Estimating current and future urban biocide emissions from building facades at the city scale

Cities are pivotal to climate change mitigation and environmental sustainability, yet controlling urban pollutant emissions from buildings remains a major challenge. Among these, biocides such as terbutryn pose significant ecotoxicological risks, particularly within stormwater systems. To quantify biocide emissions from building facades, as well as their transport and transformation, a reactive transport model that integrates national GIS datasets with municipal water service data was developed, ensuring broad applicability. Using Strasbourg, France (79 km²) as a case study, biocide emissions from building facades during rainfall were estimated, as well as their infiltration into urban soils, and their transport through stormwater systems to wastewater treatment plants and receiving rivers. Under current conditions, facades emit 5.8 mol a^-1 of terbutryn and its primary transformation product, equivalent to 177 mg ha^-1 of city a^-1 or 0.501 μmol m^-2 of facade a^-1. Runoff concentrations near facades exceed ecotoxicological thresholds by 10-20 times, and by 1.5-3 times at district outlets. While 77 % of the biocide loads infiltrate urban soils, rising to 89 % under a "sponge city" scenario, degradation in the 0-1 m topsoil remained limited. This leads to sorption-driven accumulation and potential groundwater contamination of biocides. Notably, even under a zero-biocide scenario, 42 % of previously infiltrated biocides remain in soils after 20 years, underscoring their environmental persistence and long-term contamination risks. Overall, the results highlight the urgent need for biocide-free urban design strategies. Early policy implementation is critical to limit biocide transport and accumulation in urban ecosystems and protect water quality over the long-term.