Modelling of impacts of combined sewer overflows pollution on urban and rural canal networks

The rapid and chaotic expansion of many urban areas, associated with increasing frequency of extreme events, may significantly affect the rainfall-runoff processes and pollutant fate and transport in surface waters. The consequences of such trends affect the combined sewerage systems (CSSs) where wastewater and rainwater mix, leading to a reduction of the efficiency of the existing drainage infrastructures and to the release of pollutants to the water bodies. In this work we developed a coupled hydrologic, hydraulic, and water quality model to simultaneously assess the effects of hydrologic events and CSS discharge on receiving waterbodies (RWBs) on both water quantity and water quality. The modelling framework consists of three modules: (i) the MOBIDIC-U software which is a distributed and raster-based hydrological model to simulate runoff and propagation in canal network in urban and rural areas, (ii) a reactive-transport module able to simulate the advective and dispersive transport and bio-chemical reactions of pollutants in the network, and (iii) an additional software module to assess the mitigation of pollution through the implementation of nature-based solutions (e.g., constructed wetlands). The main quality parameters in the model are: carbon, ammonia nitrogen (NH₃ and NH₄⁺), nitrate NO₃^-, total suspended solids (TSS) and dissolved oxygen. The application of the model to rural areas is particularly critical due to poor availability of data, in particular those related to the morphological characteristics of the network. To overcome this deficiency, we developed an algorithm that automatically extracts the topological/topographical data of the network (e.g., cross sections, elevations) to be provided as input to the model from the digital terrain model. We showcase the application of the model to a case study located in a suburban area of Milan (Italy) to evaluate the effects of sewerage overflows and runoff from polluted urban and agricultural surfaces on the water quality status of the RWBs. The results demonstrate that is possible to define mitigation strategies through the implementation of constructed wetlands with the purpose of obtaining a quality suitable for agricultural and environmental reuses.