FINGERPRINTING NATURAL AND ANTHROPOGENIC CONTAMINANTS WITH URANIUM ISOTOPES

At uranium mining and milling sites in the Southwestern United Sates, differentiating natural and anthropogenic sources of contaminants in water resources is challenging due to relatively high background concentrations and a lack of baseline conditions prior to mining and/or milling. An alternative to having baseline conditions is to measure various hydrologic tracers, including uranium isotopes. Unlike more established tracers, an extensive review of studies using uranium isotope measurements for the explicit purpose of fingerprinting natural and anthropogenic sources of uranium near mining and milling sites has yet to be undertaken.

This work includes a compilation and statistical analysis of ²³⁴U to ²³⁸U activity ratios (AR) for dissolved uranium in samples determined to be impacted, unimpacted, or of unknown impact from mining and/or milling of uranium ores. Data were obtained from 15 different studies, 9 of which were conducted in the Southwestern United States. All datasets were non-normally distributed, therefore the median and median absolute deviation (MAD) were used to represent central tendency. The median ± MAD values are 1.16 ± 0.25 (n = 135) for the impacted samples, 1.81 ± 0.64 (n = 206) for the unimpacted samples, and 1.82 ± 0.90 (n = 81) for the unknown samples. The median values for impacted and unimpacted datasets are significantly different based on the Wilcoxon rank sum test; however, overlap of impacted and unimpacted values is noteworthy.

Ore deposits that have reached secular equilibrium with respect to uranium radioactivity are expected to exhibit AR values near 1.0, as are the mining and milling leachates of such ore due to rapid dissolution of uranium minerals. In contrast, natural weathering of uranium minerals may allow for isotope fractionation and higher AR values. Slow and incomplete dissolution will not always be the case during weathering, particularly in highly oxidizing environments. In these cases, unimpacted AR values may resemble those expected for mining- or milling-impacted samples. Uranium AR values should be used as one of several lines of evidence for dissolved uranium origins. Previous successful studies have combined AR values with sulfur and oxygen isotopic compositions of sulfate, trace metal concentrations, water age dating, and/or other chemical tracers.