Contribution of point sources and diffuse sources to nitrogen and phosphorus loads in lowland river tributaries

Human activities may cause enhanced loads of total N (TN) and total P (TP) in running waters, which may have a serious impact on their ecological quality. Likely sources for N and P are industries, dwellings and agriculture. A GIS based model is proposed to analyse which of these sources cause the main problems. Simulated concentrations were compared with standards for TN and TP representing natural conditions in running water. The model was applied on the strongly disturbed catchment of the river Dommel (Belgium/The Netherlands). It was taken into account that river sediments may act as a sink for nutrients. Flows from point sources and dwellings were taken from databases. Diffuse sources were assumed to be a percentage of the anthropogenic input, defined as the nutrient input on the soil minus crop yield. A regression analysis on ten headwaters without point sources suggests that 5.5% of anthropogenic N and 0.02% of anthropogenic P has leached to the river network. Applying the model for all tributaries of the Dommel yielded a good fit between observed and simulated TN and TP loads (R2 of 0.88 and 0.95, respectively). All analysed tributaries exceed the standards for TN and TP. Analysis suggests that point sources are -overall- the main contributor to nutrient fluxes, although they affect few tributaries only. Most intermediate and small tributaries are affected by diffuse flows from agricultural pastures (24.2 kg TN ha-1 yr-1; 0.82 kg TP ha-1 yr-1). It can be argued that reduction of point sources may help to minimise the magnitude of the nutrient fluxes on the catchment scale, reduction of diffuse sources may help to restore a good water quality on the scale of individual tributaries.