SWASH (Surface WAter Scenarios Help) assists the user in calculating pesticide exposure concentration in the EU FOCUS surface water scenarios. It is part of the exposure calculation procedure, being part of the obligatory evaluation procedure to place an active substance on List 1 according to EU Directive 91/414/EEC. SWASH encompasses: (i) FOCUS Drift Calculator, calculating pesticide entries through spray drift deposition, (ii) PRZM-3, calculating pesticide entries through run-off, (iii) MACRO, calculating pesticide entries through drainage and (iv) TOXSWA, calculating the behaviour of pesticides in small surface waters. It is linked to SPIN, a pesticide properties tool, and prepares input for the PRZM, MACRO and TOXSWA models. Via the SWASH shell the user can enter the shells of the other models to perform the PRZM or MACRO model runs needed to assess the fate of the substance in he FOCUS water body systems using TOXSWA

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.

A sensitivity analysis has been performed for the TOXSWA model, which simulates pesticide behaviour in surface waters. For 11 pesticide input parameters, including degradation rates, sorption coefficients, saturated vapor pressure and solubility, the influence on predicted peak concentrations in water (PECwat) and sediment (PECsed), as well as Time-Weighted Averages in water (TWA7 and TWA28) have been evaluated. Advanced global sensitivity methods, including Saltelli’s Sobol sequence for sampling input parameters, were applied to ponds and streams, representing low and high flow systems, respectively. Results show that sorption coefficients are the most sensitive parameters for TWA concentrations in ponds. In addition, degradation rates are sensitive parameters. The findings refine our understanding of pesticide behaviour, enabling more targeted parameterisation, and can contribute to improving guidance for regulatory exposure assessments. Compared to prior analyses, this study incorporates variable hydrology and sediment concentrations. It is recommended to extend the analysis to system parameters and pesticide entries.

The FOCUS_TOXSWA model calculates exposure concentrations of pesticides and their metabolites in watercourses and ponds, of the so-called FOCUS surface water scenarios. These concentrations are used in the pesticide registration procedure at EU level. The model concepts of TOXSWA are described briefly. The procedure for using the TOXSWA kernel (i.e. expert use) and related input files and output files are described. Concerning FOCUS_TOXSWA, the use of the graphical user interface to access the input and output is described. Input data are stored in a database. Pesticide entries resulting from drainage or runoff/erosion are accessed from separate files generated by FOCUS_MACRO and FOCUS_PRZM. Substance properties are accessed from the SPIN tool/database. Instructions for simulating a water-sediment study and a multi-year run are given.

The user-friendly shell SWASH, acronym for Surface WAter Scenarios Help, assists the user in calculating pesticide exposure concentrations in the EU FOCUS surface water scenarios. SWASH encompasses five separate tools and models. The SWASH database contains information on projects and runs created by the user. This report gives a detailed description of the necessary flow of data between the various models, to make them communicate smoothly with each other. It also specifies the installation requirements for the MACRO, PRZM and TOXSWA models. The MACRO model uses an MS-Access database to store its substance and run information, while PRZM makes use of separate data files. TOXSWA uses the central SWASH database. After completing a SWASH session the user should manually perform simulations with the three individual models.