ИСТИНА

Cesium-137 is an isotope of technogenic origin, which is widely used as a chromomarker and tracer to assess the sedimentation rate at deposion sites and to identify the proportional contribution of various sediment sources to the river sediment yield. 137Cs fallout in the Selenga River basin occurred from 1953 to the 1980s, with a maximum in 1963. It was associated with testing of nuclear explosions in the open atmosphere. The levels of initial 137Cs fallout generally depended on the layer of precipitation during the warm season. The trend of decreasing 137Cs initial fallout from 2380 Bq/m2 to 1600 in south-east of the Selenga River basin is established based on our own and published data. Estimates of 137Сs vertical distribution in deposits were obtained for different parts of the Selenga River delta and some floodplain sections in the Mongolian part of the basin. This made it possible to determine the sedimentation rates and changes in the levels of pollution of suspended sediments over two time windows. Plots of the 137Cs vertical distribution floodplain sediments can also be used for identifying the contribution of various sediment sources to river sediment yield. However it is necessary to take into account the mechanical composition of floodplain deposits. The data obtained by us and published by other researchers on the use of 137Cs and other fallout radionuclides for assessing the intensity of erosion-accumulation processes, identifying the proportional contribution of various sediment sources to the river sediment yield, and assessing spatial-temporal changes in the transport of pollutants in various chains of the fluvial network in the Selenga River basin allows to state that the more widespread use of these tracers will allow us to reliably reconstruct the transboundary migration of heavy metals and other sediment-associated pollutants for at least two time windows.