Chromium Speciation and Distribution in the Great Lakes

A technique is described for selectively measuring the forms of chromium in freshwater samples, with detection limits of 21 ng L-1 for Cr(III), 4 ng L-1 for Cr(VI), and 8 ng L-1 for colloidal/organic Cr. Recovery of the Cr(VI) and Cr(III) spikes were consistently >90%, but only about 70% for the added organic Cr. When the method was applied to samples from Lake Ontario, it was found that Cr(VI) made up 75-85% of the dissolved Cr and that Cr(III) was consistently below the analytical detection limit. About 10% of the dissolved Cr was in the colloidal/organic form. Average concentrations of total dissolved Cr in the epilimnetic waters were found to be 69 ng L-1 in Lake Superior, 136 ng L-1 in Lake Erie, and 351 ng L-1 in Lake Ontario. The increasing concentration of Cr down the Great Lakes watershed reflects the conservative geochemical behaviour of Cr(VI) and the increasing input of Cr from industrial and municipal sources.