Sorption studies of H14CO3- on some geologic media and concrete

The investigation has shown that a significant retention of H14Co3- in a groundwater system may be expected in the presence of calcite- and calcium-containing materials. Higher distribution coefficients were obtained in the batch experiments than in the column experiments. Thus a lower flow rate would probably give a higher retention than what was observed in the present column experiments. In a potential waste repository the water flow rate would be low and calcite would be a common fracture mineral. Concrete and bentonite/quartz have been suggested as backfill materials in a repository. Concrete may be expected to retain a carbonate leakage completely, as long as the material is intact. Also a bentonite/quartz mixture would give a retention by at least a factor of 10. The 14C exchange for inactive carbon is also of importance when using the 14C method for groundwater dating. In the use of mathematical models for prediction of the carbon evolution in a groundwater system, a quantitative knowledge of possible sinks and sources of carbonate is required. The sorption of 14C on solid surfaces constitutes a sink that will, if not considered in the age determination, lead to too high estimated ages. Both precipitation and weathering may change the 14C/12C ratio in the undisturbed groundwater. Thus, the role of calcium-containing media, particularly calcite, in the repository/bedrock system must be considered in assessing 14Cd transport times from the repository to the biosphere as well as in the interpretation of 14C age determinations of groundwater