Transport modelling and contaminant fate in unsaturated soils: analysis and perspectives

Abstract

In recent years, the contamination of groundwater has become a growing threat at global scale. The improper release of chemicals from numerous and different sources (landfills, leaking storage tanks, etc.) in the subsurface has largely affected the quality of the groundwater resources. Many of the fate and transport processes take place into the unsaturated zone posing a great concern in terms of groundwater protection. Because of that, many scientific studies on models for simulating contaminant transport in the unsaturated zone have been carried out. Application of these models to field scale is a challenging task because of several findings related to the involved processes and the hydrogeological characterization of the site. The present thesis seeks to contribute to this topic, identifying features and limitations of the existing tools and, thus, developing a modelling approach useful for engineering applications to contaminated sites. In particular, a quantitative comparison of selected analytical models has been developed, in order to identify features and limitations and highlight the differences in the outcomes of the different models. The comparison highlights a significant variability of the results with respect to the variation of the considered parameters. The analysis of the models makes it possible to identify three groups, according to the assumptions on contaminant source and chemical-physical mechanisms occurring during the transport. Each group appears suitable for a different contamination scenario. A modelling procedure based on a hindcasting simulation combining a steady-state flow model and a mass transport model has been proposed. Specifically, the objectives of the procedure are to reproduce the contaminant concentration measured in the aquifer within the site, to assess the contaminant concentration of the source and, based on that, to give some insights helping to the identification of the primary sources. The procedure has been applied to a significant case study: the Taranto site The developed modelling has been able to reproduce the measured contaminant concentrations in site and the obtained contamination scenarios have been found plausible and consistent with the solubility of the analysed contaminant. [edited by Author]