Modeling the vertical distribution of carbendazim in sediments

The sorption, fate, and vertical distribution of the fungicide Derosal(R) w ith 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 deter mined by batchwise equilibration of carbendazim in a sediment suspension. F reundlich parameters were K-f = 258 mu g((1-n))L(n)/kg and n = 0.78. Analys is of different layers of sediment cores showed a slow penetration of carbe ndazim in the first 6 cm of the sediment in 60 d. The penetration was succe ssfully simulated n with a multilayer model accounting for sorption to orga nic matter (using the measured isotherm), molecular diffusion, biodegradati on, and bioturbation. The calibrated model was most sensitive to the parame ters for molecular diffusion and sorption in the sediment. Bioturbation did not affect the carbendazim profiles because of carbendazim toxicity for bi oturbators. So there was a direct feedback between carbendazim toxicity and fate. The calibrated model was validated using a dataset obtained from ano ther freshwater microcosm.