On the mechanism of the enrichment in radiocesium of near-bottom water in Lake Juodis, Lithuania.

Evidence of the thermodynamic origin of a mechanism for radiocesium enrichment of near-bottom water based on the analysis of vertical profiles of standard water variables (pH, temperature, oxygen concentrations and conductivity) in Lake Juodis is presented. This mechanism is shown to be related to disturbances in the thermohalinic stability (buoyancy effects) of sediment interstitial liquids inducing their interfacial transfer. The mechanism reveals itself even in aerobic waters during cooling processes in autumn under weather conditions inducing the formation of a steep temperature gradient in the surface sediments. These gradients may be formed due to intense sediment cooling as well as in cases of cooling interruptions owing to the arrival of warm air masses inducing temperature stratification of the water column. In the latter case, a sharp decrease in the heat flux from sediments through the stagnant water column promotes relative overheating of the sediment surface layer and the consequent initiation of buoyancy forces inducing interfacial transfer of sediment interstitial liquids enriched in dissolved material. Intrusions of interstitial liquids and their vertical thermodynamic transfer through the water column are followed by the formation of a specific vertical structure consisting of a set of evenly mixed water layers which are especially distinctly observable under ice cover in winter. The mechanism is responsible for lake water contamination by radiocesium and other pollutants dissolved in interstitial liquids. The investigation results explain the phenomenon of "super warm" lakes where temperatures of near-bottom waters under ice in winter are higher than 4 degrees C.