Hydrochemical Regime of the Kiliya Delta of the Danube River in Retrospective and Modern Conditions: II. Metal Content and Speciation

Abstract

Past and present data on the concentration and speciation of metals (Al, Fe, Cu, Zn, Pb, Cr, Cd, Mo, Ni) in the Kiliya Danube delta have been generalized, and variations in their concentrations have been compared. It has been shown that the highest pollution of the Danube lower reach has occurred from the second half of 1980s to the first half of 1990s due to intensive anthropogenic impact on the river delta ecosystem. Subsequently, decrease of the metal concentration was observed as a result of industrial crisis in the late 1990s–early 2000s. The major part of metals is transferred by water flow in the form of suspended solids, which is related to their relatively high content in water of the Kiliya part of the Danube delta. The results of studying dissolved metal species, in particular the ratio of the labile fraction that is potentially toxic to hydrobionts and complexes with dissolved organic matter, have been discussed. Data on the distribution of metals among complexes with dissolved organic substances of different chemical natures and molecular weights are given. Humic substances have been shown to contribute most to the complexation. Compounds with molecular weights of 1 to 5 and less than 1 kDa constitute the major part of anionic metal complexes with humic substances.