Combined Use of Watershed Models to Assess the Apportionment of Point and Non Point Load Sources to Surface Waters

The sustainable management of watersheds is critical since it deals with many aspects of the anthropogenic use of natural resources, The quality of water resources is a central issue for the management of watersheds. Over the last few years it has grown worldwide the need to implement tools for a knowledge-based management of the environmental resources and processes. Particularly there is the need to evaluate the response of the system to different pressures and different management alternatives. This is normally done through a Scenario Analysis which in many cases implies the use of modeling tools and the creation of a G.I.S. archive of the available knowledge. Understanding the source apportionment of pollutant loads driven through the watershed to a coastal lagoon or evaluating the socio-economical consequences of a new environmental policy or the effect of a natural or not natural catastrophe are all issues that can be studied through a sound scenario analysis. On this perspective, diffuse pollution's effect on surface and groundwaters; is often difficult to quantify. Diffuse pollutants enter the environment through a multitude of pathways at different temporal scales. During rainfall events most of the diffuse pollutant load follows the surface runoff pathways and, at the net of the plant uptake, reaches the aquifers. A fraction of this load follows the sub-surface runoff pathways and may possibly reach the surface waters after a certain time lag. Very rarely the sub-surface pollution events can be directly correlated to a specific rainfall event. In this study some case studies are presented where river and watershed modelling tools are combined with factor analysis techniques to identify the effect of the sub-surface runoff on the water quality of specific stream reaches. Through this approach the sub-surface runoff was proven to be a significant source of diffuse pollution even in dry weather conditions.