A study of trace metal contamination of urban soils in Hong Kong and agricultural soils in the Pearl River Delta

Abstract

This Master of Philosophy thesis embodies an in-depth study of trace metal contamination of urban soils in Hong Kong and agricultural soils in the Pearl River Delta in terms of degree of contamination, chemical speciation, sources of contaminants, as well as potential associations among trace and major elements. Furthermore, the study has also attempted to investigate the potential implications of trace metal contamination in the two areas, as well as other cities in the subtropical region. Urban soil profiles of six study areas, including five urban parks and one roadway divider, were analyzed for trace and major metal concentrations. The chemical partitioning and Pb isotopic composition of selected soil profiles were also determined. The analytical results showed that some urban soil profiles were contaminated with Cu, Pb and Zn, in accordance to the Netherlands Soil Regulatory Standard. Furthermore, the enrichment of these trace metals at the surface layer was generally more significant than that at the subsurface, suggesting atmospheric and/or surface deposition of the trace metals. Although soil concentrations of the other trace metals, including Cd, Co, Cr, Mo, Ni, and V, were below the "T" values, they generally exhibited a decreasing tendency with increasing soil depth, suggesting surface soil enrichment with these trace metals as well. The chemical partitioning of Cu, Pb, and Zn indicated that the percent distribution of Cu in the mobile and potentially bioavailable fraction varied with soil depth and among sampling sites. Soil Pb in the exchangeable and carbonate fractions accounted for 10-40% of total soil Pb, and the percent distribution of these fractions generally increased with increasing depth. Also, exchangeable and carbonate-bound Zn in the soil profiles, representing 5-40% of total soil Zn, decreased with increasing soil depth. Pb isotopic compositions of the selected urban soil profiles offered strong evidence of binary mixing of anthropogenic Pb at roadside study areas, where gasoline-derived Pb was likely a major source of anthropogenic Pb. Using a binary mixing model, it was roughly estimated that as high as 85% of total anthropogenic Pb at the central divider could have been derived from leaded gasoline. In the study of trace metal contamination of agricultural soils, it was found that some crop, paddy and natural soils were grossly contaminated with Cd, Cr, Cu, Ni, Pb, and Zn. Mean Pb and Cd concentrations of the crop, paddy and natural soils in the Pearl River Delta exceeded the corresponding Class A values in the Chinese Soil Regulatory Guidelines, possibly indicating a spatially extensive Pb and Cd enrichment of the soils. Moreover, the comparatively elevated trace metal concentrations of the crop soils suggested that the crop soils might have received a higher input of trace metals than the paddy and natural soils. The chemical partitioning patterns of Co, Cr, Cu, Ni, Pb, V, and Zn not only gave insight to solubility and potential mobility of the trace metals but also showed influences of different cultivation methods on soil geochemistry. Based upon Pb isotopic composition of total soil Pb and Pb in the five chemical fractions, it was evident that some of the selected soils were affected by anthropogenic Pb, possibly automobile and industrial Pb. It also revealed that anthropogenic Pb tended to reside in the exchangeable, Fe-Mn oxide and organic/sulphide fractions in the soils.