Proposed standard scenarios for a surface water model in the Dutch authorization procedure of pesticides; method to define standard scenarios for determining exposure concentrations simulated by the TOSXWA model

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).