Uranium chemical and radiological risk assessment for freshwater ecosystems receiving ore mining releases: principles, equations and parameters.

Titre du congrès :8th International Symposium on the Natural Radiation Environment (NRE-VIII)
Ville du congrès :Rio de Janeiro
Date du congrès :07/10/2007
Titre de la revue : AIP Conference Proceedings volume 1034 issue pages : 333-336

Uranium is an element that has the solely characteristic to behave as significant hazard both from a chemical and radiological point of view. Exclusively of natural occurrence, its distribution into the environment may be influenced by human activities, such as nuclear fuel cycle, military use of depleted uranium, or coal and phosphate fertilizer use, which finally may impact freshwater ecosystems. Until now, the associated environmental impact and risk assessments were conducted separately. We propose here to apply the same methodology to evaluate the ecological risk due to potential chemotoxicity and radiotoxicity of uranium. This methodology is articulated into the classical four steps (EC, 2003: problem formulation, effect and exposure analysis, risk characterisation). The problem formulation dealt both with uranium viewed as a chemical element and as the three isotopes 234, 235 and 238 of uranium and their main daughters. Then, the exposure analysis of non-human species was led on the basis of a common conceptual model of the fluxes occurring in freshwater ecosystems. No-effect values for the ecosystem were derived using the same effect data treatment in parallel. A Species Sensitivity Distribution was fitted : (1) to the ecotoxicity data sets illustrating uranium chemotoxicity and allowing the estimation of a Predicted-No-Effect-Concentration for uranium in water expressed in µg/L; (2) to radiotoxicity effect data as it was done within the ERICA project, allowing the estimation of a Predicted No-Effect-Dose-Rate (in µGy/h). Two methods were then applied to characterize the risk to the ecosystem: a screening method using the risk quotient approach, involving for the radiological aspect back calculation of the water limiting concentration from the PNEDR for each isotope taken into account and a probabilistic risk assessment. A former uranium ore mining case-study will help in demonstrating the application of the whole methodology.