What can we learn with barometric and earth tide induced groundwater level fluctuations ? From aquifer conceptualization to K and Ss assessment

Methods for understanding and predicting the impacts of groundwater extraction generally lack detailed spatial and temporal knowledge of subsurface hydromechanical properties. Estimating subsurface hydraulic properties using groundwater response to earth tides (ET) and atmospheric pressure is an alternative approach to pumping tests or “slug-tests”. These methods can be described as passive, since they use the forces of nature, as opposed to active methods requiring human intervention. These passive and inexpensive investigative techniques deserve to be developed to make analysis easily accessible. In this way, the hydromechanical properties of subsurface systems could be obtained with unprecedented spatial and temporal resolution, adding further value to commonly acquired groundwater and atmospheric pressure data.

However, assumptions concerning conceptual models, parameterization of the hydromechanical problem and the influence of drilling on the results are given little consideration. This inverse problem can also benefit from Earth diurnal tides, and not just semi-diurnal as in the literature, to identify the right models to use and to reduce uncertainties in the estimated hydromechanical parameters (K, Ss). The amplitude ratio of diurnal to semi-diurnal waves and the phase shift sign are indicators of the conceptual model to be used, and the estimated transmissivities are in agreement with those of the pumping tests in the case study. In this context, we have shown that the amplitude of terrestrial tidal signals alone cannot be used to estimate the storage coefficient Ss.

We aslo showed that barometric tides can be used to estimate the hydraulic conductivity K of aquifers when the barometric and piezometric sampling time step is adapted to the hydraulic conductivity. When permeability is not very high, there is indeed a phase shift between the tidal wave in the aquifer and that in the borehole, and this can be related to K using slug-test models, with a clear signature on both synthetics and real data. 

We thus demonstrate the potential of natural drivers induced groundwater fluctuations to better conceptualize the hydrogeological model (unconfined, leaky, confined), as well as to assess hydraulic properties such as hydraulic conductivity and specific storage.