Measuring and Modelling the Nutrients Residual Load from the Combined Sewer of the Eastern Shore of Lake Iseo

Abstract

The research is motivated by the need to understand the nutrient pollution dynamics in combined sewer overflows (CSOs) contributing to the eutrophication of Lake Iseo, the fifth largest Italian lake in terms of volume. To this end, the effectiveness of the combined sewer system along the lake’s eastern shore is assessed. The sewer’s efficiency is quantified with regard to the residual nutrients load from CSOs, which was originally reckoned at no more than 3% of the overall sewer load. A hydrodynamic model of the sewer system was developed by SWMM and calibrated by a long time series of measurements collected at two selected CSOs. This data allowed to investigate in detail the occurrence of first flush and to estimate the pollutant loads discharged during wet weather periods. The calibrated model then allowed to extrapolate the results of the year-long campaign to a 10-year simulation period providing, for the first time, quantitative information on the total residual loads to the lake. Such loads are at least 5 times larger than the design value. This research provides important insight into the potential impact of CSOs on the other deep lakes of the pre-alpine chain (e.g., lakes Como, Maggiore, and Garda in Italy), that are struggling with growing environmental stressors, opening the way to important technical and management considerations regarding remedial actions.