Aqueous chemistry of major ions and trace metals in the Clutha River, New Zealand

Abstract

Concentrations of the major ions Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃^2-, SO₄^2- and Cl^-, of silicate, and of both total and dissolved trace metals Zn, Cu, Pb and Cd were determined at sites along the Clutha River. The major ions exhibited differing behaviour both spatially and temporally. A large proportion of Cl^- and Na⁺ throughout the catchment could be attributed to atmospheric input of sea salt. Contributions from this source for the rest of the major elements were minimal, except perhaps near the mouth of the river (K⁺, Mg²⁺). The water composition was dominated by the weathering of carbonate rather than of silicate minerals. Increases in the concentrations of Mg and Na + K indicate the weathering of Mg, K and Na from rock, minerals and clays within the catchment. The concentrations of major ions and Si were relatively low compared with other rivers in New Zealand and elsewhere. Concentrations of dissolved Zn, Cu, Pb and Cd all increased consistently with distance down river, and between 40% and 100%, for a given metal, was found in the dissolved fraction. High correlation among the dissolved trace metals, with essentially constant Cu : Zn : Pb : Cd ratios of 755 : 716 : 223 : 1 (molar basis), indicates that the same weathering process or source may be occurring throughout the catchment.