An assessment of metal-humus complexes in river waters of the Upper Amur basin, Russia.

The dissolved metal concentrations have been assessed for Co, Cr, Cu, Fe, Mn, Ni, V, Zn, Ca, Mg, and Cd, and their complex compounds with humic substances (Me-HS complexes) in river waters of the Upper Amur basin using the inductively coupled plasma-mass spectrometry technique. The study shows an increase in the amount of Me-HS complexes from the southwestern steppe and forest-steppe to the northeastern mountainous taiga areas. In alkali waters of the steppe and forest-steppe areas with low concentrations of HS, the amount of Zn-HS complexes increases considerably. The observed general regularity in the distribution of metals is as follows: Fe > Cu ≥ Zn > Cr > V > Ni > Сo ≥ Mn > Ca > Mg > Cd. In acidic and neutral waters of the taiga rivers containing high concentrations of HS, 50% of metals are complexed with HS. Metals are arranged in the following order: Fe > Cu > Ni > Cr > V > Zn > Сo > Mn > Ca > Mg > Cd. Quantitative characteristics and a correlation matrix show that Me-HS complexes are dominant among toxic metals (Ni, Cu, Zn, Cr, Fe, and V) and account for 40-55% of total dissolved forms. This allows us to evaluate the important role of Me-HS complexes in satisfactory ecological situation in rivers of the Upper Amur basin. However, toxicity may increase in separate watercourses (Chita and Argun rivers) due to high concentrations of dissolved Zn, Cu, and Cr, which indicate local and transboundary sources of pollution.