The TOXSWA model version 3.3 for pesticide behaviour in small surface waters : description of processes

In the European Union (EU) the risk of plant protection products to aquatic organisms is assessed accordingto regulation 1107/2009. For this assessment the FOCUS Surface Water Scenarios have been developed. TheTOXSWA model is included in the FOCUS Surface Water Software tools to calculate the exposureconcentration in the water systems defined in those scenarios. At the national level the TOXSWA model isused to assess the exposure concentration in the water system defined in national scenarios. In this report adescription is given of the hydrology in the Dutch and EU water body systems and the fate of pesticides andtheir metabolites in water and sediment. The pesticide can enter the system by spray drift and/or drainageand run-off. The transport in the water system is described by advection, dispersion and diffusion. Theexchange at the water-sediment interface is described based on advection/diffusion. Other processes aresorption, volatilisation at the water – air interface and transformation in both the water layer and thesediment. The formation and transformation of metabolites in both compartments are described. The reportpresents an outlook on the improvements in the next version as well as on future developments.