Chernobyl fallout radionuclides in Lake Sniardwy, Poland

Abstract

Following the introduction of Chernobyl radionuclides, in May 1986, to Lake Sniardwy, the largest lake in Poland, sediment cores and fish were periodically collected through 1989 and analysed for 144Ce, 134Cs and 137Cs. Within four months of the fallout, 134Cs had penetrated down to about 10 cm in one sediment core and, in cores collected from three sites, a year later, had penetrated from 14 to 24 cm. Laboratory measurement of the partitioning of a carrier-free spike of 137Cs between sediment and pore water yielded a K d of 4.3 × 10 3 ml g −1 ruling out molecular diffusion ( D eff = 0.06 cm 2 year −1) as a significant transport mechanism. Rapid vertical transport is ascribed to mixing through strong coupling of wind-driven currents to sediments in this shallow, polymictic lake (5.8 m mean depth) and to biological activity. Profiles of 134Cs at several sites and changes in profiles over three years at one site were described by eddy diffusive mixing (3–20 cm 2 year −1) of a layer of activity initially deposited on the sediment surface. The generally discontinuous nature of profiles of 144Ce is ascribed to its association with discreet ‘hot’ (nuclear fuel) particles. The average activity of 137Cs in the flesh of bream ( Abramis brama) increased up to 120 times that of pre-Chernobyl levels (measured in 1985) 1 year after the fallout event. The delayed maximum and subsequent decrease in 137Cs activity are described by a first order kinetic model with an uptake rate constant proportional to the time-dependent concentration of the isotope in surface sediments. Model calculations are substantially improved if the uptake rate constant includes a term, measuring the ‘availability’ of 137Cs to fish, calculated in terms of radial diffusion of radiocaesium into sediment particles. The inferred residence time of 137Cs in bream was about 1 year.