County-scale Spatial-temporal Variations And Source Identification Of Heavy Metals In The Cropland Soils

This research investigated the spatial-temporal variations and their source identification of five heavy metals (Cd, Pb, Cu, Cr and As) in the cropland soils of Changxing County, Zhejiang Province.136 topsoils (0-20 cm) and 126 subsoils (20-40 cm) were collected to analyze concentrations of the five heavy metals and to investigate the county-scale spatial-temporal distribution characteristics of the heavy metals and identify their pollution sources. The results showed that:(1) The descriptive analyses of concentrations of soil heavy metals showed that the mean concentrations of the heavy metals in the topsoils were higher than those in the subsoils. The mean concentrations of Cd and As reached 0.23 mg kg-1 and 10.88 mg kg-1 in the topsoils, respectively. Cd and As concentrations in 17.65% and 2.94% of the topsoil samples exceeded the Type Ⅱ limits (pH<6.5, paddy field; Cd≤0.3 mg kg-1 and As≤30 mg kg-1) set by the China Environmental Quality Standards for Soils, indicating apparent accumulation of the two heavy metals in the soils. In the subsoils, Cd and As also showed a certain level of accumulation, with 11.90% and 0.79% of the soil samples exceeding the Type Ⅱ limits. Concentrations of Pb, Cu and Cr were lower than the corresponding Type Ⅱ limits (Pb≤250 mg kg-1, Cr≤250 mg kg-1 and Cu≤50 mg kg-1).(2) Enrichment factor (EF) analysis indicated that the degrees of accumulation for the five heavy metals in the topsoils decreased in the order of Cd>As>Cr>Pb>Cu; and in the subsoils Cd>As>Cr>Cu>Pb based on the average EFs of the five heavy metals. The ranges of EFs for Cd and As in the soils were larger with more outliers, indicating the likelihood of Cd and As anthropogenic inputs in local areas. Pollution risk evaluation for soil heavy metals indicated that the values for Nemerow comprehensive contamination index in the topsoils and subsoils were 1.299 and 1.129, respectively, showing moderate pollution level and the topsoils were more polluted than the subsoils.(3) The five heavy metals showed their distinctive spatial distribution characteristics in the cropland soils of the county. Except for As, the other four heavy metals were characterized by higher concentrations in northwest and southeast of the county. The highest As concentration was observed in Xiaopu Town and it decreased radially. As concentrations in some parts of Lijiaxiang and Hongqiao Towns were relatively high. Overall, the five heavy metals had similar spatial distribution pattern in the top-and sub-soils. Changes in the heavy metal concentrations in an interval of one decade between 2003 and 2013 indicated that there were no significant variations for Cd, Pb, Cu and Cr in 80%-90% areas in Changxing County, except for As. Concentrations for Cd, Pb, Cr and Cu in the topsoils decreased in some parts of the county. Pb and Cr concentrations in some areas of Jiapu and Hongqiao Towns increased, while Cd concentrations increased in much smaller areas. As concentrations in the topsoils showed an obvious increase, ranging from 10 to 25 mg kg"1 in some areas in Xiaopu and Huaikan Towns over the decade.(4) In the past decade, five local industries such as lead-acid battery, building material and printing and dyeing expanded rapidly in Changxing County. Different industries strongly affected the spatial distributions of various heavy metals. The spatial distributions of soil Cd and Pb were mainly controlled by lead-acid battery and building material industries, while soil Cu was affected by machine manufacturing and building material industries.To a large extent the spatial distribution of soil Cr was affected by building material and fire resistant material industries. However, the local industries made little contribution to soil As. The spatial distribution of soil As was more likely to associate with agricultural practices such as excessive and unreasonable application of As-containing pesticides, chemical and organic fertilizers. Changxing County has twice enforced strict environmental regulations in 2004 and in 2011 in order to restructure and upgrade the traditional industries led by lead-acid battery industries. The regulations have played a positive role in preventing severer contamination of heavy metals over the decade. Nevertheless, increase in heavy metals concentrations in the soils near the key industrial zones should not be overlooked.(5) The range of 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb in 19 soil samples were 18.095-18.675,15.558-15.656 and 38.414-38.907, respectively. The range of 206Pb/207Pb wag 1.163-1.197, which was much smaller than the value of 206Pb/207 for natural Pb (1.2), showing a certain level of anthropogenic Pb input in the soils. The analysis indicated that Pb isotopic composition in the soils did not fit the binary mixing model. Based on the ternary plot of 206Pb/204Pb-207Pb/204Pb-208Pb/204Pb and end-member components, it was found that Pb in the soil samples largely was associated with industrial wastes, coal burning and soil background Pb. Pb isotopic composition fitted the ternary mixing model of aforementioned three end-member components well.The findings from this research provide theoretical bases and technical supports for sustainable land use, management and protection and they are of guiding significance for pollution prevention and risk management for soil heavy metals.