Luigi Monte

Evaluation of radionuclide transfer functions from drainage basins of fresh water systems

Radionuclide transfer functions, defined as the amount of radionuclide flowing per unit time from an upstream drainage basin to a water body following a single pulse deposition of radioactive substance, were evaluated using contamination data collected by some European Laboratories in the rivers Po, Prypiat, Dnieper, Teterev, Uzh and Rhine following the Chernobyl accident. The transfer function Φr(t) (t = time) may be generally expressed as the sum of some time-dependent exponential components Φr(t) = ∑ikie−(λi + λr)t where λr + λi are the effective decay constants, λr is the radionuclide decay constant and ki are parameters depending on the deposition and the water flow. The data analysed here allow detection of two main exponential decay components. The order of magnitude of the short term effective decay constant calculated here is 10−7 s−1 (radionuclides 137Cs, 90Sr and 103Ru). The geometric means of the effective decay constants of the long term component are 1.5 × 10−8 s−1 (137Cs) and 4.9 × 10−9 s−1 (90Sr). A significant non-linearity of 90Sr transfer functions with respect to the water flow through the drainage basin was detected.

Review and assessment of models used to predict the fate of radionuclides in lakes

A variety of models for predicting the behaviour of radionuclides in fresh water ecosystems have been developed and tested during recent decades within the framework of many international research projects. These models have been implemented in Computerised Decision Support Systems (CDSS) for assisting the appropriate management of fresh water bodies contaminated by radionuclides. The assessment of the state-of-the-art and the consolidation of these CDSSs has been envisaged, by the scientific community, as a primary necessity for the rationalisation of the sector. The classification of the approaches of the various models, the determination of their essential features, the identification of similarities and differences among them and the definition of their application domains are all essential for the harmonisation of the existing CDSSs and for the possible development and improvement of reference models that can be widely applied in different environmental conditions. The present paper summarises the results of the assessment and evaluation of models for predicting the behaviour of radionuclides in lacustrine ecosystems. Such models were developed and tested within major projects financed by the European Commission during its 4th Framework Programme (1994-1998). The work done during the recent decades by many modellers at an international level has produced some consolidated results that are widely accepted by most experts. Nevertheless, some new results have arisen from recent studies and certain model improvements are still necessary.

Modelling of radiocesium in lakes : the VAMP model

Abstract The objective of this work is to present a new, state-of-the-art model for predicting radiocesium in lakes. The target variables to model and predict are Cs concentrations in lake water and in predatory fish. These results emanate from IAEA s VAMP project, initiated after the Chernobyl accident in 1986. The sites included in this work cover a wide range of lake and catchment characteristics. This new VAMP model has nine specific components (see below), which are meant to increase the predictive accuracy of the model and make it easy to apply for lakes in general. (1) A seasonal variability moderator for water discharge. (2) A dimensionless moderator for the water retention rate. (3) A seasonal variability moderator for epilimnetic and hypolimnetic temperatures, which influence lake stratification and redox-induced internal loading of radiocesium from sediments. (4) A transfer coefficient to calculate relationships between biomasses. (5) An outflow rate function for the transport of cesium from the catchment to the lake. (6) A dimensionless moderator for planktonic uptake of radiocesium, i.e. for the transfer of cesium in dissolved phase in the lake water to phytoplankton. (7) An algorithm for the lake partition coefficient ( K.d ). (8) A sub-model to express biological half-lives. (9) An approach to quantify internal loading. Empirical validations, and critical sensitivity tests, have shown that the VAMP model in many lakes yields just as good predictions as parallel sets of empirical data, and this is as good as any model can yield. It should be possible to improve the VAMP model further if and when more field data become available, for both the VAMP lakes and other sites.

Uncertainty analysis and validation of environmental models: the empirically based uncertainty analysis

Abstract In the present paper, the principles of Empirically Based Uncertainty Analysis (EBUA) are described. EBUA is based on the evaluation of ‘performance indices’ that express the level of agreement between the model and sets of empirical independent data collected in different experimental circumstances. Some of these indices may be used to evaluate the confidence limits of the model output. The method is based on the statistical analysis of the distribution of the index values and on the quantitative relationship of these values with the ratio ‘experimental data/model output’. Some performance indices are described in the present paper. Among these, the so called ‘functional distance’ (d) between the logarithm of model output and the logarithm of the experimental data, defined as d 2 = Σ n 1 ( ln M i − ln O i ) 2 n where Mi is the ith experimental value, Oi the corresponding model evaluation and n the number of the couplets ‘experimental value, predicted value’, is an important tool for the EBUA method. From the statistical distribution of this performance index, it is possible to infer the characteristics of the distribution of the ratio ‘experimental data/model output’ and, consequently to evaluate the confidence limits for the model predictions. This method was applied to calculate the uncertainty level of a model developed to predict the migration of radiocaesium in lacustrine systems. Unfortunately performance indices are affected by the uncertainty of the experimental data used in validation. Indeed, measurement results of environmental levels of contamination are generally associated with large uncertainty due to the measurement and sampling techniques and to the large variability in space and time of the measured quantities. It is demonstrated that this non-desired effect, in some circumstances, may be corrected by means of simple formulae.

Collective models in environmental sciences

Abstract The results of models for predicting the migration of toxic substances through the environment are, generally, affected with large uncertainties that are due to the complexity of the modelled environmental systems and to the difficulty of obtaining reliable values of the model parameters. Surprisingly, some models for predicting the migration of radioactive substances in water bodies, despite the complexity of the involved processes, are characterised by small levels of uncertainty. In the present paper, it was demonstrated that some of the parameters used in such sort of models show small variability, which is due to the statistical or ‘collective’ effects caused by the mutual compensation of some processes in the environment. As a consequence, the values of the ‘collective’ parameters vary within small ranges despite the wide variability of the environmental circumstances. This occurrence implies that collective models show relatively small levels of uncertainty.

MOIRA-PLUS: A decision support system for the management of complex fresh water ecosystems contaminated by radionuclides and heavy metals

The accidental release of radioactive substances into the environment leads to the necessity of applying suitable countermeasures for the restoration of the polluted environment. However, despite their obvious benefits, such interventions may result in detrimental effects of an economic, ecological and social nature that must be carefully evaluated. MOIRA-PLUS is a PC-based user-friendly, computerised decision support system (DSS) that helps decision makers to choose optimal countermeasure strategies for different kinds of aquatic ecosystems and contamination scenarios. The DSS MOIRA-PLUS is based on:(a)Validated models to evaluate the behaviour of radionuclides in contaminated water bodies and biota and to assess the effect of countermeasures on contamination levels; (b)Models to assess the radiation dose to people and biota (fish) by relevant exposure pathways, the effect of countermeasures, and the associated economic impact; (c)A multi-attribute analysis (MAA) module to evaluate the effectiveness of different countermeasure strategies by accounting for the social, ecological and economic detriments and costs in relation to their benefits; (d)A software system consisting of: (1) software realisation of the mathematical models; (2) a Geographic Information System (GIS) and associated databases to select the aquatic system of interest and, if necessary, the default environmental data required to run the models; (3) a graphical user interface (GUI); (4) an operating system connecting all the above parts. The flexible structures of the environmental models implemented in MOIRA-PLUS DSS give the potential for the application of these models to several other types of pollutants, such as heavy metals. The DSS can be applied to complex water systems comprising lakes, reservoirs and rivers. In this paper, the main principles underpinning the functioning of the DSS MOIRA-PLUS are described and discussed.

The role of physical processes controlling the behaviour of radionuclide contaminants in the aquatic environment: a review of state-of-the-art modelling approaches.

This paper is aimed at presenting and discussing the methodologies implemented in state-of-the-art models for predicting the physical processes of radionuclide migration through the aquatic environment, including transport due to water currents, diffusion, settling and re-suspension. Models are briefly described, model parameter values reviewed and values recommended. The different modelling approaches are briefly classified and the advantages and disadvantages of the various model approaches and methodologies are assessed.

A generic model for assessing the effects of countermeasures to reduce the radionuclide contamination levels in abiotic components of fresh water systems and complex catchments

Abstract This paper describes a generic model for predicting the migration of 137 Cs and 90 Sr through complex catchments and the effects of countermeasures to reduce the contamination levels. The model provides assessments of radionuclide behaviour in water systems comprised of rivers, lakes and reservoirs. It makes use of aggregate, “collective” parameters which summarise the overall effects of competing migration processes occurring in fresh water bodies. The model accounts for the radionuclide fluxes from the water column to the sediment and vice-versa, for the radionuclide migration from the catchment and for the transport of contaminated matter through the water body. It has been applied to several European fresh water systems contaminated by 90 Sr and 137 Cs due to nuclear weapon tests in the atmosphere of past decades and to the Chernobyl accident. The model can predict the effects of the following countermeasures: (a) Sediment removal; (b) Diversion of water from sub-catchments; and (c) Decontamination of sub-catchments. The results of the sensitivity and uncertainty analyses are described. Some examples of countermeasure applications are described and discussed.

Modelling of radiocesium in lakes—Lake sensitivity and remedial strategies

Abstract A given fallout of radiocesium (e.g. after the Chernobyl accident) will be distributed and taken up by biota very differently in various types of lakes. Thus, lakes have different “sensitivities” to radiocesium. Important environmental factors regulating the biouptake of 137 Cs are the lake water retention time and the K concentration of the water. Several practically useful and ecologically relevant methods exist to remediate lakes contaminated by radiocesium, e.g. liming, potash treatment and fertilization of low-productive lakes. The basic aim of this paper is to use a validated, state-of-the-art model for radiocesium in lakes, the VAMP model, first to illustrate the fact that different lakes have different ‘sensitivities’, and then to simulate the effects of alternative remedial methods. Target variables in these tests are Cs concentrations in lake water and in predatory fish. These results emanate from IAEAs VAMP project. The lakes included in this work cover a wide range of lake and catchment characteristics.

The behaviour of 137Cs in some edible fruits

Abstract The aim of this research was to study the behaviour of 137Cs in fruit-bearing plants following the Chernobyl accident. Samples of hazel-nuts were collected over a period of three years after the accident. The 137Cs content of the fruit shows an exponential decrease with time. Evaluation of the 134Cs/137Cs ratios in samples of atmospheric particulates, soils and fruit suggests that foliar translocation is the main pathway for 137Cs migration to the fruit. In addition, estimates of the effective half-times of 137Cs in hazel-nuts, walnuts, olives and apples and of the ‘translocation factors‘ for hazel-nuts, walnuts, olives, apples and chestnuts have been made.