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

¹³⁷Cs in Lake Tapeliai, Lithuania

“…Evidently, both processes are related to the radiocesium enrichment in the nearbottom waters. Nevertheless, investigations carried out in Lake Juodis [23] allowed showing that in super warm lakes the radiocesium enrichment due to the interstitial liquid intrusions might be possible under aerobic conditions and it was a precursor of the anaerobic zone formation. Presumably, under conditions of a moderately warm regime in Lake Tapeliai in winter, elevated radiocesium activity concentrations in the anaerobic near-bottom waters may be mainly due to processes of sediment organics decomposition.…”

137Cs in Lake Tapeliai, Lithuania

“…Earlier studies carried out in Lake Juodis showed that in winter these processes are of thermodynamic origin [23] and are related to the amount of heat accumulated by sediments until the time of the formation of the ice cover. Depending on the amount, Lake Tapeliai may be treated as a super warm or moderately warm lake in some years [36,37].…”

“…Thus, in Nordic lakes, average temperatures of the water column before the formation of the ice cover drop down to ~0.5 °C [41,42]. In this case, a 4 °C isotherm must be deepened below the sediment surface due to elevated thermal diffusivity of the sediment surface layer, which is affected by the buoyancy of interstitial liquids [23,43]. Therefore, the significant overcooling of the water column induces respective heat losses in the sediments.…”

“…In the case of large amounts of heat accumulated in sediments at critical heat and salinity Rayleigh numbers, the secondary buoyancy events are possible in surface sediments [23]. They are followed by the additional intrusions of highly mineralised and warm interstitial liquids to the near-bottom water and by the increase in the near-bottom water temperatures above 4 °C.…”

“…Thus, a long-term permanent study of seasonal variations of the vertical structure and radiological data (on radiocesium) in Lake Juodis (54°46'49'' N, 25°26'29'' E) made it possible to determine that carbonate deposits, which were formed in shallow areas of the lake, were acting as a radiocesium barrier [22]. Further, it became possible to evidence that in autumn and winter a thermodynamic mechanism was responsible for the enrichment in radiocesium of the near-bottom water of the lake [23]. A thick layer of organics-rich sediments and their proximity to the water surface made this lake a splendid scientific object allowing fruitful investigations of the processes at the near-bottom water-sediment interface.…”