The occurrence and bioavailability of radioactive 137Cs in small forest lakes in Southern Finland

Abstract

Following the Chernobyl accident in late April 1986, central Finland was subjected to considerable radioactive fallout. The radioactive isotope of caesium, 137Cs, was potentially the most harmful isotope in the fallout because of its long half-life (30 years). 137Cs activities remained unexpectedly low within the biota (algae, zooplankton, aquatic moss, fish) of some humic forest lakes in southern Finland compared to the clear water lakes within the same district. This observation suggested that humic substances, by binding 137Cs chemically, may reduce its bioavailability in lake water. Our laboratory experiments (throughflow systems; gel chromatography) demonstrated that binding of 137Cs by humic substances was negligible in untreated lake waters; only when most of the alkali metals had been removed with an ion exchange resin was any binding of 137Cs by low molecular humic fractions apparent. Instead, the concentrations of cations (especially potassium) were of overwhelming importance for the bioavailability of 137Cs within the lacustrine ecosystems. The concentrations of 137Cs within food chains are expected to stay at a high level for many years especially in lakes with long water renewal times.