FATE AND TRANSPORT OF POPs IN THE VENICE LAGOON: A FUGACITY-BASED MASS BALANCE APPROACH

摘要

The water quality of the Venetian lagoon, particularly with respect to persistent organic pollutants (particularly PCDD/Fs and PCBs) and heavy metals, has received much attention over the last 10 years. The area has a long history of industrial activity with oil refining and several chemical production plants around Porto Marghera. Around this industrial zone elevated PCDD/F and PCB concentrations have been measured in sediments which has aroused local concern. The Venetian population, and that of the surrounding area, consume several times the Italian national average amount of fish and shellfish. As a result, it is important that an understanding of the fate and behaviour of contaminants within and around the lagoon is achieved with particular attention being paid to exposure via local lagoon-derived foods. This study has involved the development of a mass balance model which is capable of describing the importance of routes of emission, the distribution of contaminants within compartments of the lagoon, and their ultimate fate. The model described in this paper represents an initial attempt to describe the fate and behaviour of a range of contaminants within the lagoon system. Its purpose is to identify areas for which we lack information and hence help to direct cost-effective future monitoring work. The model presented here divides the lagoon into six well mixed zones each with a bulk water column and bulk sediment compartments. Contaminants are added to the system via a combination of direct emission (i.e. discharge), freshwater inflow or via atmospheric deposition. Each zone has been constructed using the same layout and mass balance principles as the Quantitative Water Air Sediment Interaction (QWASI) lake model presented by Mackay and co-workers. However, the model has been modified to allow the calculation of the mass balance under non-steady state conditions, and, whilst it assumes that compartments are well mixed within a zone, the predicted concentration data can be used in conjunction with spatially variable data such as sediment organic properties. Hence, the model output can be displayed using Geographical Information Systems software alongside other spatially variable data. The initial modelling attempt presented in this paper has shown to be useful and warrants further effort to improve the physical description of the lagoon e.g. sediment and hydrodynamics and the need to collect more monitoring data to test the models predictions. In particular, there is a need to improve our current emission estimates for a range of persistent pollutants and to carry out measurements of ambient air concentrations and freshwater inputs.