Metal Transport Phases in the Upper Mississippi River

Six metals of geochemical and water quality interest in the upper Mississippi River were partitioned into soluble, adsorbed, solid organic, oxide coating, and crystalline phases, applying a chemical fractionation scheme to particulates and ultrafiltration to the dissolved (<0.4 µm) phase. Crystalline and oxide fractions were dominant transport phases for aluminum and iron, while solution and solid organic fractions were dominant for manganese, copper, cadmium, and lead. The fraction of metal transported in available phases (noncrystalline) ranged from 10 to 25% for aluminum and from 30 to 70% for iron, while the amount of manganese, copper, cadmium, and lead generally exceeded 90%. Ultrafiltration studies showed that the highest concentrations of copper, cadmium, and lead occurred in the 1-10K mol wt fraction and correlated with organic carbon. The magnitude of residual copper complexation capacities (~ 1.0 µM) and conditional stability constants (~ 10^(10)) suggests that natural organic ligands are likely multidentate, containing nitrogen and sulfur functional groups.