COLLOID-FACILITATED TRANSPORT OF RADIONUCLIDES THROUGH FRACTURED MEDIA

The sorption of radionuclides on natural colloids may significantly modify their transport behaviour through fractured media, since radionuclides bound to colloids may not be subject to the important retardation mechanisms of matrix diffusion and sorption onto pore surfaces. This paper reports on theoretical and experimental work aimed at assessing the relevance of colloid-facilitated transport to repository safety analyses, with specific reference to the Swiss case. Transport models are presented, developed in conjunction with field- and laboratory-based studies of deep groundwater in the crystalline basement of northern Switzerland, in which colloid size distributions, composition and sorption properties have been measured. Various potential mechanisms giving rise to both reversible and irreversible sorption are discussed. In the first case, a simple approach is examined which is based on previously reported models of colloid transport and assumes reversible, linear sorption on colloids, for which experimental data have been obtained. It is shown that transport of radionuclides would not, in general, be significantly enhanced because of this process. A more recently developed and more complex model is then described incorporating irreversible sorption, in which case the transport of radionuclides tends to be strongly dependent on the extent of colloid-fracture wall interaction,