Discharge of ¹³⁷Cs from Lake Saimaa into the Vuoksa River and Lake Ladoga

The migration of ¹³⁷Cs from the upper reaches of the Vuoksa River, Lake Saimaa, to Lake Ladoga was studied to determine the role of the river in the contamination of Lake Ladoga with ¹³⁷Cs from the Chernobyl accident. During the migration period from 1988 to 2024, the concentration of ¹³⁷Cs in the river water decreased from 113 to approximately 4.0 Bq/m³. By 2015, the ¹³⁷Cs content in the Vuoksa River had approached the pre-accident level of 4–5 Bq/m³, which was due to global ¹³⁷Cs contamination of river waters. The decrease in “Chernobyl” ¹³⁷Cs in the Vuoksa River was approximated by a two-component exponential dependence with the water purification half-lives of T₁ = 5 and T₂ = 25 years, respectively. The discharge of “Chernobyl” ¹³⁷Cs with Vuoksa’s waters from Finnish Lake Saimaa in 1986–2023 amounted to 22.5 TBq – a value roughly comparable to the deposition of “Chernobyl” ¹³⁷Cs (74.1 TBq) on Lake Ladoga in 1986. During 1986–1988, the discharge of “Chernobyl” ¹³⁷Cs from Lake Saimaa amounted to approximately 30% of the total for 1986–2024. The long-term transit of ¹³⁷Cs from Lake Saimaa has led to increased accumulation of ¹³⁷Cs (≈300 Bq/kg) in the sediment profile of the Vuoksa River and the lakes within its basin.