The Lambro-Seveso-Olona system case study; ecological quality standards, social and ecosystem costs of a Heavily Modified Water Body (HMWB).

The Lambro-Seveso-Olona system derives from the human regulation of the natural hydrology of the territory around Milan city area. The average population density in the Lambro-Seveso-Olona area is among the highest in Italy and Europe. Industry is also highly developed in this basin, chemical, textile, paper, pulp and food industries being the most important ones. Although at present the Lambro-Seveso-Olona system does not receive any more the untreated wastewaters of the Milan urban area, treated wastewaters constitute about half of the streamflow. Biotic communities in this river have a long history of poor quality status, having suffered great damage due to domestic and industrial discharges. Recently new chemical quality standards for macropollutants have been set by the Italian legislation as support for the good ecological status according to the WFD. This new index is very restrictive, and it makes extremely challenging the achievement of the water quality objectives for the Lambro-Seveso-Olona system. Aim of this study is to analyse through a modeling exercise the restoration possibilities of the Lambro-Seveso-Olona system, investigating both the source apportionment of the macropollutants, the discharge limits that should be set to achieve the good quality status and their corresponding cost.