Dissolved trace metal speciation in the Manuherikia River, Central Otago, New Zealand

Abstract

The inorganic speciation of the trace metals Cu, Ni, Cd, Zn and Pb has been calculated for the pristine, subalpine Manuherikia River. Temperature, pH, ionic strength and major-ion composition were found to be important controls on the free-ion activity of trace metals. Metal -CO₃^-2 species were the most important complexes for Pb, Cu and Ni. The metal-HCO₃^- species was most important for Cd, and Zn was intermediate. Ni, Zn and Cd were present mainly as the free divalent ion, whereas Pb (up to 81% complexed) and Cu (up to 70% complexed) were strongly affected by ionic interactions. Free-ion activities of Cu²⁺ and Pb²⁺ were essentially constant along the length of the river despite significant increases in total dissolved-metal concentrations. Thus, for these metal ions, total dissolved concentrations are a poor indicator of biological availability.