Impact assessment of urban wet-weather sewer discharges on the Vernavola river (Northern Italy)

The research concerned the ecological impact assessment of urban wet-weather sewer discharges on the Vernavola river (Pavia, Northern Italy) focusing both on the sewer system and the receiving natural environment. The complexity of the urban drainage system (combined sewer networks, pumping stations, stormwater storage tanks, etc.) was characterised through in situ inspections, measurements and numerical modelling. Various dry- and wet-weather monitoring campaigns on the Vernavola river provided the water quality tracers according to Italian standards: dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, ammonium, nitrate and Escherichia coli. The quality analyses were also devoted to the study of the macro-benthic community. Both the chemical and biological river quality appeared significantly affected by sewer overflow contaminants. During storm events the receiver ecosystem was severely loaded with organic pollution (wet-weather discharges were equivalent to a civil sewer discharge of about 68.000 population equivalent) and the biodiversity of the benthonic organisms populating the stream was reduced by the physical modification of the habitat. These sampling and modelling studies provided the priority support information for promoting solutions and management strategies on the entire chain “sewer system-treatment plant-receiving natural environment” necessary to reach the stream quality objectives.