The article presents the results of monitoring the waters of the Shagan river on the change in the specific activity of tritium for 2016–2020. The Shagan river is the longest surface watercourse on the territory of the Semipalatinsk Test Site, flowing along the eastern part of the Balapan site, where, together with the tributary, the Ashisu, flows into the “Atomic Lake”. Further, at a distance of 110 km downstream, the river forms a left-bank tributary of the Irtysh river. Previous studies have revealed and repeatedly confirmed the fact of radioactive contamination of the surface and ground waters of the Shagan river technogenic radionuclide 3H. It was revealed that the main source of pollution is groundwater entering the surface waters of the river. In this regard, in order to ensure long-term monitoring of 3H pollution of river waters, seasonal monitoring was carried out in three sections of the river located along its channel, starting from the outlet from the “Atomic Lake” heap and further downstream to the confluence with the Irtysh river.

As a result of the observations, it was found that the specific activity of 3H in surface and ground waters, depending on the observation time, varies in a wide range of values. So, in the area of maximum pollution, the content of 3H changes from the minimum – 8 Bq/kg in the spring, to the maximum 370 000 Bq/kg in the summer-autumn period. At the exit points of the Shagan river outside the landfill, the concentration of 3H varies from 90 Bq/kg to 12 400 Bq/kg. In the area of the confluence of the Shagan river in Irtysh river content of 3H does not exceed 110 Bq/kg.