Behaviour of copper and zinc added to the Tambo River, Australia, by a metal-enriched spring

Abstract

A study of the behaviour of Cu and Zn added to the upper Tambo River, Australia, by a metal enriched but neutral-pH spring (Wilga Spring) is reported. Approximately 80% of the Cu, Zn, Fe and Mn liberated in the oxidation zone associated with the Wilga orebody is deposited within the groundwater system before the spring water enters the Tambo River. Acidic water from the oxidation zone is neutralized by carbonate-rich groundwater.

The input of the spring to the Tambo River, which occurs through the base of the river over a distance of 150-200 m, causes an average sixfold increase in the Cu and Zn concentrations and a measurable increase in the sulfate concentration, but no significant decrease in the pH, in the 'active region' of the river, a distance of approximately 1.3 km. Over the 1-year study period, the spring added an estimated 120 kg of Cu (about 65% in filterable forms) and 230 kg of Zn (about 50% in filterable forms); these fluxes are approximately six times larger than those transported from the upper catchment. The spring also adds an estimated 14.5 t of sulfate, this being roughly the same as the amount (10.7 t) that is transported from the upper catchment. On average, the spring adds only small quantities of Fe and Mn.

The concentrations of filterable (or dissolved) Cu and Zn in the 'active region' are largely controlled by adsorption onto iron oxyhydroxide surfaces associated with the particulate matter. At high concentrations of filterable metals, the active surfaces apear to be saturated with Zn and possibly Cu. At river flows in excess of 5-10 ML day^-1 (the mean annual river flow is about 22 ML day^-1), little Cu and Zn appears to be lost from the water column. The difference in metal concentrations between the 'active region' and Blackfellows Flat (a distance of 10 km) can be explained mostly by dilution of the metal-enriched water in the main branch with water from the southern branch. At low river flows, considerable quantities (up to 99%, depending upon flow) of the Cu, Zn, Fe and Mn added by Wilga Spring are deposited in the 'active region' between the spring and Formfit Creek.