Study On GIS-Based Middle-Scaled Heavy Metal Spatial Interpolation Of Farmland Soil And The Pollution Appraisal

Heavy metal pulltion of farmland is in relation to the attended problems such as food safety and human health. In this thesis, the spatial interpolation method was mainly used to analyse the heavy metal content in the farmland soil samples came from Nantong set with the random sampling method.In the second chapter of the thesis, the sampling method was described in detail. And the interpolation techniques in common use was introduced in the third chapter, as well as the basic theory of spatial interpolation, the geostatistics. Two kinds of spatial interpolation technique were used in this thesis: the IDW( Inverse distance weighted) and Ordinary Kriging. And we choose two models of Ordinary Kriging, one is the Spherical and the other is Exponential model. Thses three model were particularly tested in the fifth chapter to compare with each other the effect of the agreement of Nantong sample datum.In the forth chapter 5 heavy metal factors'original datum was explored thoroughly. And we found the 5 heavy metal's content in the soil was different from each other. As, Cr, and Pb has a wider scale of datum with a considerable discrete degree. Just the opposite the Cd and Hg's content in soil is much lower than the others. The scale of data is small with little discrete degree. And the distributing of each heavy metal's data disagree with normal distribution 。 It was proved in the fifth chapter, whether the datum's structure will be a infulence to the model agreement's precision. In order to make the system error be the least which bringed by the random sampling and something in clued in the soil structure, the thesis tested the IDW, Ordinary Kriging Spherical model and Ordinary Kriging Exponential model, in the parameters grads. And the agreement results were cross-validated. The Std.error, significance probability and the agree index were calculated after the cross validation. The Nugget was also considered with those index when we decide which is the best model with the proper parameter to each of the 5 heavy metal factors.Apply the test-decided model, interpolate to As, Cd, Cr, Hg, Pb in the Arcgis software. The results were output by the images, so the spatial distribution of each heavy metal's content can be displayed directly. Also the content's isoline can show the distribution tendence of each heavy metal in Nantong. Combined with the spatial interpolate techniques the thesis appraised the heavy metal pollution of Nantong farmland soil, both in the single factor and the multi-factors way. The single factorappraisal choose the standard pollution index method. In consideration of the importance of farmland soil polltion the thesis select the semi-variance as the multi-factors appraisal method. In this appraisal method the weighted heavy metal factor's content will be given the bigger power in any one sampling point.