Heavy metals in urban soils of central Jordan: should we worry about their environmental risks?

Forty soil samples collected from central Jordan were analyzed by atomic absorption spectrophotometry for Pb, Cd, Zn, Cr, and Hg. The samples were also investigated for mineralogy using X-ray, electron, and optical microscopes. Sequential extraction procedures were used to predict the percentages of the Pb, Zn, Cd, and Cr present in each of the soil geochemical phases. The clay mineral assemblage encountered in the analyzed samples is composed of kaolinite, smectite, illite, and illite/smectite mixed-layer. The nonclay minerals of the sand-sized fraction are composed mainly of quartz and calcite as major minerals with pyroxene, biotite, and feldspars as minor minerals. The enrichment factors of the measured heavy metals Pb, Cd, Zn, Cr, and Hg in the clay fraction (<2 microm) of the collected samples are 3.1, 16.6, 1.5, 0.9, and 4.5, respectively. According to the index of geoaccumulation, the soils of the study area are considered to be moderately contaminated with respect to Cd, uncontaminated to moderately contaminated with respect to Pb, Hg, and Zn, and uncontaminated with respect to Cr. The measured metals correlated positively with the determined physicochemical factors such as pH, clay content, organic matter content, and carbonate content. The relatively high concentrations of Cd, Pb, and Hg in the soils of the study area are related to anthropogenic sources such as cement industry, fertilizers, and vehicle exhausts. It was found that Pb, Zn, and Cr are associated mainly with the residual phases and are relatively immobile. On the other hand Cd is enriched in the carbonate phase of the analyzed soil samples. It is possible to suggest the sequence of mobility for Pb, Zn, Cd, and Cr in the analyzed soil samples as the following: Cd>Pb>Cr>Zn.