Soil Cd, Cu, Pb and Zn contaminants around Mount Isa city, Queensland, Australia: Potential sources and risks to human health

摘要

This article investigates the relationship between soil Cd, Cu, Pb and Zn contaminants and the location and activities of the Pb-Zn-Ag and Cu mines at Mount Isa, Queensland, Australia. Analysis of the data focuses primarily on soil Pb distributions and concentrations because of their potential impact on children's health. The Xstrata Mount Isa Mines lease (XMIM) is Australia's leading emitter of numerous contaminants to the environment, including Cu and Pb, and the mining-related activities have been linked causally to the findings of a 2008 study that showed 11.3% of local children (12-60. months) have blood Pb levels >10μg/dL. Queensland government authorities and Xstrata Mount Isa Mines Pty Ltd maintain that contaminants within environmental systems around Mount Isa are largely the result of near-surface mineralization. The evidence for whether the contamination is derived from XMIM or other possible sources, such as the natural weathering of ore-rich bedrock, is investigated using data from surface and subsurface soil chemistry, atmospheric modelling of metal contaminants from mining and smelting operations and local geological and associated geochemical studies. Sixty surface soil samples collected from sites adjacent to houses, parks and schools throughout Mount Isa city were analyzed for their total extractable Cd, Cu, Pb and Zn concentrations in the <2. mm to >180μm (coarser) and <180μm (finer) grain size fractions. Concentrations in the finer size fraction reveal a range of values: Cd - 0.7-12.5. ppm; Cu - 31-12,100. ppm; Pb - 8-5770. ppm; Zn - 26-11,100. ppm, with several samples exceeding Australian residential health investigation guidelines. Spatial analysis shows that surface soil metal concentrations are significantly higher within 2. km of XMIM compared to more distant samples, and that more than 1000 property lots are at risk of having detrimentally high soil Pb levels. Determination of metal concentrations in 49 samples from eight soil pits shows that surface samples (0-2. cm) are enriched significantly relative to those at depth (10-20. cm), suggesting an atmospheric depositional origin. AUSPLUME air dispersal modelling of Pb originating from the Cu and Pb smelter stacks and mine site fugitive sources confirms that Pb is deposited across the urban area, during periods of the year (～20%/a) when the wind blows from the direction of XMIM towards the urban area and disperses dusts from the uncovered spoil and road surfaces, as well as from stack emission sources. Although there are some spatially restricted outcrops of Pb close to the surface in parts of the urban area, the Cu-ore body is ～244. m below the surface. However, enriched and significantly correlated surface soil concentrations of Cu and Pb (Pearson correlation 0.879, p=0.000) in and around the urban area of Mount Isa can only be explained by atmospheric transport and deposition of metals from the adjoining mining and smelting operations. The results from this study provide unequivocal evidence that both historic and ongoing emissions from XMIM are contaminating the urban environment. Given the ongoing Pb poisoning issues in Mount Isa children, it is clear that remediation, reductions in mine emissions and more stringent regulatory actions are warranted.