Abstract
Uranium oxide particles were dispersed into the environment from a factory in Colonie (NY, USA) by prevailing winds during the 1960s and '70s. Uranium concentrations and isotope ratios from bulk soil samples have been accurately measured using inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) without the need for analyte separation chemistry. The natural range of uranium concentrations in the Colonie soils has been estimated as 0.7–2.1 μg g − 1 , with a weighted geometric mean of 1.05 μg g − 1 ; the contaminated soil samples comprise uranium up to 500 ± 40 μg g − 1 . A plot of 236U/ 238U against 235U/ 238U isotope ratios describes a mixing line between natural uranium and depleted uranium (DU) in bulk soil samples; scatter from this line can be accounted for by heterogeneity in the DU particulate. The end-member of DU compositions aggregated in these bulk samples comprises (2.05 ± 0.06) × 10 − 3 235U/ 238U, (3.2 ± 0.1) × 10 − 5 236U/ 238U, and (7.1 ± 0.3) × 10 − 6 234U/ 238U. The analytical method is sensitive to as little as 50 ng g − 1 DU mixed with the natural uranium occurring in these soils. The contamination footprint has been mapped northward from site, and at least one third of the uranium in a soil sample from the surface 5 cm, collected 5.1 km NNW of the site, is DU. The distribution of contamination within the surface soil horizon follows a trend of exponential decrease with depth, which can be approximated by a simple diffusion model. Bioturbation by earthworms can account for dispersal of contaminant from the soil surface, in the form of primary uranium oxide particulates, and uranyl species that are adsorbed to organic matter. Considering this distribution, the total mass of uranium contamination emitted from the factory is estimated to be c. 4.8 tonnes.
Highlights
The by-product of uranium enrichment, where the fissile isotope 235U is artificially concentrated for use as nuclear fuel or weapons, is depleted uranium (DU)
The location is on a small terrace of the stream that flows from the National Lead Industries (NLI) site, and adjacent to an old rubbish tip on land cleared during the 1960’s
Eighteen uranium oxide particles from Colonie have been individually analysed by MC-ICP-MS, with a mean 234U/238U ratio of (7.3 ± 0.1) x10-6, which is in close agreement with the present estimate (Chapter 6 / Lloyd et al unpublished)
Summary
The by-product of uranium enrichment, where the fissile isotope 235U is artificially concentrated for use as nuclear fuel or weapons, is depleted uranium (DU). A number of other studies have characterised individual particles from environmental samples contaminated by the combustion of uranium metals and DU munitions Török et al 2004; Salbu et al 2005; Lind et al 2009) This aim of this environmental case-study is to identify the processes that affect the distribution and bioavailability of uranium from DU particulate contamination. The field site offers an accessible and large contamination footprint, with the absence of confounding contamination from DU metal fragments, and more than 25 years of environmental processing It is an attractive analogue for battlefield contamination, and potentially for epidemiological studies related to DU particulate inhalation exposure. Natural environmental samples comprise variable trace quantities of uranium, but anthropogenic contamination can be determined from 235U/238U. The dataset demonstrates the clarity for resolving DU contamination that precise 235U/238U ratios coupled with high productivity offer
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