Spatial variation of metals in the top soils of four Irish urban parks

Abstract

Urban soils are recipients of various pollutants, including metals, which can be accumulated over large timescales. The origins of urban soils are mixed and frequently unknown. As a result a strong spatial heterogeneity of metals tends to exist in soils of urban areas, making the hazard assessment of metal pollution a challenging task. In order to better understand the spatial variation of metals in urban soils, samples were collected from four urban parks: a roadside sports ground, a traditional bonfire site, a historical landfilling site and a city park in both Galway and Dublin City. Soil geochemical variables were determined using different chemical techniques (e.g. Portable X-ray fluorescence and, Inductively Coupled Plasma¿Emission Spectroscopy). The results were analyzed using conventional statistics, geostatistics and a geographic information system (GIS). Like most European countries, there are many public sports grounds and parks in Ireland; however few previous studies concerning metal contamination have been carried out in these public amenities. Strong variations in soil geochemistry were observed at all four parks. Furthermore, some elements showed multi-modal features, indicating the existence of mixed populations which proved difficult to separate. Based on the spatial distribution maps, it was found that areas with elevated concentrations of metals were close to potential pollution sources, e.g. relatively high levels of pollution were found along the roadside in both Newcastle sports ground and the Phoenix Park. Elevated concentrations of Zn, Cu and Pb in the soil were closely associated with the emissions related to bonfires which take place annually at Halloween. The hazard assessment maps clearly showed that a significant portion of South Park sports ground contained total metal concentrations greatly higher than levels known to cause environmental problems. However, the implementation of hazard assessment of metal contamination in soils requires the determination of both total (TCs) and bioavailable concentrations (BCs). Therefore, the potential BCs of metals were determined in soils of South Park using Ethylene-diamine-tetra-acetic acid (EDTA) extractants followed by ICP-OES analysis. The hazard assessment maps still indicated that a considerable portion of the study area contained elevated BCs, indicating metal contamination. Overall, in comparison with South Park, the other sites in this study are not severely contaminated. Based on the hazard assessment maps for Rahoon bonfire site, it is recommended that local communities should be advised to refrain from the burning of tyres and other hazardous metal-containing wastes in bonfires, which result in the contamination of the surrounding soil. The study provides the total and bioavailable metal concentrations in urban soils in Ireland, which not only indicated their current contaminative status, but also can be used for comparative purposes in future pollution assessment studies. These results are particularly useful for policy development and management practices in public spaces and sports grounds in urban areas.