Modeling of colloid-associated radionuclide transport in porous groundwater aquifers at the Gorleben site, Germany.

Abstract

Results are presented which have been obtained in prediction studies on the colloid-associated transport of radionuclides through porous groundwater aquifers overlying as geochemical barrier the planned nuclear repository site at Gorleben, Germany. The studies incorporate three-phase retardation factors for 227 Ac, 228 Th, 232 U and 210 Pb in site-specific groundwater aquifers containing humic colloids and efficiencies for Brownian deposition of colloidal carriers onto immobile sedimentary grains. It is shown that colloidbound radionuclide migration can be approximately modeled by using the basic advection-dispersion transport equation modified with three-phase retardation coefficients and colloid-chemical attachment factors due to the presence of multiple phases, i.e., fluid, colloid and immobile solid. Moreover, results of sitespecific assessment studies indicate that the transport of colloid-associated radionuclides in porous groundwater aquifers overlying the Gorleben salt dome may not occur over long distances, provided that the nuclides are transported by porous flow only. rock salt may reach waste packages so that nuclides might be released (Warnecke et al., 1984(Warnecke et al., , 1994. Therefore, for the performance of a longterm safety assessment the barrier function of the geological formations overlying the salt dome have been studied by batch, column and diffusion experiments (Lieser et al

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

“…If the disturbance of ionic strength occurs in combination with the presence of sorbing contaminants, the mobilized particles may also likely act as vectors for accelerated contaminant movement [41]. Saiers and Hornberger [31] also experimentally found that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate 137 Cs transport through column packed with quartz sand. Kaolinite mobility and the affinity of kaolinite for sorbing 137 Cs diminish with increasing ionic strength.…”

“…Saiers and Hornberger [31] also experimentally found that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate 137 Cs transport through column packed with quartz sand. Kaolinite mobility and the affinity of kaolinite for sorbing 137 Cs diminish with increasing ionic strength. They have also reported that mobility of colloids increased with increasing pH and increasing flow rate.…”

“…Faure et al [43] conducted packed column experiments with sand and 5% bentonite clay to study colloid and radionuclide transport induced by a salinity gradient. At the salt (NaCl) concentration >0.16 M no colloid particles were leached from the column and 137 Cs was transported as the dissolved species. When the salt concentration was decreased below the critical threshold concentration of 0.16 M, particles started to be mobilized and colloid-facilitated 137 Cs transport was observed.…”

“…137 Cs and 239 Pu) to colloidal particles consisting of clay minerals, oxides, and humic substances [66,149,[161][162][163]. Complexes with organic matter can result in humic colloids that are transported under favorable conditions of solution composition and water content [164 -167].…”

“…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].…”