Heavy Metal Pollution And Evaluation Of Dust At Xi'an Gas Station

With the development of urbanization, urban scale expands continuously and the city system is perfected increasingly, which brings new challenges to the urban environment. Heavy metal pollution is one of the focused environmental problems at home and abroad in recent years. Over the last three decades, many studies have focused on heavy metal pollution in metropolitan park dust, urban road dust, industrial districts and so on, while few information is available on gas stations which are a special and necessary part of the urban system. Taken the above mentioned fact, it's meaningful to study the physical and chemical properties, heavy metal concentration and the pollution level in gas station dust.The report analyzes systematically the physical and chemical properties (magnetic susceptibility, the pH value, particle size), characteristics of heavy metal content and sources of 30 samples gotten gas stations in Xi'an by a series of experiments. Potential ecological risk and the health risk assessment have been done in this paper. The conclusions are as follows:(l)The pH mean value of the gas station dust in Xi'an is 10.02 which belongs to the strong alkaline. The pH values near the third ring are higher than 10.0 which is the strong alkaline from the spatial distribution of the pH values. The pH values near the third-second ring in east are greater than the west, which are smaller near the first ring. Compared with other functional areas in Xi'an, the alkaline is the strongest in the gas station dust, which is likely to the multi-source of the composition and hardened cement paved roads.(2)The mean value of low frequency magnetic susceptibility of the sampling sites dusts is 441.8×10-8m3/kg, and the high frequency magnetic susceptibility is 424.6×10-8m3/k. The frequency magnetic susceptibility with the mean value 4.05% changes from 2.44% to 5.63%, which indicates that the environmental pollution sources mainly come from vehicle emissions, exhaust and non-exhaust emission.The correlation analysis between magnetic susceptibility and heavy metal shows that the correlation coefficient between the low magnetic susceptibility and frequency magnetic susceptibility is significant negative correlation, as so as the high frequency magnetic susceptibility and frequency magnetic susceptibility. The magnetic susceptibility correlates significant positively with Cr and Cu. What's more, it correlates with Mn weak positively. Frequency magnetic susceptibility correlates with Mn, Cr and Cu weak negatively, respectively. And there is a significantly negative correlation between Frequency magnetic susceptibility and Zn.(3) Rough sand particle content accounted for 48.45 percent, and powder clay content 51.55 percent in Xi'an gas station dusts. The averaged particle sizes of the gas station dusts in Xi'an range from38.40 to 269.65?m, and the distribution is characterized by better kurtosis, better symmetry and worse sorting. The analysis of particle size of Xi'an gas station dusts been silty mild clay-mealy sand indicates a bimodal distribution spectrum, which shows that the gas station dusts in Xi'an are a mixture of coarse particles derived from local area and fine particles transported over a long distance by aeolian agent.(4) The mean concentrations of Mn, Ni, Co, Cr, Cu, Cd, Pb and Zn of the gas station dusts in Xi'an are 0.69,1.15,8.13,1.19,3.94,15.62,4.42,5.09 times of the background values of the researched elements in Shaanxi soil, respectively. The spatial distributions of heavy metal content in Xi'an gas station dust show that the contents of metals in the south are higher than in the north. In addition, the concentrations of the gas station dust metals in the west are higher than the east, particularly in the southwest.(5) Three main sources of heavy metals are identified. Mn and Ni mainly originate from a natural source, Cr, Cu, Pb, Cr and Zn mainly originate from traffic source and Co mainly comes from industrial source.(6) The results of the potential ecological risk index show that Mn, Ni, Cr, Cu, Pb and Zn are all low ecological risk. Co is moderate and Cd is the very high ecological risk. Generally speaking, the potential ecological risk index of the heavy metals is 564.87, which indicates that the heavy metal pollution in the gas stations of Xi'an reaches considerable ecological risk.(7) The results of human health risk assessment of heavy metals show that the non-carcinogenic risk to children is higher than adults. HI of the heavy metals are less than the safety threshold value 1, which suggested that non-carcinogenic risk to the gas station dusts in Xi'an is light.The carcinogenic exposure dose and risk to adults are higher than children. The carcinogenic risks of heavy metals are lower than the threshold values (10-6?10-4), which indicates that heavy metals in the gas station dusts of Xi'an have no carcinogenic risks.