Colloids are particles that are < 1 pm in at least one dimension, and with a high surface area, that remain suspended in the water column.135,136 Colloids con­taining radionuclides can form either through condensation of particular radionuclide species by a hydrolytic or precipitation process, degradation of nuclear waste (intrinsic colloids), or through sorption of radionuclides onto colloids of other (inorganic or organic) material, for example, iron oxyhydr — oxides or humic acids (extrinsic or carrier colloids).44,109,137

Colloidal transport of radionuclide will be affected by the geochemical and physical properties of the water system.135,138 Geochemical conditions will affect radionuclide sorption to colloids (as with sorption to mineral surfaces), colloid formation and colloid stability.44,45 For example, high ionic strength can increase colloid coagulation, causing precipitation of the colloids from the water column.139 Colloidal transport can be retarded by colloid deposition at solid — water and air-water interfaces, and by straining in pore systems, whilst shear and hydration forces can mobilise colloids.135,139 However, binding to colloids and/or the formation of colloids by radionuclides can have a significant effect on radionuclide transport, with colloid-mediated transport often being more rapid than solution phase.135,140 In studies investigating plutonium migration in groundwater at the Mayak site, Russia, plutonium was found to be both in solution and colloid-associated at distances up to 2.15 km; further afield (up to 3.9km), 70-90 mol% Pu was associated with 1-1.5nm colloids, suggesting a key role for colloid-facilitated transport to the far-field environment.141 Mori et al. (2003)142 investigated the effect of bentonite colloids on the transport of radionuclides through granodiorite at the Grimsel test site, Switzerland. In the absence of bentonite colloids, only 20-30% of the injected Am(iii) and Pu(iv) were recovered, whilst with bentonite colloids, 70-85% were recovered; in both cases transport was faster than that of dissolved species. Cs1 was found to be transported both as a colloidal fraction and in solution, with colloid-mediated transport being more rapid, but Sr21 migration was retarded by sorption to fracture surfaces and was not affected by the presence of colloids.

2 Conclusions

Radioactive contamination in the environment is mainly caused by nuclear weapons production and testing and the nuclear fuel cycle. Historically, emissions to the atmosphere have mainly arisen from weapons testing, causing low-level global contamination from the fallout. Migration in the atmosphere will depend on the nature of the radioactive material and the prevailing meteorological conditions. Within aquatic systems, both terrestrial and surface, a more significant environmental problem is caused by localised high levels of contamination from weapons production and nuclear power. Transport in such environments will be controlled by physical processes such as advection and biogeochemical conditions in the system. In systems with significant flow, advection will be the dominant transport process, but as hydraulic conductivity decreases, chemical processes and conditions become increasingly important in controlling radionuclide migration. Factors such as solution phase chemistry (e. g., ionic strength and ligand concentrations), Eh and the nature of mineral phases in the system have a critical effect on radionuclide speciation, control­ling partitioning between solution and solid phases and hence migration. Understanding the complex interplay between these parameters is essential for predicting radionuclide behaviour and migration in the environment.