The safety assessment of a deep repository for nuclear waste poses challenging scientific and technical questions. The risks from leakage of radionuclides from the repository, including transfers to the biosphere and the food chain must be assessed. This involves complex and poorly understood interactions between groundwater, soils, plants and the atmosphere. A unique, multidisciplinary experimental and modeling program at Imperial College London has been funded by UK NIREX to develop the science and to produce modeling tools to interpret and generalize the experimental data for safety assessment. This monograph brings together for the first time the accumulated results and experience from almost two decades of research. The results have important implications for the safety assessment of nuclear waste worldwide and provide new insights into the geochemical and biological controls on the upwards migration of radiochemicals in the near-surface environment.