Abstract <p>The ecosystem approach to assessing the object of accumulated environmental damage (AED) has been implemented; it consists of the possibility of using bioassay methods together with chemical-analytical methods for the environmental assessment of subsoils and bottom sediments of the closed authorized landfill located in the Gulf of Finland basin. It has been revealed that the pollution of the basin territory is caused by the long-term storage of mixed landfill masses without observing environmental protection technologies. It has been established that the current regulatory and procedural guidelines do not provide for a reliable accounting of the factors of influence of AED objects, which leads to distortions of the parameters of eco-safety of territories and assessments of the impact on the surrounding geosystems. New pollutants with high toxicity can be formed at the objects of accumulated environmental damage. Their presence cannot be predicted only on the basis of chemical analysis. In this regard, the environmental control system should involve integral methods for assessing the toxicity of natural environments, which include bioassay. Bioassay, which performs the function of tactical control, allows us to approach the assessment of the impact of AED objects on the ecosystem and quickly establish the danger of the object for the environment. In this work, ecotoxicological studies of subsoils and bottom soils by bioassay methods have been carried out using a set of bioassay systems, including representatives of different trophic levels (crustaceans, infusoria, algae, and seeds of higher plants). The study has used eluate and contact bioassay methods, which enabled us to determine the most sensitive bioassays. The results of the research make it possible to properly justify scientific and applied solutions for the restoration of the cumulative technogenic background of natural and economic systems located in the Gulf of Finland basin.</p>