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Column experiments and reactive transport calculations of Eu3+ using pure mineral phases and natural sediments

Susan Britz1, Ulrich Noseck1, Wolfgang Durner2, Dieter Zachmann2
1 Gesellschaft für Anlagen- und Reaktorsicherheit mbH
2 TU Braunschweig

P 3.2 in From atmosphere to groundwater – hydrology of the vadose zone

Surface reactions related to transport and retardation processes in groundwater systems depend on geochemical conditions that vary in time and space. For long-term safety analyses of radioactive waste repositories it is of great interest to better understand and to realistically assess these geochemically driven surface reactions, since they predominate the long-term control of retardation processes.

To get an advanced insight into these processes column experiments are conducted representing geochemical conditions, which are as close to nature as possible. Typical sedimentary systems covering rock salt and clay formations in Northern Germany mainly consist of tertiary and quaternary sands and clays. Hence, natural sediments from the Gorleben site, Germany as well as pure mineral phases (orthoclase, muscovite, quartz) are applied in the conducted column experiments. To reflect changes in geochemical conditions over time parameters such as pH, ionic strength, ligand concentrations are varied in each experiment. Moreover, transient pH conditions are applied in selected columns.

 For reactive transport calculations of the conducted experiments the geochemical speciation code PHREEQC in combination with UCODE (parameter estimation code) is applied. So-called surface complexation parameters (SCP) such as surface site density, specific surface area, and protolyseis constants are derived from batch- and titration experiments. To calculate transport and retardation processes of potentially hazardous pollutants more realistically surface complexation models are developed that apply the priorly  derived SCP.

The chosen geochemical approach aims at describing surface complexation processes of an element (here Eu3+ as a homologue for Cm and Am) on each single mineral phase (here muscovite, orthoclase and quartz) that contributes to the sorption coefficient of the sediment bulk composition in nature (bottom-up approach). This approach may not only be applied to sites in Northern Germany but also to any other potential repository site where essential data of reactive transport processes are missing underlining the importance of this study.

last modified 2013-11-01