Spatial structure and distribution characteristics of soil heavy metals in wastewater irrigation district

Based on geostatistical methods and analysis module of ArcGIS, this paper discussed applicable conditions of Geostatistical interpolation for evaluating soil heavy metal pollution. Taken Bei Yechang wastewater irrigation district as research subject, the spatial distribution characteristics and global trends of heavy metals were analyzed in order to propose appropriate geostatistical interpolation models. The results showed that average heavy meal contents of As, Hg, Cd, Cr, Cu, Pb, Zn and Ni in Bei Yechang wastewater irrigation district were lower than the first class limit values of soil environment quality standard, and increased by 11.5%, 207.7%, 18.7%, 25%, 39.2% and 21.3% compared with soil background values. The contents of heavy metals varied from -4.6% to 10.4% compared with the same research in 1978. The spatial coefficient of variation varies from 12% to 37% with no significant spatial variation. By the histogram module of ArcGIS geostatistical analyst tool, Hg in soil was normally distributed; Cd approximately closed to the normal distributed by log transformation, while the contents of As, Cr, Cu, Pb, Zn, Ni were not normal distributed. Ordinal kriging interpolation model and Ordinal kriging interpolation model with log transformation were proper for the spatial interpolation for the content of Hg, Cd in Bei Yechang wastewater irrigation district, and universal kriging interpolation model were proper for the spatial interpolation for the content of Zn, Ni, while contents of the other facts in Bei Yechang wastewater irrigation district should choose the disjunctive kriging interpolation model. The nugget effects of Pb and Cd were 0.90 and 0.87 with weak spatial variation, respectively, indicating Pb and Cd were influenced by artificial random factors. The nugget effect values of As, Cr, Zn and Cu in Bei Yechang wastewater irrigation district were 0.52, 0.51, 0.51 and 0.46 with medium spatial variation, respectively, indicating As, Cr, Zn and Cu were influenced by the combined effects of artificial random factors and spatial structure. The nugget effect values of Hg and Ni were 0.253 and 0.06 with higher spatial correlation, respectively, indicating Hg and Ni were influenced by in structural factors and geology of the native. In the Bei Yechang wastewater irrigation district, compared with background values the heavy metal contents increased. Heavy metal contents decreased from north to south with irrigation guarantee rate decreasing along the main canal. The results indicated that long-term wastewater irrigation can lead to soil heavy metal enrichment. Compared with other irrigation districts in China, the pollution level of Bei Yechang wastewater irrigation district was slighter and wastewater should be treated to prevent soil heavy metal pollution.