Modelling the environmental fate and behavior of biocides used in façades covered with mortars and plasters and their transformation products

Façade renders containing biocides are often used on external thermal insulation composite systems (ETICS), to avoid the growth of algae, fungi, and bacteria. The aim of this study was to model the transport of four organic parent compounds (PCs), namely carbendazim (CD), diuron (DR), octylisothiazolinone (OIT), and terbutryn (TB), as well as 10 of their transformation products (TPs) within an unsaturated soil compartment (sandy or loamy/silty/clayey soil) until they reached a point of compliance (OdB). The model was developed using van Genuchten's substance transport equation and general condition assumptions with reference to a Ground Water Risk Assessment (GRA). Factors such as soil type, percolation rate, soil organic carbon sorption coefficient (KOC), and the half-life (DT50) of TPs were found to affect not only the PC and TP peak concentrations, but also the time-to-peak at the point of compliance and the time needed for the substances to leave the unsaturated soil compartment. It was found that the model delivered concentrations of PCs at the OdB are, with exception of OIT, higher than the suggested value of 0.1 μg/L, independently if it is only sandy or loamy/silty/clayey soil according to the general conditions of the GRA. However, by including an appropriate topsoil layer with an organic content of 2% the concentrations of the PCs can be reduced significantly.