Radioactivity in surface water

Analyzing radioactive materials in surface waters provides information on their time behaviour. Samples taken from different bodies of water indicate the geographic variation of radionuclides.

Surface water samples are taken once a year at the mouth of four large rivers: Kymijoki, Kokemäenjoki, Oulujoki and Kemijoki. The samples are taken either in May or October. The samples are analyzed for Cs-137 and Sr-90.

The uneven geographic distribution of the Chernobyl fallout and the different behaviours of Cs-137 and Sr-90 in waters is evident in the surface water measurements. Unlike the Chernobyl fallout, the nuclear test fallout distributed almost evenly throughout Finland. The effects of the nuclear test fallout are still detectable in the rivers of Northern Finland.

The Cs-137 concentrations in Kymijoki in Southern Finland remain clearly higher that those of Sr-90, because of the Cs-137 fallout in the catchment basin in spring 1986 was significantly higher than the Sr-90 fallout. However, in the Kemijoki catchment basin in Northern Finland, where the Cs-137 fallout was considerably lower than within the Kymijoki region, the concentrations of Cs-137 are already lower than those of Sr-90, which dissipates from water more slowly than Cs-137.

The radioactivity levels of Cs-137 and Sr-90 in surface water samples in 2019 (Bq/m3).

River Reference day Cs-137 (Bq/m3) Sr-90 (Bq/m3)
Kymijoki 14.10.2019 13 ± 2 4.6 ± 0.4
Kokemäenjoki 14.10.2019 3.2 ± 0.4 4.1 ± 0.4
Oulujoki 7.10.2019 2.3 ± 0.3 2.6 ± 0.3
Kemijoki 9.10.2019 0.73 ± 0.13 1.6 ± 0.2


The radioactivity levels of Cs-137 and Sr-90 in surface water samples in 2018 (Bq/m3).

River Reference day Cs-137 (Bq/m3) Sr-90 (Bq/m3)
Kymijoki 10.5.2018 16 ± 1 4.6 ± 0.2
Kokemäenjoki 14.6.2018 4.2 ± 0.4 4.3 ± 0.2
Oulujoki 16.5.2018 7.3 ± 0.6 2.7 ± 0.1
Kemijoki 23.5.2018 1.5 ± 0.1 1.6 ± 0.1




  • Vaaramaa Kaisa
    Head of Laboratory / MIT tel. +358975988521