Detailed study of post-Chernobyl Cs-137 redistribution in the soils of a small agricultural catchment (Tula region, Russia).

A detailed study of Cs redistribution was conducted within a small agricultural catchment in the highly contaminated Plavsk radioactive hotspot in the Tula region of Central Russia, 32 years after the Chernobyl nuclear power plant (NPP) accident, which occurred on April 26, 1986. Although more than three decades have passed since the Chernobyl NPP incident, Cs contamination is high. The Cs inventory varies from 67 to 306 kBq·m, which is 2-6 times higher than the radiation safety standard; however, the soils remain suitable for crop cultivation. The initial Cs fallout within the Plavsk radioactive hotspot was extremely heterogeneous, with a trend of decreasing Cs inventories from the NW to the SE directions within the studied territory. Contemporary Cs inventories are also very heterogeneous in the studied catchment. However, the trend of the initial Cs fallout does not appear in the contemporary Cs inventories on the slopes. Two methods of interpolation (expert-visual and automatic) were used to calculate the Cs budget, revealing high similarity in their Cs loss estimates; however, a large discrepancy was observed in their Cs gain estimates. A detailed analysis of Cs redistribution revealed the importance of hollows and "plow ramparts" (positive topographic forms on the boundaries of cultivated fields) in the transport and deposition of sediments. A quarter of the total Cs gain was deposited within the arable land, whereas a quarter was deposited within the non-plowing sides of the dry valley; the other half was deposited in the valley bottom. About 7-8 × 10 kBq of the Cs inventory flowed out of the catchment area, which was only about 2% of the Cs fallout after the Chernobyl NPP accident. About 89% of the total Cs reserve is concentrated in the top (0-25 cm) layer of soils, regardless of land use or location within the catchment.