A synthesis of worldwide sediment source tracing research including fallout radiocesium (Cs-137)
Résumé
Quantifying the main sources delivering harmful sediment loads to river systems is required to
improve our knowledge of soil erosion processes. Among these potential sources, quantifying the
contributions of surface (e.g. cultivated topsoil) and subsurface (e.g. channel bank, gully, landslide)
material to sediment transiting river systems is of particular interest. Radiocesium ($^{137}$Cs) that was
emitted during the atmospheric bomb tests that took mainly place in the 1960s and nuclear
accidents provides an effective tracer to distinguish between topsoil material exposed to the
fallout and subsoil sheltered from this fallout. A global synthesis of research articles (n=123) that
used radiocesium to fingerprint sediment sources indicated that the largest number of
publications ($\sim$55% of the total) were found in the United Kingdom, Australia and the United
States. On the contrary, very few studies ($\sim$9% of the total) were published for catchments located
in Africa or South America. Given the low proportion of fallout recorded in regions located
between 0-20°N and 0-20°S, the potential of this technique for quantifying sediment source
contributions may be limited in this part of the world. A similar conclusion may be drawn for
applying this method in agricultural areas exposed to several soil erosion during the last several
decades, such as Chinese Loess Plateau and South Africa. Overall, 94% of studies incorporating
$^{137}$Cs as a potential tracer included this property in mixing models. In the future, given the
continuous decay of the initial radiocesium fallout that peaked in the 1960s, the access to ultra-low
background gamma-ray spectrometry facilities will be increasingly necessary to measure this
important sediment tracing property. In addition, more research should be devoted to develop
surrogate tracers providing discrimination between surface and subsurface material. Based on
this extensive study review, researchers are also recommended to systematically include basic
catchment information, details on the soil/sediment sampling design and access to raw data to
facilitate the dissemination of this information among the communities of scientists and
catchment managers.
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