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 NorthEastern 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.

• The dry weather sources apportionment for the Dese-Zero watershed was quantified by means of QUAL2E model. Out of a total dry weather nitrogen load of about 510 t N y−1, QUAL2E simulations outlined a point source contribution (WWTP and industrial discharge and direct sewer discharge) of about 20%. The remaining 80% of the total dry-weather load was attributed to the loads deriving from the groundwater recharge and irrigation channel network. • The rain-driven nitrogen loads (i.e. surface runoff loads) and the total annual nitrogen load were quantified by means of the SWAT model. Runoff diffuse loads were found to be of about 200 t N y−1 out of a total annual load of about 700 t N y−1. • On an annual basis, the total source apportionment was found as follows: about 30% were surface runoff loads, about 15% were point sources and about 55% of the total annual load was found to be constituted by not raindriven diffuse sources. • These results underline the importance of diffuse loads either during dry or wet weather conditions. Environmental policies aimed to reduce the diffuse load impact on river basin should take into account these multiple diffuse sources (e.g. runoff or groundwater loads) in evaluating the response time of the abatement strategies.