Uranium isotopes in tree bark as a spatial tracer of environmental contamination near former uranium processing facilities in southwest Ohio

Abstract

Inappropriate handling of radioactive waste at nuclear facilities can introduce non-natural uranium (U) into the environment via the air or groundwater, leading to anthropogenic increases in U concentrations. Uranium isotopic analyses of natural materials (e.g. soil, plants or water) provide a means to distinguish between natural and anthropogenic U in areas near sources of radionuclides to the environment. This study examines the utility of two different tree bark transects for resolving the areal extent of U atmospheric contamination using several locations in southwest Ohio that historically processed U. This study is the first to utilize tree bark sampling transects to assess environmental contamination emanating from a nuclear facility. The former Fernald Feed Materials Production Center (FFMPC; Ross, Ohio) produced U metal from natural U ores and recycled nuclear materials from 1951 to 1989. Alba Craft Laboratory (Oxford, Ohio) machined several hundred tons of natural U metal from the FFMPC between 1952 and 1957. The Herring-Hall-Marvin Safe Company (HHM; Hamilton, Ohio) intermittently fabricated slugs rolled from natural U metal stock for use in nuclear reactors from 1943 to 1951. We have measured U concentrations and isotope signatures in tree bark sampled along an ∼35 km SSE-NNW transect from the former FFMPC to the vicinity of the former Alba Craft laboratories (transect #1) and an ∼20 km SW- NE (prevailing local wind direction) transect from the FFMPC to the vicinity of the former HHM (transect #2), with a focus on old trees with thick, persistent bark that could potentially record a time-integrated signature of environmental releases of U related to anthropogenic activity. Our results demonstrate the presence of anthropogenic U contamination in tree bark from the entire study area in both transects, with U concentrations within 1 km of the FFMPC up to ∼400 times local background levels of 0.066 ppm. Tree bark samples from the Alba Craft and HHM transects exhibit increasing U concentrations within ∼5 and ∼10 km, respectively of the FFMPC. The ²³⁶U/²³⁸U isotopic ratios in tree bark from both transects increase progressively towards the FFMPC with values as high as 2.00 × 10^-4 at the FFMPC. Tree bark sampled within 1 km of the FFMPC exhibits clear evidence for both enriched and depleted uranium with ²³⁵U/²³⁸U values from 0.00461 to 0.00736, with ²³⁴U/²³⁸U activity ratio ranging from 0.53 to 0.96, and ²³⁶U/²³⁸U from 6.05 × 10^-5 to 1.05 × 10^-4. Tree bark from transect #1 between 1 and 30 km from the FFMPC exhibits depleted and natural ²³⁵U/²³⁸U values ranging from 0.00552 to 0.00726 [²³⁴U/²³⁸U activity ratio: 0.69-1.04; ²³⁶U/²³⁸U: 2.49 × 10^-6 - 2.00 × 10^-4]. Tree bark from transect #2 sampled between 1 and ∼20 km away from the FFMPC exhibits evidence of enriched and depleted U in the environment with ²³⁵U/²³⁸U ranging from 0.00635 to 0.00738 [²³⁴U/²³⁸U activity ratio: 0.83-0.98; ²³⁶U/²³⁸U: 1.43 × 10^-5 - 2.00 × 10^-4]. Results from scanning electron microscopy with energy dispersive spectrometry provides evidence for U-rich particles as the source of contamination found in tree bark growing within 1-3 km of the former FFMPC. Such observations are consistent with the previously observed 14 μm U-rich particle identified in tree bark sampled within 1 km of the FFMPC (Conte et al., 2015). Overall, this study shows the usefulness of a tree bark sample transect to assess the areal extent of atmospheric contaminant U stemming from nuclear facilities.

Keywords: Alba Craft; Environment; Fernald; Isotope; Nuclear; Uranium.