A method is developed for the assessment of the distribution and the associated hazard due to a long-term release of long-lived alpha-emitting radionuclides into surface waters of a marine ecosystem. The methodology is designed to identify various environmental compartments and the processes occurring within them, which are of importance in affecting the behaviour and thus the distribution of actinide elements in such systems. The compartment model system (box model) chosen is applied to an imaginary coastal area. Various processes in the environmental compartments are considered separately and then assembled to show their combined interactions. Using the concept of critical nuclide-pathway-group analysis, an attempt has been made to define a number of the most important pathways by which actinides released into the aquatic environment could return to man, and especially those related to the exploitation of aquatic food resources. The concentration levels for the conditions used as an example for the critical group considered in the present work produce rather low dose rates to man of less than 3% of the maximum permissible intake. The development of the present methodology indicates that the low dose levels strongly depend upon the concentration factors of the various biological species, as well as upon source-term activities. The concentration factors used for the biological transfer of actinides relate to the water activity only. In the case where highly radioactive sediments or sedimentary-associated material were closely involved in the uptake pathway, actinide transfer to man could become more relevant. The present study shows that sedimentation and bottom sediment absorption represent the major reconcentration processes for actinides released into surface waters. In parallel to improvements of the assessment methodology, there is a need of continued radiological research with special emphasis on the benthic zone in order that the significances of the long-term sediment-biota interactions (sink-source role of sediments) on the transfer of actinides to man may be understood.