IN-SITU RADIONUCLIDE TRANSPORT NEAR THE NOPAL I URANIUM DEPOSIT AT PEÑA BLANCA, MEXICO: CONSTRAINTS FROM SHORT-LIVED DECAY-SERIES RADIONUCLIDES

For nuclear waste management, an important mechanism by which radioactive waste components are isolated from returning to the human environment, the biosphere, is by the geological barrier. The effectiveness of the barrier is characterized by in-situ retardation factor, i.e., the transport rate of a radionuclide relative to that of groundwater. As part of natural analog studies of the OSTI program of the U. S. Department of Energy, we are performing such a characterization by using naturally-occurring decay-series radioisotopes as an analog. We collected large-volume (>1000 liters) groundwater samples from three wells (PB, Pozos, and PB4, respectively) near the Nopal I Uranium Ore site at Pena Blanca, Mexico, by using an in-situ Mn-cartridge filtration technique for analysis of short-lived decay-series radionuclides. Results show that the activities of short-lived radioisotopes (Ra-228, Ra-224 and Ra-223) and activity ratios of Ra-224/Ra-228 and Ra-224/Ra-223 are higher at PB and Pozos than at PB4. In contrast, the Po-210 activity is much lower at PB and Pozos than at PB4. The high Ra activities and activities ratios at PB and Pozos are attributable to the high alpha-recoil input from the aquifer fractures, while the high Po-210 activity at PB4 is due to the enhanced colloidal transport. Based on a uranium-series transport model, we estimate that the in-situ retardation factor of Ra is (0.43±0.02)x10^3 at PB, (1.68±0.08)x10^3 at Pozos, and (1.19±0.08)x10^3 at PB4 and that the mean fracture width in the aquifer rocks is about 0.23 &mum at PB, 0.37 &mum at Pozos, and 4.0 &mum at PB4, respectively. The model-derived large fracture width at PB4 provides additional evidence to support the inference from the Po-210 measurements that particle-reactive radionuclides are transported mainly as colloidal forms through the large fractures in groundwater. Our model also suggests that in addition to alpha recoil, decay of Ra-226 from the adsorbed phases on the fractures contributes a significant source of Rn-222 to groundwater. The information obtained from this study provides useful testing and validation for the Yucca Mountain total system performance assessment model (TSPA).