Geochemical Fluxes in the Geosphere:Quantitative Understanding by Identification and Verification of Processes

An overview is given over the use of generic and site-specific information on natural geochemical fluxes and resulting concentrations in various compartments in the geosphere and surface environment for the evaluation of the possible role of a geological nuclear waste repository as a component participating in natural geochemical cycles. Use is made of information from geochemical prospecting, geochemical mapping, weathering and river catchment studies as well as case studies at natural anomalies. Following the more globally orientated prestudy an overview is given over the compilation of a generic and site-specific geochemical flux and concentration data base of selected elements in Finland for use as complementary safety indicators to be compared with calculated repository releases. In the data base site-specific data is given for the Palmottu natural analogue site (U mineralization) and four repository investigation sites. Specific geological, hydrogeological, geochemical, topographical and climatic features of the glaciated terrain in Finland are the main reason that geochemical fluxes are mostly limited to the surface environment and influences mediated by groundwater transport from the depth are difficult to identify. The methodology of the use of natural safety indicators has some potential for improvement. The importance of the geochemistry of the groundwater-rock system and the elements, in particular, in the site-specific data on U fluxes became evident. A deeper understanding of the driving forces, transport mechanisms and parameters and relevant spatial and time scales behind natural geochemical cycles in differentgeological settings is needed to enable a validating evaluation of the suitability of concepts of chemical stability and radionuclide migration, scenarios such as glacial cycles and models of coupled chemical transport and could allow confidence building into the repository safety case by evaluating uncertainties and conservativeness using complementary modelling and multiple lines of reasoning. Observations in nature comprising groundwater-mineral interactions and U mobilization and fixation could be confirmed by reactive coupled chemical transport modelling. The relevant time scales behind the transport processes could be defined by dating and preliminary conclusions were achieved concerning glacial meltwater intrusion as the triggering mechanism. The transferability of this geo-information to a larger regional scale and in the time scale was confirmed. Some additional dating and analytical data on the Palmottu water-rock system and comparable sites is still necessary. Pretests for the application of advanced concepts and modelling tools at the repository site Olkiluoto were initiated in order to improve the regulator’s capability to evaluate a safety case focussing on geochemical stability and radionuclide migration issues