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Titel |
The spatial distribution of caesium-137 over Northern Ireland from fallout from the Chernobyl nuclear accident |
VerfasserIn |
B. G. Rawlins, C. Scheib, A. N. Tyler, D. Jones, R. Webster, M. E. Young |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250021059
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Zusammenfassung |
The spatial distribution of caesium-137 (137Cs) across the land is of much interest because it
can tell us about the redistribution of the radionuclide as a result of soil erosion, differential
migration through the soil—or its complement, differential retention in the soil. Any such
inferences from survey measurements depend on the assumption of a broadly even
distribution from weapons testing fallout, and the substantial deposition of 137Cs in rain
following the Chernobyl accident on 26 April 1986. Deposition from the latter
was not uniform over large areas, of course, and measurements across northern
England showed that the magnitude of 137Cs deposition depended largely on the
distribution of convective rainfall in the days immediately after the accident. There
were too few measurements of 137Cs deposition at close spacings to estimate local
variation.
Twenty years after the deposition from Chernobyl a detailed airborne radiometric survey
of the whole of Northern Ireland was flown. Flights were made along transects 200 m apart
with recordings at 80 m intervals along the flight lines to give more than one million data in
total. We have used the data to investigate the spatial distribution of 137Cs. Our initial
geostatistical analyses suggested substantial short-range variation in the distribution of 137Cs.
We wished to determine whether soil erosion or soil type could account for this. We made
further detailed analyses using the terrain parameter compound topographic index as an index
of soil erosion and deposition and soil maps to account for the migration or retention of
137Cs.
The concentration of 137Cs in the soil is greatest where most rain fell in the few days after
the accident. However, the local variances are of similar magnitudes across the the majority
of the province. The global variogram of the radionuclide shows a large proportion of the
spatially correlated variance occurring within 700 m and a longer-range structure extending
to 15 km. Local variograms where most rain fell have the largest proportions of correlated
variance. Soil type in these regions accounts for 18% of the spatially correlated
variance, which suggests that soil controls the migration of 137Cs to some extent. This
inference accords with our independent measurements of 137Cs down through the
soil. By contrast, the terrain index accounted for very little of the variance, which
suggests that soil erosion across the largely vegetated landscape has been a much
smaller contributor to redistribution of the radionuclide. The largest short-range
variation in 137Cs concentrations occurs in the Mourne Mountains (in the south east of
the province), probably because of the numerous small patches of organic-rich
soil interspersed between shallow, raw soil over granite. This observation suggests
that differences in the capacity of the soil to trap and retain the radionuclide is
the dominant factor accounting for the observed short-range variation. We cannot
determine the relative importance of deposition in accounting for the observed
short-range variation because we do not have measurements at a sufficiently fine
resolution.
We discuss the implications of our findings for the use of 137Cs as a tracer where the
radionuclide was deposited after the Chernobyl accident. We are continuing to
process the airborne radiometric data to remove potential interferences caused by
radon. |
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