K.M. Thiessen

Comparison of interception and initial retention of wet-deposited contaminants on leaves of different vegetation types☆

Abstract Simulated rain containing both soluble radionuclides and insoluble particles labeled with a radionuclide was manually applied to several kinds of vegetation, including a conifer, a broad-leafed tree, and several herbaceous species. The fraction of each radioactive material intercepted and initially retained by the vegetation was determined for each plant type. This fraction was determined both as the mass interception factor, r Y , and the leaf area interception fraction, LAIF. Mean values of r Y ranged from 0.16 to 2.9 m2 kg−1 and of the LAIF, from 0.011 to 0.16. There was a greater range in mean retention values among radionuclide types than among plant species; the range among plant types tended to be less with the LAIF than the r Y . Significantly less interception and initial retention was measured for anions than for cations or the insoluble particles.

Quantification of the interception and initial retention of radioactive contaminants deposited on pasture grass by simulated rain

Abstract Simulated rain containing both soluble radionuclides and insoluble particles labeled with radionuclides was applied to pasture-type vegetation under conditions similar to those found during convective storms. The fraction of material in rain intercepted by vegetation and initially retained was determined for three sizes of insoluble polystyrene microspheres (3, 9 and 25 μm), soluble 7Be2+ and soluble 131I as periodate or iodide, over a range of rainfall amounts of both moderate- and high-intensity precipitation. Values for the interception and initial retention by vegetation (interception fractions) for soluble forms of 131I in simulated rain are much less than those for insoluble particles and the reactive cation 7Be2+. The interception fraction for soluble 131I is an inverse function of rain amount, varying from about 0.3 at 1 mm rain to 0.006 at 30 mm. The mass interception factor (the interception fraction normalized for biomass) of 131I is almost solely dependent on the amount of rain, with values from about 2.5 m2 kg−1 at 1 mm to less than 0.1 m2 kg−1 at 30 mm; the 131I vegetation-to-rain concentration ratio is relatively constant at approximately 2.6 l kg−1. For 7Be2+ and the insoluble particles, the interception fractions range from 0.1 to 0.6 with geometric means of approximately 0.3. For these materials there is a greater dependence on biomass than on rain amount; the geometric means of the mass interception factors for these substances range from 0.99 to 2.4 m2 kg−1, with no single variable being a major controlling factor. These results indicate that anionic 131I is essentially removed with the water once the vegetation surface becomes saturated and that the 7Be cation and the in s oluble particles are adsorbed to or settle out on the plant surface. The discrepancy between the behavior of the anionic and the cationic species is consistent with a negative charge on the plant surface.

Establishing a database of radionuclide transfer parameters for freshwater wildlife

Environmental assessments to evaluate potentials risks to humans and wildlife often involve modelling to predict contaminant exposure through key pathways. Such models require input of parameter values, including concentration ratios, to estimate contaminant concentrations in biota based on measurements or estimates of concentrations in environmental media, such as water. Due to the diversity of species and the range in physicochemical conditions in natural ecosystems, concentration ratios can vary by orders of magnitude, even within similar species. Therefore, to improve model input parameter values for application in aquatic systems, freshwater concentration ratios were collated or calculated from national grey literature, Russian language publications, and refereed papers. Collated data were then input into an international database that is being established by the International Atomic Energy Agency. The freshwater database enables entry of information for all radionuclides listed in ICRP (1983), in addition to the corresponding stable elements, and comprises a total of more than 16,500 concentration ratio (CRwo-water) values. Although data were available for all broad wildlife groups (with the exception of birds), data were sparse for many organism types. For example, zooplankton, crustaceans, insects and insect larvae, amphibians, and mammals, for which there were CRwo-water values for less than eight elements. Coverage was most comprehensive for fish, vascular plants, and molluscs. To our knowledge, the freshwater database that has now been established represents the most comprehensive set of CRwo-water values for freshwater species currently available for use in radiological environmental assessments.

Test of existing mathematical models for atmospheric resuspension of radionuclides

Atmospheric resuspension of radionuclides can be a secondary source of contamination after a release has stopped, as well as a source of contamination for people and areas not exposed to the original release. A test scenario based on measurements collected after the Chernobyl accident was used to evaluate existing mathematical models for contaminant resuspension from soil, to examine resuspension processes on both local and regional scales, and to investigate the importance of seasonal variations of these processes. Model predictions from 15 participants were compared with measured air concentrations and resuspension factors to investigate and explain the discrepancies both among model predictions and between model predictions and observations and thus to evaluate the predictive capabilities and drawbacks of commonly used generic resuspension models. The empirical models tested can give predictions within an order of magnitude of observations or better if adequately calibrated for site-specific conditions, but they do not describe the process-level events or account for spatial heterogeneity or temporal variations.

Uncertainty of the long-term resuspension factor

Abstract Resuspension of contaminated soil into the atmosphere is one of the key processes that must be considered in the estimation of inhalation doses to humans. Data for air and soil contamination collected in Ukraine over several years since the Chernobyl accident have permitted analysis of resuspension in terms of the underlying mechanisms. Various empirical models for the resuspension factor as a function of time (e.g. Linsley, Garland, Anspaugh, etc.) are compared to the observed resuspension factors over time (9 yr) at two sites; in general, these models give overestimates for the resuspension factor as a function of time. The observed values of the resuspension factor range from greater than 10−5 m−1 at early time points to around 10−10 m−1 at later points. The uncertainty in the resuspension factor is decreased to within 1 order of magnitude if annual averaging of the experimental data is used and if the resuspension factor is determined as a function of time and of the predominant regional conditions of vegetative cover and climate.

Assessment of the consequences of the radioactive contamination of aquatic media and biota for the Chernobyl NPP cooling pond : model testing using Chernobyl data

Abstract The ‘Cooling Pond’ scenario was designed to test models for radioactive contamination of aquatic ecosystems, based on data from the Chernobyl Nuclear Power Plant cooling pond, which was heavily contaminated in 1986 as a result of the reactor accident. The calculation tasks include (a) reconstruction of the dynamics of radionuclide transfer and bioaccumulation in aquatic media and biota following the accident; (b) assessment of doses to aquatic biota; and (c) assessment of potential doses and radiation risks to humans from consumption of contaminated fish. Calculations for the Scenario were performed by 19 participants using 6 different models: LAKECO-B (Netherlands); LAKEPOND (Romania); POSOD (USA); WATER, GIDRO and ECOMOD-W (Russia). For all endpoints, model predictions were compared with the test data, which were derived from the results of direct measurements and independent dose estimates based on measurements. Most of the models gave satisfactory agreement for some portions of the test data, although very few participants obtained good agreement with all criteria for model testing. The greatest level of difficulty was with the prediction of non-equilibrium radioecological processes in the first year after the accident (1986). The calculations 5 for this scenario gave modellers a unique opportunity to test their models using an independent data base and to analyse the advantages and weaknesses of different model approaches. The use of post-Chernobyl data in such a scenario is also recommended for use in training students in the field of radioecology and environmental protection.

Validation of models of radionuclide wash-off from contaminated watersheds using Chernobyl data

Abstract Based on data from the Chernobyl accident, the ‘Wash-off’ scenario was developed to provide an opportunity to test models intended to simulate the movement of trace contaminants from terrestrial sources to bodies of water. The specific objective of the test was to take into account chemical speciation, its effect on the transfer of contamination from soil to water, and the geochemical and geophysical processes that affect such transfer. Modellers were provided with descriptions of two experimental plots near the Chernobyl Nuclear Power Plant (NPP), one using simulated heavy rain (HR) and one using snowmelt (SM). They were requested to estimate the vertical distribution of total 137Cs and 90Sr and their specific forms in the soil prior to the experiments, concentrations of each radionuclide in surface runoff (separately for particulate and dissolved forms) and the total amount of each radionuclide lost from plot HR during the experiment. All predictions were to be provided as best estimates with 95% subjective confidence intervals about the best estimates. In this paper, a brief description of the modelling results is provided, together with discussions of the performance of individual models in comparison with the actual measurements and of the sources of uncertainty in the model predictions. Our conclusion is that the predictive accuracy of the mathematical models could be improved by (1) improvement of model structure to include all relevant mechanisms; and (2) further use and improvement of methods for estimation of parameter values for the situation being modelled.

Modeling the Resuspension of Radionuclides in Ukrainian Regions Impacted by Chernobyl Fallout

Following the 1986 Chernobyl event, large amounts of radioactive materials were deposited in nearby areas. Concentrations of various radionuclides were measured in air and surface soil. To study the resuspension of radioactive particulate, three different exposure situations were developed on the basis of the collected data under the auspices of the international BIOMOVS II (BIOspheric MOdel Validation Study) project. Modelers were asked to predict seasonal air concentrations and resuspension factors at several locations at different distances from Chernobyl for six successive years following the accident. Measurements of radionuclide deposition on topsoil were provided for each site along with information on soil, vegetation, land use, surface roughness, meteorology, and climate. In this paper, the three exposure situations are described, along with the initial data set provided to the modelers; two modeling approaches used to make the endpoint predictions are also presented. After the model predictions were submitted, the measured air concentrations and resuspension factors were released to the modelers. Generally, the predictions were well within an order of magnitude of the measured values. Time-dependent trends in predictions and measurements were in good agreement with one of the models, which (a) explicitly accounted for loss processes in soil and (b) used calibration to improve its predictive capabilities. Reasons for variations between predictions and measurements, suggestions for the improvement of models, and conclusions from the model validation study are presented.

Opportunities for the testing of environmental transport models using data obtained following the Chernobyl accident.

The aftermath of the Chernobyl accident has provided a unique opportunity to collect data sets specifically for the purpose of model testing, and with these data to create scenarios against which environmental transport models may be tested in a format constituting a blind test. This article serves as an introduction to three test scenarios designed for testing models at the process level: (1) surface water contamination with radionuclides initially deposited onto soils; (2) contamination of different aquatic media and biota due to fallout of radionuclides into a body of water; and (3) atmospheric resuspension of radionuclides from contaminated land surfaces. These scenarios are the first such tests to use data sets collected in the former Soviet Union. Interested modelers are invited to participate in the test exercises by making calculations for any of these test scenarios. Information on participation is included.

Modeling the Washoff of 90Sr and 137Cs from an Experimental Plot Established in the Vicinity of the Chernobyl Reactor

After the Chernobyl event, a large area of land was contaminated following the deposition of radionuclides. This area became a continuing source of radionuclides to natural waters and aquatic ecosystems. In 1986, an experimental plot was constructed in a contaminated area near the Chernobyl Nuclear Power Plant to study the washoff of radionuclides by surface runoff. Concentrations of 137Cs and 90Sr were measured in the top 10 cm of the soil prior to the experiments. During two separate experiments, intense artificial rainfall was applied to the plot. A washoff scenario was then prepared with site-specific information on initial soil contamination, duration and quantities of rainfall and runoff, physicochemical properties of the topsoil, and some climatological data. Modelers were asked to predict (a) the vertical distributions of the initial concentrations of 137Cs and 90Sr in various chemical forms in the topsoil, (b) concentrations of these radionuclides in various chemical forms in the runoff water during each experiment, and (c) the total amounts of these radionuclides that were washed off during each experiment. Stochastically generated local rainfall data were used in a water budget model to generate annual average runoff and infiltration rates. A vertical, one-dimensional, multiphase, multispecies transport model was then developed to simulate the movement of contaminants in the topsoil during the 160-d period between the Chernobyl event and the experiments as well as the washouts of contaminants by runoff during the experiments and during the 24-h period thereafter. The model provided very good predictions of the vertical distributions of total contaminant concentrations in the top 10 cm of the soil; however, the concentrations in individual chemical forms were not predicted as accurately. Initially, the model overpredicted the washout of contaminants for the two experiments and the 24-h period thereafter. Fraction of runoff that flows as interflow and average sediment loading in the runoff were identified as parameters responsible for the overprediction. Calibration of the interflow fraction and adjustment of the average sediment loading in runoff to a level representative of Eastern Europe considerably improved these predictions. The complete modeling approach and comparisons of model predictions with measurements and with predictions from other modelers are presented.