A Revised Conceptual Model to Reproduce the Distribution of Chlorinated Solvents in the Rho Aquifer (Italy)

Authors

  • Daniele Pedretti,

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    • D. Pedretti, corresponding author, is a postdoc researcher at the Department of Geotechnical Engineering and Geosciences, Universitat Politécnica de Catalunya UPC-BarcelonaTech, E-08034 Barcelona, Spain and Dipartimento di Scienze della Terra “A. Desio,” Universita degli Studi di Milano, I-20133, Milano, Italy; daniele.pedretti@upc.edu.

  • Marco Masetti,

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    • M. Masetti is an associate professor at the Dipartimento di Scienze della Terra “A. Desio,” Universita degli Studi di Milano, I-20133, Milano, Italy.

  • Giovanni P. Beretta,

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    • M. Vitiello is an external consultant for the Dipartimento di Scienze della Terra “A. Desio,” Universita degli Studi di Milano, I-20133, Milano, Italy.

  • Mariangela Vitiello

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    • G. P. Beretta is a full professor at the Dipartimento di Scienze della Terra “A. Desio,” Universita degli Studi di Milano, I-20133, Milano, Italy.


Abstract

To make cost-effective decisions regarding aquifer protection against chlorinated solvent contamination, a conceptual site model explaining the hydrodynamic behavior of these solvents needs to account for soil heterogeneities in sufficient detail. Among the key requirements, it is necessary to perform site-specific characterization of (1) the architecture and type of chlorinated solvent sources, (2) the architecture, nature, and distribution of aquitards (bodies of low permeability), and (3) local groundwater dynamics. Local-scale data are often lacking and regional descriptions are therefore adopted with the consequence of losing small-scale features that control local chlorinated solvent migration which, in turn, limits the effectiveness of safeguard facilities. This article addresses these arguments, focusing on the Rho aquifer (Italy). We show that the local conceptual model must integrate small-scale architectural and hydrodynamic anomalies of the aquifers and aquitards with regional trends and the sedimentary origin of the aquifers. New evidence was found by processing stratigraphic logs, piezometric heads and solute concentrations from local wells. The revised conceptual model should account for the occurrence of secondary chlorinated solvent sources located in depressions on the aquitard surface separating the two upper and most contaminated Rho aquifers. While the spatial distribution of depressions is in accordance with regional sedimentological dynamics, the direction of local groundwater flow at various levels within the main hydrogeological unit is remarkably opposite to the regional groundwater flow direction. These findings, along with evidence of vertical heterogeneous soil texture and stratification of chlorinated solvents in various phases, are supported by a simple particle-tracking model and suggest how to redefine the conceptual model and, consequently, how to design safeguard facilities to effectively prevent further plume migration.

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