Natural organic substances in granitic groundwater and their implications for nuclear waste disposal

Natural organic substances such as humic and fulvic acids can influence the chemistry of radionuclides in groundwater which is of interest for the long time performance of a repository for radioactive waste. Organic complexes and colloids might impair the function of the rock as a retention barrier to radionuclide transport. This paper presents an overview of the studies made to assess the importance of this issue for disposal of high level waste in crystalline rock in Canada, Finland, France, Japan, Sweden and Switzerland. Groundwater have been sampled and analyzed, experiments with radionuclides have been made and safety assessment studies have been performed. The humic substances do indeed form relatively strong complexes with some radionuclide cations such as + III actinides. It has also been demonstrated that colloid formation is influenced by dissolved humic substances. However, in granitic groundwater the concentration of dissolved organic matter is not more than about a few mg/L and humic substances are just a fraction of that quantity. According to laboratory experiments, the mobility of some radionuclides will be influenced at the low concentration levels of humic substances, but the effects are less significant from the safety point of view. In recent performance assessments of high level radioactive waste such influences as reduced sorption are either included in the uncertainty of the sorption coefficients or compensated for by means of a minor adjustment downwards of these parameters. However, in addition to the simplified treatment in performance assessment, studies are being carried out in order to deepen our understanding of the phenomena involved and increase our confidence in the measures taken. (authors). 30 refs