Colloid-Facilitated Transport of Radionuclides through Fractured Chalk Aquifers

In deep geological disposal facilities, bentonite backfill is designed to sorb any leaked radionuclides and prevent their release into the environment. However, when colloidal-sized particles of bentonite are eroded from the backfill, any radionuclides sorbed to them may subsequently be transported at a rate equivalent to or even greater than dissolved species, which are subject to retardation through sorption and diffusion. In this research, the influence of bentonite colloids on the mobility of U(VI), Cs and Pu(IV) is investigated. Radioisotopes of each element, along with the conservative tracer tritium, were injected into a 30-cm naturally fractured chalk core drilled from the Avdat chalk formation in the northwestern Negev Desert. Experiments were conducted in the presence and absence of 0.025 g/L suspensions of bentonite colloids prepared in a background solution of artificial rain water representative of the region. All suspensions were allowed to stir and equilibrate for at least a week prior to running the experiment. In parallel, batch experiments were conducted to calculate the sorption coefficient (K_d) of each element to both the colloids and the chalk fracture material. All laboratory work was conducted at Lawrence Livermore National Laboratory in California, USA.

Geochemical models reveal that under these conditions, nearly all (>99%) of the U(VI) remains stable as a dissolved complex with calcium and carbonate. However, because of the high surface area of the chalk and the clays contained within, only 6% of the U(VI) was recovered in the absence of colloids. This recovery increased to 25% when colloids were injected, though it still seemed that most of the U(VI) was transported as a dissolved species. In contrast, the very sorptive Pu demonstrated marked colloid-facilitated transport, where an early breakthrough of Pu relative to the tritium was observed. Additionally, most of the mobile Pu was sorbed to the colloids, and only a small fraction was recovered as a dissolved species. However, the total Pu recovery remained at only 7%, indicating that most of the Pu was sorbed to the fracture surface or flow-through cell components. Cesium exhibited very low recovery (1%), partially due to its high sorption to illite components within the rock matrix. Cesium transport in the presence of colloids has not yet been investigated. Ultimately, this indicates that sorptive species such as Pu(IV) and Cs would be expected to undergo colloid-facilitated transport, while the more stable calcium-uranyl-carbonate ternary complexes have the ability to diffuse into the pore spaces and come into contact with more available surface complexation sites, resulting in high retardation.