| Summary: | For a better understanding of the previously observed increased natural radioactivity at the confluence of the Varzob and Sioma rivers, Central Tajikistan, the activity concentration of natural radioactive <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mn>40</mn></msup></semantics></math></inline-formula>K, Th and U daughters as well as of the anthropogenic <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mn>137</mn></msup></semantics></math></inline-formula>Cs were determined in nine locations covering the entire Sioma River. The radiometric determinations evidenced, in addition to increased activity concentrations of Th and U daughter radionuclides, an anomalous accumulation of anthropic <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mn>137</mn></msup></semantics></math></inline-formula>Cs and natural <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mn>210</mn></msup></semantics></math></inline-formula>Pb in the middle of the Sioma gorge. Complementarily, the mass fractions of natural Th and U determined by instrumental neutron activation analysis evidenced an increased presence of Th and U, which, in some places, overpasses six-times the average mass fraction found in the upper continental crust. The possible influence of active air transport from the southern Tadjik plane during the Afghani storms, as well as the presence of actinide-rich minerals, were analyzed and discussed.
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