Speciation and Transport of Radionuclides from the Chernobyl Accident within the Gideå Site

Doctoral thesis, 1993

The transport and redistribution of radionuclides originating from the Chernobyl accident were studied within a 1 km2 area in the northeastern part of Sweden (the Gideå site). Physical and chemical speciation of the fallout radionuclides were investigated on water samples collected within the study site using techniques developed especially for this purpose. The concentration of cesium activity in the creek water had become established at a rather constant level of 0.1 Bq/L after 5 years. Of the cesium activity being transported by creek water, ~ 10% is transported as particulates and the rest as cations. The transport rate of the deposited radionuclides in the soil was studied over a period of 5 years. At the end of this period, the major portion of the deposited cesium activity was found in the upper 5 cm of the soil. The distribution of radionuclides in respect to the different soil grain size fractions was determined and the results showed a greater abundance of cesium activity in the silt-clay fractions. The ability of the soil to retard the deposited radionuclides was studied by sequential leaching. This leaching investigation showed that, on average, ~ 50% of 137Cs is tightly bound to the grains, except in of the A horizon, in which ~ 70% is strongly sorbed. The less tightly bound, i.e. mobile, part of the 137Cs activity makes up approximately 15%. Laboratory experiments were performed to investigate the sorption of the monovalent cations sodium, rubidium and cesium on soil collected within the Gideå site. Batch experiments on soil from different horizons showed a substantially greater retardation of cesium than of sodium and rubidium. The sorption of cesium was most significant in the A horizon (Kd value ~ 10 m3/kg). Sorption of sodium, rubidium and cesium on different grain size fractions showed a strong sorption on clay particles, even though a notable sorption was observed on soil particles larger than 1 mm. The sorption of cesium could not be coupled to the leaching of Na+, K+, Ca2+, Mg2+ and NH4+ from the soil particles. Column experiments were performed using undisturbed and partly unsaturated soil columns. The hydrodynamic properties in the soil columns were determined by using tritiated water added to the column as a pulse. The nuclides studied were 82Br-, 22Na+, 86Rb+ and 134Cs+. The results show that 82Br- can be used as a nonsorbing tracer. Of the cations studied, only sodium could be eluted from the soil column. A one-dimensional advection-dispersion model was applied to the break-through curves obtained for tritiated water and sodium. The Kd value obtained from the column experiment for sodium was similar to the Kd value obtained from batch experiments. The model applied was found to be applicable for weakly sorbing elements transported through an undisturbed and unsaturated soil column.