Modelling nitrogen transformations in the lower Seine river and estuary (France): impact of wastewater release on oxygenation and N₂O emission

A model of the ecological functioning of a drainage network (RIVERSTRAHLER: Billen, G., J. Garnier & Ph. Hanset, 1994. Modelling phytoplankton development in whole drainage networks: the RIVERSTRAHLER model applied to the Seine river system. Hydrobiologia, 289:119-137; Garnier, J., G. Billen & M. Coste, 1995. Seasonal succession of diatoms and Chlorophyceae in the drainage network of the river Seine: Observations and modelling. Limnology and Oceanography. 40: 750-765), has been developed to describe nutrient (N, P, Si) transfer processes at the scale of the whole Seine Basin taking into account human activities such as agricultural practices, waterscape and urban wastewater management. Whereas the upstream basin is strongly influenced by intensive agriculture, leading to high nitrate concentrations, the lower Seine River and estuary are densely populated. Paris and its suburbs represent alone up to 60% of the population in the basin (10.10⁶ inhabitants), causing large amounts of ammonium to be released by domestic effluents discharged downstream from Paris (of which the Achères wastewater treatment plant -WWTP- treated up to 80%). The ammonium loading is completely nitrified in the upstream fluvial estuary (300 km farther the effluent outlet), which leads to a strong oxygen deficit in summer. A conceptual representation of nitrification was constructed in which microbial compartments were taken explicitly into account, and the intermediate production of N₂O included. On this basis a physiological analysis of the two stages of the nitrification by nitrifying bacteria (ammonium and nitrite oxidizing bacteria) was carried out, as a function of the controlling factors (O₂, NH ₄ ⁺ , NO ₂ - ; Brion, N. & G. Billen, 1998. Une réévaluation de la méthode d'incorporation de 14HCO3 pour mesurer la nitrification autotrophe et son application pour estimer les biomasses de bactéries nitrifiantes. Revue des Sciences de l'Eau, 11: 283-302; Cébron, A., J. Garnier & G. Billen 2005. Nitrous oxide production and nitrification kinetics by bacteria communities naturally present in river water (the lower Seine, France). Aquatic Microbial Ecolology, 41: 25-38). A mathematical formulation of the kinetics and the parameters values were incorporated into the general model of ecological functioning of the fluvial sector and freshwater estuary of the Seine River. N₂O emissions due to denitrification were also considered. Results from summer field studies between 1998 and 2003 were used to validate the model which is able to reproduce the main spatial and temporal patterns of the activities of the microbial nitrifying communities as well as the levels of oxygen and nitrogen forms (NH ₄ ⁺ , NO ₂ - , NO ₃ - N₂O). Once validated, the model is used to examine the planned installation of a tertiary treatment at the Achères WWTP, scheduled for 2007 (a 90% reduction by nitrification of the presently discharged ammonium, and a 30% reduction of the nitrate by denitrification). The model shows that a nitrification treatment leads to a significant improvement in the oxygenation and a reduction of N₂O emission. However, only further denitrification of urban effluents, expected in 2015, would significantly reduce the nitrogen delivery to the coastal zone.