Environmental risk assessment of the radioactively contaminated forests includes definition, localisation and evaluation the criteria determining formation and evolution of the exterior and interior dose.

The main factors of the exterior dose formation are these of secondary redistribution of the radionuclides at the environment, including their horisontal and vertical migration and biogenic accumulation. The biogenic accumulation of the radionuclides is mostly important factor of the interior dose formation.

Because of spatial character of the problem, the necessary element for the evaluation of the environmental risk assessment at the radioactively polluted territories is the cartographic modelling and GIS application. It allows to define areals and estimate intencity of redistribution and biogenic accumulation the radionuclides. On that basis radioactive contamination of the forests and can be identified in evolution and, in turn, approaches on the dose rates can be developed, as well as the environmental risk factors can be evaluated.

As the actual contamination of the forest is determined mostly by the root income of the radionuclides, depending of the local soil and other environmental conditions, at least following information layers (basic maps) are needed for its cartographic modelling:

• map of soil radionuclide contamination, which reflects character of initial radioactive fallout;
• map of the forest ecosystems, including the types and age structure of the woodstands;
• landscape map of the environmental factors which determine the root income and horizontal redistribution of the radionuclides.

To manipulate effectively these cartographic information layers special radioecological GIS have been created on the basis of the landscape approach (Davydchuk, Palko, Glieca, 1997). It was developed with the programming tools of SPANS GIS, an ITERA TYDAC product, using a SUN SPARC 10 Workstation. This hardware and software represent part of Canadaôs assistance to Ukraine in dealing with the impact of the Chernobyl disaster (Palko et al., 1995).

Complex landscape (geosystem) approach was proposed as structural base of the radioecological GIS created. The maps of the Cs-137 biologically available, forest ecosystems contamination by Cs-137 and Cs-137 balances in the landscape areals can be prepared by GIS. These maps generalise co-influence of main environmental factors of the irradiation riscs formation and evolution at the forested territories. So they can be proposed as the background for the visualisation, analysis and presentation spatial aspects and factors of the environmental risk assessment.