Kolloidtransport i vattenförande sprickor
Forskare:
Colloids, particles in the size range of 1-1000nm, have due to their large specific area high affinity for different contaminants. Colloidal clays have because of their negatively charged surfaces in neutral to alkaline pH, high sorption capacity for metal ions. Due to colloids potential impact on radionuclide mobility and transport, it is very important to study colloids impact on radionuclide transport in a deep bedrock repository. In deep granitic groundwater colloids originating from clay, silica, iron oxides, organic degradation products and micro-organisms can be present, although at extremely low concentrations at present-day conditions. In the beginning of the lifetime of a deep bedrock repository in Sweden, it is safe to say that colloidal transport will not be important, since the conditions are such, mainly due to high salinity in the groundwaters, that colloids will not be stable. However, due to climate changes, the conditions may change with time.
In Sweden, glacial cycles can be expected, with the first for coming in the magnitude of 10 000 years from now. When the ice melts, dilute glacial water might intrude to repository depth. In the worst case scenario the melt water will intrude with a high flow and exchange fast with the existing old saline groundwater. No mixing will take place but the glacial water will intrude with a front and press away the old groundwater. Long times are needed to equilibrate the mineral surfaces with the dilute water to increase the salinity. Bentonite buffers in contact with dilute water in water bearing fractures can give plausible release of colloids. The colloids have been shown to be stable in dilute waters, whereas if the conditions are fulfilled for bentonite erosion, the colloid transport will be the limiting step. If large amounts of bentonite colloids are transported away from the buffer the functionality as a barrier is in danger. Also eroded colloids can if stable transport sorbed radionuclides from a leaching canister, out towards the biosphere. However, it is not necessary so that bentonite erosion will give bentonite colloid transport away from the buffer. Retardation processes in the system depend on fracture characteristics such as flow, aperture distribution, surface roughness and mineral composition. They will also depend on the colloid characteristics namely size distribution, conformation, surface charge and density. Experiments are performed in lab concerning colloidal stability, colloidal interactions, and colloidal transport in small scale. Field experiments are performed at the Grimsel test site in Switzerland.
