Spatio-temporal distributions of soil heavy metals in vegetable soil in Nanjing, China

Recent years, China's urbanization process is taking place at a very high rate. The acceleration of urbanization created wealth and opportunities for society, but it also intensified environmental problems, especially soil pollution as some studies reported. But how did urbanization process affect the soil pollution? Our study concentrated on heavy metals contents variation due to urbanization in vegetable soil. With spatio-temporal variation of soil heavy metals contents in vegetable soil during the course of urbanizations in Nanjing, the capital of Jiangsu province, as a case for study, laws and causes of the spatio-temporal variation were analyzed. The levels of Cu, Zn, Pb, Cd and Hg in vegetable soil samples were detected. The transverse and vertical spatio-temporal variation of soil heavy metals were analyzed on the base of field investigations and laboratory analysis. The conclusions showed that: (1) In vegetable soil, the contents of heavy metals went down gradually from urban to rural area; the contents of main heavy metals in urban are significantly higher than suburban and rural area; (2)The mean values of heavy metal contents, which were calculated by the top-layer (0-15cm) subtracting the sub-layer (15-30cm), are all up zero and large in absolute value in urban area, but in suburban and rural area, the means are all up or down zero and small in absolute value. (3) And the contents of Zn and Pb in 1999 were significantly higher than those in 1985. The causes of the spatio-temporal variation were analyzed as follows: One was associated with mellowness of the soil and soil cultivated history; the other with population density and industrials intensity decreasing along the urban-rural gradient, i.e. the urbanization level can explain the distribution of soil heavy metals to some extent.