Influence of humic substances on the migration of 237Np, 238Pu and 241Am in a weak loess aquifer

The influence of humic substances on the migration of radionuclides 237Np, 238Pu and 241Am in a weak loess aquifer was investigated in situ with bisectional experimental columns. The experimental columns were constructed by connecting two undisturbed aquifer medium columns together, and inserted into the aquifer horizontally through holes pre-prepared in the wall of a vertical Experimental Shaft in an Underground Research Facility at the CIRP's field test site. Quartz containing radionuclides 3H, 237Np, 238Pu and 241Am was introduced as point source and plane source at the central parts of the experimental columns, respectively. Outflow water of the experimental columns were collected and 3H in the outflows were analyzed for calculating the water flow velocity in the experimental columns. When the experiment was completed, the experimental columns were dismounted and cut into slices and the activities of 237Np, 238Pu and 241Am in the sliced samples were determined. The water flow velocity in the point source experimental column and in the plane source experimental column was calculated as 1.15±0.05 cm/d and 1.06±0.05 cm/d, respectively. The concentration distribution of 237Np, 238Pu and 241Am in the experimental columns indicate that, adjacent to the source, the concentration of each radionuclide is very large and decreases abruptly as the distance increases. In addition, some other high concentration regions/points were observed. This phenomenon may suggest that multiple species of these radionuclides with different mobility may exist in the in-situ aquifer condition. The calculated apparent distribution coefficients (Kd, ml/g) of the main specie of these radionuclides with and without additional humic substances are 8.90 x 101 and 8.90 x 102 for 237Np, 1.93 x 102 and 9.48 x 102 for 238Pu, 7.60 x 102 and 9.79 x 102 for 241Am, respectively. These data suggest that the introduction of additional humic substances into the aquifer may enhance the mobility of 237Np and 238Pu, while it has no obvious influence on the migration of 241Am. (orig.)