights
It is important for us to continuously monitor the confinement of aquifers as the confinement may be transient and can be destroyed by various disturbances, such as earthquakes, which will cause the wastewater leakage and pollute the environment [1, 2]
According to the above simulation, the unsaturated zone above the watertable maybe is a reason which leads to the phase shift increasing and amplitude ratio decreasing when the watertable approaches 2 m below the ground surface
We prove that the unsaturated flow can largely influence the tidal response of leaky aquifers; the assessment of aquifer leakage from the observed tidal response cannot be ignored
Summary
It is important for us to continuously monitor the confinement of aquifers as the confinement may be transient and can be destroyed by various disturbances, such as earthquakes, which will cause the wastewater leakage and pollute the environment [1, 2]. The response of water level in wells to Earth tides has recently become popular to evaluate the permeability of leaky aquifers [3–5]. These studies are flawed as they did not consider the effect of unsaturated zone, which might change the water movement in the saturated zone and thereby induce watertable fluctuations [6–8]. To interpret the response to Earth tides in a multilayer model, a two-dimensional Richards equation coupling poroelastic deformation of the aquifer to groundwater flow is essential. We establish a two-dimensional multilayered finite element model to investigate the influence of unsaturated flow on the leaky aquifer to the Earth tides. We illuminate the tidal response of a well installed in Lijiang, Yunnan province, China, by employing the numerical model
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