Transient flow and transport modelling of an historical CHC source in North-West Milano

Legislative Decree 152/2006 requires Public Authorities to identify the subjects who are responsible for soil and groundwater contamination. In highly urbanized areas with a long industrial history and an elevated number of potential contaminant sources, as in N-W Milano Functional Urban Area (FUA), their identification can be difficult. Since the groundwater flow has showed consistent changes in the last 30 years as in Milan, the problem became even more complicate. The Public Authorities put in charge by the law, i.e., Regione Lombardia and Città Metropolitana Milanese, need new methodologies to assist them in finding the source locations and implementing remediation actions. The aim of this study is, coupling unsteady flow with fate and transport model of Chlorinated Hydrocarbons, to reconstruct the potential impact of a former chemical plant on public wells in the N-W area of Milano. The proposed methodology consists in (a) reconstruction of the piezometric trend over time (1980-2018) by means of a transient flow model (MODFLOW2005 + Parameter Estimation - PEST) and (b) simulation of transport as a function of the flow variations in time. The obtained results were compared with the previous ones obtained with a quasi-steady model (no changes in time-dependent parameters). Finally, a predictive scenario was performed to assess the potential evolution of tetrachloroethylene (PCE) in groundwater; on this frame, strategies to monitor and remediate the contamination were proposed.