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TOXSWA is a computer model that simulates pesticide concentrations in the water and sediment layers of a water body. The theoretical concepts are briefly discussed. A sample simulation is presented to guide the user in running TOXSWA 1.2. The input and output files are described. The use of a graphical user interface for writing the input, running the program and creating the graphical output are discussed. Guidelines are given for estimating the input parameters.

The TOXSWA model will be used to estimate exposure concentrations for aquatic organisms in the Dutch pesticide registration procedure. Applying a model in the registration procedure requires standard scenarios. A gross number of 24 scenarios were defined, based on worst case values of three sensitive system parameters (water depth, flow velocity, and macrophyte mass). In concurrency with developments at EU level on standard `waterbodies' we here propose scenarios representing ditches with a water depth of 0.30 m, without macrophytes, and a flow velocity of 10 m d-1 for spring and summer and 100 m d-1 for autumn. For these standard scenarios graphs have been constructed in which the momentary exposure concentration is given as a function of the coefficient describing sorption to sediment and suspended solids, based on the organic matter content. Other graphs give the average exposure concentrations as a function of the dissipation half-life (comprising transformation in water and volatilization).

The TOXSWA model describes the fate of pesticides entering field ditches by spray drift, atmospheric deposition, surface run-off, drainage or leaching. It considers four processes: transport, transformation, sorption and volatilization. Analytical andnumerical solutions corresponded well. A sample simulation showed that sorption to macrophytes can considerably reduce the concentration in the water phase. However, sorption to macrophytes leads to a slower transport and thus to longer residence times for the pesticide in the water layer. Sedimentation and resuspension of suspended solids were not considered, so that the model can only be applied for periods shorter than one month.

TOXSWA is a computer model that simulates pesticide concentrations in the water and sediment layers of a field ditch. The theoretical concepts are briefly discussed. A sample simulation is presented to guide the user in running TOXSWA. The input and output files are described. Writing the input files with the aid of a user-friendly input editor, running the program and creating the graphical output are discussed. Guidelines are given for estimating the input parameters.