Integrated Modeling Framework for Assessing Dynamics of Façade Pollutants in the Urban Hydrologic Response

The concern for the presence of micropollutants in surface waters is nowadays increasing. Among them, biocides leached from façades have demonstrated to be particularly ecotoxic. This study deals with these chemicals from a new viewpoint. Instead of analyzing biocides wash-off at wall scale, as done so far, we present a mathematical framework to compute their release and transport at basin scale. The approach adopted firstly requires the development of a hydrodynamic model, accounting for “flashy" response of impervious surfaces. Secondarily, a nonpoint solute transport model is constructed and integrated into the hydrologic model, through the approximation of stirred reactor. Finally, a façade leaching model is upscaled and coupled to the hydrologic pollution model, in order to provide it with biocides flowrates in response to rainfall forcing. The integrated model is applied to two meso- scale urban hydrosystems in western Switzerland, the Lausanne sewer catchment and the overlapping Vuach ere river basin. Results obtained display that concentrations of studied biocides (Terbutryn, Carbendazim, Diuron) almost never exceeds the PNEC for the riverine system. Simulations also show that yearly load arriving to Geneva Lake from the two catchments is about 1600 g for each pollutant. The proposed framework indicates that, despite heterogeneities, a dynamic yet computationally “light" and parsimonious approach can be successfully employed to simulate the hydrograph and chemograph response of engineered hydrosystems. The model developed turned out to be a valuable and flexible tool for assessing impact of biocides on aquatic ecosystems and to optimize downstream protection strategies.