137Cs in Lake Tapeliai, Lithuania

Abstract

The results of an integrated study of the radiocesium behaviour in Lake Tapeliai by using not only conventional data on radiocesium activity concentrations in lake water and sediments but also a complex data set on seasonal variations and vertical profiles of standard water variables are presented. Radiocesium activity concentrations in lake water and a vertical structure of the water column considerably depend on radiocesiumenriched inflows of the coloured waters from the lake’s swampy watershed. The global fallouts are mainly responsible for radiocesium inventory in lake sediments, where maximum values of radiocesium are found in the upper part of the water column above the ~5.4 m depth. The maximum values of radiocesium inventories in lake sediments are consistent with the respective densities of its deposits in the nearest forest soils. The main process of sediment activation is a direct sorption of radiocesium onto the sediment surface. Sedimentation rates in the lake mainly vary in the range of 3.5–5 mm yr–1. The lake bottom feeding sources located mainly on the southern terrace as well as their related near-bottom flows reduce respective sedimentation and radiocesium inventories in sediments. The thermal regime of the near-bottom water in Lake Tapeliai in winter strongly depends on the meteorological conditions in autumn and may be classified as varying from super warm to moderately warm. Under conditions of a super warm regime, the elevated radiocesium concentrations in the near-bottom waters may be partially due to the thermodynamic mechanism of its release from the bottom sediments.