Anthropogenic ²³⁶U at Rocky Flats, Ashtabula river harbor, and Mersey estuary: Three case studies by sector inductively coupled plasma mass spectrometry

²³⁶U (t_1/2=2.3×10⁷ y) is formed as a result of thermal neutron capture by ²³⁵U. In naturally occurring U ores, where a high neutron flux is present from spontaneous fission of ²³⁸U, ²³⁶U/²³⁸U atom ratios are ∼10^-4 ppm. In the natural Earth's crust, unaffected by nuclear fallout, these ratios are expected to be on the order of 10^-8 ppm. Reactor-irradiated U, however, exhibits high ²³⁶U/²³⁸U atom ratios approaching 10⁴ ppm. As a result, the presence of very small quantities of reactor-irradiated U will significantly enhance the 'background' ²³⁶U/²³⁸U atom ratio. When sufficiently elevated ²³⁶U/²³⁸U ratios are present, the determination of ²³⁶U/²³⁸U by rapid inductively coupled plasma mass spectrometric (ICPMS) methods is attractive. We have used sector ICPMS at medium resolving power (R=3440) to measure ²³⁶U/²³⁸U atom ratios with a determination limit of 0.2 ppm. The limiting factors in the measurement are the ²³⁵U¹H⁺ isobar and background signal at m/z 236 arising from the ²³⁸U⁺ peak tail. Based upon the analysis of replicates and considerations of possible systematic errors, uncertainties of ±5% are found for ²³⁶U/²³⁸U atom ratios of 1-100 ppm. This procedure has been demonstrated in studies of anthropogenic ²³⁶U in the environment at three locations: (a) offsite soils from the vicinity of the Rocky Flats Environmental Technology site (Golden, Colorado, USA); (b) sediments from the Ashtabula River (Ohio, USA); and (c) sediments from the Mersey estuary (Liverpool, UK). In each of these three locations, definite plumes of elevated ²³⁶U/²³⁸U are identified and characterized. Maximum ²³⁶U/²³⁸U atom ratios observed in RFETS-vicinity soils, the Ashtabula River, and the Mersey Estuary are 2.8, 140, and 4.4 ppm, respectively.