On trace metal geochemistry in the Danube River and western Black Sea

Trace metal concentrations were measured in the Danube River estuary and in the shelf area of the north-western Black Sea. Total dissolved concentrations (<0.45 μM) of the freshwater in the river end-member were found to be: Cd=117 pM, Pb=81 pM, Co=266 pM, Zn=6 nM, Mn=19 nM, Ni=15 nM, Cu=36 nM, Al=384 nM and Fe=20 nM. These concentrations are surprisingly low for an area where serious contamination has been suspected. This is particularly true of Cd, Pb and Zn, which are generally anthropogenic in polluted regions. The observed low concentrations, in particular for Fe, Mn, Co and Al can be attributable to the precipitation of hydrated oxides as a consequence of high pH of the Danube River. Concentrations of metals in the particulate fraction are similar to those in other major rivers of the world. The evolution of the total dissolved trace metal concentrations in the surface waters within the salinity gradient suggests: (1) no noticeable exchange between the particulate and the dissolved fraction for Cu, Ni and Zn; and (2) evidence for a low solid to liquid exchange for the other metals (Cd, Fe, Mn, Co and Pb) which brings about a two-fold increase in their concentrations from lower to higher salinity. Another source of dissolved material is suspected at around salinity 15. This source may be from localized-patches in the sediment where specific redox conditions may induce a vertical flux of dissolved metals: the bottom waters above these patches are characterized by a diminution of the dissolved O₂ and high concentrations of metals. The colloidal fraction (between 10 kD and 0.45 μm) was established on a limited number of samples for Cd, Cu, Mn and Fe and was found to be significant: 40% of Cu, 40% of Cd, ~ 50% of Mn and 60% of Fe of the so-called dissolved phase are actually associated with colloidal material. It represents at least 40% of the total dissolved concentration in the fresh water and in the mixing zone. This proportion decreases linearly with salinity for Cd and Cu. Fe is mostly in colloidal form with a maximum (~80%) between salinity 5 and 10. The surface distribution of the total dissolved metals throughout the study area show a limited influence of the Danube on the concentrations observed for the 'open sea' (salinity= 18). Most of the riverine flux seems to follow the general circulation to the south-west area. These 'open Black Sea' concentrations are the same order of magnitude-as average surface concentrations in the Mediterranean Sea.