par Billen, Gilles ;Somville, Martine ;De Becker, E.;Servais, Pierre
Référence Netherlands journal of sea research, 19, 3-4, page (223-230)
Publication Publié, 1985-12-01
Article révisé par les pairs
Résumé : A nitrogen budget including nitrite, nitrate, ammonium and organic nitrogen is presented for the western Scheldt estuary. The nitrogen entering the estuarine zone is evaluated from measurements of NO2−, NO3−, NH4+ and organic nitrogen concentration at Rupelmonde. These results are part of 10 years survey (1973–1983) of water quality in the Scheldt estuary. The origin of this load in the Scheldt estuary is further investigated by the evaluation of the contribution of domestic, industrial sewages, agriculture and breeding in the nitrogenous load of the upper Scheldt drainage basin. Domestic load is evaluated from the watershed population. Industrial sewages are quantified by use of the evaluation of specific nitrogen spoilage by the various industries as a function of their number of workers. Nitrogen leaching of agricultural soils has been measured by determining the nitrogen concentration in small river draining agricultural areas, upstream any domestic or industrial discharges. Cattle-farming wastes are for the biggest part spread on soils. A fraction however is directly rejected in rivers. Denitrification in the tributaries of the Scheldt is important in the control of nitrate entering the estuarine zone. Its evaluation will be presented. In the estuarine part of the Scheldt (Rupelmonde-Vlissingen), the nitrogenous load is important due to the upstream load and to the sewages of the Antwerp district. These sewages (domestic, industrial, agricultural) have been evaluated as described above for the upper Scheldt basin. The important load carried at that moment by the Scheldt gives rise to an important bacterial activity which results in anaerobic conditions. Denitrification then takes place. This process reduces NO3− to N2O and N2, i.e. eliminates a substantial fraction of the nitrate load in the Scheldt. The importance of this process will be quantified both by measurement of in situ denitrifying activities and by analysis of NO2− + NO3− profiles in the river. When reoxidation of the water occurs by reaeration and mixing with well aerated seawater, the total mineral nitrogen has a conservative behaviour as indicated by the (straight) linear relationships between Σ Nmin and chlorinity, in spite of the primary production, bacterial activities and sediment influence. This conservative behaviour of Σ Nmin is used in this work for evaluating Nmin exportation by the Scheldt to the North Sea. The straight line relation extrapolated at low salinity gives a “fictive nitrogen concentration” in fresh water. The product of this “fictive concentration” and the upstream discharge gives an accurate evaluation of the exportation flux of mineral nitrogen to the sea. This work shows the predominant role of denitrification in tributaries of the drainage basin and in the estuary itself as a nitrogen sink which reduces the amount of nitrogen exported by the Scheldt to the North Sea. It is suggested that the pursuit of the present waste water treatment policy, only based on the elimination of the organic load without any tertiary treatment, could result in increasing the nitrogen output into the Belgian-Dutch coastal zones by a factor 2–3.