Methodology of preparation of radioactive contamination maps of the territory

Formation of the field of radioactive contamination of the territory is conditioned by a number of factors first of all by dynamics and spectrum of discharged radionuclides and meteorological conditions (Section I).

On condition of precise reconstruction of spatiotemporal map of air flows it is possible to determine quite precise structure of the field and take into account field anisotropy choosing the methodology of preparation of maps. However approach based on antecedent determination of the structure of contamination field has a number of shortcomings. Field macrostructure can be considered as clearly determined only for western trace. For other traces the field structure is fuzzy. Data representing meteorological conditions at time of formation of contamination field are less precise and representative than direct observation data of this field. The latter can be updated as a result of new measurements while information on meteorological conditions can be updated only using mathematical simulation methods. This problem is even more complicated since set of flows determines the character of fallouts ambiguously and depends on the mesoscale level factors – status of atmospheric boundary layer, precipitation, landscape etc. For the majority of mentioned factors only qualitative assessments of influence are known of those that could be used for clarification of the structure of contamination field. Some quantitative assessments of the influence of mentioned factors are less precise than direct measurements. In general field have been formed during several phases as a superposition of fallouts separated in time. Therefore the field structure is more complicated, heterogeneous and utilization of information on the influence of other factors on mapping becomes more difficult.

Hundreds of thousands of contamination levels measurements have been executed on separate spots or surface patches after the accident at Chornobyl NPP. For mapping of contamination maps the values of field contamination levels on separate spots have to be expanded to adjacent territories. Different interpolation methods are often used for reconstruction of continuous variable fields by separate dimensions. Majority of these methods are realized in available commercial geostatic packages and geoinformation software tools. For mapping of contamination maps it was necessary to determine the sequence of the tasks on data processing and preparation, data interpolation and mapping, its verification and editing. Both formal algorithmic methods and methods with utilization of expert assessments have been used while executing these tasks.

The procedure used while preparing the maps of radioactive contamination is briefly described below. General procedure for mapping was the same for different contamination maps however peculiarity for each index has been taken into account. Majority of peculiarities is conditioned by ambiguity of available observation data regarding every radionuclide. Specificity of preparation of contamination maps has been considered after description of general procedures. Drafting of contamination maps consisted of successive execution of the following operations.

Observation data acquisition and processing. Despite the fact that there are a lot of measurements of radioactive contamination values executed by different organizations the radioactive contamination fields are not completely supported by primary data. On the other hand some data sets often contradict each other. Therefore one of the main tasks during mapping was to choose the precise and representative data and to integrate data from different sources to one single array. During preliminary data processing major attention has been paid to its verification using known quantitative ratio among different radionuclides of Chornobyl discharge and its expert evaluation. Anomalous data by the results of verification have been discarded.

Statistical direct observation data analysis. Data analysis have been executed for long-lived radionuclides of Chornobyl discharge. Statistical peculiarities of every isotope have been determined and taken into account during mapping.

Data interpolation and plotting of the grids of contamination fields. Methods proven during mapping of the Atlas of contamination of Europe by cesium [45] have been used for data interpolation, in particular “inverse distance weights”. Application of this method ensures good results for detailed data sets. In case of data incompleteness it can result in misrepresented picture especially for anisotropic fields. For probabilistic reconstruction of contamination field along air flows of Chornobyl discharge it is necessary to execute measurements directly in the flows and on adjacent territories. In case of unavailability of these measurements the field structure cannot be precisely reconstructed. Therefore verification procedure has been applied after mapping for territories with high-gradient fields with subsequent corrections.

During interpolation the contamination level values were calculated in square lattice nodes. This lattice is a basis for plotting of the contamination level isolines. Also lattice for separate contamination indices has been used for recalculation of contamination field levels at different dates.

Plotting of isoline maps. The lattice of contamination field obtained during interpolation has been used while plotting the isoline maps. Restoration of isolines has been executed using the system ArcInfo (ESRI). During restoration work the generalization has been executed consisting in smoothing of lines and deleting of small outlines.

Map verification. Utilization of formal methods for mapping does not always precisely reconstruct the structure of contamination field in particular along the "traces” and on the insufficiently investigated territories. Anomalies detected during verification have been analyzed with the purpose to determine the causes of its developments. In principle the question was whether the anomalies have been caused by inaccuracy of interpolation procedure or by the nature of input data. It should be emphasized that the majority of problematic positions have been caused by input data and more often the problems occur during integration of data from different organizations. Therefore for some territories detailed survey has been executed and after resolution of the problems with data the mapping procedure has been repeated once again. In some cases incorrect restoration of contamination field occurred due to incorrect field interpolation by inverse distance method on the grounds with significant anisotropy. Expert editing following the detailed analysis of data, additional and auxiliary information has been executed for such cases.

Summarizing the basic principles of mapping the contamination maps are as follows:

• application of primary observation data for mapping;
• utilization of additional data for map verification and searching inaccuracies;
• mapping using computer methods realized on well known commercial software;
• expert methods for verification, generalization and editing of maps;
• comparison of maps of contamination by different radionuclides and its mutual verification;
• plotting of maps of contamination by different radionuclides for the same territories by the same or similar methods and algorithms.

Specificity of plotting the contamination maps of territory by separate radionuclides is described below.

Cesium-137. Surveys on cesium-137 have been quite complete and enable to restore the structure of contamination field by direct interpolation of observation data. The following basic sets of data are used for mapping:

survey of settlements of Ukraine;
survey of agricultural lands;
results of aerial gamma-spectrometry

Other set of data has been used as auxiliary during verification and comparison of the basic data sets.

The main problem during data preparation was during integration of the data from aerial gamma survey and gamma-spectrometrical measurement of soil samples. Examination of these data sets has shown that they reconstruct close picture of contamination field for forests however for built-over and agricultural lands some deviations may occur. Therefore aerial gamma survey results have been used only for forest lands. It should be mentioned that with the exception of some grounds there are no other reliable data of radiological survey for forests. Following the analysis and integration of data sets by cesium-137 subsequent plotting of contamination maps has been done in accordance with above mentioned scheme.

Strontium-90. While studying the structure of radioactive contamination fields it is important to pay attention to proportion of the levels for different nuclides. Observations by strontium-90 are less complete than by cesium-137 especially for clean territories and territories with low level contamination. Therefore for determination of quantitative proportions among levels of contamination by different nuclides statistical examination of available primary observation data measured in one sample has been executed. Analysis of observation data has been executed on big amount of uniform grounds chosen on different distances from reactor along all known traces and radionuclide transport flows. Examination of available data arrays has shown that despite the correlation between the levels of Chornobyl fallouts of cesium-137 and strontium-90 the procedure of restoration of strontium-90 values on the basis of cesium-137 does not ensure the precision and confidence necessary for cartography. Generally speaking direct measurements of strontium-90 are more precise and reliable than calculations of levels of strontium-90 on the basis of levels of cesium-137. Therefore only direct observations data have been used in plotting of contamination maps by strontium-90. The following data sets have been used:

• survey of settlements of Ukraine;
• survey of agricultural lands.

Additional data sets have been used for verification of final maps.

Plotting of contamination maps has been executed by interpolation methods described earlier in this section.

Transuranium elements. Database containing data both by sum of plutonium isotopes and by plutonium-238 has been used for plotting of maps of contamination by isotopes of plutonium and americium-241. This database contains verified measurement results executed by different organizations. In total the database contains about five thousand records. Majority of data related to contamination by americium-241 covers close zone of ChNPP and for the rest of the territory of Ukraine these data have selective nature.

There are a lot of objective and subjective factors complicating direct application of single formalized criteria and approaches in relation to verification and plotting of contamination maps by transuranium elements:

• both intensity of discharges and its radionuclide and disperse content have changed during the time span when contamination of territory occurred. Presence of condensate and fuel discharge components, turbulent nature of atmospheric processes of transportation and complicated nature of fallouts to Earth surface formed contamination picture with high heterogeneity.
• in determination of contamination of a major part of the territory of Ukraine by transuranium elements it was necessary to take into account the global fallout levels that had not been thoroughly determined;
• Geographical data have been bound basically to nearest settlement and it did not allow to account objectively existent gradients of contamination fields that significantly changed even within the same settlement.

These factors stipulated development of methods for analysis, verification and plotting of respective contamination maps. The data from organizations that have determined a wide spectrum of long-lived radionuclides and were more reliable have been taken as a basis.

Experimental grounds both in close zone of Chornobyl NPP and in remote regions along western and south-western traces have been used for development and justification of proposed methods. Surveys have been executed on these territories during 1988-1997 taking into account Chornobyl fallouts. As a result of these surveys the data on levels of contamination by cesium-137, strontium-90, plutonium-238, -239, -240 and americium-241 have been simultaneously collected. Analysis of proportion of cesium-137 to plutonium-239, -240 for western and south-western traces of discharge has been executed. This enabled to make conclusions on constant proportion and similarity by absolute value to fuel character of contamination along the axis of western trace and discover stable trend in relation to increasing of proportion up to the values 40 000 – 50 000 along the south-western trace confirming the condensation character of discharge on this territory. Proportion among different radionuclides has quite significant and stable trends observed in general for separate traces and contamination spots.

So the plotting of maps of contamination of the territory of Ukraine and its separate regions by transuranium element has been executed step by step:

• acquisition and preliminary analysis of data on transuranium element contamination;
• analysis of additional materials (meteorological data for active discharge phase, landscape peculiarities, data on contamination by other radionuclides and respective maps etc.).
• analysis and verification of primary observation data (geostatistical analysis, general trends of proportion among different radionuclides, verification and correction of data based on detected trends, development of predictive estimates for territories where respective data are unavailable, complementing of direct determination data by predictive data for territories highly contaminated by cesium-137 where data on transuranium elements are unavailable);
• preparation of basic materials (modeling of proportions among radionuclides on the basis of verified data and additional field lattices, modeling of generalized lattice of cesium-137 contamination densities using all available data, plotting of the maps of transuranium element contamination densities by superposition of proportion fields and generalized lattice of cesium-137 contamination density;
• analysis of obtained materials, expert assessment and correction taking into account the nature of forming of contamination field at different territories.

This section contains the map of stations of radiometric observation network of Hydrometeorological Service of Ukraine .

Map of soils is included as an illustration to show the complexity of such a phenomenon as radioactive contamination since the types of soils determine the coefficients of transition of radionuclides to plants that are the basis for human trophic chains. Part of contaminated territory is represented by meadow-mud, peat-muck and soddy-podzolic soils. Under conditions of increased humidity of the territory these types of soils facilitate to biological availability of radionuclides and its migration by trophic chains. For example transition of cesium from soil to plants on overwet meadows and in flooded places is several times higher than on upland meadows with similar soils. It is proven that major part of radiocesium in peat soil is distributed in upper layer 0-6 cm and in soddy-podzolic soils cesium migrated in vertical layer to the depth of 20 cm.

Maps of this section have been developed by the following group of authors: O.Ye. Lytvynenko, S.V. Davydchuk, V.I. Resetnyk, V.P. Razov

L.Ya.Tabachnyi, O.V. Gaidar, S.V. Davydchuk, O.Ye. Lytvynenko,