The concentrations of 1,2-dichloroethane due to a spill into the Rhine River are calculated with a one-dimensional dispersion-advection model. Particular attention was given to the hydrologic variety along the river by a careful selection and estimation of the governing parameters. The calculated results are in good agreement with observed concentrations for three sites of measurement at the middle Rhine. At two neighboring sites in the lower Rhine there is a striking discrepancy. Using Monte-Carlo simulations, it can be shown that uncertainties of the input parameters cannot explain the overestimated concentrations. Possibly, the elimination is increased by shallow water zones in the lower Rhine.