| Over accumulation of heavy metals in soil will adversely affect the original ecological balance, alter soil components, structure or function, lower the yield and quality of the agricultural products, and pollute the air and waters as secondary pollution source. Recent research shows that heavy metal (cadmium for example) enrichment zones which extend for hundred kilometers are present along with the plain area from the Tuotuo River, origin of Yangzi River, to the sea entrance.In this enrichment zone, heavy metal contents in a considerable proportion of agricultural product exceed the standard amount. However, there is no further research currently in causes for heavy metal enrichment zone formation and little sense of the potential ecological risks. In this study, we sample in surface and underlying soil layers along the Yangzi River within territory of Southest China Anhui province, and study distribution features of contents in arsenic (As), lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd) and mercury (Hg) with purpose of revealing formation causes and regularities of element transfer or transformation and evaluating potential econogical risks. The area is also typical to discuss formation causes of enrichment in treace elements along the River.(1) The spatial distribution of Cd, As, Pb, Zn and Hg along Yangzi River in Anhui province is determined by the natural factors, whileas variation in parent material and soil formation circumstances is a guiding factor to determine the special distribution of soil polluting elements. The Fluvo-Aquic soil of this area is featured with higher pH value, high content of Ca, Mg, Zn, Cd and Hg but lower content of Si.(2) In the study area, the As contents in surface soil layer are lower than that in parent layer, showing that the transformation of As is significant. But the contents of Cd, Pb, Cu, Zn, and Hg are higher in surface layer because of the accumulation of clay mineral and organic matter.(3) The multivariate statistical analysis method is applied to identify relationship between the microelement and macroelement, which is efficient in distinguishing the enrichment of microelements for natural causes.(4) We estimate the background values of 6 soil microelements calculated on the Geometric mean plus the twifold Geometric deviation, which is effective to determine the soil background values in large area and therefore further to distinguish effects of natural or human factors based on distribution abnormity in research area.(5) Mineral formation leads to abnormity of microelements distribution in local area, which pollution spread fastened by mining or smelting has caused abnormal accumulation of microelements (As, Pb, Cu and Zn) in parent materials. The pollution of paddy soil around the mining area is caused by irrigation with polluted water.(6) In addition to mineralization and mining or smelting, the close relationship of Cd abnormal distribution with water systems indicates presence of other industrial pollution sources in the research area.(7) The accumulation of Hg is rarely affected by mineralization, but determined by the sedimentation of air from the population-intensive cities of medium or large size. The effect of prevailing wind direction is significant to the distribution of Hg.(8) As shown in analysis of microelement configuration, Cd in sampled soils primarily exists in form of Exchangeable fraction and is subject to polluting soil ecology. Despite weak enrichment, Pb, existing in form of exchangeable and Acido-soluble fraction, is more risky ecologically. The ecological risks of Hg in the form of Humate-bound and Organic-bound fraction, shall also be highlighted.In this study, through high intensive surface soil sampling and analysis of typical area, we, based on multiple technological means to make comprehensive analysis, which, for the first time, studies the distribution features of treace elements in areas along the Yangtze River, reveals the causes for enrichments and regularities of transfer or transformationm, and assesses potential ecological risks. We use the multivariate statistical analysis method to analyze the combination relationship between the microelement and macroelement, which is efficient in distinguishing the source of pollution. Selection of a proper method is done for determining background distribution of treace elements in a large scale, and further highlighting human-caused distribution abnormity, which provides another way to analyze causes for enrichment of soil microelements. A comprehensive evaluation of pollution ecological effects, based on abnormal distribution features, configuration analysis and other results of analog tests, has revealed in term of mechanism, a problem in respect of agricultural products safety in the research area. |
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