Evaluation of accumulation and concentration of heavy metals in different urban roadside soil types in Miranda Park, Sydney

The overall objectives of this study were to examine the relationship between the concentrations of heavy metals such as Pb, Zn, Cu, and Cd in roadside soil derived from three different geological parent materials, Hawkesbury Sandstone, Wianamatta Shale, and Mittagong Formation and also to examine the influence of rainfall events on heavy metal concentrations in both the topsoil and the subsoil in all three soil types. In this paper, the focus is on lead and zinc. The results obtained from the samples taken from an initial transect were used to select the location of the study sites. Soil samples were collected using a stainless steel auger at distances of 1, 5, and 10 m from the edge of two major roads of similar traffic volumes bordering a suburban park. At each of five study sites, samples were collected at depths of 0-10 and 10-30 cm, three times pre-rainfall (after extended periods of no rain) and three times post-rainfall (after intensive rainfall periods). The modified aqua regia digestion method was applied for heavy metal concentrations measurement. To determine the temporal dynamics of trace elements in the soils, sequential extractions were applied to all the topsoil samples according to the modified three-step sequential extraction procedure. The corresponding concentrations of Pb and Zn were different for the soil derived from Hawkesbury Sandstone and Wianamatta Shale and also Mittagong Formation. The highest concentration of Pb was in the soil from Wianamatta Shale, 159.32 mg/kg and the highest concentration of Zn was in the soil from the Mittagong Formation, 254.12 mg/kg, all at a distance of 1 m from the roadside. From the sequential chemical extraction results, the rainfall substantially influenced the exchangeable fraction (F1) of Pb at a distance from the road of 1 m. A significant reduction of F1 was found for the soil derived from Mittagong Formation which also had the most significant reduction of total Zn concentration. The interpretation of the results showed that there was a clear correlation between the concentration of Pb and Zn with the distance from the roadside and depth in all soil types. However, the results also showed that there are variable concentrations between the soil types. The heavy metal concentrations at the same distance for the three soil types are different. The rainfall events do influence the heavy metal concentration differently in both topsoil and subsoil of the three soil types at the same distance from the roadside.