Estimation of spatial extent of mature hydrocarbon contamination in the subsoli: the Trecate case study.

The characterization of contaminated sites is composed of both the definition of the regions impacted by contamination and by the pathways that can transport this contamination offsite thus substantiating the risk of contact with possible receptors. Both goals may benefit from the application of non-invasive techniques, provided that solid relationships are established between the geophysically-measured quantities (e.g. electrical conductivity) and the state variables of interest, being them related to water presence and flow (for pathway characterization) or to contaminant presence. The latter is by far the most challenging aspect of non-invasive characterization of contaminated sites. The reason is twofold: (a) the physical signal induced by the presence of contaminant is usually very small per se, and (b) spatial and temporal variations of geophysical parameters in the subsurface may be linked to other factors than contamination itself, such as lithology and hydrological state of the system. Therefore, in order to exploit the information content of geophysical data, ancillary information is needed, relevant to both the spatial heterogeneity in lithology and hydrogeological state and to the contamination state, where measured. The advantage of geophysics, however, lies in its capability of providing high-resolution spatial coverage, potentially in 3D, thus zoning reliably the different portions of the subsurface, including contaminated versus uncontaminated, and avoiding the pitfalls of insufficient spatial sampling, typical of direct investigations: spatial aliasing in geophysical data is minimal, as opposed to its being the norm in direct investigations. A combination of direct and non-invasive investigations is thus optimal. In this contribution we describe the spatial mapping of contamination at a well-known contaminated site in Trecate, NW Italy, which was affected in 1994 by crude oil contamination. Ever since, in-depth analyses of extensive data have shown how a correlation exists between residual oil contamination in the smear zone and a measurable increase in electrical conductivity, linked to the biological activity triggered by the carbon source availability. The use of spatially extensive Electromagnetic Induction (EMI) and ERT surveys allows a full detailed mapping of the contamination in the deep vadose zone in correlation with the sparse direct investigation results. This study was developed in the frame of “The Geosciences for Sustainable Development” project (Budget Ministero dell’Università e della Ricerca–Dipartimenti di Eccellenza 2023–2027 C93C23002690001).