Pulsative discharge of dissolved metals from copper slag emplacements into the Windang unconfined sandy aquifer adjacent to Lake Illawarra

Abstract

Granulated copper slag is a waste residue of copper smelting activities. Copper slag is mainly composed of a ferro-silica matrix (Fayalite) embedded with numerous, submicron sulfide particles of various metals originating from the parental copper ore. In Wollongong, some 120,000 to 150,000 tonnes of slag produced annually by a local smelter has been disposed of in landfills. A large quantity of granulated copper slag has been emplaced within the Windang Peninsula’s unconfined sandy aquifer since 1945. A portion of the emplacement, situated within the fluctuating water table zone (unsaturated zone of the aquifer), exhibits intensive biological and/or chemical weathering, resulting in the release of soluble metals such as zinc, copper and cadmium into the aquifer. Groundwater quality data recorded from 1993 to 2003 suggests that in the aquifer beneath the slag emplacement, zinc occurs totally in the soluble form. Concentrations of zinc in groundwater beneath the emplacement rise after each rainfall event and gradually decrease in dry weather due to diffusion and dispersion processes. The pulsative nature of metal discharge into the aquifer is partially due to continuous biological and chemical reactions in the unsaturated zone in dry weather and the lack of advection forces in the Windang Peninsula’s sandy aquifer. Continuous oxidation of sulfides in slag produces iron and silica oxy-hydroxides which coat slag granules or precipitate in the inter-particle pore space. Ferrous oxyhydroxides act as a temporary trap to adsorb metal ions, solubilized in the sulfide oxidation process in dry weather. In the next wet period, the infiltrating rainwater removes these adsorbed metals and drives them through the unoxidised mass of slag below the water table, into the deeper part of the aquifer.