Characterization of saturation and copper concentration in sand and clay with SIP measurements.

Adsorption by clay minerals, oxides and organic matter is commonly the most effective mechanism controlling the mobility and bioavailability of heavy metals in soil. As contamination processes of natural systems commonly present an important variability with time and space, we aim to show if non-invasive and imaging geophysical methods, specifically Spectral Induced Polarization (SIP), are sensitive to mobility and/or bioavailability of copper. Promising works have recently shown that SIP is sensitive to the different ions dissolved in soil water thanks to their different adsorption behavior. To the best of our knowledge, these works have used clean sand as medium; that is why we need to reach a more generic comprehension of natural soil by including the relevant adsorbents. In this paper, we focus on the copper SIP signature accounting for the presence of two types of clay (montmorillonite and kaolinite), different saturation levels and representative copper concentrations which have been chosen on the base of Cu chemical extractions from soil samples taken in a Cu polluted test site. During the set of SIP measurements one single variable at time is changed: soil components, saturation and solution Cu concentration. At the same time pH and temperature are monitored. A successive modeling will consist of two parts. 1) In order to correctly interpret the SIP measurements and make sure that the signals are only influenced by matrix and fluid composition, thanks to very recent publications, we will model the EM inductive coupling, the effect of the electrodes used and the Maxwell-Wagner effect. 2) Geochemical modeling characterizing the electrical double layer (EDL) state at the different experimental conditions; then we will try to theoretically link the EDL state to the SIP results. This last step could provide an important insight about the polarization mechanisms under the investigated conditions.