Cesium-137 Runoff from Lake Saimaa to Vuoksa River and Lake Ladoga

The 137Cs migration from the upper reaches of the Vuoksa River, Lake Saimaa, to Lake Ladoga was studied to reveal the role of the river in the Ladoga contamination with 137Cs from the emissions during the Chernobyl accident. The 137Cs migration in 1988–2024 led to a decrease in its concentration in the river water from 113 to ≈4.0 Bq/m3. By 2015, the 137Cs concentration in the Vuoksa River water approached the level of 4–5 Bq/m3, corresponding to the contamination of river waters with global 137Cs prior to the accident. The decrease in the concentration of “Chernobyl” 137Cs in the Vuoksa River is approximated by a two-component exponential dependence with the water half-clearance times Т1 = 5 and Т2 = 25 years, respectively. The runoff of “Chernobyl” 137Cs from Lake Saimaa with the Vuoksa River water in 1986–2023 amounted to 22.5 TBq. This value is comparable in the order of magnitude with the fallout of “Chernobyl” 137Cs on Lake Ladoga in 1986 (74.1 TBq). In 1986–1988, the runoff of “Chernobyl” 137Cs from Lake Saimaa amounted to ≈30% of the total runoff in 1986–2024. The many-year transit of 137Cs from Lake Saimaa caused increased accumulation of 137Cs (≈300 Bq/kg) in the profile of silt sediments of the Vuoksa River and lakes of its catchment area.