THE CHROMIUM CYCLE IN A SEASONALLY ANOXIC LAKE

The Cr cycle was investigated in seasonally anoxic Greifensee, Switzerland. Cr(III), Cr(VI), and total dissolved (<0.45-mu-m) Cr and total Cr concentration profiles were measured from January until November 1989 together with other redox sensitive species of N, Mn, Fe, and S. Cr(VI) was the predominant species even under anoxic conditions. Concentrations of Cr(VI) ranged from 1 to 4 nM. Reduction of Cr(VI) could be observed only in extremely reducing conditions in the presence of S(-II) and Fe(II). It was concluded that a suitable reducing agent had to be present for the reduction of CR(VI) to Cr(III) to take place and that organic reductants most probably played a minor role. In addition, calculations indicated the occurrence of a slow removal process, with a half-time of 100-230 d, that reduced Cr(VI) concentrations by either adsorption or reduction in both oxic and suboxic waters. Dissolved free Cr(III) species were not detected, which was explained by the strong tendency of this species to bind with surfaces and organic ligands and colloids. Even in oxic conditions, the sediments did not appear to be a source of Cr(VI).