[en] The paper presents the data on Chernobyl radiocesium bound to suspended matter and bottom sediments at different locations along the sampling rout from Rivers of Chernobyl zone to upper Reservoirs of Dnieper River. These data were collected as a result of joint Ukrainian-Italian field exercises in the frame of ECP-3 project. It was found out that total ¹³⁷Cs concentration in the water column decreases downstream the Chernobyl zone while K_D in situ values substantially increase with approach to the Kiev HPS dam. Taking account of uniform hydro-chemical conditions in investigated area one can explain this phenomenon only by gradual elimination of coarse sandy component with low sorption capacity from the river flow by sedimentation. In contrary, radiocesium which is selectively sorbed and fixed on fine clay particles travels much longer distances and ensures observed higher K_D in situ values. This conclusion is supported by the analyses of three sediment cores taken in upper, middle and lower parts of Kiev reservoir

[en] Cooling Pond (CP) of the Chornobyl Nuclear Power Plant (ChNPP) is one of the most radioactively contaminated large water bodies over the globe. During the active phase of the ChNPP accident, radionuclides got into the CP in result of atmospheric deposition, release of highly contaminated water from system of accidental cooling, and water used to extinguish the fire. In the years after the accident, the contamination was distributed in the CP due to currents. For this period, three types of hydrological conditions dominated in the CP. Initially, the currents were forced by the cooling system of the ChNPP, which caused a circular movement of water. After the decommissioning of the ChNPP, the natural circulation took place in the CP. Starting from the end of 2014, when pumps that continuously fed the CP with water from the Prypiat River were shutdown, a gradual decrease of water level began. Now the water level has dropped by about 6 m leading to the transformation of the whole reservoir into several small lakes and redistribution of radionuclides in them. The objectives of the study were to calibrate models, which were customized for the CP, using data for the whole post-accident period including data collected during the drawdown period by the joint efforts of Ukrainian and Japanese researchers, and then to provide model based predictions of the future radionuclide concentrations in new water bodies. During field studies that were carried out in November 2020, the current state of radioactive contamination of the CP was investigated. Samples of water, suspended and bottom sediments and biota were taken in 9 closed or semi-closed water bodies formed after partial drying of the CP. Concentrations of Cs-137 and its distribution in dissolved and particulated forms were measured in the laboratory. For simulations, the modeling system that consists of the 3D model of thermohydrodynamics and radionuclide transfer THREETOX and the box model POSEIDON-R was created. The THREETOX model was used for the obtaining currents in the CP for each type of hydrological conditions. The POSEIDON-R model was applied for the long-term simulations of the changes of activity concentration in the water, bottom sediments and biota starting from the 1986. The system of boxes in the POSEIDON-R model includes shallow and deep-water boxes. It was built in such a way that after the water level in the CP fell, the calculations were performed only in deep-water boxes. Fluxes of water between boxes were calculated based on currents from the THREETOX model. Seasonal changes in distribution coefficient Kd describing the partition of Cs-137 concentration between water and sediments were also taken into account. Calculated concentrations of Cs-137 in water and bottom sediments agree well with measurements for all boxes and for entire modeling period. It has been shown that POSEIDON-R model is able to reproduce changes in the concentrations of Cs-137 in freshwater fish occupying different levels the food chain. Scenarios for the potential changes of Cs-137 concentrations were considered by the variation of basic parameters.

No abstract available

[en] Sampling and sample preparation are known to carry large, but typically unknown uncertainty contributions to the final analytical data and there is a lack of qualitative and quantitative data on the comparability of results achieved by different sampling methods. To this end an intercomparison exercise was carried out to compare different methods for the collection of suspended material used for the monitoring of environmental radioactivity in freshwater bodies. This paper presents the results of this intercomparison exercise in which 'in situ' particulate sampling devices were compared in field exercises performed in the Kiev Reservoir (Ukraine) and in the Po River (Italy). The main criterion for this intercomparison was the agreement among the ¹³⁷Cs activity concentrations associated with the suspended particles expressed as Bq x g^-1 and among the (C/N) molar ratios measured on the suspended particles. In addition, an estimate of the uncertainties associated with each measuring method has been performed. (author)

[en] Full text: The evaluation of the pattern of environmental contamination is vitally dependent on reliable data, which derive from complex analytical processes, in which each step of the process, i.e. sampling, sample transportation, treatment, analysis and evaluation and interpretation of the results can affect to some degree the accuracy and precision of the final analytical data. Advances in analytical techniques and improved laboratory practice have reduced many sources of uncertainties which can originate during the laboratory analytical procedures, but the assessment of uncertainties associated with sampling of environmental components has not been fully considered in the past, since collaborative field studies require noteworthy organisational efforts. Sampling and sample preparation/processing are known to carry large, but typically unknown uncertainty contribution to the final analytical data and there is a lack of qualitative and quantitative data on the comparability of results achieved by the different sampling methods. To this end an intercomparison programme was founded by the European Commission, in which Institutions from the European Union and from Ukraine participated using their own in situ methods for collection of suspended material and fixation of dissolved radiocaesium in fresh water bodies. This paper presents the outcome of this study: limits and field applicability of the most widely used sampling methodologies for water and suspended particle in aquatic environments. (author)