THE OCCURRENCE AND BIOAVAILABILITY OF RADIOACTIVE CS-137 IN SMALL FOREST LAKES IN SOUTHERN FINLAND

Following the Chernobyl accident in late April 1986, central Finland w as subjected to considerable radioactive fallout. The radioactive isot ope of caesium, Cs-137, was potentially the most harmful isotope in th e fallout because of its long half-life (30 years). Cs-137 activities remained unexpectedly low within the biota (algae, zooplankton, aquati c moss, fish) of some humic forest lakes in southern Finland compared to the clear water lakes within the same district. This observation su ggested that humic substances, by binding Cs-137 chemically, may reduc e its bioavailability in lake water. Our laboratory experiments (throu ghflow systems; gel chromatography) demonstrated that binding of Cs-13 7 by humic substances was negligible in untreated lake waters; only wh en most of the alkali metals had been removed with an ion exchange res in was any binding of Cs-137 by low molecular humic fractions apparent . Instead, the concentrations of cations (especially potassium) were o f overwhelming importance for the bioavailability of Cs-137 within the lacustrine ecosystems. The concentrations of Cs-137 within food chain s are expected to stay at a high level for many years especially in la kes with long water renewal times.