Environmental and geochemical analysis of lead contamination: its sources and pathways of exposure and impact on children
thesisposted on 2022-03-28, 13:24 authored by Chenyin Dong
Resolving the source of environmental contamination is a critical first step in planning remediation and intervention strategies to adduce exposure prevention. However, a major challenge in established mining and smelting communities is that the sources of contamination and related potential health risks are often minimised by polluters and government agencies, often using unsupported scientific claims. A significant and longstanding example of this vexed issue is the protracted dispute over contamination sources and causes of childhood blood Pb exposures in Broken Hill (New South Wales), Australia's longest-operating Pb-Ag-Zn mining city. In order to unravel the sources, causes and impacts of contemporary Pb contamination in Broken Hill, this study applied a multiple lines of evidence approach using a combination of geochemical analyses and human health data. The methods applied included analysis of trace element concentrations in soil, dust, tailing dumps and weathered ores, their Pb isotopic and chemical compositions and the morphological characteristics of individual Pb-rich particles. The results demonstrated that Broken Hill contemporary dust Pb deposits were primarily sourced from current mining emissions as opposed to naturally weathered or legacy source sheld in soil and dust around the city. Children living closest to the mining operations were found to be at an increased risk of Pb exposure due to higher dust Pb loading (mean 255μg/m2/day) and its high bioaccessibility (75% of total Pb). The effects of current mining emissions on the contemporary dust Pb loading were quantitatively estimated. The analysis showed that a 1% increase in distance away from the current mining operations was associated with -0.501% (95% CI: -0.728, -0.275) reduction in dust Pb loading, while a 1% increase in production intensity at the mine increased the expected amount of dust Pb loading by 1.487% (95% CI: 0.537, 2.437%). Consistent with the dust Pb decreasing with distance away from current active mining operations, contemporary blood Pb concentrations (2011-2015) also decreased with distance (i.e. 1% increase in distance was associated with a 0.173% reduction in blood Pb concentration). Soil Pb concentrations, which are a proxy for legacy contamination from atmospheric dust depositions across the city also showed a decreasing trend away from the mining operations. Consequently, analysis of the long-term relationship between environmental Pb and blood Pb (1991-2015) revealed that both soil and dust Pb were significantly correlated with blood Pb concentrations, complicating the specific attribution of these individual sources to blood Pb exposure. To quantify the relevant sources of Pb, two 'natural experiments' were examined to separate the specific role of dust and soil Pb on children's blood Pb exposures: (a) the city's northerly prevailing wind direction; (b) the 2009 dust storm that blanketed Broken Hill soil with desert borne sands, rich in crustal elements Al, Si and Fe. The analyses showed that children living within the prevailing wind direction were at the greatest risk of elevated blood Pb compared to those living in a non-prevailing wind direction, even after adjusting for effects of residential soil Pb contamination. The role of current emissions on driving exposures was further corroborated by the fact that children's blood Pb shifted systematically with mine Pb production volumes irrespective of remedial interventions. The second natural experiment, the 2009 dust storm, showed blood Pb concentrations still decreased with distance even though soil Pb-distance gradient was reduced after the dust storm. Generalised linear regression analysis of contemporary outdoor dust Pb deposits and blood Pb concentrations corroborated dust as being a significant driver of childhood Pb exposures. This thesis research confirms that soil Pb on its own cannot explain blood Pb distributions and that dust Pb depositions are a significant factor in driving contemporary exposures. This finding provides evidence-based support for the mitigation and remediation approaches that are likely to be the most effective in terms of reducing blood Pb exposures in children-these being a clear need to control current mining emissions in Broken Hill as has been identified in Australia's other mining and smelting communities of Mount Isa and Port Pirie. The adverse consequences of not implementing evidence-based and targeted mitigation and remediation strategies on childhood development outcomes were also evaluated in the thesis. Early childhood development indices (i.e. developmental vulnerabilities-Australian Early Development Census data; school test scores-National Assessment Program Literacy and Numeracy) showed that children from the most contaminated areas in Broken Hill had lower educational performance and higher developmental vulnerabilities, even when normalised for socio-economic status. Similarly, poor scores were identified for Australia's other Pb processing locations of Mount Isa (Queensland) and Port Pire (South Australia), indicating the urgent need to implement the highest standards to limit the impact of mining and smelting Pb emissions on adjoining communities. Finally, application of a multiple lines of evidence as used in this thesis research to establish more precisely the source(s) of exposure in mining and smelting communities could be applied to other locations. Such an approach would help to address unequivocally extant disputes over the sources, causes and impacts of environmental Pb contamination in order to better target remediation and prevention strategies.