Relationship between suspended sediment concentration and total nitrogen, and comparison among basins for some Apennine rivers

Suspended sediment yield is well known as a cause of several detrimental consequences like reservoir siltation, filter clogging and water pollution. Surface runoff and groundwater are among the main vectors of pollutants and nutrients: many studies have already proved how nitrogen content has been constantly rising in aquifers and surface waters [Böhlke and Denver, 1995] and how the natural cycle of nitrogen has been modified because of fertilizers and sewage [Vitousek et al., 1997]. Finer particles and their aggregates have been proved to be the preferential vehicle for particulate nitrogen [Droppo et al., 1997; Ongley et al., 1992]. In the current study an attempt was made to gain some knowledge of nitrogen concentration patterns in a typical Italian Apennine environment. Four mountain basins ranging from 8.7 to 139 Km2 were monitored via automatic sampling devices. The gauging stations were built at the outlet of the mountain basin of some Reno river’s tributaries: the Sillaro (139 Km2), the Lavino (83 Km2) and the Savena (124 Km2), and at the outlet of the minor Gaiana torrent (8.7 Km2). Each monitoring station was equipped with an ultrasonic flow meter, interfaced to an automatic pumped sampler. Discharge data were collected on a half-hourly frequency and suspended sediment samples were taken accordingly to a volume-based sampling strategy. The collected samples were analysed to characterise suspended solids in terms of their grain size distribution and total nitrogen and with respect to the source catchment. The data were then inspected in order to assess the existence of relevant regression relationships between the measured variables. Total nitrogen in the solid fraction of samples was measured via the combustion method. First results seem to point out the existence of a direct relationship between nitrogen concentration and finer particle concentration (<20 μm), whereas inverse proportionality seems to occur between total nitrogen concentration and the presence of coarser fraction (20-50 μm). As a result, on one hand it might be concluded that maximum nitrogen loss values are in relation to factors like the presence of clayey formations, their position within the catchment and the availability of suspended particles. On the other hand it seems that agricultural practices do not affect nitrogen loss to a relevant extent, while a stronger source factor might be considered to be the existence of severe linear erosion features, as gullies and badlands.