Use of uranium isotopes as a temporal and spatial tracer of nuclear contamination in the environment

The Fernald Feed Materials Production Center (FFMPC) was established in 1951 to process natural uranium (U) ore, enriched uranium (EU) and depleted uranium (DU). This study tests the utility of U isotopic ratios in sediment cores and lichens as indicators of the aerial extent, degree and timing of anthropogenic U contamination, using the FFMPC as a test case. An 80-cm-long sediment core was extracted from an impoundment located approximately 6.7 km southwest of the FFMPC. Elemental concentrations of thorium (2.7–6.2 μg g−1) and U (0.33–1.33 μg g−1) as well as major and minor U isotopes were analyzed in the core. The lack of measurable 137Cs in the deepest sample as well as a natural 235U/238U signature and no measurable 236U, are consistent with pre-FFMPC activity. Anomalously elevated U with respect to Th concentrations occur in seven consecutive samples immediately above the base of the core (62–76 cm depth). Samples with elevated U concentrations also show variable 235U/238U (0.00645–0.00748), and all contain measurable 236U (236U/238U = 2.1 × 10−6–3.6 × 10−5). Correspondence between the known releases of U dust from the FFMPC through time and variations in sediment core U concentrations, 235U/238U and 236U/238U ratios provide evidence for distinct releases of both DU and EU. Furthermore, these relationships demonstrate that the sediment core serves as a robust archive of past environmental U contamination events. Samples in the upper 40 cm display natural 235U/238U, but measurable 236U/238U (236U/238U = 5.68 × 10−6–1.43 × 10−5), further indicating the continued presence of anthropogenic U in present-day sediment. Three local lichen samples were also analyzed, and all display either EU or DU signatures coupled with elevated 236U/238U, recording airborne U contamination from the FFMPC. -- Highlights: • Sediment core near Fernald Feed Materials Production Center has non-natural U isotopic ratios. • DU and EU signatures and highest 236U/238U correlate temporally with known U releases from Fernald. • 236U in sediment with natural 235U/238U demonstrates sensitivity of 236U as contaminant tracer. • Lichens are effective biomonitors of airborne uranium contamination