U, Th, Eu and colloid mobility in a granite fracture under near-natural flow conditions

Abstract

Laboratory core migration experiments were performed in a granite fracture from the Grimsel Test Site (GTS, central Swiss Alps). The flow velocity was varied (46 m yr −1 , 94 m yr −1 , 187 m yr −1 ) and solutions with 10 −6 mol L −1 U(VI), 10 −8 mol L −1 Th(IV) and 10 −8 mol L −1 Eu(III) without (cocktail I) and with addition of 2 mgL −1 bentonite colloids (cocktail II) have been injected. Results are compared with those obtained in a field study at the GTS. Flow field-flow fractionation and ultrafiltration measurements show that U(VI) is not colloid borne, different from Th(IV) and Eu(III) which are associated with colloids in both spiked natural groundwaters with and without bentonite colloid addition. The partly unretarded U(VI) migration observed at short contact times (1-2 h) diminished under near-natural groundwater velocities (∼ 46 m yr −1 ) and only a weak retardation (R f = 19.5) could be observed. Furthermore, the U(VI) mass recovery decreased with increasing contact time and was as expected unaffected by bentonite colloid addition. A groundwater colloid mediated Th(IV)/Eu(III) migration without bentonite colloid addition could be confirmed in the core experiments but only at fast groundwater flow rates. Experiments at a water flow rate of 94 m yr −1 with cocktail II demonstrated a partially bentonite colloid facilitated transport of Th(IV) and Eu(III) with 27% and 37% mass recovery, respectively. At long tracer residence times in the flow field, no breakthrough of colloidal Th(IV)/Eu(III) could be observed in both runs, indicating the strong dependence of reaction kinetics on their mobility. Reference colloid experiments using fluorescence dye labeled carboxylated polystyrene microspheres (25 nm, 50 nm and 100 nm) revealed in general a higher recovery of smaller colloid size classes increasing with groundwater velocity. Additionally, an earlier peak arrival time of colloids and colloid-associated Th(IV) and Eu(III) was observed with respect to the conservative tracer 3 H in all experiments due to pore-size or charge exclusion effects. In general, the colloid recoveries found in the laboratory core experiments are lower than those obtained in the field studies. The decrease of colloid mobility with increasing residence time suggests the occurrence of colloid attachment to the rock surfaces even under the colloid stabilizing groundwater conditions.

Size dispersion and colloid mediated radionuclide transport in a synthetic porous media

“…3) shows a peak arrival time of colloids significant earlier than that of the conservative tracer HTO. This behavior has been observed in the literature [32,33] and is interpreted as a size chromatography effect. The peak arrival times of Pu-244 and Am-241 correspond to the colloid breakthrough showing a colloid mediated transport.…”

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media

“…The study of radionuclide migration through the subsurface is an important issue in the safety assessment of nuclear waste repositories or radionuclide migration at former nuclear test sites. A number of investigations suggest that colloids dispersed in the groundwater provide an additional phase for radionuclides and enhance the transport of radinuclides [5,6,67,[136][137][138][139][140][141][142][143][144][145][146]. Radionuclide elements, especially the actinides (e.g., U, Np, Pu, Am), have a strong tendency to undergo hydrolysis and to form colloidal particles by precipitation [65 -67,142,147,148].…”

Interaction of bentonite colloids with Cs, Eu, Th and U in presence of humic acid: A flow field-flow fractionation study

“…This technique has been previously used to characterize clay and humic colloids (Plaschke et al, 2001;Bouby et al, 2008) and to study their interaction with actinides or lanthanides as their chemical homologues Schäfer et al, 2004). Actually, the AsFlFFF-ICP-MS experiments allowed the study of the evolution of metal ion -colloid systems over a time period of 3 years at trace element concentrations.…”

“…The barrels were transferred directly to the laboratory. The chemical composition of the groundwater is characterized by a low ionic strength, low carbonate concentration and high pH (Duro et al, 2000;Schäfer et al, 2004). Details can be found in Table S1 of the Supplementary data file, see Appendix A.…”