Passive characterization of aquifers hydro-mechanical properties using tidal signals

Following the evolution of the hydrodynamic parameters (hydraulic conductivity and storativity) of an aquifer over time is difficult and does not exist for an aquitard. The primary reason is the small number of observations, which are mainly pumping or slug tests. However, solutions exist to recover these parameters using only groundwater level monitoring data at a sampling rate of 1 hour, data which is extensively available. These solutions take advantage of the boreholes’ response to different tidal phenomena, including oceanic, earth and atmospheric tides.

Martinique Island, in the Lesser Antilles, is a very interesting field to study these techniques, since 16 years of piezometric level data have been recorded on this volcanic island in a monitoring network of 29 boreholes. The variety of aquifer geometry and geology enables to study different types of responses, in addition the island is submitted to seismic activity that may impact aquifers and particularly their hydraulic conductivity.

The method consists in computing amplitude and phase response of aquifers to both atmospheric, earth and ocean tides. Then, the response of semi-confined aquifers to different loading sources at the tidal frequencies (between 1 and 2 cycles per day) is modelled. The model is a general and adaptable one, in order to consider the various geometries (including the presence or not of an aquitard) and aquifer boundary conditions (confined, semi-confined, connected to the atmosphere or not). Finally, a careful inversion is done to obtain the characteristics of the aquifer.

Our results show that the Martinique aquifers present responses to different sources of loading. We show that using the dominant source response alone, we are able to recover aquifer or aquitard hydraulic conductivity and its evolution over the last 16 years, which we compare with pumping tests results. Using a combination of sources, we are also able to recover elastic properties like the evolution of the loading efficiency and shear modulus. Finally, comparing different sites responses, we are able to assess which aquifer is more vulnerable to pollution through vertical leakage within their aquitard.