A PRELIMINARY INVESTIGATION OF THE EXISTENCE OF RADIOCOLLOIDS IN LEACHATE FROM THE NRC FIELD LYSIMETER INVESTIGATIONS

Leachate samples collected from sand and soil-filled lysimeters (of the Field Lysimeter Investigation: Low-Level Waste Data Base Development Program) were examined to determine if radiocolloid particles were present. Radiometric analyses were performed on 1.0 l volumes of leachate. Samples of these leachates were passed through polycarbonate filters with pore diameters from 0.01 mu m to 0.40 mu m One set of filters was analyzed to determine if radioactive material was trapped on their surfaces. A second set of filters was analyzed with a scanning electron microscope and energy dispersive x-ray analysis. Three radionuclides, Sr-90, Cs-137, and Co-60, were measured at 33.7 +/- 0.52, 0.20 +/- 0.03, and 0.13 +/- 0.03 Bq/L, respectively, in the sand filled lysimeter leachate. Only Sr-90 at a concentration of 8.58 +/- 0.37 Bq/l was measured in the soil filled lysimeter leachate. Amorphous particles < 0.20 mu m in diameter were observed on the filters used with the sand filled lysimeter leachate. X-ray dispersive analysis was used to quantify the fractions of high atomic number materials present in these particles. Although stoichiometrically appropriate quantities of oxygen and perhaps carbon are also present, x-ray dispersive analysis indicated that these particles were composed of Si (9%), Ca (87%), and Mg (4%). Amorphous particles less than or equal to 0.20 mu m in diameter composed primarily of Si, and spherical particles about 0.2 mu m in diameter composed of low atomic number material, were observed on the filters used with the soil filled lysimeter leachate. Beta radiation emitting radioactive material was detected on the filters. The relative activity of beta emiting radionuclides detected on the filters was observed to increase as filter pore sizes decreased. This observation provides evidence of the presence of radiocolloids. It also indicates that a minor radiocolloid transport mechanism may be occurring within these lysimeters.