EVIDENCE FOR RADIUM MOBILITY AT THE NOPAL I URANIUM DEPOSIT, PEÑA BLANCA, MEXICO

The Nopal I uranium deposit in Chihuahua, Mexico, is an excellent analogue for evaluating the fate of spent fuel, associated actinides, and fission products over long time scales at a Yucca Mountain high-level nuclear waste repository. Our previous U-series studies have shown that U, Pa, and Th have remained undisturbed in fractures in the unsaturated zone near the deposit for at least the past 200,000 years, whereas Ra shows recent (< 8,000 y) open-system behavior. Groundwater samples can provide complementary information to the fractures by revealing a more recent and instantaneous picture of U-series transport. Based on the fracture data, low U, Th, and Pa concentrations and high Ra concentrations in water samples from the unsaturated zone are expected.

 

During the last year, we have analyzed adit water samples from the unsaturated zone within the uranium deposit and saturated zone samples from wells adjacent to and several kilometers away from the deposit. Groundwater U concentrations are generally greatest for samples near the deposit. Well samples near the deposit have 0.1-18 ppm U, unsaturated zone adit water samples have 0.2-68 ppb U, and well samples distant from the deposit have 0.2-10 ppb U. However, the elevated U concentrations (ppm levels) for newly drilled well samples adjacent to the deposit may be an artifact of drilling operations and need to be verified.  Radium-226 concentrations measured by TIMS range from 5-470 fg/g for unsaturated zone waters collected within the deposit, whereas well waters distant from the deposit have lower concentrations (~0.1 fg/g). Unsaturated zone water ²²⁶Ra concentrations correspond to large activity excesses relative to ²³⁸U (²²⁶Ra/²³⁸U AR=4-113), ²³⁴U, and ²³⁰Th. These data suggest large preferential mobility of ²²⁶Ra relative to U and Th within the unsaturated zone, with a relative order of mobility of Ra>>U>>Th, in agreement with the fracture data. In contrast, limited Ra data for saturated zone waters indicate that ²²⁶Ra/²³⁸U activity ratios are less than one, suggesting a relative order of mobility in this environment of U>Ra>>Th.  This change in Ra mobility between unsaturated and saturated conditions may be due to differences in Ra solubility/complexation or kinetic effects over long transport distances.