Influence of Different Urban Structures on Metal Contamination in Two Metropolitan Cities

The influence of urban structures and land use patterns of metropolitan cities on the distribution of contaminants is not well understood. In this study, two metropolitan cities [Jeddah (a typical corridor city) and Madinah (a typical compact city)], featuring different spreading patterns and urban structures, were selected to investigate the contamination level and potential risk caused by metals (i.e., Pb, Zn, Co, Fe, Al, Cr, Cu, Ni, and Mn) associated with urban dust. The findings of this study show that a metropolitan city with a limited variety of activities and a polar center (e.g., Madinah) displays a typical distribution pattern of metals, i.e., concentrations of metals increase gradually toward the center of the city. In contrast, a metropolitan city with multiple major activities (e.g., Jeddah) displays a different distribution pattern, controlled by multiple key actors (e.g., seaports, oil refineries, and desalination/power plants) able to shift the location of highest contamination away from the city center. The above findings are supported by the results of several contamination and health indices. In Jeddah, the highest Pb contamination was found near an oil refinery based on geoaccumulation index (Igeo), contamination factor (CF), enrichment factor (EF), and ecological risk (E(i)) values; whereas, the highest Zn contamination was found near a seaport, based on EF, CF, and E(i) values. However, in Madinah, the contamination indices indicate that the most contaminated locations are near the city center. The highest non-carcinogenic health risk in Jeddah was found near an oil refinery and in the city center; whereas in Madinah, it was found mainly in the city center. Although there is no significant risk of cancer due to metals associated with dust in the two cities, Cr, representing a health risk contribution of >24%, was the major contributor of non-carcinogenic health risk in the two metropolitan cities.