Modelling the point and non-point nitrogen loads to the Venice Lagoon (Italy): the application of water quality models to the Dese-Zero basin

The study analyzes the source apportionment of a river basin, the Dese-Zero river, located within the North-Eastern part of the Venice Lagoon Watershed (VLW), Italy. The Dese-Zero river basin is characterised by a very intensive agriculture and landfarming system. To assess the source apportionment, two different models have been applied: the QUAL2E model for the simulation of the dry weather scenario and the BASINS-SWAT model for the simulation of the wet weather scenario. Out of a total nitrogen load of 512 t N y−1, for a surface of 293 km2, in dry weather conditions the apportionment was found as follows: 20% point sources (i.e. WWTP and industrial discharge and direct sewer discharge into surface waters) and about 80% attributed to not rain-driven diffuse loads (i.e. groundwater/spring recharge and tributary/irrigation channels coming from bordering watersheds). As far as rainy events are concerned, the surface runoff load was of about 200 t N y−1, over a total annual nitrogen load of about 700 t N y−1. Therefore, on an annual basis, the total source apportionment was found as follows: about 30% surface runoff loads, about 15% point sources and about 55% of the total annual load was found to be constituted by not rain-driven diffuse sources.