Northern Suburbs Of Xi 'an Sewage Irrigation Pollution Characteristics And Assessment Of Heavy Metal Element

In this paper, six sampling points of the crop and vegetable land are selected in the Xitangzhai, Zhongchazhai and Dongchazhai of northern suburb of Xi’an. On one hand, the pH value, soil particle composition is tested. On the other hand, the concentration of the main heavy metal elements as As, Co, Cr, Cu, Mn, Ni, Pb and Zn is tested by XRF and the speciations of these heavy metals are measured by atomic absorption spectrophotometry. The heavy metal migration depth and enrichment layer are analyzed in this paper, and the main influencing factors of the distribution of heavy metal speciations are also tested. Moreover, the scientific assessment of soil environment is analyzed. The main conclusion and recognition as follows:1. The pH value of sewage irrigation area is about8.5, so the soil is alkaline. The section’s pH value of0-0.1meters depth is lower than the deeper layer. The main compositions of soil sections are clay and silt, and the silt content of wastewater irrigation of vegetable land is higher than that of crop land, but it is lower than the rinsing irrigation of crop land. The major elements’concentrations of soil section have little variation.2. The heavy metal contents of crop land of Zhongchazhai are lower than that of crop land of Dongchazhai and Xitangzhai. The heavy metal contents of crop land of Dongchazhai are the highest. The heavy metal contents of vegetable land of Zhongchazhai are smaller than that of Dozngchazhai and Xitangzhai, except As, Pb and Zn. Besides Pb and Zn, the heavy metal contents of vegetable land of Xitangzhai are apparently higher than that of Xichazhai and Dongchazhai. Because the heavy metal concentrations in the inrrigation water source of Zhongchazhai are low except Pb and Zn. The heavy metal concentrations in the wastewater irrigation of vegetable land are higher than that of crop land, because the wastewater irrigation of vegetable land is frequently irrigated by sewage.3. The migration depth and enrichment layer of the heavy metals are different, and they are decided by the kind of heavy metal and the soil type. The migration depth of As in the sewage irrigation area is deeper than that in the rinsing irrigated area. Besides in the Xitangzhai, the enrichment of Co decreases with the increase of section depth. The enrichment of Cr is obvious above the depth of0.5meters, and Cu is obvious above upper part of section.While the enrichment of Mn is very obvious in the whole section, and the migration depth of Ni in rinsing irrigated area is lower than sewage irrigation area. The enrichment of Pb is obvious above the depth of0.6meters. And the enrichment of Zn in different types of soil mainly appears above0.5meters except the vegetable land of Dongchazhai.4. The main speciation of Mn and Pb is reducible fraction, while the main speciation of Co, Cr, Cu, Ni and Zn is residual fraction. The content of speciations of Co and Ni is in the order of decreasing as follows:weak acid soluble fraction<reducible fraction<oxidizable fraction<residual fraction, and Cr, Cu, Zn is in the order of decreasing as follows:weak acid soluble fraction<oxidizable fraction<reducible fraction<residual fraction. The value of coefficient of variation of residual fraction is smaller than that of the others. This result shows that the percentage of the second-phase fraction in heavy metals is easily distracted by outside environment. The percentage of the second-phase fraction in heavy metals is in the order of decreasing as follows:Pb> Mn>Zn>Cr>Ni>Cu>Co. Judging by percentage of the second-phase fraction in heavy metals, Pb, Mn, Zn have the highest bioavailability and transfer ability in all heavy metals, while Cr, Cu, Ni and Co is lower bioavailability.The speciations of heavy metals are affected by the content of heavy metal, MnO and Fe2O3, pH, colloidal particle, clay, silt and sand. The total concentration of heavy metals is the main effect factor of its form distribution, follows are the pH, and Fe2O3content is the main factor of weak Acid soluble fraction of Co, Pb, Mn and reducible fraction of Cu, clay content is the main factor of weak Acid soluble fraction distribution, and MnO content is the main factor of oxidizable fraction of Pb.5. Using the principal component analysis, the first principal factor is Cr, Cu, Ni, Pb and the second is Co. Two principal factors accounted for83.7%of total samples. The results of composite score of principal components show that the composite score of principal components is decreasing in the order of as follows:the crop land of Zhongchazhai<the crop land of Dongchazhai<the crop land of Xitangzhai<the vegetable land of Zhongchazhai<the vegetable land of Dongchazhai<the vegetable land of Xitangzhai. The soil environmental quality of the crop land of Zhongchazhai is the best, and the vegetable land of Xitangzhai is the worst. From analyzing the land use patterns, the soil environmental quality of the vegetable land is worse than that of the crop land. This result is because the vegetable land is irrigated by more sewage. 6. The level of heavy metal pollution is assessed by the combination assessment of potential ecological risk index and pollution load index and the combination assessment method of the single factor index and the Nemerow index. The analysis results show that the ecological environment is under the condition of middle ecological risk. If the soil is irrigated by sewage, the ecological environment will contaminate easily.