The spatial distribution of caesium-137 over Northern Ireland fromudfallout from the Chernobyl nuclear accident

摘要

The spatial distribution of caesium-137 (137Cs) across the land is of much interest because it can tell us aboutudthe redistribution of the radionuclide as a result of soil erosion, differential migration through the soil—or itsudcomplement, differential retention in the soil. Any such inferences from survey measurements depend on theudassumption of a broadly even distribution from weapons testing fallout, and the substantial deposition of 137Csudin rain following the Chernobyl accident on 26 April 1986. Deposition from the latter was not uniform overudlarge areas, of course, and measurements across northern England showed that the magnitude of 137Cs depositionuddepended largely on the distribution of convective rainfall in the days immediately after the accident. There wereudtoo few measurements of 137Cs deposition at close spacings to estimate local variation.udTwenty years after the deposition from Chernobyl a detailed airborne radiometric survey of the whole of NorthernudIreland was flown. Flights were made along transects 200 m apart with recordings at 80 m intervals along the flightudlines to give more than one million data in total. We have used the data to investigate the spatial distribution ofud137Cs. Our initial geostatistical analyses suggested substantial short-range variation in the distribution of 137Cs.udWe wished to determine whether soil erosion or soil type could account for this.We made further detailed analysesudusing the terrain parameter compound topographic index as an index of soil erosion and deposition and soil mapsudto account for the migration or retention of 137Cs.udThe concentration of 137Cs in the soil is greatest where most rain fell in the few days after the accident. However,udthe local variances are of similar magnitudes across the the majority of the province. The global variogram of theudradionuclide shows a large proportion of the spatially correlated variance occurring within 700 m and a longerrangeudstructure extending to 15 km. Local variograms where most rain fell have the largest proportions of correlatedudvariance. Soil type in these regions accounts for 18% of the spatially correlated variance, which suggests that soiludcontrols the migration of 137Cs to some extent. This inference accords with our independent measurements ofud137Cs down through the soil. By contrast, the terrain index accounted for very little of the variance, which suggestsudthat soil erosion across the largely vegetated landscape has been a much smaller contributor to redistribution ofudthe radionuclide. The largest short-range variation in 137Cs concentrations occurs in the Mourne Mountains (inudthe south east of the province), probably because of the numerous small patches of organic-rich soil interspersedudbetween shallow, raw soil over granite. This observation suggests that differences in the capacity of the soil to trapudand retain the radionuclide is the dominant factor accounting for the observed short-range variation. We cannotuddetermine the relative importance of deposition in accounting for the observed short-range variation because weuddo not have measurements at a sufficiently fine resolution.udWe discuss the implications of our findings for the use of 137Cs as a tracer where the radionuclide was depositedudafter the Chernobyl accident. We are continuing to process the airborne radiometric data to remove potentialudinterferences caused by radon.