TOXSWA is a quasi-two-dimensional numerical model of pesticide behaviour in a small surface water system, including its sediment. It describes the behaviour of pesticides in a water body at a small scale, i. e. a ditch, pond or stream adjacent to a single field. It calculates pesticide concentrations in the water layer in horizontal direction only and in the sediment layer in both horizontal and vertical directions. TOXSWA considers four processes: (i) transport, (ii) transformation, (iii) sorption and (iv) volatilisation. The transformation rate covers the combined effects of hydrolysis, photolysis and biodegradation and is a function of temperature. It does not simulate formation of metabolites. Sorption to suspended solids and to sediment is described by the Freundlich equation. Sorption to macrophytes is described by a linear sorption isotherm but this feature is not used for the FOCUS scenarios. Pesticides are transported across the water-sediment interface by diffusion and by advection (upward or downward seepage, zero for FOCUS scenarios) (Aadriaanse, 1996, 1997, Adriaanse and Beltmann, 2009).