Emissions of the urban biocide terbutryn from facades to soil and stormwater system estimated over three decades

Urban biocides, such as terbutryn, are widely used in facade paints and renders to reduce algae growth and are released with each rain event. Cities are increasingly adopting sustainable stormwater management associated to the sponge city concept, involving infiltration and retention systems and less water in sewer systems. Although biocides and their transformation products have been detected in groundwater, knowledge of the distribution and the infiltration of biocides and transformation products on the district scale is currently missing. Here we aimed at comparing classical and sustainable stormwater management (i.e., infiltration trench and pond) in terms of distribution and hot spots of biocide infiltration at the district scale (2.4 ha). In addition, we assessed the transport of biocides and their transformation products (TP) from facades to soil and stormwater retention systems. We combined a field campaign (6 months), including regular soil and water sampling and description of the water balance using hydroclimatic parameters and land use data, with a modelling approach to estimate and predict biocides leaching and degradation over three decades. The concentrations of terbutryn and TP (from 3 to 300 ng.L^-1)in water regularly exceeded the predicted no effect concentrations (PNEC) of terbutryn. Our modelling approach underscored prevailing (27-73%) biocide infiltration towards groundwater close to facades while a smaller biocide fraction (7-39%) reached the infiltration trench and the pond. The model enabled estimating the distribution of biocides and transformation products among urban compartments, i.e., topsoil close to facades, infiltration and retention systems and deeper soils toward groundwater, for various urban surface-soil interfaces. The interfaces included infiltration through gravel layer close to facade, pavements and vegetated soils and infiltration through trenches, ponds and wells. By comparing integrated scenarios of water management and painting of facades, our results are a first step to evaluate the chronic emission of biocides and TP from facades to evaluate risks and benefits of transition scenarios towards a biocide-free city.