Characterisation of airborne uranium and thorium contamination in Northern England through measurement of U, Th and 235U/238U in tree bark

Samples of tree bark were collected from four locations in Northern England (a typical rural site, a coal-fired power station, a uranium (isotopic) enrichment plant and a nuclear fuel fabrication facility), to assess the nature and extent of airborne uranium and thorium contamination. The U and Th concentrations of bark were determined by inductively coupled plasma mass spectrometry after conventional nebulisation of bark digests, whilst measurement of U-235/U-238 isotopic ratio utilised high efficiency nebulisation. Uranium concentrations varied between and within the sites (range, 0.01-12 mug g(-1)), with maximum values recorded within 1 km of the nuclear fuel fabrication plant (Springfields). In comparison, the concentration of Th in bark was low (mean, 0.018 mug g(-1)) at all sites with the exception of the area affected by coal combustion (0.2-0.8 mug g(-1)). The U/Th ratio varied from 0.5 to 3900 compared with the average crustal ratio of 0.3. Low values (<2) were recorded at the 'coal' and 'rural' sites whilst Capenhurst and Springfields showed high values indicating the relative magnitude of uranium elevation. Significant enrichment of the natural U-235/U-238 ratio (0.00725) was observed near the nuclear installations, in particular, the enrichment plant (Capenhurst).