Effectiveness of cracker dust as a capping material for Pb-rich soil in the mining town of Broken Hill, Australia

• Pb-free cracker dust has been installed on top of Pb-rich topsoil in Broken Hill.
• We investigated the efficacy of cracker dust in suppressing topsoil Pb.
• High Pb concentrations are found in natural soils near the Broken Hill orebody.
• Low Pb concentrations are found in most cracker dusted topsoils near the orebody.
• Pb-rich dust deposition has only a slight effect on the efficacy of cracker dust.

Lead (Pb)-poor ‘cracker dust’, commonly used in kerb and gutter construction, has been installed on top of Pb-rich soil surfaces in Broken Hill over the last few decades, with the aim of reducing the risk of human exposure to Pb particulates. In this study, topsoil (0–0.1 m) and subsoil (0.3–0.5 m) samples were collected along rays spanning all parts of the Broken Hill urban area to investigate the effectiveness of remediation with cracker dust. Total Pb concentrations in the fine earth fraction (< 2 mm) of topsoil and subsoil samples and the dust-sized fraction (< 100 μm) of topsoil samples were measured. Undisturbed subsoil Pb concentrations are high near the ‘Line of Lode’ (LoL) orebody, especially on its southern flank, but are similar to background Pb concentrations at locations more than 0.8 km from the LoL. Topsoil Pb concentrations at locations near the LoL are very high, but tend to decrease rapidly with increasing distance from the LoL. At most sampled locations the topsoil is significantly enriched in Pb relative to the subsoil, while Pb concentrations in the dust-sized fraction of topsoils are generally twice as high as those in the fine earth fraction of topsoils. Notably, where cracker dust has been applied, most topsoil Pb concentrations are significantly lower, indicating that remediation with cracker dust has been generally effective. Although contemporary dust deposits in Broken Hill can be very rich in Pb, the relatively small amounts of dust deposited ensure that Pb accretion from fugitive dust deposition is a very gradual process; undisturbed cracker dust should have a ‘clean lifespan’ of at least 100 years. However, a small number of cracker dusted sites are found to have Pb-enriched surfaces, suggesting that the efficacy of cracker dust is curtailed where fluvial transport of Pb-rich sediment and/or mixing of the applied cracker dust with the underlying Pb-rich soil occurs.

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