Floodplain sedimentation is one of the most dynamic geomorphic processes within plain and lowland landscapes. There is generally a good understanding of longer term floodplain evolution, but quantitative information on overbank deposition rates for recent shorter timescales is lacking. This paper describes the application of Chernobyl-derived 137 Cs to quantify floodplain aggradation rates for the River Plava (a small river draining a severely contaminated part of the upland region of Central European Russia), based on detailed sampling of four representative floodplain study sites. Two approaches have been employed for estimating post-Chernobyl (1986-2009) floodplain accumulation rates. The first was based simply on locating the Chernobyl fallout-associated 137 Cs peak in overbank sediment sections. The second involved quantification of the increase in the total 137 Cs inventory at individual sampling points associated with the post-Chernobyl deposition of contaminated suspended sediment. It has been shown that considerable local-scale variability of overbank deposition rates exists, with aggradation rates on the low level floodplain (6 AE 1.2-14 AE 2.8 mm year À1 ) exceeding by 1.5-3 times the values for the middle level floodplain (2 AE 0.4-9 AE 1.8 mm year À1 ) and by 3-6 times the values for the upper level floodplain (1 AE 0.2-5 AE 1.0 mm year À1 ) floodplain levels. Combining these estimates with information on the areas occupied by different floodplain levels within the 54 km long valley section, derived from detailed geomorphic surveys of the selected reaches, it has been estimated that about 9700 AE 1950 t of sediment have been stored on floodplain during the 1986-2009 period. The role of floodplain storage in the overall basin sediment budget and conveyance losses within the main channel system have been evaluated. a First figure -assuming maximum rate of downward migration for the 1986 peak, second figure -assuming minimum rate of downward migration for the 1986 peak. 816 V. R. BELYAEV ET AL.

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The estimation of modern parameters of 137 Cs root uptake was conducted in natural meadow and agricultural ecosystems of post-Chernobyl landscapes of Tula region. The agrosystems with main crops of field rotation (barley, potatoes, rape, maize) occupying watersheds and slopes with arable chernozems are contaminated at a level 460-670 Bq/kg (4.7-6.0 Ci/km 2); natural meadow ecosystems occupying lower parts of slopes and floodplains are contaminated at a level 620-710 Bq/kg (5.8-7.6 Ci/km 2). In the arable soils 137 Cs uniformly distributed to a depth of Ap horizon (20-30 cm of thickness), while in meadow soils 70-80% of the radionuclide is concentrated within the top Ad horizon (9-13 cm of thickness). These topsoil layer accords with rhizosphere zone, where >80-90% of plant roots are concentrated, and from which 137 Cs is mostly consumed by vegetation. Total amount of 137 Cs root uptake depends on the level of soil radioactive contamination (correlation coefficient 0.61). So 137 Cs activity in meadow vegetation (103-160 Bq/kg) is generally more than one in agricultural vegetation (9-92 Bq/kg). The values of 137 Cs transfer factor in the studied ecosystems vary from 0.01 (rape) to 0.20 (wet meadow), that confirms the discrimination of the radionuclide's root uptake. The larger are the volume of roots and their absorbing surface, the higher are the values of transfer factor from soil to plant (correlation coefficients 0.71 and 0.64 respectively). 137 Cs translocation from roots to shoots is also determined by biological features of plants. At the same level of soil contamination aboveground parts of meadow herbs accumulate more 137 Cs than Gramineae species, and in agrosystems above-ground parts of weeds concentrate more 137 Cs than cultivated cereals. Thus, the level of soil radioactive pollution and biological features of plants are determinants in the process of 137 Cs root uptake and translocation and should be considered in land use policy.

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