Temporal and spatial dynamics of organic chemicals in the environment with a multimedia fate model.

Once chemicals are released into the environment, their environmental fate is determined by their usage and discharge, partitioning tendencies, and by transport and transformation processes, which may vary greatly in space and time. In the last three decades, an increasing number of models based on the concept of “communicating media” have been developed to understand and predict the fate of chemicals in the environment. The tendency emerging from recent experimental and modelling efforts is to consider the fate of chemicals more closely related to the dynamics characterizing the “environmental scenario”. This thesis aims to give a contribution to the investigation of the role played by temporal and spatial dynamics of key chemodynamic processes and environmental properties in influencing the fate of chemicals in the environment. The focus of the thesis was the development of a new, dynamic and site specific multimedia fate model (MFM) of organic chemicals in the air-soil system. The model, named SoilPlus, is described in Chapter 1 and includes two dynamic air compartments, a multicompartmental litter, acting as an “intermediate” compartment between the air and a multicompartmental soil. The model also performs a complete water balance, accounting also for dissolved organic carbon (DOC) fluxes in the litter/soil system. The model underwent a process of benchmarking and then was used to investigate the role of the litter as a buffer compartment between soil and air, modelling hexachlorobenzene as an example of persistent organic pollutant (POP). SoilPlus uses the same two air compartments developed for the AirFug model, described in Chapter 2. AirFug integrates a new MFM, characterized by two dynamic-air compartments and a single soil compartment, and a meteorological preprocessor; after the model benchmarking, it was evaluated against experimental data from the literature regarding diel (less than 24 hours) variations of PCBs concentrations in urban air. The process of development of these two models prompted to investigate a suitable numerical solution for the equation system describing the chemical mass balance. Chapter 3 is focused on a critical evaluation of the performance of three different numerical methods suitable for models based on adjacent compartments. One of the tested methods was then implemented in the spatial-explicit version of SoilPlus developed and described in Chapter 4. This version of the model was applied to a geographic explicit scenario (a small agricultural basin) for which measured concentrations of pesticides in water were available, in order to evaluate the benefits arising from considering a more spatially resolved scenario (up to the field-scale) on the calculation of environmental concentrations. Chapter 5, finally, summarizes the results of a recent workshop held at the University of Lancaster in 2009 focused on the need to consider cross-fertilization between the POP-cycle and C-cycle communities in order to more accurately predict future exposure scenarios for POPs.