Spatial Variability Of Heavy Metal In Cropland Soils At County Scale

Geostatistics combined with GIS were used in this study for the analysis of the spatial variability and influencing factors of Copper (Cu), Zinc (Zn) and Chromium (Cr) of cropland soil in Shuangliu county Sichuan province and the sampling numbers, pollution status, risk assessment and the prediction of the exceeding possibilities were thoroughly analyzed.The conclusions were as follows:(1) The concentrations of Copper (Cu), Zinc (Zn) and Lead(Pb) in the study area were affected by the exogenous factors, and very obviously accumulated with wide distribution range. The concentrations of Cu, Zn and Cr in soil had moderate spatial correlation, and their spatial variability were caused by structural factors and random factors;but the concentration of Pb had weak spatial correlation,and it was mainly caused by random factors. In anisotropic conditions,the spatial correlation of these four metals had increased to a certain extent. The spatial distribution trends of concentrations of Cu, Zn and Cr in soil were quite similar,high in the northwest and low in the southeast,and it had a highly significant positive correlation.The distribution of Pb concentration was not obvious and the high-value areas distributed in different topographic conditions.The cross-validation showed the high interpolation precision of Cu, Zn and Cr concentration and low interpolation precision of Pb,which was mainly caused by the spatial correlation.(2) Compared with conventional random sampling method, co-kriging method ensured the spatial correlation after the samples reduction. In the same sample volumes, prediction accuracy of the co-kriging method was higher than that of ordinary kriging. Under the condition that the initial samples were reduced by 10%,the prediction accuracy of Cu and Cr of co-kriging was still higher than that of ordinary kriging interpolation of the initial 623 samples, and the spatial distribution map of the two methods was highly similar.(3) According to classification limits of soil environmental quality from" Farmland environmental quality evaluation standards for edible agricultural products ", the single pollution index of sample Cu and Zn mainly belonged to the clean level,while the single and multi pollution index of Pb just belonged to the clean level.The area statistics showed that the pollution in the study area was not serious, many of which were still the clean level. Among them, the area which exceeded the single pollution index of Pb concentration was comparatively large; Potential ecological risk assessment indicated that the potential ecological risk of Cu, Zn and Pb was small, and at a slightly ecological risk level. The probability that the concentration of the three kinds of heavy metals content exceeded the critical value was low;meanwhile the probability range that the Pb concentration exceeded the critical value was mostly centralized, and the probability distribution regularity that the Cu and Zn concentration exceeded the critical value was relatively excessive while that of Pb was discrete.(4) The variation orders of soil Cu, Zn and Cr concentration under the three kinds of geomorphologic types and parent materials demonstrated consistently, respectively, but with different impact. The geomorphologic types were an important factor affecting Cu concentration, while the parent materials had a deep impact on the Cu and Zn concentration; the application of organic fertilizer had a profound impact on the spatial variability of soil Cu, Zn and Cr; in the comprehensive agricultural conditions, the basic conditions of agriculture had a big impact on the spatial variability of soil Cu, Zn and Cr, whereas the spatial variability of Pb concentration was mainly affected by the application dosage of pesticide and the scale of transport; buffer analysis showed that when the distance from the road increased,the Pb concentration first increased, then decreased.