Abstract For the risk assessment of substances the dilution of effluent from the majority of waste water treatment plants in The Netherlands has been estimated. The results of this study have been applied in the risk evaluation of ditallow dimethyl ammonium chloride (DTDMAC) to estimate exposure concentrations in the surface waters. The information from a national database has been combined with a dilution model to estimate dilution factors at these waste facilities. The surface water system in The Netherlands is for an extensive part made up by polders. The results of the model show that the median dilution factor in The Netherlands is about 30. However, the distribution of the dilution factor varies from 1 for polder waters up to 100,000 in the River Rhine. In 20 percent of the cases the dilution factor is less than 5, mostly in the narrow polder waters and small tributaries. Application of the dilution factors in the case of DTDMAC results in a medium exposure concentration of 3 μg/l but it ranges from 0.01 up to 250 μg/l, showing the importance to consider the large variation of the dilution factor. Risk assessment of substances applying medium or average values will result in an under-estimation in about 50 percent of the cases of the concentration in the aquatic environment.