Migration of colloids may facilitate the transport of radionuclides leaked from near surface waste sites and geological repositories. Intrinsic colloids are favorably formed by precipitation with carbonates in bicarbonate-rich environments, and their migration may be enhanced through fractured bedrock. The mobility of Ce(III) as an intrinsic colloid was studied in an artificial rainwater solution through a natural discrete chalk fracture. The results indicate that at variable injection concentrations (between 1 and 30 mg/L), nearly all of the recovered Ce takes the form of an intrinsic colloid of >0.45 μm diameter, including in those experiments in which the inlet solution was first filtered via 0.45 μm. In all experiments, these intrinsic colloids reached their maximum relative concentrations prior to that of the Br conservative tracer. Total Ce recovery from experiments using 0.45 μm filtered inlet solutions was only about 0.1%, and colloids of >0.45 μm constituted the majority of recovered Ce. About 1% of Ce was recovered when colloids of >0.45 μm were injected, indicating the enhanced mobility and recovery of Ce in the presence of bicarbonate.
Current
research on radionuclide disposal is mostly conducted in
granite, clay, saltstone, or volcanic tuff formations. These rock
types are not always available to host a geological repository in
every nuclear waste-generating country, but carbonate rocks may serve
as a potential alternative. To assess their feasibility, a forced
gradient cross-borehole tracer experiment was conducted in a saturated
fractured chalk formation. The mobility of stable Sr and Cs (as analogs
for their radioactive counterparts), Ce (an actinide analog), Re (a
Tc analog), bentonite particles, and fluorescent dye tracers through
the flow path was analyzed. The migration of each of these radionuclide
analogs (RAs) was shown to be dependent upon their chemical speciation
in solution, their interactions with bentonite, and their sorption
potential to the chalk rock matrix. The brackish groundwater resulted
in flocculation and immobilization of most particulate RAs. Nevertheless,
the high permeability of the fracture system allowed for fast overall
transport times of all aqueous RAs investigated. This study suggests
that the geochemical properties of carbonate rocks may provide suitable
conditions for certain types of radionuclide storage (in particular,
brackish, high-porosity, and low-permeability chalks). Nevertheless,
careful consideration should be given to high-permeability fracture
networks that may result in high radionuclide mobility.
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