[en] Most countries producing nuclear energy are considering, or actively pursuing, a deep geologic repository for radioactive waste. As part of the assessment of the performance or safety of such a repository, radionuclide transport through the heterogeneous, geologic environment must be modelled. In most cases, an important migration mechanism is transport in groundwater and developing an understanding and modelling capability for how radionuclides might migrate away from the repository through the surrounding geosphere is an integral part of making the safety case for a repository. This understanding has been significantly improved through the NEA GEOTRAP Project, the results of which have been documented in a final synthesis report summarizing the outcomes of the five GEOTRAP workshops held between 1996 and 2001