Assessment of pollution loads and toxicity effects on urban streams due to combined sewer overflows

The Navile Channel (Bologna, Italy) is an ancient artificial water course derived from the Reno river and it is the main receiving water body for the urban catchment of Bologna and also for the WWTP main outlet. Discharges usually range from few cubic meters per second, especially during dry periods in summer times, up to several cubic meters per second during wet weather. The WWTP outflow increases the Navile Channel’s flow by up to 1800 liters per second, during dry weather period. During storms the urban catchment contributes mainly through CSOs, whose harmful effect was object of previous studies (Artina et al., 2004a). To collect flow and water quality data in both dry and wet weather conditions, on the Navile Channel two measuring and sampling stations were installed, right upstream and downstream the WWTP outflow. The study shows that CSOs activate even when light rain events occur, spilling a huge amount of pollutants into the Navile Channel, with concentrations of orders of magnitude higher than in dry weather flow, showing also an acute toxicity behaviour.