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The spread of radioactive contamination in the subsurface medium near Lake Karachai is consid ered. The complexity of this process requires a comprehensive approach to its study. The source of radioactive contamination is overviewed. The map of faults in subsurface medium is considered in order to determine the prevailing direction of contaminated groundwater flow. Photometry in observation wells has been used for structural geological estimation of transport properties of the shallow aquifer, where contaminated ground water is moving. This study was carried out along with hydrochemical logging, which makes it possible to esti mate the dynamics of contamination of subsurface medium and vertical distribution of groundwater contam ination. Special attention is paid to transport of radionuclides in the form of radiocolloid particles. Ground water samples were taken from various depths corresponding to different contamination levels near Lake Karachai. The depth intervals of sampling were determined from the data of hydrochemical logging. Ultra filtration through membranes with a specific pore size in combination with gamma spectrometry was used to characterize radionuclide transfer with colloidal particles differing in size. The local elemental composition of the radiocolloid surface was studied by Auger spectroscopy. The chemical composition and structure of radiocolloid particles were determined by X ray photoelectron spectroscopy with consecutive etching of the particle surface by argon ions for a certain depth. The data obtained indicate that radiocolloid particles are heterogeneous and an organic shell consisting of humic and fulvic acids and technogenic organic compounds coat their surface.