The Chernobyl Nuclear Power Plant accident was the largest nuclear accident in human history. Influenced by the wind direction, most of the dust drifted towards Belarus, but the water, it carried the nuclear radiation all the way south.
The Dnieper River has traces of radiation, making swimming in it inadvisable. This is common knowledge for the people of Kyiv, so in the sections of the Dnieper River flowing through the Kyiv City District, you won't see anyone swimming, playing in it, or eating fish from it.
For the people of Kyiv, the closest interaction they have with the Dnieper River is probably just taking a boat ride to enjoy the views, and that's it.
So seeing a bunch of naked people bathing by the river, there's no need to ask, no need to think, you don't even need to guess; it's definitely Djo Malthael's men.