The transport of radionuclides in groundwater is a function of their stability in solution and their retention. In solution, radionuclides may be transported by diffusion or with groundwater flow in ionic or particulate/colloidal form. Retention may be the result of reversible sorption on sediment, inclusion in pre-existing minerals or by inclusion during mineral formation. Microorganisms may be important, especially in retention and chemical conversion in biofilms.