Characterising source terms for uranium series elements in waste-related and natural geological settings

Predictions of contaminant migration rely on detailed characterisation of the transport medium, usually groundwater, and potential sources-sinks for pollutants along the transport pathway. Given the long half-lives of the actinides, it is accepted that changes in the groundwater flow regime will occur over time and this is factored into the model calculations. However, a key parameter, the source term, that is, the concentration of a contaminant released, is often treated as invariant with time or as a simple function of water velocity. This paper tests such assumptions by examining a number of actual and potential sources of uranium contamination from around the world. Evidence is presented which demonstrates that the source term is a function not only of the flow system but also of the physical-mineralogical characteristics of the solid phase(s) and, above all, the composition of the leachant. Factors which influence the release of uranium are discussed and suggestions made for evaluating source terms in both mining and waste disposal contexts. (orig.)