Dynamic modelling of the fate of DDT in Lake Maggiore: Preliminary results


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Release of chemical substances significantly impacts European waters. Impacts are both direct and indirect, through degradation products, acute and/or chronic toxicity, and/or long-term effects via bioaccumulation in aquatic food chains. As the number of substances is large and measures to reduce emission are costly and time demanding, there is a necessary prioritisation of the efforts through risk assessment studies requiring exposure data, which are both scarce and difficult to obtain. This is complicated by the fact that contaminants may reach the aquatic environment and hence biota and sediments, through several routes ranging from atmospheric deposition (airshed), diffuse and/or point sources in the watershed. Furthermore, a contaminant may arrive constantly during the year or in a pulse due to an accidental release or periodically driven by environmental fluctuations.

The role of mathematical models in assessing the fate and effects of contaminants in aquatic ecosystems is continuously increasing, for a recent review see Koelmans et al., (2001). Mathematical models, once properly validated, offer the possibilities to simulate process that would not be possible to measure in reality; to forecast the results of different plausible scenarios; and to extract information about the global dynamic behaviour of the studied system which would be impossible from any field experiment. However, to develop and validate such as model a considerable amount of experimental data is required.

For these reasons, as a first step, we have developed a model to simulate the fate of DDT family, i.e. pp’DDT, pp’DDD, pp’DDE, op’DDT, op’DDD, op’DDE, in Lake Maggiore. The lake has been extensively studied from the limnological view point (Commission for the Protection of the Italian-Swiss Waters, CIPAIS, 1999a; 2000; 2001; 2002a) and also, after that in 1996 very high levels of DDT - beyond the legal limit for several edible fish species- were found, experimental data on DDT exists (CIPAIS, 1999b; 2002b; 2003) for different environmental compartments, e.g. water column, sediments, biota. This increase in DDT concentrations was responsible for the banning in June 1996 of the commercial fishing of some species. The source of the pollution was traced back to a chemical plant (CIPAIS, 1999b), which both produced and discharged DDT over several years into River Marmazza, a tributary of River Toce, which is in turn one of the major affluent of Lake Maggiore. The model is 0D and consists on a dynamic mass balance that includes a time-variable chemical transport and fate model for calculating DDT concentrations in the water column as well as and in sediments based on Farley et al., (1999) model for PCBs in Hudson river and estuarine. An overview of the processes included in the DDT model is represented in fig. 1.

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