Investigation of the trace element vertical distribution in Danube Delta lake sediments

The aim of the present study is to quantify the pollution levels in the active sedimentation zones of the Danube Delta and to reconstruct the history of their input in lake sediments. Four lakes were selected for this study: Furtuna, Lung and Mester in the western part of Delta and Matita lake in north-eastern part of Delta, in order to determine the sediment origin. Instrumental neutron activation analysis was used for the element content determination. The sediments were dated by using the dating methods with 210 Pb and 137 Cs, developed in our department. Based on the obtained values of sedimentation rates, the vertical distribution of pollutant elements was obtained and the history of these elements' input into the lake sediments could be reconstructed. Increased concentration values of these pollutants before 1987 might be due to the industrialisation of central and eastern European countries. Decreasing trends of the pollutant concentration after 1990, could be explained by the changes of environmental policy in these countries. The average values obtained for the last 50 years and the values obtained for the core collected in Sulimanca channel are presented. Increased values for Br, Zn, Sb (three times larger than their natural concentration) are observed in Mester L. and for As, Be, Sb in Matita L. In Furtuna L.. and Lung L., increased values for As, Cr and Zn were obtained. In the sediments collected from Sulimanca channel, only Cr and Zn have slightly increased concentration. As a conclusion, we estimate that the Danube Delta lake sediments are slightly polluted with these elements. For the sediment core collected from Matita L. (250 cm length), characterised by a low sedimentation rate, the vertical profiles of rare earth elements (REE) present some peculiarities, which can be well correlated with the core structure. A positive correlation could be observed between the total concentration of REE and Sc concentration. The patterns of the investigated REE, normalised to the Upper Continental Crust (UCC) concentration of the same elements show a ratio close to 1, which is consistent with the continental origin of the Danube Delta sediments. We investigated also the La/Th ratio for the whole core. This ratio is considered to be very important in establishing the nature of sediments. The superior part of the core (0-170 cm), corresponding to lacustrine and brackish period of Delta's evolution, is characterised by an average value of this ratio of 3.49 ± 0.17. The same ratio has a value of 2.99 ± 0.15 for the inferior part of the core (170 - 250 cm), very close to about 2.8 (reported for the world-wide provenance loess), showing the continental fundament of the Danube Delta. (authors)