MINOR ISOTOPES OF URANIUM AS SENSITIVE SOURCE TRACERS OF ENVIRONMENTAL CONTAMINATION

Tree bark has the potential to serve as an archive of environmental contamination, including airborne dust and aerosols from a variety of sources. In this study, we have analyzed thirteen tree bark samples from southwest Ohio in the vicinity of the Fernald Feed Materials Production Center (FFMPC), in order to investigate the utility of the minor isotopes of uranium as sensitive source tracers of environmental uranium contamination. Tree bark was sampled along an ~35 km transect from near the FFMPC towards the NNW. Concentrations of uranium in dry ashed tree bark sampled >6 km from the FFMPC range from 66 ppb to 529 ppb and do not exhibit a correlation with distance from the FFMPC. However, samples from ≤5 km from the FFMPC have substantially higher uranium concentrations, ranging from 1.4-6.5 ppm. ²³⁵U/²³⁸U ratios likewise exhibit variations with distance from the FFMPC, ranging from natural values in samples >30 km from the FFMPC to depleted uranium signatures with ²³⁵U/²³⁸U as low as 5.09x10^-3 in samples close to the FFMPC. ²³⁴U/²³⁸U activity ratios also decrease towards the FFMPC, from values slightly higher than secular equilibrium to ratios as low as 0.69, further consistent with a component of depleted uranium even in some samples with natural ²³⁵U/²³⁸U. Most notably, measurable ²³⁶U above global fallout levels was found in all samples, and ²³⁶U/²³⁸U ratios increase with increasing proximity to the FFMPC, reaching values as high as 1.2x10^-4. These results suggest that the minor isotopes of uranium, including both ²³⁶U and ²³⁴U/²³⁸U activity ratios, are significantly more sensitive tracers of environmental uranium contamination than the major isotopes (²³⁵U/²³⁸U). The presence of ²³⁶U in all samples, and the negative correlations of ²³⁶U/²³⁸U with ²³⁵U/²³⁸U ratios and ²³⁴U/²³⁸U activity ratios, further suggest that the contaminant includes a component of reprocessed nuclear fuel and is a mixture between depleted and enriched uranium. Calculations of the fraction of contaminant uranium based on ²³⁵U/²³⁸U ratios alone would therefore underestimate the influence of the contaminant uranium in the environment.