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

U-236 (t(1/2) = 2.3 x 10(7) y) is formed as a result of thermal neutron capture by U-235. In naturally occurring U ores, where a high neutron flux is present from spontaneous fission of U-238, U-236/U-238 atom ratios are similar to10(-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 U-236/U-238 atom ratios approaching 101 ppm. As a result, the presence of very small quantities of reactor-irradiated U will significantly enhance the 'background' U-236/U-238 atom ratio. When sufficiently elevated U-236/U-238 ratios are present, the determination of U-236/U-238 by rapid inductively coupled plasma mass spectrometric (ICPMS) methods is attractive. We have used sector ICPMS at medium resolving power (R = 3440) to measure U-236/U-238 atom ratios with a determination limit of 0.2 ppm. The limiting factors in the measurement are the (UH+)-U-235-H-1 isobar and background signal at m/z 236 arising from the U-238, peak tail. Based upon the analysis of replicates and considerations of possible systematic errors, uncertainties of +/-5% are found for U-236/U-238 atom ratios of 1 - 100 ppm. This procedure has been demonstrated in studies of anthropogeniC U-236 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 U-236/U-238 are identified and characterized. Maximum U-236/U-238 atom ratios observed in RFETS-vicinity soils, the Ashtabula River, and the Mersey Estuary are 2.8, 140, and 4.4 ppm, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.