Distribution characteristics of soil heavy metal content in northern slope of Tianshan Mountains and its source explanation

Pedosphere constitutes an important part of the earth surface and is the base for most lives. Sustaining better quality soil is a prerequisite for healthy ecosystem. In recent decades, however, soil conditions have been heavily affected by rapid agricultural and industry activities. Heavy metal soil contamination and pollution from enhanced human activity have become a major threat to the health of regional ecosystems. As a result, research on heavy metal contamination of soils (e.g., discriminating the sources and evaluating environmental risks of heavy metals) has become a hot-field in geography, pedo-science and environmental science. Located in central Xinjiang Uygur Autonomous Region with relatively plenty rainfall, Tianshan Mountains is an important water source and ecological barrier in China. Variations in ecological and environmental conditions in the mountains have exerted an obvious pressure on the oases in the lower reaches. In August 2011, a field investigation was carried out in order to gain a comprehensive insight into soil heavy metal contamination conditions of Tianshan Mountains. A total of 153 soil samples were collected from three sections - the eastern (Balikun-Yiwu), middle (Xinyuan-Bayinbuluke) and western (Zhaosu-Tekesi)-in that northern slop of Tianshan Mountains. The samples were sent to the laboratory in the Institute of Ecology and Geography, Chinese Academy of Sciences for analysis for heavy metals. Five heavy metals (Cr, Cu, Ni, Pb and Zn) were analyzed using standard methods. Using single factor pollution index and comprehensive pollution index, quantitative comparison was done for soil heavy metal pollution in different sections with the soil background value in Xinjiang and the secondary standard of soil environmental quality (GB15618-1995). Moreover, the distribution features and pollution sources of soil heavy metals in the different sections were analyzed and illustrated using principal component analysis. The results showed that soil heavy metal content and the related single factor pollution index and comprehensive pollution index for the eastern section of the northern slope of Tianshan Mountains (Balikun-Yiwu) were higher than those in the middle (Xinyuan-Bayinbuluke) and western sections (Zhaosu-Tekesi). The soil heavy metal pollution level in the middle section was higher than that in the western section. Heavy metal pollution in the eastern section of the northern slope (Balikun-Yiwu) was caused mainly by human factors such as traffic, industry, agriculture, tourism. Although human factors (traffic and industry) had some effects, soil heavy metal pollution in the middle section (Xinyuan-Bayinbuluke) was mainly from soil parent materials. In the western section (Zhaosu-Tekesi), soil parent materials were main resources of soil heavy pollution with some effects from agriculture.