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

The influence of humic substances on the migration of radionuclides Np-237, Pu-238 and Am-241 in a weak loess aquifer was investigated in situ with bi sectional experimental columns. The experimental columns were constructed b y connecting two undisturbed aquifer medium columns together, and inserted into the aquifer horizontally through holes pre-prepared in the wall of a v ertical Experimental Shaft in an Underground Research Facility at the CIRP' s field test site. Quartz containing radionuclides H-3, Np-237, Pu-238 and Am-241 was introduced as point source and plane source at the central parts of the experimental columns, respectively. Outflow water of the experiment al columns were collected and H-3 in the outflows were analyzed for calcula ting the water flow velocity in the experimental columns. When the experime nt was completed, the experimental columns were dismounted and cut into sli ces and the activities of Np-237, Pu-238 and Am-241 in the sliced samples w ere 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 Np-237, Pu-238 and Am-241 in the experimental columns indicate that, adjacent to th e source, the concentration of each radionuclide is very large and decrease s abruptly as the distance increases. In addition, some other high concentr ation regions/points were observed. This phenomenon may suggest that multip le species of these radionuclides with different mobility may exist in the in-situ aquifer condition. The calculated apparent distribution coefficient s (K-d, ml/g) of the main specie of these radionuclides with and without ad ditional humic substances are 8.90 x 10(1) and 8.90 x 10(2) for Np-237, 1.9 3 X 10(2) and 9.48 x 10(2) for Pu-238, 7.60 x 10(2) and 9.79 x 10(2) for Am -241, respectively. These data suggest that the introduction of additional humic substances into the aquifer may enhance the mobility of Np-237 and Pu -238, while it has no obvious influence on the migration of Am-241.