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

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 (15×50 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.