The sorption, fate, and vertical distribution of the fungicide Derosal with active ingredient carbendazim in sediments were investigated in indoor freshwater microcosms. Carbendazim penetration in the sediment was measured as a function of time and depth. Freundlich sorption parameters were determined by batchwise equilibration of carbendazim in a sediment suspension. Freundlich parameters were K f ϭ 258 g (1Ϫn) L n /kg and n ϭ 0.78. Analysis of different layers of sediment cores showed a slow penetration of carbendazim in the first 6 cm of the sediment in 60 d. The penetration was successfully simulated with a multilayer model accounting for sorption to organic matter (using the measured isotherm), molecular diffusion, biodegradation, and bioturbation. The calibrated model was most sensitive to the parameters for molecular diffusion and sorption in the sediment. Bioturbation did not affect the carbendazim profiles because of carbendazim toxicity for bioturbators. So there was a direct feedback between carbendazim toxicity and fate. The calibrated model was validated using a dataset obtained from another freshwater microcosm.

Abstract-The sorption, fate, and vertical distribution of the fungicide Derosal with active ingredient carbendazim in sediments were investigated in indoor freshwater microcosms. Carbendazim penetration in the sediment was measured as a function of time and depth. Freundlich sorption parameters were determined by batchwise equilibration of carbendazim in a sediment suspension. Freundlich parameters were K f ϭ 258 g (1Ϫn) L n /kg and n ϭ 0.78. Analysis of different layers of sediment cores showed a slow penetration of carbendazim in the first 6 cm of the sediment in 60 d. The penetration was successfully simulated with a multilayer model accounting for sorption to organic matter (using the measured isotherm), molecular diffusion, biodegradation, and bioturbation. The calibrated model was most sensitive to the parameters for molecular diffusion and sorption in the sediment. Bioturbation did not affect the carbendazim profiles because of carbendazim toxicity for bioturbators. So there was a direct feedback between carbendazim toxicity and fate. The calibrated model was validated using a dataset obtained from another freshwater microcosm.