Environmental concern due to handling of hazardous chemicals is growing. This issue draws stakeholder attentions more than before to risks associated with accidental spillage in industry or traffic. This study aims at addressing the risks resulting from the spillage of one metric tonne nonylphenol from an imaginary traffic accident. The environmental risk assessment approach outlined in this study attempts to address the concern for the potential impact of hazardous substances on the environment by examining both exposures and effects of such incidents on the structure and function of the ecosystem. Nonylphenol has been selected as the discharged contaminant in this thesis for these reasons. It is an organic liquid with low vapour pressure. It is not produced in Sweden. About 2400 tonnes are imported yearly. It is mainly used for making nonylphenol ethoxylates, which have a wide use as detergents, emulsifiers, lubricants and additives in a variety of industries. It is released from the ethoxylates in waste water. There are some published reports on its toxicity as well as endocrine property to species. In this study the exposure concentrations are predicted through developing a multimedia fateexposure model for the Göta älv fresh water ecosystem. It is a dynamic version of QMXfugacity model applicable for river basins. This fate model is integrated with a simplified food web model in order to quantify the extent of nonylphenol concentration in organisms. Moreover the dose response correlation derived from the most validated experimental studies is utilized to estimate Predicted No Effect Concentration for aquatic ecosystem. The probability of accidental spillage of nonylphenol is extremely low and is not part of this study. On the other hand the consequence of spillage affecting the ecosystem is treated from several aspects, mainly by using the PEC/PNEC ratio. In the aquatic ecosystem pelagic (free water) and benthic (bottom zone) organisms are studied. Estimated risk concerning the spillage suggests that acute toxicity among pelagic organisms is plausible up river especially in the Trollhättan region. However sub-lethal effects such as reproduction and growth inhibition will probably be observed all along the river with most concern in up river. In the sediment phase the benthic organisms are shown to be put at risk for a prolonged period of time and organisms may suffer from chronic toxicity. In addition the sediment acts as a sink for contaminant with potential release of the hazardous substance. However, it is difficult to predict a full extent of adverse consequences. But it seems that sublethal effects on benthos and consequent side effects on other populations should be concluded as the most important direct consequence of a nonylphenol spillage.