Comprehensive conceptual model of an Italian megasite combining numerical flow and transport modeling, MLS and CSIA of nitrogen and chlorinated solvents

Chemical, geological and hydrogeological investigations were conducted in a polluted site in Italy, with the aim of improving and validating a conceptual model of groundwater flow and contaminant transport and of providing a rough evaluation of contaminant evolution with time in the context of a permanent remediation strategy. Various sources of pollution were detected, including petroleum hydrocarbons, chlorinated solvents and nutrients (phosphorous and nitrogen). The collected data facilitated the construction of a numerical model, which takes into account groundwater flow interaction with salt water intrusion by a density-dependent model. Simulation results agree with the field data, showing that the dissolved contaminants are completely intercepted by a P&T system, the salt wedge intrusion has reached the pumping wells, and that approximately 25% of pumped groundwater comes from losses from a surface seawater canal at the site. The three dimensional flow model was validated by data collected in 2009-10, and it is now used to forecast and manage the pumping rates of more than 70 wells. New monitoring by multilevel sampling has been performed in 2011-12 to build a transport model, including dispersive/diffusive components, to be used for a preliminary evaluation of the mass of total dissolved contaminants in groundwater. It has been demonstrated that the effect of a vertical physical barrier has created a stagnation zone downgradient of the barrier, where contaminant concentration does not decrease with time, except by natural attenuation. The conceptual model of the contamination evolution takes into account the processes influencing locally and at the site scale natural attenuation coupled with pump & treat strategies. Isotope analyses of the main contaminants have been performed to evaluate the processes that are involved in their attenuation. In particular, they were focused on nitrogen compounds, related to past industrial activities and to a sewage system, and chlorinated solvents, from industrial processes in limited zones of the megasite and migrated downgradient with time