School of Earth & Environmental Science
Trajcevski, Michael, Heavy metal concentration and transport from existing copper slag emplacements along the Windang Peninsula, through the unconfined sandy aquifer to Lake Illawarra, BEnviSc Hons, School of Earth & Environmental Science, University of Wollongong, 2017.
Up until recently, copper slag was widely considered to be chemically inert, and was thought to pose no significant environmental risks when deposited. Since the late 20th century there have been many reports claiming that copper slag can pose a risk to groundwater systems once it has degraded in its depositional environment over time.
Wollongong City Council is interested in examining the possible environmental impacts of a copper slag emplacement located on the Windang peninsula, a narrow strip of estuarine sand on the south coast of NSW. In the past, high concentrations of zinc, iron, cadmium and copper have been observed in the emplacement groundwater. The concerns were the possibility of dissolved metals in the groundwater increasing over time due to increased metal leaching from weathering, and also the transport of these dissolved metals through the Windang unconfined sandy aquifer to nearby Lake Illawarra.
Dissolved metal concentrations in groundwater were analysed in this study from a number of new and existing bores within the slag emplacement, and results were compared with existing data gathered in past reports. Weathered and unweathered slag samples were also inspected visually using reflected light microscopy and scanning electron microscopy (SEM) to determine structural differences in the outer perimeter of the slag granules. The slag samples also underwent elemental analysis using EDS to determine the distribution and abundance of various elements throughout the slag granules, and help determine which metals leach out fastest.
The groundwater analysis yielded results similar to background levels for most samples, with the exception of zinc in groundwater from BH9 only, which on all four sampling rounds exceeding the ANZECC (2000) guidelines. When compared to past results from equivalent bores, the concentration of most metals in the groundwater decreased on average over time. The only metals which increased over time contained concentrations equal to or lower than background levels in all samples, suggesting the slag is not a major source of these metals.
XRF and EDS results consistently displayed significant depletion of zinc from the weathered zones of slag. As the main source of zinc in groundwater is the weathered outer rim of the slag granule, it suggests that any new zinc being leached into the groundwater must be originating from the weathering of the unweathered, inner zone of the slag granule. With the production of this weathering rind around the perimeter of partly weathered slag granules, it is hypothesised that the rate of weathering of the unweathered inner section of the slag granule is greatly decreased due to the reduced exposure to air and moisture as a result of the “shielding” effect of this weathering rind. The reduced supply of zinc from slag into groundwater through leaching, and the dispersion of existing dissolved zinc throughout the aquifer, provides a possible explanation for the reduced zinc concentrations in the Windang aquifer over time. With the likely continuation of this trend, as long as the emplacement site is not agitated or disturbed, no action is necessary regarding metal immobilisation within the groundwater.