The Application of Heavy Metal Distribution and Pb Isotopic Ratios to Determine the Pollution Source of Surface Soils in Mashhad Parks

Anthropogenic activities have a high impact on urban areas and lead to severe pollution in large cities. Urban soils are huge ‘basins’ for accumulation of various pollutants. Thus, they also serve as informative media. Therefore, the study of urban soils has developed in recent years. Mashhad is the second metropolitan city in Iran which has more than 3 million residences and over 20 million annual pilgrimage tourists. This city is surrounded by numerous factories and the high growth of urbanization has led to accelerated air pollution. In this research study, new analytical data for heavy metals (Cd, Co, Cr, Cu, Ni, Pb, Sn and Zn) in soil of parks in Mashhad are presented and they are compared with unpolluted non-urban soils. Then, the origin of soil pollution in park soils is discussed by statistical methods and Pb isotope values.

The main aim of this work is to focus on the urban areas of Mashhad and study park soil geochemistry. Twenty three parks were selected for this purpose. At each selected park, top soils (5-20 cm) were collected. In addition to park samples, 4 soil samples were collected from areas which were far from the urban areas and they were considered to be unpolluted. The concentration of heavy metals was defined by ICP-MS at the accredited Activation Laboratories (Actlabs.), Canada. To determine available and bioavailable fractions of Co, Cr and Ni, soil samples were analyzed by the DTPA method (Lindsay and Norvell, 1978). Pearson correlation coefficients and PCA are used to investigate elemental associations and extract latent factors for analyzing relationships among the observed variables. In this regard, these statistical analyses of park soils data were done with the SPSS 16.0 software package for Windows. The geochemical maps were plotted by a geographical information system (GIS) using ArcMap v.10.0 (ArcGIS) to show the overall spatial distribution patterns of total and available heavy metal concentrations. The Pb isotopes were measured in all samples by a ThermoFinnigan Neptune MC-ICP-MS instrument. Isotopic measurements were performed by the procedure proposed by Álvarez-Iglesias et al.’s (2012).

Park soil samples have a higher concentration of heavy metals than non-urban soils. PCA analysis and Pearson correlation coefficient indicate that anthropogenic sources are the main factors controlling the Cd, Pb, Sn and Zn concentrations (PC1 component) in park soils. In other words, both anthropogenic and natural sources are responsible for Cr, Co and Ni (PC2 component) distribution in these soils. The spatial variation of heavy metals in the soil of parks in Mashhad confirm the statistical results and PC1 and PC2 heavy metals show different spatial variations. Furthermore, there is clear correlation between metals distributions, so that central parks generally have a higher content of all heavy metals which could be related to the heavy traffic and many traffic jams and air pollution in these areas. The lead isotopic ratios of the sample studied show that park soils show a distinct composition from non-urban soils. These samples are less radiogenic than non-urban soils with lower 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 206Pb/207Pb and higher 208Pb/206Pb ratios. The wide range of isotope ratios in park soils indicate that the Pb content in these soils is produced by the combination of different sources including natural and anthropogenic origins and that it has also been accumulating over time because of the enormous use of Pb in fuel, industrial activities, etc. (Galušková et al., 2014). The low 206Pb/204Pb values in park soils confirm a possible anthropogenic origin from the use of fossil fuels. The three-end-member model was used to determine possible Pb sources in the soil of parks in Mashhad. The average contribution was 6.6% and 93.4% for natural and anthropogenic (industrial and leaded petrol) sources, respectively. The detailed investigation of Pb isotope and Pb content of soil of parks suggests that industrial source is the main origin of samples with relatively low Pb content (<90 mg kg-1). The contribution of leaded petrol increases in the samples with high Pb. These soils were sampled from the central parks of Mashhad which have high traffic intensity.

The concentrations of potentially toxic elements in the soil of parks in the city of Mashhad are highly enriched relative to non-urban soils. The Pb isotope composition of non-urban soils indicate that they have a natural source while park soil samples have originated from anthropogenic sources. The soils which are sampled from the central parks of Mashhad have shown the highest heavy metal pollution due to high traffic congestion in these areas.