Characterization of geological heterogeneity through time-domain induced polarization at contaminated sites: the case of a former gravel pit filled with waste

In the recent years, time-domain induced-polarization (TDIP) surveys have been successfully employed in a wide range of environmental applications. This technique allows for characterizing the electrical properties of the ground, not only in terms of electrical conductivity, but also in terms of polarization effects at the interface between rock matrix and water. This makes TDIP particularly well suited for contaminated sites, where a detailed description of geological heterogeneity is need to predict the contaminant spreading via accurate groundwater flow modeling. In this study, 9.8 kilometers of 2D TDIP profiles have been carried out in the surroundings of a former gravel pit, filled with municipal and industrial wastes after its depletion, in the Province of Brescia. TDIP data have been inverted via traditional resistivity inversion and with full-decay spectral TDIP inversion, directly in terms of subsoil hydraulic conductivity (K), using petrophysical relations that link electrical properties and hydraulic properties derived in the laboratory. Reasonable 2D K models are retrieved, and the resistivity models improve significantly when inverted along with TDIP data.