Vertical distribution of 40K, 232Th, and 137Cs mass activities in lake sediment (Vransko Lake, Croatia) and their relationship with the source material and sedimentation

Abstract

The goal of the study was to research the relationship between the source rocks and the sediment deposited in the Vransko Lake in the Ravni Kotari area and to study the sedimentation pattern in the lake. Sediment was classified as the sandy silt and the phase analysis showed the presence of the calcite, quartz, micas, and pyrite. It is possible that the anoxic conditions formed in the sediment due to the pyrite presence. Carbonate component predominates in the sediment composition. CaCO3 mass fractions in the range 72–94% with the mean values in the range 82–88% were determined. 40K mass activities were in the range 31–166 Bq/kg, 232Th in the range 3.1–18 Bq/kg, and 137Cs in the range 0.3–68 Bq/kg. Negative correlations between CaCO3 mass fractions and radionuclides mass activities were determined. Correlations between the radionuclides mass activities were positive. In the NW part of the lake sedimentation rates of 10.0, 4.1 and 5.0 mm/a were estimated for the 1954–1964, 1964–1986, and the 1986–2010 periods, respectively. At the SE part sedimentation rates for the respective periods were as follows: 8.0, 2.3 and 2.5 mm/a. Average sedimentation rates were 6.4 mm/a for the NW part of the lake and 4.2 mm/a for the SE part. Sedimentation rate in the Vransko Lake varies in time and showed a significant decrease after the 1964. The input of the carbonate and silicate material into the lake is also variable.