ACTINIDES IN THE SOIL CHRONOSEQUENCE OF THE AMUR RIVER FLOODPLAIN

For the first time in the Russian Far East, a study was conducted to assess the rate of accumulation of gross and mobile forms of actinides (U and Th) in a 5000-year-old soil chrono-sequence embedded within the floodplain of the middle reaches of the Amur River. The relationships between actinides and the properties of alluvial and residual alluvial soils are characterized using regression models. It was found that during the evolution of soils, the content of the gross form of actinides in soils of the automorphic series increased for U from 1 to 2 mg/kg, for Th from 4 to 10 mg/kg. In the soils of the hydromorphic series, over a shorter period of time (2600 years), the increase was from 1 to 3 mg/kg for U and from 4 to 12 mg/kg for Th. The content of the mobile U form in automorphic and hydromorphic soils increased on average from 0.1 to 0.4 mg/kg, for Th from 0.02 to 0.2 mg/kg. In automorphic soils, accumulation of U is observed while the flood regime is in effect, Th continues to accumulate even after the floodplain leaves the flood zone. In hydromorphic soils, the accumulation of actinides continues over the entire chronological range. The results obtained show that the main soil properties determining the accumulation of actinides in soils are the content of clay minerals and iron oxides. The intake of actinides into the soils of the Amur River floodplain is carried out mainly due to the weathering of melanocratic granitoid minerals in the composition of alluvium. The mobilization of actinides is influenced by pH in automorphic soils and Eh in hydromorphic soils.