Actinides in the Soil Chronosequence of the Amur River Floodplain

For the first time, a study was conducted at the Russian Far East to assess the rate of accumulation of gross and mobile forms of actinides (U and Th) in a 5000-year-old soil chronosequence 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, the content of the gross form of actinides in the soils of the automorphic series increased from 1 to 2 mg/kg for U and from 4 to 10 mg/kg for Th. In the soils of the hydromorphic series, the increase over a shorter time period (2600 years) 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, and that of Th, from 0.02 to 0.2 mg/kg. In the automorphic soils, the accumulation of U is observed as long as the floodplain is regularly flooded, while Th continues to accumulate even after the floodplain leaves the flood zone. In the 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 occurs mainly due to the weathering of melanocratic granitoid minerals in the alluvium. The mobilization of actinides is affected by pH in automorphic soils and Eh in hydromorphic soils.