Characterization of a heterogeneous limestone vadose zone based on a multimethod and multiscale geophysical approach

Abstract Water resources are a critical societal issue. As they are more and more limited, their distribution, usage and contamination are of vital importance. In the Beauce region, there are concerns on both the water quality and quantity because of farming and the related groundwater contamination. Since water is a vector of contamination, these questions require a precise vadose zone water content characterization considering the site lithology and water content heterogeneities. We apply a multiscale and multimethod geophysical approach to integrate the limestone heterogeneity at various scales into the characterization. In the first step, at a multi‐decametric scale, the joint interpretation of a 3D surface nuclear magnetic resonance water content, electrical resistivity and chargeability models allows to distinguish zones with limestone heterogeneities (clay content, fractures) related to geophysical anomalies. At a decimetric scale, the joint interpretation of borehole nuclear magnetic resonance water content, borehole electrical resistivity and density logs in each zone allows us to precise the spatial fractures and karstification levels in the lower saturated and unsaturated limestone. The second step is to analyse the water content variations to build a water content model of the Beauce limestone. This model goes further into the site porosity description by illustrating how the different limestone water‐filled porosities are sensed by geophysical investigations. It is compatible with the known regional specific yield range, with laboratory data and with a previous dynamic evaluation of water column variation during a recharge seasonal episode at field scale. This shows that our preliminary model may be exportable to other calcareous settings and can be used as a basis to study and simulate the vadose zone dynamic.