An integrated modelling approach to forecast the impact of human pressure in the Seine estuary

Within the framework of the European Water Framework Directive, the Seine- Normandie Water Agency has defined prospective scenarios describing possible trends of evolution of the pressures on water resources. In order to evaluate the resulting water quality improvement or degradation of water bodies in the Seine river basin, an integrated modelling was proposed. The approach consisted in coupling three models, the SENEQUE model for upstream sub- basins, the PROSE model for the Seine river and main tributaries and finally the SiAM1D model for the downstream estuary. After consistency verification, the integrated model was applied to scenarios proposed by the Seine- Normandie Water Agency. As a result of improvement in the nitrogen treatment by waste water treatment plants, the annual load of ammonia at the basin scale will be reduced by 65%. The oxygen and ammonia criteria in the estuary will improve from '' bad '' to '' good ''. However the nitrate criteria will remain '' poor '', given the strong influence of non- point sources. Despite a 70 - 75% drop of the point orthophosphate loads, the criteria for this variable will also remain '' poor ''. The nutrient levels will be high enough to maintain eutrophication in the system; a general trend to a shift from N- limitation to P- limitation will be accentuated.