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Title: Soil migration, plant uptake and volatilisation of radio-selenium from a contaminated water table

item Ashworth, Daniel

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2006
Publication Date: 8/2/2006
Citation: Ashworth, D.J., Shaw, G. 2006. Soil migration, plant uptake and volatilisation of radio-selenium from a contaminated water table. Science of the Total Environment. Vol 370:506-514

Interpretive Summary: Selenium-79 is a radioactive isotope with a half-life of around 1 million years. It is found in the waste from nuclear power stations. One option for its disposal is burial in a geological repository, i.e. several hundred metres underground. However, if such a repository were to leak, the potential for long half-life radionuclides to be transported upwards, and hence become an environmental and human concern, needs to be studied. The processes that control the upwards movement of radionuclides were considered in this experiment, using soil columns in the laboratory. Radioactive selenium solution was introduced to the soil through a water table at the base of the column and its: upward movement through the soil; its transfer into grass growing on the column surface; and its volatilisation (emission as a gas) from the soil surface and grass leaves, were determined. Water was able to move freely from the water table and upwards through the soil column, due to evaporation from the soil surface and transpiration from the grass leaves. However, anoxic conditions at the base of the soil columns (within the water table) led to the radioactive selenium converting from a soluble form to an insoluble form. As a result, the radioactive selenium was found to accumulate at the bottom of the column. Because most of the grass roots did not penetrate to this depth, only small amounts of radioactive selenium were found in the grass leaves. These processes indicate that 79Se from a geological repository is unlikely to significantly contaminate the surface environment, and hence the human food chain. Nevertheless, volatilisation of the radioactive selenium occurred from both the soil and the grass leaves; a process by which atmospheric contamination may occur. The findings will be useful in predicting the risk associated with radioactive waste disposal in a geological repository

Technical Abstract: The properties of 79Se make it of likely potential importance in safety studies for geological disposal of radioactive wastes. Despite a substantial literature on toxic and nutritional aspects of selenium in the environment little consideration has been given to the behaviour of radioactive selenium and its potential transfer from a radioactive waste repository to the biosphere. Column experiments (15 x 50 cm), using a sandy loam soil, indicated that the upwards migration of 75Se (as a surrogate for 79Se) from a contaminated water table was dependent upon the redox status of the soil. Low redox conditions within the water table strongly limited upwards 75Se soil migration, presumably due to the immobilisation of reduced Se species. Under natural conditions, 79Se from a radioactive waste repository is therefore likely to accumulate at considerable depth. As a consequence, its absence from the rooting zone is likely to limit its transfer into plants. Nevertheless, the column experiments indicated that when an overlap between roots and soil contamination occurs, uptake into the plant is observed. Quantification of 75Se volatilisation from the column surfaces suggested that this is a significant pathway by which 79Se may move either directly from soil to the atmosphere, or from soil to plants and then to the atmosphere.