Coupled hydrologic, hydraulic, and surface water quality models for pollution management in urban–rural areas

Urban expansion and increasing frequency of extreme events threaten both urban drainage and receiving waterbody water quality. Combined sewer systems are particularly vulnerable, as they mix wastewater and stormwater, making them susceptible to reduced drainage efficiency and potential release of pollutants. Nature-based solutions, such as constructed wetlands, offer an effective means of pollution mitigation by providing sustainable and cost-efficient methods for enhancing water quality and reducing pollution in urban environments. This study presents a numerical modelling framework to simulate the impact of pollution and to design effective remediation strategies in mixed urban–rural networks subjected to storm events. The model consists of three main modules integrating hydrological, hydrodynamic, and reactive transport components to simulate the water and pollutants dynamics. The first module simulates runoff based on a distributed hydrological model and propagation within the canal network. A reactive transport module simulates advective–dispersive transport and biochemical reactions of pollutants. A third module evaluates the effectiveness of horizontal flow constructed wetlands for pollution mitigation. The model is applied to a suburban area near Milan, Italy, where sewer overflows enter a network of irrigation canals. The results demonstrate that the model was effective in identifying a suitable location for implementing a constructed wetland and in determining its proper size to achieve improved water quality. According to the simulations, the designed treatment system can achieve a contamination reduction of up to 23% for ammonium, 84% for nitrates, and 85% for carbonaceous BOD.