Distribution of some major and trace elements in Danube Delta lacustrine sediments and soil

Abstract

Sediment cores collected from lakes Mesteru and Furtuna (eastern part), Sontea channel and soil samples collected from Caraorman bar, all located in the Danube Delta, were analyzed for 42 elements (Ag, Al, As, Be, Na, Mg, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Mo, Ag, Cd, In, Sn, Sb, Cs, Ce, Hf, Hg, Tl. Pb, Bi, Th. U) by instrumental neutron activation analysis (INAA), thick target proton induced X-ray emission (TT-PIXE) and inductively coupled plasma-mass spectrometry (ICP-MS). The INAA and TTPIXE yielded total concentrations whereas the ICP-MS data reflected the fractions soluble in 14M HNO3. The ICP-MS data exhibited surface enrichment relative to the lower part of the sediment core of Cu, Zn, As, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb, and Bi, most prominently by Cd and Hg. Their vertical distribution in the investigated cores generally reflected the pollution history of recent sediments in Danube delta, showing a steady increase until the end of the 1980s followed by a slow decrease after 1990. The vertical profiles of most remaining elements were characterized by a relatively uniform distribution along the cores. In some cases, the concentrations of As, Cd, Cu, Cr, Mn, Ni and Pb exceeded minimum thresholds of safety, as defined by the Romanian regulations. The elemental composition of the sediment below 20 cm depth (total concentrations) was similar to that of the upper continental crust (UCC) for most elements. Values distinctly higher than UCC were observed for As, Sb (factor ~5) and Cr, Ni, Cu (factor 2 to 3). The nitric acid soluble element concentrations in the soil samples in some cases showed increased values at the surface as compared to 30 cm depth, either due to air pollution or to the action of plants. In no case a large contribution to the topsoil from atmospheric deposition was evident, indicating that the surface contamination of the sediments was mainly by riverine transport.