On the shapes of deep slopes of radiocesium vertical profiles in lake sediments.

The shapes of the deep slope (below the maximum values) of the vertical profiles of radiocesium activity concentrations in the sediment samples taken in 2003-2012 in the Lakes Juodis, Tapeliai and Red lake were studied. The Gaussian shape of the deep slope indicates the migration of radiocesium into the depths of the sediments, and this process is significantly enhanced in some places due to bioturbation caused by tench preparing for hibernation. The exponential shape of the deep slope is typical for sediments in which in winter, in the presence of an ice cover on the lake, thermodynamic mixing occurs in the surface layer caused by the effects of pore water buoyancy. In these sediments, radiocesium dissolved in the pore fluid migrates upwards into the near-bottom water, becoming a source of secondary pollution of the water column. In winter, the presence of such a process is easily determined by the emergence of a layered structure of the water column in the lake and the temperatures of the near-bottom waters exceeding 4 °C. In this case, each layer is characterized by constant standard water parameters (temperature, conductivity, concentrations of oxygen, and trace elements). Complex forms of the deep slope of the vertical profile of radiocesium activity concentrations, combining elements of the exponential and Gaussian forms, indicate the episodic presence of both migration mechanisms. A method is proposed for identifying sediments that are a source of secondary pollution of lake waters by estimating the differences between the normalized logarithms of the radiocesium activity concentrations of the deep slopes (below the maximum concentrations) of its vertical profiles in the sediments of the studied samples and the sample of the carbonate barrier sediments, which were discovered in the shallow part of Lake Juodis in 2003.