Earthquakes and Heavy Rainfall Influence on Aquifer Properties: A New Coupled Earth and Barometric Tidal Response Model in a Confined Bi‐Layer Aquifer. Issue 4 (5th April 2023) - Thomas, A.
Abstract: Among the impacts of earthquakes on aquifers, permeability change is one of the most challenging to quantify, since techniques to measure permeability evolution are scarce. The study of tidal response of boreholes is one of the most promising, yet complex to use in practice. We used 14 years of piezometric level measurements and two concurrent source signals, earth tidal strain and barometric pressure, for which we separated the respective contribution in a state‐of‐the‐art tidal analysis. We developed a new general analytical hydrogeological model, based on geological observations of a confined bi‐layer aquifer. It is able to match combined observations of earth and barometric tide phase lags which could not be explained by existing models. We demonstrate that its relative complexity can be overcome thanks to the results of tidal analysis, yielding a simpler model adapted to the Fond Lahaye site of the Martinique Island. The resulting evolution of diffusivity and loading efficiency, was validated independently with several pumping tests occurring all along the studied period. The transient diffusivity increases and decreases indicate which earthquakes impacted the aquifer, enabling to establish an empirical magnitude‐distance relationship criterion. This criterion confirms the suspected dependence on dynamic stresses, which decrease as the square of the hypocentral distance. Additionally, we investigate two other factors of diffusivity changes: heavy rainfall events and aquifer withdrawals, which demonstrates the sensitivity of volcanic aquifers properties to environmental and anthropogenic influence. Key Points: A model for the tidal response of bi‐layer aquifers was developed, focusing on pressure diffusion and exchange between two confined layers Using 14 years of hourly piezometric data, the temporal evolution of aquifer diffusivity was inferred and confirmed independently by pumping tests The change of permeability is shown to be due to earthquakes, extreme rainfall events and aquifer withdrawals in upstream borehole