STUDIES OF COLLOIDS AND SUSPENDED PARTICLES, CIGAR LAKE URANIUM DEPOSIT, SASKATCHEWAN, CANADA

Cigar Lake U deposit is located in northern Saskatchewan in the easter n part of the Athabasca Sandstone Basin, and consists of a high-grade ore body (up to 55% U) located at a depth of approximately 430 m. As p art of a study to evaluate the analog features of this deposit with re spect to a disposal vault for waste nuclear fuel, colioids (1-450 nm) and suspended particles (>450 nm) in groundwater have been investigate d to evaluate their effect on element transport through the U deposit. Tangential-flow ultrafiltration was used to concentrate particles fro m 501 groundwater samples in order to characterize the size distributi on, concentration, composition and natural radionuclide content of par ticles in representative parts of the U deposit. Although Cigar Lake g roundwaters contain particles in all sizes ranging from 10 nm to sligh tly larger than 20 mum, most samples contained a relatively high conce ntration of colloids in the 100-400 nm size range. Particle compositio ns are similar to the composition of minerals in the sandstones and or e body, suggesting that particles in groundwater are generated by the erosion of fracture-lining minerals. As a result, particle concentrati ons in groundwater are affected by the integrity of the host rock. In some piezometers the high initial concentrations of suspended particle s, which may have been drilling artifacts, decreased during the collec tion of the first 350 1. Although colloid concentrations fluctuated du ring sampling, there are no indications that these concentrations will be permanently reduced by continued groundwater pumping. The observed colloid and suspended particle concentrations in the deep groundwater s are too low to have a significant impact on radionuclide migration, provided that radionuclide sorption is reversible. If radionuclides ar e irreversibly sorbed to particles they cannot sorb to the host rock a nd their migration can only be evaluated with an understanding of part icle mobility. The data for dissolved and particulate U, Th and Ra wer e used to calculate field-derived distribution ratios (R(d)) between p articles and groundwater. The wide range of observed R(d) values indic ates that these radionuclides in particulate form are not in equilibri um with groundwater. U-series isotope data indicated that most of the U and Ra on particles was derived from groundwater. Some particles cou ld have retained their U for as long as 8000 a. The U and Ra contents of particles in the ore and surrounding clay zones are significantly h igher than in particles from sandstone, suggesting that the clay has b een an effective barrier to particle migration.