Distribution of Cs-137 and Sr-90 in the southern part of Lake Paijanne

Lake Paijanne is a large, oligotrophic lake in the middle of Finland. It is important both for freshwater fishing and as a source of drinking water for about 1 million inhabitants in the capital area. Asikkalanselka is the southernmost basin of Lake Paijanne, the surface area of which is 52.8 km(2). The fallout of Cs-137 from Chernobyl in 1986 to this area was about 64 000 Bq/m(2), and that of Sr-90, about 420 Bq/m(2). After the fallout the Cs-137 concentration in the water of the lake rapidly decreased. The longer the time after the fallout, the longer was the halftime of Cs-137 in water, being now about four years. The relative distribution of Cs-137 between water, particles suspended in water, and sediment has gradually changed, so that the longer the time after fallout, the greater was the fraction transferred to bottom sediment and the less was the fraction detected in water. For example, in 1994 Cs-137 occurred almost totally (96%) in sediment, and only a minor amount occurred in water and seston (4%), while in August 1986, 55% of the Cs-137 occurred in water, about 30% in sediment, and the rest in particles suspended in water. The Cs-137 content in nonpredatory fishes was the highest in the year of the accident, 1986, while predatory fishes had the maximum content of Cs-137 two years after the fallout: in pikes on the average 4200 Bq/kg fresh weight. In 1994 the values for pikes varied from 330 to 1100 Bq/kg. The Sr-90 content in pikes was low, about 0.3 Bq/kg, but was higher in perches. Transfer of radiocesium to fishes was rather effective in Asikkalanselka, which is typical of oligotrophic lakes. The highest estimated transfer factors of Cs-137 in predatory fishes in 1988 were about 0.1 m(2)/kg. The respective values for Sr-90 in the edible parts of various species varied from 0.001 to 0.03 m(2)/kg during 1986-1994. The longer the time after the deposition, the longer are the halflives of Cs-137 in fishes, being now, almost nine years after the fallout, typically 3-7 years for various species.