This paper reports on the radioecological properties of environmental components in the Mzymta and Sochi River valleys on the Russian Black Sea coast. The environmental components covered are river sediments, alluvial soils, mosses, and fluvial water. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to determine the distribution patterns of Th and U stemming from two geochemical alkaline barriers present in the river valleys. One is a calcium barrier resulting from the metamorphic transition from neutral mudstones to alkaline carbonate rocks. The other is a sodium barrier formed in estuaries under the influence of Na-rich tidal seawater. The sodium barrier was responsible for an increase in Th and U concentrations in river water, a slight decrease in soil and sediments, and strong sorption on mosses. The calcium barrier reduced Th and U concentrations in river water, but increased them in soil and mosses. In the Sochi River valley, the content of calcium in the components of the natural environment is higher compared to the Mzymta River valley and, on average, 4 times higher in the river water. This factor determines the relatively high Th and U content in the natural environment of the Sochi River valley, as these radionuclides are easily extracted from the mineral component of river water by carbonate solutions. On the whole, the radioecological status of the river valleys was recognized as favourable. Gamma radiation exposure dose rate near the surface (20-27 mu R/h or less in mountainous areas and 9-17 mu R/h in the lowlands) was below the natural level typical for open mountainous areas of central Russia. River sediments in both valleys had low radionuclide concentrations based on the specific activities of naturally occurring radionuclides Ra-226, Ra-228, Ra-224, Th-232, and K-40. In similar river basins that may be targeted for future development, accumulation of Th and U in soil and mosses on calcium barriers and elevated concentrations in river water on sodium barriers should be expected.
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